CA1290021C - Trunked radio repeater system - Google Patents

Abstract

TRUNKED RADIO REPEATER SYSTEM

ABSTRACT
A digitally trunked radio repeater system provides substantial improvements in timeliness of channel acquisition and channel drop, and in reliability of critical control signalling. The system uses a much higher digital signalling rate than is typically found in prior art systems, and uses a control channel to convey digital channel request and assignment messages between the central site and mobile transceivers. The mobile radio transceivers transmit channel requests on the control channel (if no response is received, the mobile retries during a retry time window which increases in duration in dependence on the number of retries). The mobile transceiver switches to a working channel in response to an assignment message received on the control channel. Subaudible digital signals transmitted on the control channel and on active working channels allow late entry, shifting to higher priority calls, and other advanced functions. Message and transmission trunking capabilities are both present so as to maximize working channel usage without compromising channel access for high priority communications. During transmission trunking, called and calling transceivers return to the control channel after each transmission (and called transceivers may be inhibited from transmitting) but grant higher priority to calls from the other transceivers being communicated with to ensure continuity over an entire conversation. Additional functions and fault tolerant features further increase the versatility and reliability of the system.

Description

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TRUNKED RADIO REPEATER SYSTEM

This invention is generally directed to the art of trunked radio repeater systems. It is more particularly directed to such systems using digital control signals transmitted over a dedicated control channel while also using plural working channels which are assigned temporarily for use by individual radio units.

The trunking of radio repeaters is well-known.
Early trunking systems used analog control signals while some more recent systems have utilized digital control signals. Control signals have been utilized on a dedicated control channel and/or on different ones of the working channels for various different reasons and effects. A non-exhaustive but somewhat representative sampling of prior art publications and patents describing typical prior art trunked radio repeater sy~tems is identified below:

U.S. Paten~ No. 3,292,178 - Magnuski (1966) ~ .

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U.S. Patent No. 3,458,664 - Adlhoch et al (1969) U.S. Patent No. 3,571,519 - Tsimbidis (1971) U.S. Patent No. 3,696,210 - Peterson et al (1972 U.S Patent No. 3,906,166 - Cooper et al (19753 U.S. Patent No. 3,936,616 - DiGianfilippo (1976) U.S. Patent No 3,970,801 - Ross et al (1976) U.S. Patent No. 4,001,693 - Stackhouse etal (1977) U.S. Patent No. 4,010,327 - Kobrinetz et al (1977) U.S. Patent No. 4,012,597 - Lynk, Jr. et al (1977) U.S. Patent No. 4,022,973 - Stackhouse etal (1977) U.S. Patent No. 4,027,243 - Stackhouse etal (1977) U.S. Patent No. 4~029,901 - Campbell (1977) U.S. Patent No. 4,128,740 - Graziano (1978) U.S. Patent No. 4,131,849 - Freeburg et al (1978) U.S. Patent No. 4,184,118 - Cannalte et al (1980) U.S. Patent No. 4,231,114 - Dolikian (1980) U.S. Patent No. 4,309,772 - Kloker et al (1982) U.S. Patent No. ~,312,070 - Coombes et al (1982) U.S. Patent No. 4,312,074 - Pautle et al (1982) U.S. Patent No. 4,326,264 - Cohen et al (1982~
U.S. Patent No. 4,339,823 - Predina et al (1982) U.S. Patent No. 4,347,625 - Williams (1982) U.S. Patent No. 4,360,927 - Bowen et al (1982) U.S. P~tent No. 4,400,585 - Kamen et al (1982) U.S. Patent No. 4,409,687 - Berti et al (1983) U.S. Patent No. 4,430,742 - Milleker et al (1984) U.S. Patent No. 4,430,755 - Nadir et al (1984) U.S. Patent No. 4,433,256 - Dolikian (1984) U.S. Patent No. 4,450,573 - Noble (1984) U.S. Patent No. 4,485,486 - Webb et al (1984) U.S. Patent No. 4,578,815 - Persinotti (1985) 1~900~

Bowen et al is one example of prior art switched channel repeater systems which avoid using a dedicated control channel -- in part by providing a handshake with the repeater site controller on a seized "idle" working channel before communication with the called unit(s) is permitted to proceed.

There are many actual and potèntial applications for trunked radio repeater systems. However, one of the more important applications is for public service trunked (PST) systems. For example, one metropolitan area may advantageously utilize a single system of trunked radio repeaters to provide efficient radio communications between individual radio units within many different agencies. Each agency may, in turn, achieve efficient communication between individual units of different fleets or sub-units (e.g., the police department may have a need to provide efficient communications between diferent units of its squad car force, different portable units assigned to foot patrolmen, different units of detectives or narcotics agents and the like).
Sometimes it may be important to communicate simultaneously to predefined groups of units (e.g., all units, all the squad cars, all of the foot patrolmen, etc.~. At the same time, other agencies (e.g., the fire department, the transportation department, the water department, the emergency/rescue services, etc.) may be in need of similar communication services. As is well-known to those familiar with trunking theory, a relatively small number of radio repeaters can efficiently service all 12~10~
~5MR 0056 of these needs within a given geographic area if they are trunked (i.e., shared on an "as-needed" basis between all potential units).

This invention also is especially adapted for special mobile radio (SMR) trunked users. Here, an entrepreneur may provade a trunked radio repeater system at one or more sites within a given geographic area and then sell air time to many different independent businesses or other entities having the need to provide effici~nt radio communication between individual units of their particular organization. In many respects, the requirements of an SMR user are similar to those of a PST user.

In fact, the potential advantages of trunked radio repeater systems for public services is so well recognized that an orqanization known as the Association of Public-Safety Communications Officers, Inc. (formerly the Association of Police Communications Officers) (APCO) has developed a set of highly desirable features for such a system commonly known a~ the "APC0-16 Requirements." A
complete listing and explanation of such requirements may be found in available publications known to those in the art.

One of the APC0-16 requirements is that any user must have voice channel access within one-half second after engaging a push-to-talk (PTT) switch. This same requirement must especially be met in emergency situations -- and that implies that the ~o~

system must be able to actively drop lower priority users also within a very short time frame. And, of course, the ability to quickly and efficiently drop channel assignments as soon as channel usage is terminated is also important for efficient usage of the trunked facility even in non-emergency situations.

Prior trunked radio systems have attempted to more or less "just meet" such APC0-16 requirements of timeliness. For example, publi3hed specifications of one such prior system indicates an ability to achieve channel update (in a 19 channel system) within 450 milliseconds and channel drops within 500 milliseconds. To achieve this, it utilizes 3,600 bits per second (bps) digital signalling over a dedicated digital control channel. Unfortunately, although theoretically the APC0-16 requirements of timeliness should be met by such a prior system, in reality, the APC0-16 timeliness re~uirements are often not met --or, are met only at the expense of suffering with the obviously adverse effects of somewhat unreliable digital control signalling (which are, at best, annoying even in non-emergency situations).
Accordingly, there is considerable room for improvement.

The present invention provides substantial improvements -- both in timeliness and in reliability of critical control signalling in a digitally trunked radio system of this general type. To begin with, a much higher digital signalling rate (9600 bps) is 9(3~

utilized. However, rather than using all of the increased signalling rate to provide a 9600/3600 =

2.67 improvement factor in timeliness, a large portion of the increased signalling rate capacity is utilized to improve signalling reliability. Accordingly, the increased timeliness of 19 channel updating capa~ility, for example, is improved by a factor of approximately 1.58 (e.g., 285 milliseconds versus 450 milliseconds) while the rest of the increased signalling capacity is utilized to increase the reliability of control signalling. At the same time, virtually all of the increased signalling capacity is utilized to improve the timeliness of channel drop ability (e.g., 190 milliseconds versus 500 milliseconds~.

~ s previously demonstrated by Bell System Technical Journal articles on the AMPS system (e.g.
"Voice and Data Transmission", by Arredondo et al, The Bell S~stem Technical Journal, Vol. 58, No. l, January 1979, pp 97 - 122), digital data rates on radio channels should be either very low (e.g., 200 hert7) or as high as the channel bandwidth permits. The present invention utilizes the maximum high speed data rates (e.g., 9600 bps on the typical 25 KHz bandwidth radio channel~ for critical control channel signalling and control signalling on the working channels both immediately before and immediately after the user communication interval. In addition, subaudible low-speed digital data is also utilized on the ~orking channel during user communications so as to assure additional signalling reliability -- and ~9~
45MR ~056 to also permit implementation of additional features.
B~rsts of high-speed data are occasionally inserted into the subaudible low-speed digital data stream transmitted on the working channels to further increase signalling capacity.

In the exemplary embodiment, all channels (the control as well as working channels) are fully duplexed so that there may be simultaneous i~bound and outbound signalling on all channels In general, this invention achieves reliable and prompt communication within a trunked ~-adio repeater system having a digital control channel and plural working channels, which working ~hannels are assigned for temporary use by individual radio units as specified by digital control signals on the control channel.

Channel assignment is initially requested by a calling radio unit passing digital request signals to a control site over the active control channel. In accordance with channel availability, a controller at the central site assigns a specific then-available working channel to the requested communication and passes digital assignment signals outbound over the control channel. Both the calling radio unit and the called unit(s) detect the ~orking channel assignment and switch their transmitter and receiver operations over to the proper working channel. Thereafter, digital handshake signals are again exchanged between the control site and at least one of the radio units (e.g., the calling unit) over ~he assigned working channel. In response to a successful handshake on the ~2~

assigned working channel, the central site then transmits digital release signals over the assigned working channel so as to release the appropriate units for communication thereover.

As one technique for increasing reliability, the initial request signals may include three-fold data redundancy (at least for critical data) while the channel assignment signals subsequently transmitted over the control channel may include as much as six fold redundancy of data ~e.g., at least of critical data such as that representing the called party and the assigned channel). The handshake signals subsequently exchanged on the assigned working channel may also include three-fold data redundancy of at least critical data. In this manner, some of the increased signalling capacity made available by the high-speed data rate ~e.g., 9600 bps) is sacrificed in favor of more reliable channel allocation and communication functions -- while still comfortably exceeding all APC0-16 requirements.

To insure responsiveness to higher priority calls, subaudible digital channel assignment update messages are also transmitted over the assigned working channel. These are monitored in each unit then residing on the working channels. Accordingly, if a higher priority call is directed to some unit already engaged in a communication, that unit is enabled to promptly switch operations to a new assigned working channel so as to immediately receive the higher priority call.

~z9~
45MR 005~0 In addition, to accommodate late entry of called parties to an ongoing communication, digital channel assignment "late entry" messages continue to be transmitted on the control channel even after a succe~sful channel assignment process has been effected 80 that late entrants (e.g., tho~e just turning on their radio, jU8t passing out of a tunnel or from behind a building or otherwise back into radio communication after temporary interruption, completion of a higher or equal priority call, etc.) may nevertheless be ~witched onto the proper assigned working channel as soon a8 possible. (The late entry ~eature, er se, is related to co~monly assigned U.S. Pa~ent No. 4,649,567 issued March 10, 1987).

To effect prompt and reliable termination of channel assignments, when the PTT switch of a calling unit is released, it sends a digital unkeyed mes&age on the assigned working channel and, in response to reception of this unkeyed message at the control ~ite, a digital drop signal i 9 transmitted over the assigned working channel 50 as to immediately drop all units therefrom and thus free that working channel for reassig~ment. (As will be appreciated, a given radio unit will automatically revert to monitoring the control channel upon dropping from an assigned working channel.) The ~ystem of this invention is sometimes termed a "digitally" trunked system because trunking control is effected by digital signals passed over a L2~
45~R 00560 continuously dedicated ''control" data channel. All units are programmed so as to automatically revert to the predetermined primary control channel upon being turned on or being res~t. If the expected control chann~l data format i~ not there discovered, then alternate pos~ible control cha~nels are succe~sively monitored in a predetermined equence un$il an active control channel i 8 di~covered. In thi 8 mann~r, the usual contro1 channel apparatus at the central control site may b~ temporarily removed from service (e.g., for maintenance purpose~). This same feature also permits continued trunked 8y8tem operation in the ~v~nt that the regular control channel unexpectedly m~lfunction~ or i8 otherwise taksn out of service.

The exemplary embodiment of thi8 invention i8 de~igned in such a way that it meets and, in many areas, surpasse~ of the existing APC0-16 r2quirements. It may also support available voice encryption techniques, mobile digital data ~erminals (digital data may be passed in lieu of an~log voice data ~uring a trunked radio communication se~sion) and/or available automatic vehicular lo~ation systems. Preferably, a fault toler~n~ architecture is used [see related co~monly aseigned Canadi n Application Serial No. 5S6,663 ~iled May 12, 1988]
so as to maintain trunked system operation even if the central processor happens to fail at ~he control site. If digital da~a is to be communicated between radio units and/or the central site, then it may be processed through the system in a manner similar to analog voice signals.

~ ~ 9 ~ 45MR 00~0 ~ 1 1 -In particular, such digital data communication will be carried out at a rate accommodated within the existing audio pass band and will be trunked just like desired voice communications (i.e., no dedicated digital data communication channels are required). To help increase reliability of digital data communications, data transmissions (and analog voice transmissions, as well) may be voted in voting systems employing satellite receivers connected to the central control site.

In the exemplary embodiment, digital control signalling messages of the following types are utilized:

Channel TYPe Direction Rate Control Channel INBOUND 9600 pbs -Group Call -Special Call : -Status -Status Request/Page ~ -Emergency Alert/Group Call ; -Individual Call -Cancel Dynamic Regroup -Dynamic Regroup - Forced Select -Dynamic Regroup - Option Deselect -Dynamic Regroup - Option Select -Login/Dynamic Regroup Acknowledge -Logical ID Request -Programming Request ~9~0~ 45MR 00~6~

OUTBOUND9600 bps -Channel Assignment -Channel Update -Enable/Disable Unit -Dynamic Regroup -Preconfiguration -Alias I D
-Unit Keyed/Unkeyed -Emergency Channel Assignment -Cancel Dynamic Regroup Dynamic Regroup - Forced Select -Dynamic Regroup - Optional Select -Dynamic Regroup - Optional Select -Assign Group ID An Alias ID
-Assign Logical ID An Alias ID
-Status Acknowledge/Page -Time Mark -Emergency Channel Update -Site ID
-System Operationa Made -Site Status -Logical ID Assignment : Programming Channel Assignment ; Working Channel INBOUND 9600 bps -Initial Handshake -Special Call Signalling : -Unit ID-PTT and Reverse PTT
-Miscellaneous OUTBOUND 9600 bps 1;~9~

-Initial handshake -Channel Drop -Status Messages INBOUND Low Speed -Confirm Unit PTT
-OUTBOUND Low Speed -Priority Scan -Falsing Prevention Some of the general features of the exemplary embodiment and expected benefits are summarized below:

FEATURE BENEFIT

VERY S~ORT AYERAGE Practically instantaneous CHANNEL ACCESS TIME access doesn't cut off syllables.
Normal Signal Strength Provides operation which Areas: 280 Milliseconds is faster than most coded Weak Signal Areas (12dB squelch systems.
Sinad: 500 Milliseconds LATE ENTRY Minimizes missed Should a mobile turn on conversations, keeps during the period in which police up to the minute, its group is involved in minimizes call backs.
a conversation, the mobile will automatically be directed to join that conversation.

AUTOMATIC CHANNEL SWITCHING Frequency coordination 12~1 45~R 00560 Mobiles, portables, control of the fleets and stations, and consoles subfleets requires automatically switch to no action on the part of the appropriate channel. any field personnel.
-HIGH SPEED CALL PROCESSING Dedicated control channel Proc~ssor assigns unit provides more rapid initiating the call, and channel assignments on all called units, to an laxger systems. System appropriate working channel. size does not impact Initial channel assignment upon channel aGqUiSitiOn communication between site time. Control channel is controller and radio units available for additional occurs on the control functions such as status channel. and unit ID.

CALL RETRY Eliminates the need for Calling unit will repetitive PTT operations automatically repeat its by the operator in weak request up to eight times signal situations.
if no response is received. Terminating retries also Retries terminated upon shortens the signalling system response. time.

UNIT DISABLE In hostage situations, Trunked units can be these units can be disabled on an individual assigned to a special basis. These disabled units group for communication continue to monitor the with the criminals.
control channel and can be Also, with automatic polled to determine their vehicular location, status. these units could be ...

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tracked for apprehension.

SUPPLEMENTARY CONTROL Provides user and system CH~NNEL FUNCTIONALITY operator with system In addition to providing manager features not channel assignments, the available on other type~
control channel is used of systems.
for: status messages, polling, system status, logging, late entry, dynamic regrouping, system testing and other system functions.

GROUP PRIVACY Each group has the same Each group hears only his privacy as having their own group, unless own channel with the specifically programmed additional benefits of otherwise. Dispatcher being regrouped with can override for individual other complementing units or groups of mobiles functions for emergency at any time. operations.

CALL QUEUING Maintains orderly entry When all channels are procedure for busy busy, call requests will system. Call requests be queued until a channel are accepted in the order becomes available. Unit they are r~ceived except requesting a channel will higher priority users go be notified to prevent to the head of the line.
call backs. Members of groups already in the queue ~2~0~
45~R 00560 will not be reentered in the queue.

DATA COMMUNICATIONS This feature greatly System ha~ the optional enhance~ the value of the capability of using 9600 ~y~tem becau~e it avoids bps data on the working the expense of additional channel~. Data RF cha~nel~.
commu~ic~tions will taXe placa on any equipped working channel and they are trunked, ~ust as volce communication.

VOIC~ ENCRYPTION Voice encryption offers Sy~tem has the optional the ~ame en~rypted range capability o~ u3~ng a~ ~or clear voice available 9600 baud voice transmission~. Voice can encryption. ~See, e.g. be pas~ed from each site commonly a~signed US through conventional Patent No. 4,622,680, voice grade phone lines U~S. Patent No. or microwav~ links. Only 4,817,146 and U.S. minor modlfication~ are Patent No. 4,757,536.) needed to the base skation interface eguipment to accommodate such sophi~ticated voice ~ecurity system~.
Any mobile can be upgraded to thi~
sy~tem. Any mobile can be upgraded to thi 8 ~29(~
45MR ~0560 capability by adding an external module. No - internal changes to the radio are required.

UNIT IDENTIFICATION Each unit on each All units are automatically transmission is identified when they identified by the same transmit. This is true ID regardless of the regardless of whether the agency, fleet or transmission takes place subfleet in which he is on the control channel currently operating.
or on a working channel. 16,384 is many times The system is able to more than the logical accommodate 16,384 discreet number of users in a addresses independent of fully loaded twenty fleet and subfleet. channel system so there is more than sufficient capacity.

TRAFFIC LOGGING Statistics on system All system information is usage, such as peak logged. Each unit trans- loading, individual and mission causes the units group usage versus time, ID, agency, fleet, sub- as well as many other fleet, channel, time, site system parameters, are and list of sites involved available for tabulation to be logged for manage- and analysis.
ment reports.

TELEPHONE INTERCONNECT All mobiles and portables Authorized mobiles have in the system can be ~oo~
45~R 00560 the ability to place and interconnected to the receive calls and patch telephone system. Those them to individuals or mobiles not specifically groups of mobiles which equipped for this can be may or may not ~e equipped patched by their dis-for telephone interconnect. patcher to maintain adequate control of system loading façtors.

GROUP PRIORITY ASSIGNMENT System manager can set Eight priority levels are individual group provided in the system. priorities according to Each group ~as well as the criticality of their each individual) is service.
assigned a priority The flow of traffic is more easily maintained by providing recent users priority over nonrecent users of the same priority level.

SUBAUDIBLE SIGNALLING/PRIORITY SCAN
; Subaudible signalling on Mobiles and portables working channels in use can be immediately updates called and calling directed to higher units with recent channel priority assignments -- allowing communications while them to change to a working involved in a lower channel carrying higher priority communications priority communications. on another working High-speed data bursts channel.
embedded in the subaudible -` ~L2~0~

signalling or ensure rapid receipt of priority information.

In the exemplary system, 11 bits are available to determine the address of a unit within an agency, fleet or subfleet. Fourteen bits are available to determine the individual identity of a particular unit (i.e., its "logical ID"). The use of 11 bits for determining group addresses within an agency, fleet or subfleet provides great flexibility such that each agency may have many different fleet and subfleet structures. Furthermore, unit identification is not limited by a particular fleet and subfleet structure. The 16, 384 unit identification codes can be divided among the subfleets in a manner that best suits a particular system.

Some features of this exemplary system which are believed to be particularly unique and advantageous are summarized briefly below (order of appearance not reflecting any order of importance -- nor is this list to be considered in any way exhaustive or limiting):

a) Widenina the Retry Window If a requested working channel assignment is not achieved, the request is automatically retried -- and the time window in which such a retry is attempted is increased in duration as a function of the number of prior unsuccessful retries. This significantly decreases the ~X9{)0Z~
45M~ 00560 average channel access time where noise is the real problem rather than request collisions --while still providing a recovery mechanism for request collision problems as well.

b) Better Use of Subaudible Siqnallinq Because the data rate of subaudible signalling on the working channels is low compared to the high-speed signalling occurring on the control (and working~ channels, subaudible messages may be substantially delayed relative to corresponding high-speed signals. For this reason, subaudible signalling may sometimes be unreliable in reporting or confirming events which occurred more than two seconds earlier -- and subaudible signalling may not effectively convey messages to mobile units rapidly enough for them to react to real-time events (e.g., a higher priority transmission trunked call on another working channel). In the exemplary embodiment, subaudible signalling reports most events ~e.g., channel assignments) only a limited number of times (e.g., twice) and for only a limited time period -to prevent outdated information from being reported. Additionally, windows are defined in the subaudible signalling stream which are reserved for bursts of hiqh-speed signalling. The high-speed signalling bursts assure that mobile units operating on working channels receive important information very \ ~290021 rapidly. The mobile units are programmed to temporarily mute during these high-speed bursts so that users are not disturbed by the noise which might otherwise be heard.
-c) Minimizinq PrioritY Communique Fraqmentations bY Dvnamically Alterina Scan ~unctions Ater initiating a priority call, a radiotemporarily (e.g., for two seconds) disables the usual multiple group scan on the control channel -- in favor of looking for the highly probable r~turned higher priority call. This reduces the possibility of getting diverted momentarily into an ongoing lower priority communique -- and also perhaps missing a fragment of the next higher priority communique. ~ similar temporary (e.g., two seconds) scan preference (except for priority calls) for a just-previously involved call group also helps prevent fragmentation of non-priority communiques.

d) Use of Transmission-Trunked Bit in Channel Assiqnment The trunking system has two trunking modes:

a) Transmission Trunked Mode in which the working channel is de-allocated as soon as ; the calling unit unkeys, and 45~R 00~60 b) Message Trunked Mode in which the wor~ing channel is de-allocated "n" seconds following a unit's unkeying, unless another unit keys onto the channel within such "n"
seconds. "n" is called the "hang time".

, By dynamically insuring that both called and calling units "know" that a transmission-trunking mode is in effect, the calling unit may immediately revert to the control channel upon PTT release -- thus immediately freeing the working channel for drop channel signalling from the control sike. The called units can also be positively prevented from ever transmitting on the working channel -thus avoiding multiple keying of radio units on the working channel.
.
e) Automatic Addressinq of Immediatelv Returned Calls Both the called and calling units/groups are identified in the initial channel assignment signalling. The called unit captures the calling unit ID and is enabled to automatically address a return call to the just calling radio if the PTT switch is depressed within a predetermined period (e.g., 5 seconds) after the just completed communique even if the system is in the Transmission Trunked Mode. Not only does this simplify the necessary call back procedures and minimize access times, by allowing greater 12~
45~R 0056 application of the Transmission Trunked Mode it also increases the probability of successful message exchanges -- especially in poor signalling areas f) 9600 bps Permits "Loose" SYnchronization Use of higher rate 9600 bps signalling permits simpliied bit synchronization to be rapidly achieved by simple "dotting" sequences (i.e., a string of alternating ones and zeros 101010...). Thus, there is no need to Xeep information transfers precisely synchronized across all channels. This not only reduces hardware requirements system-wide, it also facilitates a more fault tolerant architecture at the control site.

g) Improved Channel DroP Siqnallin~

The drop channel signalling is simply an extended dotting sequence. Therefore, each radio may easily simultaneouslv look for drop channel signalling and channel assignment conirmations.
This means that the control site may more immediately consider a given working channel available for reassignment -- and, if "loaded up," immediately interrupt the drop-channel signalling to issue fresh channel assignment confirmation signals on the working channel (which each individual radio will ignore unless properly addressed to it) As a result, a :

.~

-"~ 1290~2~ -4~R 00560 "loaded" system (i.e., one where existing channel requests are already queued) may drop a working channel within about only 100 msec -- and immediately reassign it to a queued request.
Radios that happen to enter late into the call being dropped can detect that fact and properly drop from the channel because of the ability to simultaneously look for drop channel signalling and channel assignment confirmation signalling.

h) Eeature Proaramminq To avoid cumbersome feature programming (and reprogramming to add features) by factory or distribution personnel, novel procedures are employed which safely permit the end user to perform all such "programming." All units are i programmed at the factory to perform all available functions. A function enable bit map and a unique physical ID are together encrypted at the factory and provided to the user as "Program Codes." When the user programs each device, its encrypted "Program Codes" are input to a Radio Programmer which, in turn, properly sets the feature enable bit map in a connected radio unit -- and the decoded physical ID --and a "Just Programmed" bit). The "just programmed" radio device logs into the central controller with a request for a logical ID --based on its apparent physical ID If illegal copying of function enabling Program Codes occurs, then the same logical ID will be 2900~
4~MR 00560 assigned -- and the usefulness of the radio within the trunked repeater system will be diminished.

i) Double Channel Assianment Handshake -- One Being on the Assiqned Workina Channel ..
A first 9600 bps channel assignment signalling exchange occurs on the control channel. ~owever, a confirmation (i.e., a second handshake~ then ocçurs on the assigned working channel. Thus, it is assured that the desired channel has been successfully assigned and locked onto before the central controller unmutes the called units on the assigned channel. To decrease system access time even further, the calling unit does not wait to receive and decode the confirmation on the assigned working channel before transmitting its key message (which is preceded by dotting). That is, the calling unit begins transmitting synchronization signals (dotting~ on the inbound working channel at the same time the called units are decoding the information message transmitted by the site on the outbound working channel. This reduces system access time by as m~ch as 40 ms, and reliability is not adversely affected because of redundancy in the fields of the channel assignment message on the control channel.

These as well as other objects and advantages o this invention will he more completely understood and t ~29~

appreciated by carefully studying the following detailed description of the presently preferred exemplary embodiment taken in conjunction with the accompanying drawings, of which:

FIGURE 1 is a general explanatory diagram o trunked radio repeater system in accordance with this invention;

FIGURE 2 is a simplified block diagram of a central control site (as well as a satellite receiver site) in the trunked repeater system of FIGURE 1;

FIGURE 3 is a simplified block diagram of the general site architecture of the main controller for the central control site;
`.
FIGURE 4 is a simplified block diagram of the channel architecture used within each channel of the central site architecture shown in EIGURE

3;

FIGURE 5 is a simplified general block diagram of a technical mobile/portable radio unit to be utilized for communication within the trunked repeater system of FIGURE l;

FIGURE 6 is a simplified flowchart of typical call processing sequence in the exemplary ; embodiment from the perspective of a calling unit;

29~02~
45~R 00560 FIGURE 7 is a simplified flowchart of a call processing sequence within a called unit;

FIGURE 8 illustrates the trunked channel dropping procedure and typical required drop times;

FIGURE 9 generally illustrates call initiation signalling within the exemplary system as well as typical timing requirements;

EIGURE 10 is a graphical illustration of the control signalling protocol utilized to initiate and terminate trunked r~dio communications on either an individual or group basis in the exemplary system;

FIGURE 11 is a simplified flowchart of suitable computer programs which might be utilized at the site controller, the calling unit and the called unit so as to achieve the ; signalling protocol of FIGURE 10; and FIGURE 12 is a graphical illustrati~n of subaudible control signalling protocol transmitted on active working channels in the exemplary system.

An exemplary trunked radio repeater system in accordance with this invention is generally depicted at FIGURE 1. As illustrated, individual units of various groups communicate with each other (both 0~
45~R 00560 within and possibly outside of their own group) via shared radio repeater channels located at a trunked repeater control site 100. The dispatch console 102 may be housed directly at the repeater station site 104 or may be remotely located via other communication facilities 106 as will be appreciated by those in the art. There also may be multiple dispatch consoles 102 ~e.g., one for each separate fleet) and a master or supervisory dispatch console ~or the entire system as will also be appreciated by those in the art.

The central site is depicted in somewhat more detail at FIGURE 2 in conjunction with one or more satellite receiver sites 100-1. As will be appreciated, the satellite receiver sites are displaced spatially from the central site 100 such that radio reception may temporarily be better at one or the other of the chosen antenna sites. Thus, received signals from the satellite sites as well as the central sites are combined in "voter" circuitry so as to ~hoose the best available signal for control or communication processes.

At the central site, a transmitting antenna 200 and receiving antenna 202 (which may sometimes be a common antenna structure) may be utilized with conventional signal combining/decombining circuits 204, 206 as will be apparent to those in the art. The transmitting and receiving RF antenna circuitry 200-206 thus individually services a plurality of duplex RF channel transmit/receive circuits included in a plurality of R~ repeater "stations" 300, 302, ;

129~

45~R 00560 304, 306, etc. Typically, there may be 20 ~uch stations. Each station transmitter and receiver circuitry i 6 typically controlled by a dedicated control ~helf C5 (e.g., a microprocessor-based csntrol circuit) as i B al80 generally depicted in FIGURE 2. Such control shelf logic circuits associated with each station are, in turn, controll~d by trunking card~ TC (e.g., further microproce~sor-based logic control circuits) 400, 402, 404 and 406. All of ~he trunking oards 400-406 commu~icate w~th sne ano~her a~d/or with a primary ~ite controller 410 via control data bus 412. The primary 8~ te contr~ller (and optional backup controller~ if d2si-ed~ may be a csmmerrially aYai~ e g~neral purps~e proce~80r (~.g. ~ a PDP
ll/73 pro~e~or w~th l8 MHz-Jll chip set). Al~hough-the major "intell~gence" a~d control capability for-~he ent~re 8y8tem resides within oontroller 410, alternate backup or "fail soft" control funotions ~ay.
al~o b~ incorporated within the tru~king cards 400-406 so as to provide conti~u~d trunked repeater service even in the event that controller 410 malfunction~ or i8 otherwi~ taken out of ~ervice.
(Nore detail on such fail soft features may be found in commonly assigned Canadian Application Serial No.
5~6,563 filed May 12, 1988 en~itled "~ail Soft Architecture For Public Trunking System.") An optional t~lephon~ in~erco~ect 414 m~y al~o be provided to the public switched telephone network.
Typically, a sy~tem ma~ager termin~l, printer, etc., 416 i~ al80 provided for overall system management and `- ~290~2~

control (together with one or more dispatcher consoles 102). A special test and alarming facility 418 may also be provided if desired.

The signal "voter" circuits 502, 504, 506 and 508 are connected so as~to receive a plurality of input digital or analog signals and to selectively output therefrom the strongest and/or otherwise most reliable one of the input signals. I'hus, received signals from the central site 100 are input to respective ones of the channel voter circuits 502-508 while additional similar input signals are generated from receivers in the satellite receiver site 100-1 and also input to the appropriate respective voter circuits. The results of the voting process are then passed back to the trunking card circuits 400-406 where they are further processed as the valid "received" signals.

A slightly more detailed view of the site architecture or control data communication is shown in FIGURE 3. Here, the PDP 11/73 controller 410 is seen to communicate over a 19.2 Kilobit link 412 with up to 25 trunking control cards TC
controlling respective duplex repeater circuits in those individual channels. Another high-speed 19.2 Kilobit link 420 is used to communicate with the hardware that supports the down link to~from the dispatch console 102. Other data communication with the central processor 410 is via 9600 baud links as shown in FIGURE 3. The central processor 410 may include, for example, a 128 Kilobyte code PROM, 1 Megabyte of RAM and 32 DHV-ll/J compatible RS-232C

~ X 9~ 0~ ~ 45~R 00560 ports. It may typically be programmed using Micropower Pascal to provide a multi-tasking, event-driven operating system to manage all of the various data communication ports on an acceptable real time basis.

At each controlled repeater channel, the 19.2 Kilobit data bus 412 (as well as that from an optional backup controller if desired) is monitored by an 8031 processor in the TC module. The TC
trunking control module exercises control over the control shelf CS of its associated repeater with audio, signalling, and control busses as depicted in FIGURE 4, and may typically also receive hard-wired inputs providing clock synchronization and a ~fail soft" indication (e.g., indicating that normal control by the central controller 410 is not available and that an alternate distributed control algorithm should then be implemented within each of the trunking control modules TC).

The general architecture of a suitable mobile/portable radio unit for use within the exemplary system is also microprocessor based as depicted in FIGURE 5. Here, microprocessor 550 is provided with suitable memory 552 and input/output circuits 554 so as to interface with the radio unit display, keypad, push-to-talk (PTT) switch as well as audio circuits 556 which provide basic analog audio outputs to the speaker and accept analog audio inputs from the microphone. Auxiliary control over a modem 558 as a digital interface (e.g., to voice 9o~

encryption, vehicle location or other types of digital communication subsystems) may also be provided if desired. And, of course, the I/0 circuits 554 also permit suitable programmed control over RF receiver 560 and transmitter 562 which, via conventional signal combiners 564 permit two-way f~lly duplexed communication over a common antenna 566 as will be appreciated by those in the art.

A detailed and in-depth description of all units and sub-units of such a sophisticated system would necessarily be e~tremely voluminous and complex. However, since those in this art are already generally familiar with digitally controlled trunked repeater systems with suitable RF transmitter and receiver circuits, programmed general purpose computer controllers, etc., no such exorbitantly detailed description is believed necessary. Instead, it would only serve to obscure the subject matter which constitutes the invention. Accordingly, the remainder of this description will concentrate on the signalling protocol utilized to initiate and terminate calls within the system since this is believed to constitute a significant improvement (both in reliability and speed) -- while still facilitating the retention of many highly desirable system features and meeting or exceeding all ~PC0-16 requirements.

Call placement begins by the calling unit transmitting a special digital channel request signal on the dedicated control channel to the central site.
In return, the central site transmits, outbound on the ~ lXg~o~
45~R 00560 control channel, a special digital channel assignment signal. The calling unit then responds by switching immediately to the assigned working channel. The central site sends an assignment confirmation message (also in high-speed digital form) over the assigned working channel to confirm to called units responding to the channel assignment signal that they are properly on the working channel (preventing a unit erroneously decoding a channel assignment message from unmuting for a working channel messagz not intended for it). Meanwhile, the calling unit does not wait for the confirmation message ~ut instead immediately begins transmitting a synchronization signal. The synchronization signal allows the central site to confirm the calling unit has properly switched to the assigned working channel, in response to ~hich the central site releases the called units to begin the requested communication session on the working channel (by transmitting a unit-keyed/unmute message in the exemplary embodiment). Alternatively, if during this process the calling unit receives a channel update message on the control channel addressed to it, then the channel request call is temporarily suspended (unless the channel request under way is an emergency or higher priority request) and the calling unit then reverts to the called state so as to receive the incoming call. If the calling unit receives no response (or an improperly completed response handshake sequence), it automatically waits a random period before retrying to successfully place the call request (up to a maximum of 8 tries).

~9(~

The called unit initially resides in a standby configuration where it continually monitors the digital messages appearing on the control channel outbound from the central site. If it detects a channel assignment message addressed to it as the called party (or perhaps as one party of a called group), then the called unit immediately switches its operations onto the assigned working channel. There, it also detects the confirmation signals outbound on the working channel from the central site and, if successfully confirmed, awaits a release or unsquelching signal on the working channel (e.g., transmitted from the central site in response to successful completion of a handshake with the calling unit on the working channel). The called unit may also receive a channel update message indicating that the group is already operating on the correct channel.

The programming for a calling unit is generally depicted in the simplified flowchart of FIGURE 6.
~ere, upon entry into the calling mode, a call request is sent on the control channel CC at step 600. A
test is made at 602 for call queuing. If queued, transfer is made to wait loop 603 (including a test for a detected assignment at step 604 followed by a check for expiration of a 30 second timer at 606 (whereupon control e~fectively is passed back to a manual requirement to restart the calling process via exit 607).

If the call request is not queued, then a test is made at 606 to see ii this partic~lar unit has already ., 9~0~1 previously been requested as a called party. If so, then transfer is made at 610 to the called mode of operation. If not, then a check for a returning channel assignment is made at 612. If not received at the expected time, then a random wait is interposed at 614 before a test i~ made at 616 to see if eight tries have yet been made to complete this particular call.
If so, then the subroutine is exited at 618. If not, then the subroutine is re-entered at 600.

If a channel assignment is successfully detected at either 612 or 604, then the unit operation is immediately switched to the assigned working channel at step 620. The calling unit transmits an elongated sequence of dotting at 624 followed by a transmission of voice at 626 (or data if a digital communication session has been requested) over the assigned working channel before exit from the subroutine is taken at 628 (e.g., to a standard monitoring routine which looks for release of the PTT switch and transmits an unkeyed signal at 627).

The protocol followed by a called unit is generally depicted at.FIGU~E 7 (e.g, representing a suitable computer program for controlling the unit in this mode of operation).

Upon entry, the control channel is simply monitored at 700 for any "good" message ~e.g., one addressed to this particular unit). If such a message is detected, a check is made at 702 for an "update"
type of ~essage. If the message is of this type, then 290~

a check is made at 704 to see if it is repeated within about 1.0 seconds. If not, then reentry into the called mode is made. However, if the update of a higher priority incoming call is repeated within such period, then an immediate switch to the there-assigned working channel is made at 706. If signalling i5 not confirmed at 707, ~hen an immediate switch to unsquelching (716) is made and that channel is thereafter monitored. If, on the other hand, signalling confirmation at 707 is achieved, it is an indication that normal channel assignment is, in reality, taking place and control is passed to block 714 to look for an unmute message.

If no channel update message is detected at 702, then the message is checked to see if it was a channel assignment at 708. If not, then return is made to the beginning of the subroutine. However, if a proper channel assignment has been received, then a switch to the assigned working channel is made at 710 and a check for proper confirmation signalling on the working channel is then made at 712. If a proper unmuting message is thereafter also received on that assigned working channel at 714, then the called unit unsquelches at 716. If no unmuting message is received at 714, then a check ~or a drop message is made at 718. If there is no drop message but high-speed signalling is still present on the working channel (as detected by 720), then a further check is made for the unmute message at 714. However, if there is no drop message and the high-speed signalling 9 0 0~ 1 45~ 0560 has ceased at 720, then the called unit is nevertheless unsquelched at 716.

At the conclusion of a desired audio call, the calling radio transmitter transmits a special release PTT signal as depicted graphically in FIGURE 8.
After a suitable transmission and detection delay period, the assigned working channel responds by transmitting a drop channel signal on the worXing channel. As shown in FIGURE 8, this results in a typical working channel availability in only 167 milliseconds after the release PTT signal is initiated.

Typical timing of calling protocol signals is depicted graphically in FIGURE 9 where it can be seen that typical calling protocol can be completed and communication begun over the desired working channel within about 245 milliseconds.

Some bit-level maps of some relevant message formats (and other related signalling formats and protocol) are graphically illustrated at FIGURE 10.
The control channel transmits an outbound continuous transmission repeating the format 800 depicted in FIGURE lO. As will be seen, each 40 bit message is transmitted three times (including one inverted transmission where all O's are changed to l's and vice versa) and there are two such messages transmitted per recurring message time "slot." As will be appreciated, the optional dotting prefix (if used) insures continued bit synchronization by 9~0~:~
45~R 00560 -3~3-receiving units and the unique Barker code permits frame synchronization so as to define bit boundaries between the 40 bit-level messages which follow.
Since the control channel transmits these message slots continuously, no dotting prefix is needed and but one transmission of the word framing Barker code will suffice for each recurrent transmission cycle.
Of course, if desired, a relatively short dotting prefix may be used to even further insure continued bit synchronization.

Inbound messages on the control channel CC are of the format 802 shown in FIGURE 10 and may comprise, for example, group/individual channel assignment requests transmitted from a calling unit. Here, the dotting prefix is considerably longer and the word framing Barker code is repeated three times so as to insure that the receiving circuits at the central site are properly synchronized before the 40 bit messages (again with three-fold redundancy) are transmitted.
Preferably, suitable transmission timing circuits are utilized so as to make such incoming control channel messages synchronously time "slotted" -- meaning that the messages on the inbound portion of the control channel occur during the same time slot as outgoing messages from the central site on the control channel (as generally indicated by dotted lines in FIGURE 10).

A group call request message format is shown in expanded scale in FIGURE 10. It includes a 3-bit message type (MT) code (the message-type field may be extended in a tree-logic fashion to include -` lX900~
45~R 0056 additional bits as will be appreciated). This MT-A field thus distinguishes a group call from an individual call, for example, and may also indicate the type of communication session being requested.
(If desired, a priority field of l-bit also ma~ be ~sed to indicate if-a highest priority emergency call is being requested.) The called identification code of 11 bits (representing either a group or an individual unit) is followed by a 14-bit field representing the identity of the calling unit ("logical ID"). The 40-bit message concludes with 12 bits of standard BCH code for error detection and correction purposes as will be appreciat~d.

The returning channel assignment message actually comprises a two message pair also having a format as shown in expanded scale at FIGURE lO. The first three bits identify the message type (MT) and the type of communication session which is to take place. The identity of the calling unit is next represented by an eight-digit field, with the eight most significant bits being transmitted in one message of the two-message pair and the eight least significant bits being transmitted in the other of such messages (this information can be used to prevent collisions of radios calling the same group and might otherwise transmit on top of each other on the working channel). The next 5-bit field identifies the assigned working channel. This 5-bit field is followed by a l-bit field indicating whether the call is transmission or message trunked -- so that the calling unit may determine this trunking status before ~9~o~
45~R 00560 returning to the assigned working channel The yroup or individual identity of the called unit(s) is contained within the next 11 bits followed by 12 bits of BCH error detection/correction code.

Once operation~reverts to the assigned working channel, the central site transmits a confirmation message of format 804 outbound on the working channel. As will be observed, it is of the same general form as the continuous transmissions on the control channel CC except that the message length has been reduced to 32 bits on the working channel.
Once again, the message is sent with three-fold redundancy (one being inverted~. Preferably, the confirmation message is timed in the working channel so as to be within the same time slot as messages being transmitted on the control channel. The format of the 32 bit confirmation message is also depicted in expanded Eorm at 806 in FIGURE 10. Here, 4 bits are devoted to the message type (MT) code. A succeeding single bit indicates whether the communication session is transmission trunked or message trunked. Another bit of the confirmation message 806 identifies the call as being either of a group or an individual unit while the identity of the called group or individual unit is contained within the following 14 bits. The confirmation message 806 concludes with 12 bits of BCH error detection/correction code.

As soon as the calling unit has successfully tuned to the assigned working channel, the calling unit begins transmitting 384 bits of dotting followed ~.~9~0~1 45MR 00~60 by audio (in the case of a requested audio communication session) as is also depicted at FIGURE
10 .

The elongated dotted sequence transmitted by the calling unit on the working channel constitutes an acknowledgment of the successfu] handshake sequence and, in response, the central site transmits an outbound digital message on the working channel to positively unmute the called unit(s). The format 808 of such an unmute message is depicted in FIGURE
10. Once again, the message type code uses the first four bits while the next bit is currently unused in the exemplary embodiment ~but instead may be used for other optional purposes). The next bit denotes trunked status or non-trunked status (e.g., regular hang-time). The identity of the unit(s) to be unmuted is set forth in the next 14 bits followed by 12 bits of standard BCH error detection/correction code.

At the conclusion of a communication session on the working channel, the calling unit again transmits 384 bits of dotting followed by 4 data blocks of 128 bits each. Each such data block includes 16 dotting bits and a 16 bit Barker code Isome of which bits may be "filler" as will be appreciated) as prefix followed by 8-bit bytes, each of which is transmitted with three-fold redundancy (one inverted) -- thus constituting a 32-bit message characteristic of digital messages being transmitted on the working channel. The format of the 32-bit unkey message ~ 9~C~

810 is also shown in ELGURE 10. Here, a 4-'oit message type code is followed by 2 bits for a block count (used to indicate repeats of the message in the data block). The identity of the calling unit is set forth in the ne~t 14 bits followed by 12 bits of standard BCH error detection/correction code.

Finally, in response to receipt of the unkey message at the central site on the working channel, an outbound digital message on -the working channel of a super-extended dotting sequence (e.g., 89~ to 2816 bits) is transmitted from the central site as depicted at 812 in FIGURE 10 -- and in response, all units then on that channel drop from that particular working channel and revert to the active control channel.

The sequence of programmed events occurring at the site controller, the calling unit and the called unit(s) during a typical call origination/termination sequence is depicted in the parallel flowcharts of FIGURE 11.

Each programmed unit has a quiescent control channel (CC) monitor routine where, in a quiescent state, all units and the site controller reside. When the calling unit enters the calling subroutine from the CC monitor at 1100, a test is made at 1102 to see if this calling attempt is a retry. If not, the retry counter is set to a maximum content of 8 at 1106 and then decremented by one at 1108 (which step is directly taken from test 1102 if a retry is in progress). If the retry counter has been decremented ~290~21 to zero as tested at 1110, then a failed acquisition audible beep is generated at 1112 and exit is taken back to the CC monitor. On the other hand, if the maximum number of retries have not yet been made, then a channel assignment request is transmitted on the control channel and slot synchronization at 111 (e.g., at time tl~.

Upon detecting an inbound message, the site controller will receive and store the channel aqsignment request and assign a free working channel at step 1200. In the exemplary system, a response to an inbound request may be supplied within a predetermined delay. The outbound channel assignment messages (i.e., a message pair) are transmitted on the control channel as soon as possible at step 1204 (time t2). The two message channel assignment pair is then received and stored from the control channel in the calling unit at step 1118 (the unit will look ~or the messages up to the maximum number of slots). As previously explained, if a channel update is received in the interim, then an exit may be taken to a called state (assuming that the call request under way is not an emergency). If a valid channel assignment message has not been received as tested at 1120 and the maximum number of slots have been observed, then a suitable delay is loaded at 1122 and exit is taken back to the CC monitor (from which a return entry to the calling subroutine will soon be taken).

; The process of loading in a suitable delay before retrying may be thought of as a progressive "widening"

~29~

of the retry window -- in a consciously controlled manner. There are three reasons why an inbound data message rom a radio would not get a response: (1) the inbound message was not successfully detected; (2~
the outbound message was not successfully detected; or (3) a collision occurred (two or more mobiles sent in a request on the same inbound control channel slot).

Given that a collision has occurred, unless mobiles randomly retransmit their requests, collisions will continue to occur. Consequently, when a radio fails to receive a response to an inbound message, it waits a "random" period of time to retransmit its request. However, if case (1) or (2) has occurred, there is really no reason to randomize the retry.
Unfortunately, the radio cannot determine the cause of a failed response.

But, the longer a mobile waits to retransmit, the longer the average access time becomes in poor signalling areas since that is where the majority of retries take place. Since often it is noise and not collisions that cause missed responses, randomizing retries is often wasteful.

To correct this problem, the present invention takes some corrective action. First, non-channel-acquisition messages are caused to have a much slower retry rate than channel request messages. Access time for the former is not critical (whereas it is for the latter). So, if a collision occurs between a radio sending in a non-channel 45~R 00560 request message and a radio sending in a channel request me~sage, the former's retry rate will be 510w enough ta guarantee no chance of a collision with the latter on the next retry.

Second, the random retry rate varies with the retry number. The retry algorithm (or channel acquisition messages only~ widens the width of the retry window with each succeeding retry. This decreases the average access time in the presence of noise but still provides a recovery mechanism should the cause for a missed response be a collision.

The preferred embodiment uses the following simple rule:

1st retry 2 slot random variability 2nd retry 4 slot random variability succeeding retries 8 slot random variability It also is possible to vary the retry window width as a function of the received bit error rate in order to gain still greater efficiency.

If a valid working channel assignment has been received as tested at 1120, then the calling unit switches immediately to the assigned working channel at 1124 and begins to transmit 384 bits of dotting on the working channel at 1132 followed by voice transmissions (or other desired communication session~
at 1134 Meanwhile, a confirmation message is transmitted on the working channel by the site ~900~
45~R 0056 controller at step 1206 at time t3. The calling unit need not decode the confirmation message in the exemplary embodiment before transmitting its key message (block 132) because much of the information obtained in the two messages of the channel assignment message pair is redundant -- and therefore the probability that the calling unit will "false" and go to an incorrect working channel is very small Back at the site controller, a check is made at 1208 for the acknowledgment dotting of extended duration on the working channel. If it is not received, then exit is taken. Ho~ever, if it is properly received, then two unit-keyed/unmute messages are transmitted outbound on the working channel at step 1210.

While all of the above has been taking place, the called unit has (if everything is workiny properly) received arld stored the two-message channel assignment pair from the control channel at time t2 (at substantially the same time as the calling unit) as depicted at 1300. In response, the called unit is also switched to the assigned working channel at 1302 and has thereafter monitored the assigned working channel for the proper confirmation thereon at step 1304 (and at time t3). Only if the proper confirmation message has been received does the called unit then look for and receive the unmute message transmitted from the site controller on the working channel at time tS and, in response, unmutes the 1 Z900~1 45~R 00560 receiver of the called unit on the working channel at 1306.

During the ensuing communication session on the assigned working channel between the calling and called unit(s), the site controller (via the TC's) continues to send subaudible new channel assignment (and drop) data to all units on all working channels at 1212 (thus enabling higher priority calls to be promptly received and accepted by all units). The site controller (via the proper TC) also continues to transmit channel update messages periodically on the control channel at 1214 (e.g., so as to permit late entrants to immediately go to the proper working channel). The site controller informs all TC's of the channel assignments and drops and, in response, each TC generates suitable subaudible signalling for its L channel.

All subaudible signalling occurs at a baud rate of 150 bits per second in the exemplary embodiment --equivalent to a 75 Hz tone when dotting is being sent. Whenever a unit is transmitting in clear voice on a working channel, it transmits dotting for subaudible signalling. The central site uses the absence of this dotting in conjunction with key and unkey high-speed inbound working channel signalling to determine if a radio is transmitting on the channel.

The entire time a working channel is "on the air," a continual stream of subaudible data frames are transmitted by the central site (via the 452~R 00560

-4~-appropriate TC) to inform all units on the channel of other channel assignments taking place on the system This mechanism is used to implement the priority scan feature -- as will be explained shortly. A background process executed by the site controller continually sends channel update messages (via the proper TC) in channel order (non-busy channels are excluded). This process is interrupted anytime a new channel assignment is made, and the new channel assignment is transmitted at high priority (similar to the way a channel assignment interrupts the channel update message seguence on the control channel).

In the exemplary embodiment, su~audible channel updates are discarded and no longer transmitted after being transmitted twice (once at priority, once in background). This prevents the subaudible signalling from reporting stale information that is no longer accurate (e.g., channel assignments for a transmission-trunked message that has already terminated) -- preventing units from switching to working channels that were but no longer are in use.
By transmitting channel assignments via subaudible signalling for only a limited period of time (e.g., two seconds in the exemplary embodiment), stale information is prevented from propagating over the worXing channels.

Due to the high speed at which events occur in the system, the subaudible signalling data rate may be too slow to report events in time for a desired ~L~9~3(3~

response to occur (e.g., move a called unit from one working channel to another in time to hear an emergency transmission). In the exemplary embodiment, this problem is overcome by providing pairs of bits in the subaudible siynalling stream that are not actually used but are instead set aside for the bursting of high speed (9600 baud) data. This bursting is performed in a manner which prevents the high-speed data from corrupting the low speed data.

The subaudible signalling frame has a duration of 0.693 seconds and includes an 8-bit synch string (10111011 in the exemplary embodiment) followed by 96 bits of duplicated and interleaved data (see FIGUR~ 12). The 96 bits of data actually consists of a 48-bit message in the exemplary embodiment --where every two bits are inverted and repeated (this maintains a steady DC point while effectively high pass filtering the signal for improved detection, and also increases the likelihood of successful decoding even when bit errors occur).

The use of the 48 bits is shown below:

hhssi ggggggggggg hhssi ggggggggggg hhuu bbbbbbbbbbbb Where:
h Bit space reserved for High-Speed Signalling (9600 baud~
s Spare Bit Idefined as 0) i Idle Frame Bit g Group Id Bits u Bit space reserved for High-Speed 452~R 00560 - Confirmation b BCH Code Bits Note: h and u bits are defined to be O for BCH
calculation.

Each frame contains two groups which are "on channel." If no groups are on channel, the frame is "idle" and has i set to 1 and the group field set to 7FF hex (all ones). If a single group is on channel, its id is placed in both slots of the frame.

Should an individual call or dynamic regroup take place, the high speed data will go out identically in all three high-speed slots of the subaudible frame.

Overflow from subaudible updates is transmitted via high speed signalling. Any of the three windows may be used. The message is not repeated (though all three windows may be used if subaudible is continuously overflowing).

For updating which group belongs on the channel, the special update high-speed slot is used in the first frame and in each eighth frame thereafter.

Note that the high-speed slot is located on even bit boundaries so that when the bits are duplicated and inverted as part of the overall subaudible transmission scheme, a 4-bit interval (26 milliseconds) is actually set aside for the high speed data.

1;~9~

The mobile units are preproyrammed to mute their audio during the 4~bit time high-speed slots to prevent high-speed signalling from generating noise in the audio heard by the user. In addition, the units are programmed to "force" these 2-bit values to preset values (O is--the exemplary embodiment) for purposes of error checking, and of course, to decode the high-speed signalling at the high speed rate when it comes in. The short high-speed signalling slots in the subaudible signalling frames do not interfere with background subaudible signalling and yet provide a mechanism by which the control site can very rapidly (e.g., at 9600 baud) transmit messages to units operating on working channels.

In existing systems subaudible signalling typically is used only as a validity check by 1 mobiles. When a mobile is on a working channel it monitors the subaudible signalling to make sure it belongs on that channel. One reason why a radio might get onto a channel where it does not belong is that while monitoring the control channel, it incorrectly decodes a message and goes to an incorrect channel.
;

There is a sufficiently high probability of incorrectly decoding outbound control messages on existing systems that a quick way to redirect radios ~rom channels where they do not belong is typically provided. To do this, subaudible signalling typically is used exclusively for this purpose.
However, with this invention, advantage is taken of the high information rate on the control channel, and ~9~3~
45r*R 005~0 a mobile is required to see an update message twice before going to a working channel. There is a negligible increase in the late entry time, but the probability of going to an incorrect channel is virtually eliminated. As a result, subaudible data can also be used for another purpose... e.g., a priority scan -- and high-speed data bursts provided in the subaudible data from in the exemplary embodiment ensure that priority information is received by affected units as soon as possible.

At the conclusion of the desired communications session, the unkeying of the PTT switch in t~e calling unit is detected at 1136 resulting in the sending of an unkeyed message on the working channel at 1138 Itime t6). If in a transmission-trunked mode, the calling unit may immediately revert to the control channel -- thus immediately freeing the working channel (the calling unit was informed that the call was transmission trunked during receipt of the channel assignment message and does not wait to receive a responsive unkey signal from the site controller before reverting to the control channel).
In response to receipt of the calling units' unkey message, at 1216, the site controller at 1218, sends a super elongated dotting string (896 to 2816 bits on the working channel at time t7). The called unit has, of course, also received the unkeyed message on the working channel at time t6 and, in response, has already muted the receiver at 1308. The called unit receives the super-elongated dotting string outbound ~rom the site controller on the working channel at 0~

time t7 and, in response, reverts to the control channel at 1310.

A special priority scan sequence is used (in the preferred embodiment) to minimize communique fragmentation.

When a radio unit scans for multiple groups and a call is made to its priority group, the radio automatically disables the multiple group scan (in favor of a priority group only scan) for a two second interval upon returning to the control channel. Since the priority group was just communicating, the probability is high that another communique will take place within this interval. If the radio immediately scanned into another (non-priority) group call (which by definit~.on is of a lower priority), and another communique then occurs on the priority group, the radio would hear a communique fragment from a non-priority group -- and would have its entry into the next priority group communique delayed (priority scan typically may take between 1.0 and l.S seconds to get the radio into the priority group).

Another unique feature used to minimize communique fragmentation is a preference priority the radio automatically assigns to a just-previously monitored non-priority group. In essence, if a non-priority communique is monitored, for two seconds following the communique the radio will ignore all other scanned calls (except to the priority group of course~...similarly to priority group communiques 0~
-45rIR 00560 -~4-In addition, the radio always remembers the last non-priority group monitored. Upon returning from a priority group communique, the radio will prefer the last non-priority group monitored over any other groups being scanned.

In the example below, a '--' means the group is involved in a communique channel, and it is assumed that Group A is the priority group and that its j communi~ues are separated by less than 2 seconds:
,~

Group A ---- ---- -- -----Group B ---------------------------------roup C -------------------------------------Radio monitors: BBBAAA AAAA AA AAAAA BBBB CCCC

There is a bit (i.e., the message/transmissiontrunked bit) in the working channel confirmation signalling that informs the radios as to whether the communique is transmission-trunked or message-trunked. This unique feature offers greater frequency efficiency.

The calling radio is guaranteed to see the message/transmission trunked bit when it receives the channel assignment message. If the bit is set to "Transmission Mode," the calling mobile knows the channel will be removed as soon as it stops ~Z90(~Zl 45~$R 00560 transmitting. Consequently, ~hen its PTT is released, the calling radio automatically and immediately goes back to the control channel. This gains channel usage efficiency because the working channel TC can begin channel drop signalling as soon as it detects the ca-lling mobile's unkey message.
That is, one does not have to extend the signalling to make sure the transmitting mobile finishes transmitting, gets its receiver on channel and then has plenty of time to be guaranteed that the channel drop signalling is detected in the calling mo~ile.

Radios that are called also look at this message/transmission trunked bit (in the channel assignment message and also in the working channel confirmation and unit keyed/unmute messages~, but for an entirely different reason. If the communique is message-trunked, radios that were called must be able to key on the assigned working channel in case they must offer a response before the channel drop.
However, if the communique is transmission-trunked, none of the called radios should ever transmit on the assigned working channel. Therefore, if the bit is set to "Transmission" mode, called radios will not be permitted to key on the working channel. This is a very useful feature since it prevents radios from keying on top of each other.

The message/transmission trunked bit thus offers three system advantages: It makes transmission trunking more frequency (i.e , channel) efficient by decreasing the channel drop time (by a factor of three 9~

from typical prior systems), it r~duces the dead time between transmission where users can not key (e.g., on typical existing systems, if a radio is keyed during the .5 second drop seqùence it must wait until the sequence is complete), and it offers absolute protection from radios keying on top of each other on a working channel.

To make an Individual Call on the exemplary system, the calling radio uses a single inbound slot of the control channel to identify itself and to specify the radio being called. Both radios are referred, via an outbound control channel message, to an availa~le working channel where the confirmation signalling takes place. The unmute message to the called radio (at the completion of the high speed confirmation signalling) also specifies the ID of the calling radio. The called radio automatically stores the ID of the calling radio and, if the PTT switch of the called radio is depressed within 5 seconds of the last PTT release of the calling station, will automatically place an individual call back to the original calling. This capability allows easy user-convenient transmission trunking, and therefor better frequency (i.e., channel) efficiency, during individual calls. It also allows the calling radio to contact a called radio and converse with no channel hang time even though the called radio is not previously programmed to initiate a call to the calling radio.

- 1~9~V~
4~DR 00560 The signalling in the exemplary embodiment is extremely efficient, minimizing the channel drop time and there~or increasing system efficiency. It is unique in that, for example, it is high speed signalling as opposed to low speed typically used on all other existing systems. In addition, the signalling is designed specifically for minimizing message traffic in a distributed architecture site.
\

Without such novel channel drop signalling, as the channel starts dropping, a message would have to be sent from the working channel TC to the control channel TC Ivia the site controller) stopping all updates on the outbound control channel (i.e., those that are referring radios to the working channel now being dropped). Once off the air, the channel TC
would have to send an additional message to the site controller informing it of such so it can reassign the dropped working channel when appropriate. Besides additional messages within the central site slowing the channel drop process, such prior techniques also incur additional loading on the site controller.
Another aspect of the problem is that the drop channel signalling that is transmitted on the working channel must be of sufficient duration to guarantee that timing ambiguities don't permit a radio to enter late onto the channel once it is down....or even worse to enter late onto the channel after the next call has already started to take place on that channel.

The exemplary embodiment uses unique drop channel signalling, a unique radio signalling detection ~9~

algorithm and timing of when the channel TC sends the drop channel message to the site controller.

By malcing the drop channel signalling 9600 bps dotting, not only can the drop channel signalling be detected ~nd muted in radios prior to the radio operator hearing the signalling, but the detection algorithm places a light enough processor loading on the radios that they can simultaneously look for ths dotting and for confirmation signalling.

The following rulPs are followed by a wor~ing channel TC as it drops:

1) Transmit 100 msec of dotting 2) Without interrupting the dotting, send a channel drop message to the site controller.
3) Transmit an additional 200 msec of dotting...BUT... stop it and start sending a confirmation message should a channel assignment message be received from the site controller.

The following rules are followed by the site controller when it receives a drop channel message from a given channel TC:

1) Immediately inform the control channel TC
so it can stop transmitting updates to the working channel TC.
2) Consider the channel immediately available for reassignment.

. ~ .
. .

lXg~z~

The following rules are followed by a radio as it leaves a working channel:

1) For 1/2 second ignore all channel updates to the group and channel of the communique being left.

The follo~ing rules are followed by a radio as it arrives on a working channel:

1) Look for dotting (i.e., of sufficiently long duration to constitute a drop channel signal) and confirmation signalling simultaneously.
2) If drop-channel dotting is seen, leave the channel.
3) If confirmation is seen then leave the channel if the ID is not correct, otherwise lock onto the signalling and do not unmute until told to do so.
4) If confirmation signalling stops~ or no signalling is seen on the channel, loo~ for suba~dible and unmute.
.
To understand the significance of the net effect of these procedures, consider two cases: (1) when the channel is nGt immediately reassigned and (2) when it is immediately reassigned.

It is only possible for the radio to enter late onto the drop channel signalling 100 msec following the point when the drop channel message was sent to -, ~x~
45~R ~0~60 the site controller. So if the channel is not being reassigned, a late entering radio will see the additional dotting being transmitted and will know to drop off the channel. On the other hand, if the system is loaded (e.g, call requests are queued in the site controller) the channel immediately gets assigned to the first group in the queue. A radio that attempts to enter late into the call just dropped will see the confirmation message with the group of the next call starting to take place and will know to drop off the channel.

The bottom line is that dropping a channel in a loaded system requires only 100 ~sec of signalling and only one message to the site controller. Radios that happen to enter late into the call being dropped detect that fact because of the radio's ability to look for the drop channel signalling and the confirmation signalling simultaneously.

For an increase in the radio price, the PST
radio manufacturer may program additional of these "features" into radios. The typical prior way to do this is to burn a unique PROM or an EEPROM at the factory. One disadvantage of this approach is the expense of uniquely programming each radio before it leaves the factory -- and it is inefficient to upgrade a radio should a customer subsequently desire additional features.

However, the exemplary embodiment permits one to eliminate factory programming costs. Since each ~29~ 2~

radio is programmed in the field (e.g., groups, systems, etc.) by the customer, features should be programmed into the radio at that time. The problem is how to control the programming task sufficiently to make sure the customer only programs purchased features.

Every shipment of radios that goes to a customer will have a sheet of paper which lists a set of Programming Codes and Physical IDs (one pair for each radio). Each Programming Code is an encryption of a "feature enable bitmap" and the Physical ID of the radio.

When a customer programs a radio he/she must do two things. First, he/she programs the radio. To do this, he/she selects the Programming Code representative of the features he/she has purchased for the radio and enters it in the Radio Programmer.
He/she then programs the radio using the Radio Programmer while the Programming Code prevents him/her from programming disabled features. Second, the user enters the radio onto the system database via the system manager. The radio's Physical ID must be specified in order to get the radio into the database.

Anytime the Radio Programmer writes data into a radio, it sets a "Just Programmed" bit inside the radio's personality. Whenever a radio is turned on, it checks that bit. If set, the radio will use its Physical ID to request a Logical ID from the site controller before allowing its user to communicate on ~900~

the trun~ed system. The site controller will go to the system manager data base to determine the Logical ID to be assigned to the radio. Notice that if a customer tries using the same Prograrnming ID for programming different radios he/she will end up with the same Logical ID in each radio which means unique identification capability is lost. This is the same consequence suffered if a customer copied the PROM
used in existing systems.

The result is that one has the same level of -protection, -- while avoiding the need to program radios in the factory. Adding features to a radio involves issuing an updated Programming ID, ambiguities in programming radios are eliminated (e.g., in existing systems a radio could be programmed to do something it was not enabled to do...so when a customer programs it and it doesn't work he/she cannot tell whether the radio is erroneously programmed or the feature is disabled), and no special software is written in the mobiles...just the Radio Programmer.
This last benafit is nice since fixing a software bug relative to feature enable/disable would involve changing code in just a few computers rather than for all radios in the field.

Detailed descriptions of the signalling protocols and formats involved in many different types of call origination sequences are summarized below:
f I. Radio Oriaination, Loqical ID Acquisition Sequence -~300~

A. The CC transmits a continuous stream of control messages which all inactive mobiles recei~e.
The messages are sent two messages to a 30-msec frame in the following frame format:

Dotting =~16 bits Barker = 16 bits (e.g., 11 bits Barker code plus 5 bits dotting preamble~
Dotting = 16 bits Message #1 = 40 bits Message #1 (inverted) = 40 bits Message #1 = 40 bits Message #2 = 40 bits Message #2 (inverted) = 40 bits Message ~2 = 40 bits B. When mobile power is turned on, it receives a site ID message from the Control Channel (CC)in the following format:

MT-A = 3 bits (e.g.,111) MT-B = 3 bits (e.g.,111) MT-D = 5 bits (e.g.,OlOdd) Channel = S bits Priority = 3 bits Homesite = 1 bit Failsoft = 2 bits Site ID = 6 bits BCH code = 12 bits Delay (dd in MT-D) specifies tne maximum number of control channel slots before a control -45~R 00~6 channel responds to an inbound transmission. Channel specifies the channel number for the active control channel. Priority prohibits mobiles with lower priority from transmitting on the inbound control channel. Home site bit specifies whether the site ID
is the home (=0) or adjacent (=1) ID.

C. If desired, and if priority allows, mobile optionally transmits a login request on the control channel in synchronism with the received control channel messages. The frame form is as follows:

Dotting = 152 bits Barker Code (repeated three times) = 48 bits (including filler) Message = 40 bits Message (inverted) = 40 bits Message = 40 bits The login message is coded as follows:

MT-A = 3 bits MT-B = 3 bits MT-C = 2 bits Physical ID = 20 bits Logical ID = 14 bits BCH Code = 12 bits T-C definition: 00 ==> Requesting programming 01 ==> Requesting Logical ID
10 ~=> Logging into the site (Logical ID) `` lX9002~
45~R 00560 11 ==> Undefined PURPOSE:

These commands allow a radio which roams into a system the ability to request programming information. They also allow a radio which is already programmed for a particular system to request a logical ID for temporary usage on the system.
Finally, if the need to track key radios between sites ever becomes necessary, the cc=10 message is used to notify the site that the radio has just moved to another site.

II. Radio Call Sequence - Radio Oriqination, Grou~
Call A. Control channel transmits a continuous stream of control messages which all inactive mobiles receive. The messages are sent two messages to a 30-msec frame that has the following format:
, Dotting = 16 bits Barker = 16 bits Dotting = 16 bits Message #1 = 40 bits Message #l (inverted~ = 40 bits Message #1 = 40 bits Message #2 = 40 bits Message ~2 (inverted~ = 40 bits Message #2 = 40 bits ~9~
45~R 00560 B. Mobile that wishes to originate a group call transmits a group channel assignment request on ~he control channel in synchronism with the received control channel messages. The frame format is as follows:

Dotting = 108 bits Barker (repeated three times) = 48 bits Message = 40 bits Message (inverted) = 40 bits Message = 40 bits The group call request message is coded as follows:

MT-A = 3 bits Group ID = 11 bits Logical ID = 14 bits ! BCH Code = 12 bits C. The control channel responds with a channel-assignment two-message pair. Coding is as follows:

MT-A Code = 3 bits 1/2 Logical ID = 8 MSBs or LSBs Channel = 5 bits Transmission Trunked = 1 bit Group ID = 11 bits BCH Code = 12 bits ~' . ;,:

~L29~
45~R 00560 D. All mobiles of the called group switch to assigned working channel and receive a confirmation message. Slotted working channel messages are transmitted using ~he following frame:

Dotting = 32 (16) bits Barker = 16 bits Message = 32 bits Message (inverted) ~ 32 bits Message = 32 bits The group call confirmation message is coded as follows:

MT Code = 4 bits Message/Transmission Trunking = 1 bit Group/Logical ID = 1 bit Group ID = 11 bits Logical ID = 14 bits BCH Code = 12 bits E. Originating mobile recei~es confirmation message and transmits 384 bits of dotting, then audio.

F. Working channel receives dotting and transmits two unit-keyed/unmute messages.

MT Code = 4 bits Undefined - 1 bit Message/Transmission Trunking = 1 bit Logical ID = 14 bits ~X9~0~1 BCH Code = 12 bits Called ~obile receives unmute message and unmutes audio.

G. Active mob-iles sn other working channels receive a subaudible channel assignment message.

H. Control channel transmits channel update message for late entry mobiles.

I. Transmitting mobile unkeys and sends a non-slotted unkey message. All non-slotted message formats are:

Dotting = 128 bits Data Block #3 = 128 bits Data Block #2 = 128 bits Data Block #1 = 128 bits Date Block #0 = 128 bits Data Block #3, #2, #1 and #0 are identical -- except for a two bit block count -- (each block is repeated four times) and each has the following format:

Dotting = 16 bits Barker Code = 16 bits Byte 1 = 8 bits Byte 1 (inverted) = 8 bits Byte 1 = 8 bits Byte 2 = 8 bits Byte 2 (in-~erted) = 8 bits 129~02~.
g5MR 0056 Byte 3 = 8 bits Byte 4 = 8 bits Byte 4 (inverted) = 8 bits Byte 4 = 8 bits The unkey message i~ coded as follows:

MT Code = 4 ~its Block Count = 2 bits Logical ID = 14 bits BCH Code = 12 bits .
J. Working channel transmits 896 to 2816 bits of dotting to drop all mobiles from the channel.

III. Radio Call Sequence-Radio Oriqination, Individual Call , A. Control transmits a continuous stream of control messages which all inactive mobiles receive.
The messages are sent two messages to a 30-msec.
frame that has the following format:

d Dotting = 16 bits Barker = 16 bits Dotting = 16 bits Message #1 = 40 bits Message #l (inverted) = ~0 bits L2900Z~

Message #1 = 40 bits Message #2 = 40 bits Message #2 (inverted) = 40 bits Message #2 = 40 bits B. Mobile that wishes to originate an individual call transmits an assignment re~uest on the control channel in synchronism with the received control channel messages. The frame format is as follows:

Dotting = 152 bits Barker (repeated three times) = 48 bits Message = 40 bits Message (inverted) = 40 bits Message = 40 bits The individual call request message is in two messages and is coded as follows:

(First Message) MT-A Code = 3 bits . MT-B = 3 bits : MT-D = 5 bits Priority = 3 bits Logical ID (caller) = 14 bits BCH Code = 12 bits (Second Message) MT-A Code = 3 bits MT-B Code = 3 bits MT-D Code = 5 bits ~9~21 MT-E Code = 3 bits Logical ID = 14 bits BCH Code = 12 bits C. The control channel responds with a channel-assignment two-message pair with the logical ID of the called unit sent in the first message and the logical ID or the caller unit sent in the second. Coding is as follows:

MT-A Code = 3 bits MT-B = 3 bits Undefined = 2 bits Channel = 5 bits ~ Qualifier = 1 bit ; Logical ID = 14 bits BCH Code = 12 bits D. Both calling ~last logical ID) and called mobile switch to assigned working channel and receive a confirmation message. Slotted working channel messages are transmitted using the following frame:

Dotting = 16 bits Barker = 16 bits Message = 32 bits Message (inverted) = 32 bits Message = 32 bits The individual call confirmation message is coded as follows:

~L290~1 MT Code = 4 bits Message/Transmission Trunking = 1 bit Group/Logical ID = 1 bit Group ID = 11 bits Logical ID = 14 bits BC~ Code = 12 bits E. Originating mobile receives confirmation message and transmits 384 bits of dotting, then audio.

F. Working channel receives dotting and transmits two unit-keyed/unmute messages:

MT Code = 4 bits Unused bit = 1 bit Message/Transmission Trunking = 1 bit Logical ID = 14 bits BCH Code = 12 bits Called mobile receives unmute message and unmutes audio.

G. Active mobiles on other working channels do not receive a subaudible channel assignment message.

H. Control channel transmits channel update message for late entry mobiles.
:
I. Transmitting mobile unkeys and sends a non-910tted unkey message. All non-slotted message formats are:

"` ~Z90~

Dotting = 128 bits Data #3 = 128 bits Date #2 = 128 bits Data #1 = 128 bits Data #0 = 128 bits Data #3, #2, #1, and #0 are identical (repeated four times) and each has the following format:

Dotting = 16 bits Barker = 16 bits Byte 1 = 8 bits Byte 1 (inverted) = 8 bits Byte 1 = 8 bits 8yte 2 = 8 bits Byte 2 (inverted) = 8 bits Byte 3 = 8 bits Byte 4 = 8 bits Byte 4 (inverted) = 8 bits Byte 4 = 8 bits The unkey message is coded as follows:

MT Code = 4 bits Block count = 2 bits Logical ID = 14 bits BCH Code = 12 bits ~o~
45MR ~0560 IV. Radio Call Sequence-Radio Origination, Emerqency Group Call A. Control channel transmits a continuous stream of control messages which all inactive mobiles receive. The messages are sent two messages to a 30-msec. frame that has the following format:

Dotting = 16 bits Barker = 16 bits Dotting = 16 bits Message #1 = 40 bits ; Message #1 (inverted) = 40 bits Message #1 = 40 bits Message #2 = 40 bits Message #2 (inverted) = 40 bits Message #2 = 40 bits B. Mobile that wishes to originate an emergency group call transmits an assignment request on the control channel in synchronism with the received control channel messages. The frame format is as follows:

Dotting = 152 bits Barker (repeated three times) = 48 bits Message = 40 bits Message (inverted) = 40 bits Message = 40 bits The emergency group call request message is coded as follows:

MT-A Code = 3 bits Group ID - 11 bits Logical ID = 14 bits BCH Code = 12 bits C. The control channel r~sponds with two channel-assignment messages that are coded as follows:

MT-A Code = 3 bits 1/2 Logical ID = 8 MSBs or LSBs Channel = 5 bits Transmission trunked = 1 bit Group ID = 11 bits BCH Code = 12 bits D. All mobiles of the called group switch to assigned working channel and receive a confirmation message. Slotted working channel messages are transmitted using the following frame:

Dotting = 16 bits Barker = 16 bits Dotting = 16 bits Message = 32 bits Message (inverted) = 32 bits Message = 32 bits The emergency group call confirmation message is coded as follows:

MT Code = 4 bits Message/Transmission TrunXing = 1 bit Group/Logical ID = 1 bit Group ID = 11 bits Logical ID = 14 bits BCH = 12 bits E. Originating mobile receives confirmation message and transmits 384 bits of dotting, then audio.

F. Working channel receives dotting and transmits two unit-keyed/unmute messages.

MT Code = 4 bits - Unused bit = 1 bit Message/Transmission Trunking = 1 bit Logical ID = 14 bits BCH Code = 12 bits Called mobile receives unmute message and unmutes audio.

G. Active Mobiles on other working channels receive subaudible channel assignment message.

H. Control channel transmits channel update message for late entry mobiles.

~ ~ ~ ~ ~ 45~R 00560 I. Transmitting mobile unkeys and sends two non-slotted unkey messages. All non-slotted message formats are:

Dotting = 128 bits Data #3 = 128 bits Data #2 ~ 128 bits Data #1 = 128 bits Data #0 ~ 128 bits Data #3, #1, #1 and ~0 are identical (repeated four times) and each has the following format:

Dotting = 16 bits Barker = 16 bits Byte l = 8 bits Byte 1 (inverted3 = 8 bits Byte 1 = 8 bits Byte 2 = 8 bits Byte 2 (inverted3 = 8 bits ' Byte 3 = 8 bits Byte 4 = 8 bits Byte 4 (inverted) = 8 bits Byte 4 = 8 bits The unkey message is coded as follows:

MT Code = 4 bits ,~

~9~o~

Block Count = 2 bits Logical ID = 14 bits BCH Code = 12 bits V. Radio Call Sequence-Radio Oriqination, Status Call A. Control channel transmits a continuous steam of control messages which all inactive mobiles receive. The messages are sent two messages to a 30-msec. frame that has the following format:

Dotting = 16 bits Barker = 16 bits Dotting = 16 bits Message #1 = 40 bits Message ~1 (inverted) = 40 bits Message #1 = 40 bits Message #2 = 40 bits Message #2 (inverted) = 40 bits Message #2 = 40 bits B. Mobile that wishes to originate a status call transmits a status request on the control channel in synchronism with the received control channel messages. The frame format is as follows:

Dotting = 152 bits Barker (repeated three times) = 4a bits Message = 40 bits Message (inverted) = 40 bits Message = 40 bits 12~(:)0Z~

The status request message is coded as follows:

MT-A Code = 3 bits MT-B Code = 3 bits MT-C Code = 2 bits Status = 6 bits Logical ID = 14 bits BCH Code = 12 bits C. The control channel responds with a status page message that is coded as follows:

MT-A Code = 3 bits MT-B Code = 3 bits - MT-C Code= 2 bits Status = 6 bits Logical ID = 14 bits BCH Code = 12 bits D. Called mobile transmits a control channel status message that is coded as follows:

-` MT-A Code = 3 bits MT-B Code= 3 bits MT-C Code= 2 bits Status = 6 bits Logical ID = 14 bits BCH Code = 12 bits E. Control channel responds With a status acknowledge message that is coded as follows:

` ~LZ9~2~

MT-A Code = 3 bits MT-B Code = 3 blts MT-c Code = 2 bits Status = 6 bits Logical ID - 14 bits BCH Code - 12 bits Originating mobile receives status message.

VI. Radio Call Sequence-Radio Oriqination, SPecial Call A. Control channel transmits a continuous stream of control messages which all inactive mobiles receive. The messages are sent two messages to a ~0-msec. frame that has the following format:

,1 Dotting = 16 bits Barker = 16 bits Dotting = 16 bits Message #1 = 40 bits Message #1 ~inverted) = 40 bits Message #I = 40 bits Message #2 = 40 bits Message #2 (inverted) = 40 bits Message #2 = 40 bits B. Mobile that wishes to originate a special call transmits a special call request on the control channel in synchronism with the received control channel messages. The frame format is as follows:
.~ .

12.~00~1 Dotting = 152 bits Barker (repeated three times) = 48 bits Message = 40 bits Message (inverted3 = 40 bits Message = 40 bits The special call request message is coded as follows:

MT-A Code = 3 bits MT-B Code= 3 bits MT-C Code= 2 bits 3 bits (unused) Priority Code = 3 bits Logical ID = 14 bits BCH Code = 12 bits C. The control channel responds with a channel-assignment two-message pair. Coding is as follows:

MT-A Code = 3 bits . MT-B = 3 bits MT-C = 2 bits , Channel = 5 bits Individual/Group = 1 bit Logical ID = 14 bits Group ID = 11 bits : BCH Code = 12 bits D~ Mobile switches to the assigned working channel and receives a confirmation message. Slotted ~2~00~

working channel messages are transmitted using the following frame.

Dotting = 16 bits Barker = 16 bits Dotting = 16 bits Message = 32 bits Message (inverted~ = 32 bits Message = 32 bits BCH Code = 12 bits The special call confirmation message is coded as follows:

MT Code = 4 bits Hang/time/Trunked = 1 bit Group/Logical ID = 1 bit Group ID = 11 bits Logical ID = 14 bits BCH Code = 12 bits E. Originating mobile receives confirmation ~-~ . message and transmits a multi-block special call i message. The message frame (shown below) can have from 1 to 16 blocks.

Dotting = 96 bits : Data #3 Block #l = 128 bits Data #2 Block ~1 = 128 bits Data #1 Block #1 = 128 bits Data #0 Block #l = 128 bits Data #3 Block #2 = 96 bits ~ ~9~

Data ~2 Block #2 = 96 bits Data #3, #2, #1, #0 are identical in each block (repeated four times3. Coding for Block ~1 Data is:

Dotting = 16 bits Barker = 16 bits Byte 1 = 8 bits Byte 1 = (inverted) = 8 bits Byte 1 = 8 bits Byte 2 = 8 bits Byte 2 (inverted) = 8 bits . ~ .
.
. , Byte 3 = 8 bits Byte 4 = 8 bits Byte 4 (inverted) = 8 bits Byte 4 = 8 bits Data in blocks after block #1 do not have Barker code but do carry a block number field. If telephone interconnect is required, block #1 data is coded as follows:

Digital/IDs = 1 bit Spare - 2 bits ~` 12900~
45M~ 0~560 Phone Digit Count = 5 bits Phone Digit #1 = 4 bits MSD
Phone digit #2 = 4 bits Phone Digit #3 = 4 bits BCH Code = 12 bits If no interconnect is required, block #l is coded as Unused ~ 4 bits Group/Individual = 1 bit Group/Logical ID = 14 bits BCH Code = 12 bits Subsequent blocks are coded with either one group ID, one logical ID, or five telephone digits as required to satisfy the blocX #l counts. The telephone digits are first, then the logical ID's, then the group ID's. Digit coding is one digit per nibble (null=1010). ID coding is 8 Bite = 10101010 (8 bits) Group/Logical ID = 14 bits BCH Code = 12 bits F. Working channel transmits a slotted working channel special call receive bitmap message.
Slotted working channel messages are transmitted using the following frame:

Dotting = 16 bits ~29~0~

45~R 00550 Barker = 16 bits --Dotting = 16 bits Mes~age = 32 bits Mes~age ( in~erted) ~ 32 bits Me~3sage ~ 32 bit~
BCH Code = 12 bit~

The special call receiYe bit map is coded as follows (use of similar acknowledgmenk bitmaps is the subject of related commonly as~;igned U.S. Patent No. 4 , 905 , 234 issued February 27 , 1990):
,~

M~ Cs)de = 4 bits Block #l bit = 1 bit ( e . g., OK) Block #2 bit = 1 bit (e.g., OK) Block #3 bit = 1 bit (e.g., O = repeat~
Block #4 bit = 1 bit Block #16 bit = 1 BCH Code = 12 bitæ

(:. Or~gi~ating mobile r~ceives bi~map me~sas~e and transmits a multi-block special call me~s~age.
T~e me~age frame ( 0hown bslow) can have from 1 to 16 blocks .

Dotting = 96 bi'cs Data #3 Block #1 = 128 bits ,~ ," i :
. .~

.

Data #2 Block #l = 128 bits Data #1 Block #1 = 128 bits Data #0 Block #1 = 128 bits Data #3 Block #2 = 96 bits Data #2 Block ~2 = 96 bits Data #3, ~2, #1, #0 are identical in each block (repeated four times). Coding for block #l is Dotting = 16 bits Barker = 16 bits Byte 1 = 8 bits Byte 1 (inverted) = 8 bits Byte 1 = 8 bits Byte 2 = 8 bits Byte 2 (inverted) = 8 bits :

Byte 3 = 8 bits Byte 4 - 8 bits Byte 4 (inverted) = 8 bits Byte 4 = 8 bits Data in blocks after block #l do not have dotting or Barker code.

~Z~

Only blocks that have a "O" in their bitmap bit are transmitted. For example in step F, block #3 in step E would be the first block retransmitted. If no bit map is received within 100 msec. after steps E or G, all blocks are retransmitted.

Steps F and G are repeated until all blocks are received correctly (BITMAP = l's).

H. Control channel transmits a continuous stream of control messages which all inactive mobiles receive. The messages are sent two messages to a 30-msec. rame that has the following format:

Dotting = 16 bits Barker = 16 bits Dotting = 16 bits Message #1 = 40 bits Message ~1 (inverted) = 4G bits Message #1 = 40 bits Message #2 = 40 bits Message #2 (inverted) = 40 bits Message #2 = 40 bits I. The control channel sends from O to 16 channel-assignment two-message pairs as required for the special call. Coding for each message is as follows:

MT-A Code = 3 bits 1/2 Logical ID = 8 MSBs or 6 LSBs 293~0~

Channel = 5 bits Transmission Trunked = 1 bit Group ID = 11 bits ~CH Code = 12 bits J. All called mobiles go to the assigned working channel and unmute (same as late entry).
From this point the working channel messages are the same as a group or individual call.

VII. Radio Call Sequence-Console Oriqination, Group Call A. Control channel transmits a continuous stream of control messages which all inactive mobiles receive. The messages are sent two messages to a 30-msec. frame that has the ~ollowing format:
~ ' ; Dotting = 16 bits Barker = 16 bits Dotting = 16 bits Message #1 = 40 bits Message #1 (inverted) = 40 bits Message ~1 = 40 bits Message #2 = 40 bits Message #2 (inverted) = 40 bits Message #2 = 40 bits B. Console that wishes to originate a group call transmits a group call message to the downlink. The group call message is coded as follows:
:

:' '''^`` 1.~0~1 MID = 1 byte (#0) = 8 bits # bytes = 1 byte (#1) = 8 bits Source destination = bytes #2 and #3 = 16 bits Not Used = 4 bits MT-A Code = 3 bits Group ID = 12 bits Logical ID = 14 bits Parity = 1 byte (#8) = 8 bits C. The control channel responds with a channel-assignment two-message pair. Coding is as follows:

MT-A Code = 3 bits 1/2 Logical ID = 8 MSBs or LSBs Channel = 5 bits Transmission or Message Trunking = 1 bit Group ID = 11 bits BCH Code = 12 bits D. All mobiles of the called group switch to assigned working channel and receive a confirmation message. Slotted working channel messages are transmitted using the following frame:

Dotting = 16 bits Barker = 16 bits Message = 32 bits Message (inverted) = 32 bits Message = 32 bits :
,~

- ~2900~

The group call confirmation message is coded as follows:

MT Code = 4 bits Hang Time/Trunked = 1 bit Group/Logical ID = 1 bit Group ID = 11 bits Logical ID = 14 bits BCH Code = 12 bits E. Originating console receives channel-assignment from the downlink and switches : audio to the specified channel. Console message is coded as follows:

MID = 1 Byte (#0) = 8 bits # Bytes = 1 Byte (#1) = 8 bits S/D = Bytes #2, #3 = 16 bits Not Used = 4 bits MT Code = 3 bits Logical ID - 14 bits Not Used = 2 bits GR/L ID = 1 bit Channel = 5 bits Group ID = 11 bits Parity = 1 Byte (#9) = 8 bits F. Working channel transmits two unit-keyed/unmute messages:

MT Code = 4 bits ;~ Unused = 1 bit --` 1.2~9~)0~

Hang-time/trunked = 1 bit Logical ID = 14 bits BCH Code = 12 bits Called mobiles receive unmute message and unmute audio.

G. Active mobiles on other working channels receive a subaudible channel assignment message.

H. Control channel transmits channel update message for late entry mobiles.

I. Console sends unkey message that is coded as follows:

MID = 1 Byte (#0) = 8 bits # Bytes = 1 byte = 8 bits Source Destination = Bytes #2 ~ ~3 = 16 bits Not used = 4 bits MT Code = 4 bits Not Used = 2 bits Logical ID = 14 bits Parity = Byte #7 = 8 biks J. Working channel transmits 896 to 2816 bits of dotting to drop all mobiles from the channel.

K. Console receives unkey message:

MID = Byte ~0 = 8 bits ~ Bytes = Byte ~1 = 8 bits ~;~900~
45~R 00560 Source Designation = Bytes #2 ~ ~3 = 16 bits MT-A/B/C = 9 bits Drop Cr = 1 bit Not Used = 1 bit Channel = 5 bits ~ Logical ID = 14 bits : Parity = Byte #8 = 8 bits VIII. Radio Call Sequence-Console Ori~ination, Individual Call A. Control channel transmits a continuous stream of control messages which all inactive mobiles receive. The messages are sent two messages to a 30-msec. frame that has the following format:

Dotting = 16 bits Barker = 16 bits Dotting = 16 bits Message #1 = 40 bits Message #1 (inverted) = 40 bits Message #1 = 40 bits Message #2 = 40 bits Message ~2 (inverted) = 40 bits Message #2 = 40 bits -~ B. Console that wishes to originate an individual call transmits an individual call message to the downlink. The individual call message is coded as follows:

MID = Byte #O = 8 bits ' ' ~9oo~

# Bytes = Byte #1 = 8 bits Source Destination = Bytes #2 & #3 = 16 bits Not Used = 2 bits MT-A Code = 3 bits Logical ID = 14 bits Logical ID = 14 bits Parity = Byte #8 = 8 bits C. The control channel responds with a channel-assignment two-message pair. Coding is as follows:

MT-A Code = 3 bits 1/2 Logical ID = 8 MSBs or LSBs Channel = 5 bits Transmission or Message Trunked = 1 bit Logical ID = 14 bits BCH Code = 12 bits D. Called mobile switches to assigned working channel and receives a confirmation message. Slotted working channel messages are transmitted using the . following frame:

Dotting = 16 bits Barker = 16 bits Message = 32 bits Message inverted = 32 bits Message = 32 bits The individual call confirmation message is coded as follows:

~ (10~
452~R 00560 MT Code = 4 bits Hang Time/Trunked = 1 bit Group/Logical ID = 1 bit Group ID = 11 bits Logical ID = 14 bits BCH Code = 12 bits E. Originating console receives channel-assignment message from the downlink and switches audio to the specified channel. Console message is coded as follows:

MID = Byte #0 (8 bits) # Bytes = Byte #1 = 8 bits Source Destination = Bytes #2, & ~3 = 16 bits ; Not Used = 4 bits r, MT Code = 3 bits Logical ID = 14 bits Not used = 2 bits GR/L ID = 1 bit Channel = 5 bits Logical ID = 14 bits . Parity = Byte #9 = 8 bits F. Working channel transmits two unit-keyed/unmute messages.

MT Code = 4 bits Unused = l bit Hang-Time/TrunkPd = 1 bit Logical ID = 14 bits BCH Code - 12 bits , ~2~00~
452~R 00560 Called mobile receives unmute message and unmutes audio.

G. Active Mobiles on other working channels do not receive a subaudible channel assignment message.

H. Control channel transmits channel update message for late entry mobiles.

I. Console sends unkey message that is coded as follows:

MID = Byte #0 = 8 bits # Bytes = Byte #1 = 8 bits Source Destination = Bytes #2 ~ #3 = 16 bits Not Used = 4 bits MT Code = 4 bites Not Used = 2 bits Logical ID - 14 bits Parity = Byte #7 = 8 bits J. Working channel transmits 896 to 2816 bits of dotting to drop all mobiles from the channel.

K. Console receives unkey message:

MID = Byte #0 = 8 bits # Bytes = Byte #1 = 8 bits Source Destination = Bytes #2 and ~3 = 16 bits MT-A/B/C = 9 bits Drop Ch = 1 bit ~2~
45MR 0~560 Not Used = 1 bit Channel = ~ bites Logical ID = 14 bits Parity = Byte #8 = 8 bits IX. Radio Call Seauence Console Oriaination, Activate Patch : A. Control channel transmits a continuous stream of control messages which all inactive mobiles receive. The messages are set two messages to a 30-msec. frame that has the following format:

Dotting = 16 bits Barker = 16 bits Dotting = 16 bits - Message #1 = 40 bits ' Message #1 (inverted) = 40 bits Message #1 = 40 bits Message #2 = 40 bits . Message #2 (inverted) = 40 bits Message #2 = 40 bits : . , B. Console that wishes to establish a patch transmits a patch ID assignment message to the downlink. Th~ patch ID assignmen' message ~s variable length depending upon the group and individual ID counts:

MID=29 = Byte #0 = 8 bits # Bytes = Byte #1 = 8 bits Source/Destination = Bytes #2, #3 = 16 bits l~9QO~l 45MR 005Ç~

Not Used = 4 bits Group Count = 4 bits Individual Count = 4 bits Logical ID = 14 bits Not Used = 10 bits Logical ID = 14 bits Not Used = 10 bits Logical ID = 14 bits Not Used = 11 bits Group ID = 11 bits Not Used - 13 bits Group ID = ll bits Not used = 13 bits Patch ID = 11 bits Parity = 8 bits C. Console receives an acknowledgement of the patch request from the site controller using a special group ID code (1000 0000 0000):

MID=12 = Byte #0 = 8 bits # Bytes = Byte #1 = 8 bits Source/Destination = Bytes #2, #3 = 16 bits Not Used = 3 bits : MT-A Code = 3 bits MT-B Code = 100 = 3 bits Patch ID = 11 bits Group ID = ll bits Parity ~ Byte ~8 = 8 bits D. When console wishes to activate the patch it transmits a patch activate message to the downlink.

~L~9~0Z~.

45~R OU560 MID=27 = Byte #0 = 8 bits # Bytes = Byte #1 = 05 = 8 bits Source/Destination = ~ytes #2, & #3 = 16 bits Not Used = 4 bits MT Code = 4 bits (1110) Not Used ~ 5 bits Patch ID = 11 bits Parity = Byte #7 = 8 bits E. The control channel responds with alias ID
assignment messages. Group assignment messages are coded:

MT-A Code = 3 bits . MT-B Code = 3 bits (110) Alias Group ID = 11 bits Not Used = 1 bit ; Group ID = 11 bits BCH Code = 12 bits ; Group alias ID messages are repeated in the control channel background mode and not acknowledged by the mobiles.

Individual alias ID assignment messages are coded as follows:

MT-A Code = 3 bits MT-B Code = 3 bits ~101) Alias Group ID = 11 bits Logical ID ~ 14 bits BCH Code = 12 bits ~Z9~
45~R 00560 _99_ F. All mobiles receive assignment messages but only individual call mobiles acknowledge message.

MT-A Code = 3 bits , MT-B Code = 3 bits (110) Alias Group ID = 11 bits Logical ID = 14 bits BCH Code = 12 bits G. ConsoIe receives patch assignment/activate - messages to confirm patçh. One message for each assignment:

MID=12 = Byte #Q - 8 bits # Bytes = Byte #1 = 8 bitg Source/Destination = Bytes #2 & #3 = 16 bits Not Used = 4 bits MT-A Code = 3 bits (11) MT-B Code = 3 bits (100) Patch ID = 11 bits Group ID = 12 bits Parity = Byte #8 = 8 bits ; MID=13 - Byte #0 = 8 bits # Bytes = Byte #1 = 8 bits Source/Destination = Bytes #2 & #3 = 16 bits Not Used = 1 bit MT-A Code = 3 bits MT-B Code = 3 bits (lOl) : Patch ID = 11 bits Logical ID = 14 bits Parity = Byte #8 = 8 bits " lZ~Of~l H. Console originates a patch call by transmitting a group call message using the patch ID:

MID=24 = Byte #0 = 8 bits # Bytes = Byte #1 = 8 bits Source/Destination = Bytes #2 & ~3 = 16 bits Not Used = 3 bits MT Code = 3 bits (00) Patch ID = 12 bita Logical ID = 14 bits - Parity = Byte #8 I. Control channel transmits group call message. Subsequent steps are same as console originated group call.

-While only one exemplary embodimént of this - 'invention has been described in detail, those skilled in the art will recognize that many variations and modifications may be made in this embodiment while still retaining many of its novel features and advantages. Accordingly, all such modifications and variations are intended to be included within the scope of the appended claims.

Claims (66)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A method for achieving reliable and prompt communications with a trunked radio repeater system having a digital control channel and plural working channels, which working channels are assigned for temporary use of individual radio units specified by digital control signals on the control channel, said method comprising the steps of:
(1) operating a first radio unit on a first working channel in response to digital assignment signals passed over said control channel;
(2) operating a second radio unit on a second working channel in response to further digital assignment signals passed over said control channel;
(3) passing a stream of digital signals over said first working channel to said first radio unit indicating said second working channel assignment; and (4) operating said first radio unit on said second working channel in response to said stream of signals transmitted by said transmitting step (3).

2. A method as in claim 1 wherein said passing step (3) comprises:
(a) continuously passing subaudible digital signalling over said first working channel; and (b) interrupting said subaudible digital signals with at least one burst of high-speed digital signalling embedded within said subaudible signals, said high-speed signalling indicating said second working channel assignment.

3. A method as in claim 2 wherein said passing step (3) further includes:
(c) subsequent to said passing step (b), indicating said second working channel assignment with said subaudible digital signals.

4. A method as in claim 3 wherein said indicating step (c) is performed only once.

5. A method as in claim 1 wherein said passing step (3) is repeated over a preset time period.

6. a method as in claim 1 further comprising the step of: transmitting digital signals indicating a working channel assignment exactly twice on one of said first and second working channels each time a working channel different from said one channel is assigned, once at a high data rate, and the second time at a lower data rate.

7. A trunked radio repeater system for achieving reliable and prompt communications, said system having a digital control channel and plural working channels which working channels are assigned for temporary use of individual radio units specified by digital control signals on the control channel, said system comprising:
means for operating a first radio unit on a first working channel in response to digital assignment signals passed over said control channel;
means for operating a second radio unit on a second working channel in response to further digital assignment signals passed over said control channel; and means for passing a stream of digital signals over said first working channel to said first radio unit indicating said second working channel assignment, means wherein said first-mentioned operating means is also for operating said first radio unit on said second working channel in response to said stream of digital signals passed over said first working channel by said passing means.

8. A system as in claim 7 wherein said passing means comprising:
means for continuously passing subaudible digital signalling over said first working channel;
and means for interrupting said subaudible digital signals with at least one burst of high-speed digital signalling embedded within said subaudible signals, said high-speed signalling indicating said second working channel assignment.

9. A system as in claim 8 wherein said subaudible signalling passing means encodes subaudible digital signals with an indication of said second working channel assignment.

10. A system as in claim 9 wherein said subaudible signalling passing means encodes said subaudible signals with said second working channel assignment indication only once.

11. A system as in claim 7 wherein said passing means passes digital signals indicating said second working channel assignment for a preset time period less than the time period said second working channel remains assigned.

12. A method of priority scanning in a trunked radio repeater system having a digital control channel and plural working channels, which working channels are temporarily assigned for use of individual radio units specified by digital control signals on the control channel, said method including the steps of:
(1) assigning a first working channel to a first radio unit by passing digital assignment signals over said control channels;
(2) passing a burst of digital signals substantially at 9600 bits per second over all assigned ones of said plural working channels indicating the assignment of said first working channel;
(3) subsequently to said passing step (2), passing digital signals substantially at 150 bits per second over all assigned ones of said plural working channels indicating the assignment of said first working channel; and (4) retuning a further radio unit from a working channel assigned thereto to said first working channel in response to said signals passed by said passing step (3) and/or by said passing step (4).

13. A method as in claim 12 wherein said passing step (3) includes passing a subaudible digital signalling frame in which every two bits are inverted and retransmitted.

14. A method as in claim 12 wherein:
said passing step (3) includes reserving time slots within said 150 bits per second digital signalling for high-speed data bursts; and said passing step (2) includes filling at least one of said reserved time slots with high-speed digital signalling.

15. A trunked radio repeater system having a digital control channel and plural working channels, which working channels are temporarily assigned for use of individual radio units specified by digital control signals on the control channel, said system including:
means for assigning a first working channel to a radio unit by passing digital assignment signals over said control channel;
means for passing a burst of digital signals substantially at 9600 bits per second over all assigned ones of said plural working channels indicating the assignment of said first working channel;
means cooperating with said first-mentioned passing means for passing subaudible digital signals substantially at 150 bits per second over all assigned ones of said plural working channels indicating the assignment of said first working channel; and means for retuning a further radio unit from a working channel assigned thereto to said first working channel in response to said digital signal burst and/or subaudible digital signals passed over said working channel assigned thereto.

16. A system as in claim 15 wherein said subaudible signal passing means includes means for transmitting a subaudible digital signalling frame in which every two-bits are inverted.

17. A system as in claim 15 wherein:
said subaudible signal passing means includes means for reserving at least one time slot within said 150 bits per second subaudible digital signalling for high-speed data bursts; and said digital signal burst passing means includes means for filling at least one of said reserved time slots with high-speed digital signalling.

18. A method of communicating control signals over active working channels of a trunked radio repeater system having a digital control channel and plural working channels, which working channels are temporarily assigned for use of individual radio units specified by digital control signals on the control channel, said method including the steps of:
(1) receiving subaudible digital signalling over a working channel and substantially simultaneously receiving other signals sent over said working channel at a rate which is higher than the rate of said subaudible signalling, said subaudible signalling defining a frame structure;
(2) receiving high speed data bursts embedded within said subaudible digital signalling at predetermined positions within said signalling frame structure; and (3) muting audio receiver circuitry during the times said high-speed data bursts are received.

19. A method as in claim 18 wherein said subaudible signalling defines a frame structure wherein every two bits are inverted and repeated, and said high-speed data bursts occupy 4-bit time slots within said frame structure.

20. A method as in claim 19 wherein said subaudible signalling frame structure has the following format:
(a) 2-bit times reserved for said high-speed data bursts;
(b) two additional bits;
(c) an idle frame bit which is set whenever no radio units are on the working channel;
(d) an 11-bit group identification field;
(e) 2-bit times reserved for said high-speed data bursts;
(f) two additional bits;
(g) a further idle frame bit which is set whenever no radio units are on the working channel;
(h) a further 11-bit group identification field;
(i) 2-bit times reserved for said high-speed data bursts;
(j) 2-bit times reserved for high-speed confirmation signals from radio units on the working channel; and (k) a 12-bit error-checking field.

21. In a trunked radio repeater system of the type having a digital control channel and plural working channels, which working channels are temporarily assigned for use of individual radio units specified by digital control signals on the control channel, an improvement comprising:
means for receiving subaudible digital signalling over a working channel and substantially simultaneously receiving other signals sent over said active working channel at a rate which is higher than the rate of said subaudible signalling;
means for receiving high-speed data bursts embedded within said subaudible digital signals at predetermined positions within said signalling; and means cooperating with said high-speed data bursts receiving means for muting audio receiver circuitry during times said high-speed data bursts are being received.

22. A system as in claim 21 wherein said subaudible signalling defines a frame structure wherein every two bits are inverted and repeated, and said high-speed data bursts occupy 4-bit time slots within said frame structure.

23. A system as in claim 22 wherein said subaudible signalling frame structure has the following format:
(a) 2-bit times reserved for said high-speed data bursts;
(b) two additional bits;
(c) an idle frame bit which is set whenever no radio units are on the working channels;

(d) an 11-bit group identification field, (e) 2-bit times reserved for said high-speed data bursts;
(f) two additional bits;
(g) a further idle frame bit which is set whenever no radio units are on the working channel;
(h) a further 11-bit group identification field;
(i) 2-bit times reserved for said high-speed data bursts;
(j) 2-bit times reserved for high-speed confirmation signals from radio units on the working channel; and (k) a 12-bit error-checking field.

24. A method for achieving reliable and prompt communication within a trunked radio repeater system having a digital control channel and plural working channels, which working channels are assigned for temporary use of individual radio units specified by digital control signals on the control channels, said method comprising the steps of:
monitoring digital signals passed over an assigned working channel after operation of a radio unit has been established thereon, said digital signals being passed over said working channel first substantially at 9600 bits per second and subsequently at a subaudible data rate, and dropping operation of a radio unit from said working channel in response to a detected change in said digital signals.

25. Apparatus for achieving reliable and prompt communication within a trunked radio repeater system having a digital control channel and plural working channels, which working channels are assigned for temporary use of individual radio units specified by digital control signals on the control channel, said apparatus comprising:
means for monitoring digital signals transmitted over an assigned working channel after operation of a radio unit has been established thereon, said digital signals including a subaudible signal stream having 9600 bits per second high-speed digital signal bursts embedded therein; and means for dropping operation of a radio unit from said working channel in response to said subaudible signal stream and/or said high-speed signal bursts.

26. A method for achieving reliable and prompt communication within a trunked radio repeater system having a digital control channel and plural working channels, which working channels are assigned for temporary use of individual radio units specified by digital control signals on the control channel, said method comprising the steps of:
requesting assignment of a working channel by passing digital request signals from a first radio unit to a control site over said control channel;
assigning a specific working channel to said first radio unit and to at least a second radio unit by passing digital assignment signals over said control channel;

passing a digital handshake signal over said assigned working channel from said first radio unit to said control site;
substantially simultaneously with said passing step, passing a confirmation message from said control site to said second radio unit; and releasing said first and second units for communication over said assigned working channel by sending digital release signals on said assigned working channel in response to successful exchange of said handshake signal.

27. A method as in claim 26 wherein said digital assignment signals include a channel assignment message pair, one message in said pair identifying said first radio unit, the other message in said pair identifying said second radio unit.

28. A method as in claim 26 wherein all said digital signals are transmitted at substantially 9600 bits per second.

29. A method as in claim 26 wherein said control signals, request signals and handshake signals include triple data redundancy and said assignment signals include six-fold redundancy of data representing the called party and the assigned channel.

30. A method as in claim 26 further comprising the steps of:
passing digital, new channel assignment updated messages over the assigned working channel first at a high data rate and subsequently at a lower subaudible data rate; and monitoring said new channel assignment messages in each radio unit and responsively dropping off the assigned working channel if a higher priority incoming message is directed thereto.

31. A method for achieving reliable and prompt communication within a trunked radio repeater system having a digital control channel and plural working channels, which working channels are assigned for temporary use of individual radio units specified by digital control signals on the control channel, said method comprising the steps of:
requesting assignment of a working channel by passing digital request signals from a first radio unit to a control site over said control channel;
assigning a specific working channel to said first radio unit and to at least one second radio unit by passing digital assignment signals from said control site to said radio units over said control channel;
confirming said assignment by sending digital confirmation signals on said assigned working channel from said control site;
passing a digital handshake signal over said assigned working channel between said first radio unit and said control site substantially simultaneously with said confirming step; and releasing at least one of said units for communication by sending digital release signals over said assigned working channel in response to said handshake signal.

32. A method as in claim 31 wherein said second unit is enabled to communicate by receipt of said release signals.

33. A method as in claim 31 wherein said assigning step includes the step of passing a channel assignment message pair over said control channel, one message in said pair identifying said first radio unit, the other message in said pair identifying said second radio unit.

34. A method as in claim 31 wherein said request signals, assignment signals, acknowledgement signals, digital handshake signals and release signals are all digitally encoded and transmitted at substantially 9600 bits per second over said control channel and assigned working channel, respectively.

35. A method as in claim 31 wherein:
said control signals, said request signals and said confirmation signals each include triple data redundancy, and said assignment signals include six-fold redundancy of data representing the called party and the assigned channel.

36. A method as in claim 31 further comprising the steps of:

transmitting digital new channel assignment update messages on the assigned working channel first at substantially 9600 bits per second and subsequently at a much lower, subaudible data rate, and monitoring said update messages in each unit and responsively dropping off the assigned working channel if a higher priority incoming message is directed thereto.

37. A method for achieving reliable and prompt communication within a trunked radio repeater system having a digital control channel and plural working channels, which working channels are assigned for temporary use of individual radio units specified by digital control signals on the control channel, said method comprising the steps of:
A. sending, from a first radio unit to a control site over the control channel, request signals including a. a string of dotting bits to establish bit sync, b. a predetermined word sync code repeated three times, and c. a request message of 40 bits repeated three times, each message including i. 3 bits identifying the message type, ii. 11 bits identifying the called unit, iii. 14 bits identifying the calling unit, and iv. 12 bits of BCH error detection and correction data;
B. sending, from said control site to said first radio unit over said control channel, assignment signals including a. a predetermined word sync code, b. a pair of assignment messages, each being repeated three times and having 40 bits including i. 3 bits identifying the message type, ii. 8 bits providing one-half the identity of the calling unit;
iii. 5 bits identifying the assigned working channel, iv. one bit identifying whether transmission trunking is in effect, v. 11 bits identifying the called group or individual unit, and vi. 12 bits of BCH error detection and correction data;
C. sending, from said control site over the assigned working channel, confirmation signals including a. a predetermined word sync code, and b. a confirmation message of 32 bits repeated three times including i. 4 bits identifying the type of message, ii. 1 bit identifying whether trans-mission trunked operation is in effect,

Claim 37 continued:
iii. 1 bit identifying whether a group or individual call is being made, iv. 11 bits identifying the called group or individual unit, and v. 14 bits identifying the calling unit, and vi. 12 bits of BCH error detection and correction data;
D. sending, from said first unit over the assigned working channel, digital acknowledgement signals including a. a relatively longer string of dotting bits, followed by b. communications over the working channel to the called-group or individual unit, and E. in response to step D(a) and prior to step D(b), sending, from said control site over the assigned working channel, digital release signals including a. a predetermined word sync code, and b. two unit-keyed/unmute messages, each repeated three times and being 32 bit messages including i. 4 bits identifying the type of message, ii. 1 bit identifying whether trunked operation is in effect, iii. 14 bits identifying the calling unit, and iv. 12 bits of BCH error detection and correction data.

38. A method as in claim 37 further comprising the step of:
subsequent to step E, sending digital signals over said assigned working channel including new channel assignment messages first substantially at 9600 bits per second and subsequently at a subaudible data rate, said digital signals being monitored by each unit residing on that working channel for higher priority calls directed to that unit or a predefined group including such unit.

39. A method as in claim 38 further comprising the step of:
subsequent to step E, sending a channel alignment update message over said control channel from the control site to permit late entry of additional called units to the assigned working channel.

40. A method as in claim 39 further comprising the step of at the conclusion of communication by said first unit, the transmission from the first unit over said assigned working channel of an unkeyed message including a. a string of dotting bits, b. a block of 128 bits repeated four times, each data block being of the form i. a string of dotting bits, ii. a predetermined word sync code, iii. four 8 bit bytes, each transmitted with triple redundancy and defining a 32 bit message having 4 bits identifying the type of message, a 2-bit block count, 14 bits of BCH
error detection and correction data.

41. a method as in claim 40 further comprising the step of:
responsive to said unkeyed message, sending, from said control site over said assigned working channel a substantially elongated string of dotting bits, and all units then assigned to the working channel dropping therefrom and reverting to the control channel in response to said elongated string of dotting bits on the working channel.

42. A system of apparatus for achieving reliable and prompt communication within a trunked radio repeater system having a digital control channel and plural working channels, which working channels are assigned for temporary use of individual radio units specified by digital control signals on the control channel, said system comprising:
means for requesting assignment of a working channel by passing digital request signals from a first radio unit to a control site over said control channel;

means for assigning a specific working channel to said first radio unit and to at least a further radio unit by passing digital assignment signals over said control channel;
means for confirming the operation of said first radio unit on said assigned working channel by passing a string of synchronization signals from said first radio unit to said control site over said working channel;
means for confirming said working channel assignment to said further radio unit by passing digital confirmation message over said assigned working channel from said control site to said further radio unit at substantially the same time said string of synchronization signals are being passed from said first radio unit to said control site; and means for releasing at least one of said first and further unit for communication over said assigned working channel by sending digital release signals on said assigned working channel in response to successful receipt of said synchronization signals.

43. A system as in claim 42 further comprising:
means for passing new channel assignment message over the assigned working channel, and means for monitoring said update messages in each unit and responsively dropping off the assigned working channel if a higher priority incoming message is directed thereto.

44. A system as in claim 42 wherein said assigning means includes means for passing a channel assignment two-message pair over said control channel, the first message in said pair identifying said first radio unit, the second message in said pair identifying said second radio unit.

45. A system as in claim 42 wherein both messages in said pair partially identify said first radio unit.

46. A system as in claim 44 wherein said passing means includes means for passing at least one high-speed data burst in addition to subaudible digital signals over said assigned working channel both indicating assignment of another working channel.

47. A system as in claim 42 further comprising:
means for sensing, from the calling unit on the assigned working channel, a digital unkey message, and means responsive to said unkey message, for sensing, from said control site on said assigned working channel, a digital drop signal which causes all units to drop off the then-assigned working channel thus freeing it for immediate further assignment.

48. A system for achieving reliable and prompt communication within a trunked radio repeater system having a digital control channel and plural working channels, which working channels are assigned for temporary use of individual radio units specified by digital control signals on the control channel, said system comprising:

means for requesting assignment of a working channel by passing digital request signals from a first radio unit to a control site over said control channel;
means for assigning a specific working channel to said first radio unit and to at least one further radio unit by passing digital assignment signals from said control site to said radio units over said control channel;
means for confirming said assignment by sending digital confirmation signals on said assigned working channel from said control site;
means for acknowledging successful receipt of said assignment signals by at least one of said units sending digital acknowledgement signals to said control site over said assigned working channel in response to said confirmation signals; and means for releasing at least one of said units for communication by sending digital release signals over said assigned working channel in response to said acknowledgement signals.

49. A system as in claim 48 wherein said request signals, assignment signals, acknowledgement signals and release signals are all digitally encoded and transmitted at substantially 9600 bps over said control channel and assigned working channel, respectively.

50. A system as in claim 48 wherein:

said control signals, said request signals and said confirmation signals each include triple data redundancy, and said assignment signals include six-fold redundancy of data representing the called party and the assigned channel.

51. A system for achieving reliable and prompt communication within a trunked radio repeater system having a digital control channel and plural working channels, which working channels are assigned for temporary use of individual radio units specified by digital control signals on the control channel, said system comprising:
A. means for sending, from a first radio unit to a control site over the control channel, request signals including a. a string of dotting bits to establish bit sync, b. a predetermined word sync code repeated three times, and c. a request message of 40 bits repeated three times, each message including i. 4 bits identifying the message type, ii. 11 bits identifying the called unit, iii. 14 bits identifying the calling unit, and iv. 12 bits of BCH error detection and correction data;

B. means for sending, from said control site to said first radio unit over said control channel, assignment signals including a. a predetermined word sync code, b. a pair of assignment messages, each being repeated three times and having 40 bits including i. 3 bits identifying the message type, ii.8 bits providing one-half the identity of the calling unit;
iii. 5 bits identifying the assigned working channel, iv. 1 bit identifying whether transmission trunking is in effect, v. 11 bits identifying the called group or individual unit, and vi.12 bits of BCH error detection and correction data;
C. means for sending, from said control site over the assigned working channel, confirmation signals including a. a predetermined word sync code, and b. a confirmation message of 32 bits repeated three times including i. 4 bits identifying the type of message, ii. 1 bit identifying whether trans-mission trunked operation is in effect, iii. 1 bit identifying whether a group or individual call is being made, iv. 11 bits identifying the called group or individual unit, v. 14 bits identifying the calling unit, and vi. 12 bits of BCH error detection and correction data;
D. means for sending, from said first unit over the assigned working channel, digital acknowledgement signals including a. a relatively longer string of dotting bits, followed by b. communications over the working channel to the called group or individual unit, and E. means operative in response to said longer string of dotting bits for sending, from said control site over the assigned working channel, digital release signals including a. a predetermined word sync code, and b. two unit-keyed/unmute messages, each repeated three times and being 32 bit messages including i. 4 bits identifying the type of message, ii. 1 bit identifying whether trunked operation is in effect, iii. 14 bits identifying the calling unit, and iv. 12 bits of BCH error detection and correction data.

52. A method for achieving reliable and prompt communication within a trunked radio repeater system having a digital control channel and plural working channels, which working channels are assigned for temporary use of individual radio units specified by digital control signals on the control channel, said method comprising the steps of:
requesting assignment of a working channel;
monitoring digital working channel assignment signals passed over said control channel issued in response to said requested assignment;
confirming said channel assignment by passing confirmation digital signals over said assigned working channel between said control site and a first of said radio units and by passing different confirmation signals between said control site and a second of said radio units; and releasing said assigned working channel for communication by sending digital release signals over said assigned working channel in response to successful exchange of said confirmation signals.

53. A method as in claim 52 wherein said release signals and said different confirmation signals are sent from said control site, and said first-released confirmation signals are sent from said first radio unit.

54. A method as in claim 52 wherein all said digital signals are transmitted at substantially 9600 bits per second.

55. A method as in claim 52 wherein said confirmation and release signals include triple data redundancy and said assignment signals include six-fold redundancy of data representing the called party and the assigned channel.

56. A method as in claim 52 further comprising the steps of:
passing new channel assignment messages on the assigned working channel, and monitoring said assignment messages in each radio unit and responsively dropping off the assigned working channel if a higher priority incoming message is directed thereto.

57. A method as in claim 56 wherein said passing step includes:
transmitting each new channel assignment message over said working channel at substantially 9600 bits per second; and subsequently transmitting each new channel assignment message at a subaudible rate.

58. A system of apparatus for achieving reliable and prompt communication within a trunked radio repeater system having a digital control channel and plural working channels, which working channels are assigned for temporary use by individual radio units specified through digital control signals on the control channel, said system comprising:
means for requesting assignment of a working channel;

means for assigning a specific working channel by passing digital assignment signals over said control channel;
means for confirming said channel assignment by passing confirmation digital signalling over said assigned working channel between said control site and a first of said radio units and passing different confirmation signals between said control site and a second of said radio units; and means for releasing said assigned working channel for communication thereover by sending digital release signals on said assigned working channel in response to successful exchange of said confirmation signals.

59. A system as in claim 58 further comprising:
means for transmitting subaudible new channel assignment messages on the assigned working channel, and means for monitoring said subaudible assignment messages in each unit and responsively dropping off the assigned working channel if a higher priority incoming message is directed thereto.

60. A system as in claim 59 wherein said confirming step includes:
passing each new channel assignment message over said working channel at substantially 9600 bits per second; and subsequently passing each new channel assignment message only once at a subaudible rate.

61. A system as in claim 58 further comprising:
means for subsequently transmitting digital channel assignment late entry messages on the control channel to permit the late entry of a called unit to an ongoing working channel communication.

62. A system as in claim 58 further comprising:
means for sending, from the calling unit on the assigned working channel, a digital unkey message, and means responsive to said unkey message, for sending, from said control site on said assigned working channel, a digital drop signal which causes all units to drop off the then-assigned working channel thus freeing it for immediate further assigned.

63. A radio unit for achieving reliable and prompt communication within a trunked radio repeater system having a digital control channel and plural working channels, which working channels are assigned for temporary use of individual radio units specified by digital control signals on the control channel, said radio unit comprising:
means for requesting assignment of a working channel by passing digital request signals to a control site over said control channel;
means for detecting digital working channel assignment signals over said control channel;
means for passing digital synchronization signals over said assigned working channel, and means for detecting digital release signals on said assigned working channel after successfully passing said synchronization signals and for unmuting communication over the assigned working channel in response to said detecting release signals;
means for detecting new channel assignment digital signals on the assigned working channel, said assignment signals including one of (1) a high-speed burst embedded in a subaudible digital signalling frame on said working channel, and (2) subaudible signals on said working channel subsequent to said high-speed burst and part of said subaudible signalling frame, and means for responsively dropping off the assigned working channel if a higher priority incoming message is detected.

64. A system as in claim 63 further comprising:
means for sending, from the calling unit on the assigned working channel, a digital unkey message in response to release of a PTT switch, and means responsive to a digital drop signal received on the assigned working channel for dropping off the then-assigned working channel thus freeing it for immediate further assignment.

65. A radio unit for achieving reliable and prompt communication within a trunked radio repeater system having a digital control channel and plural working channels, which working channels are assigned for temporary use of individual radio units specified by digital control signals on the control channel, said radio unit comprising:
A. means for sending, to a control site over the control channel, request signals including a. a string of dotting bits to establish bit sync, b. a predetermined word sync code repeated three times, and c. a request message of 40 bits repeated three times, each message including i. 3 bits identifying the message type and type of requested communication, ii. 1 bits identifying the called unit, iii. 14 bits identifying the calling unit, and iv. 12 bits of BCH error detection and correction data;
B. means for detecting on said control channel, assignment signals including a. a predetermined word sync code, b. a pair of assignment messages, each being repeated three times and having 40 bits including i. 3 bits identifying the message type, ii. 8 bits providing one-half the identity of the calling unit;
iii. 5 bits identifying the assigned working channel, iv. one bit identifying whether transmission trunking operation is in effect, v. 11 bits identifying the called group or individual unit, and vi. 12 bits of BCH error detection and correction data;
C. means for detecting on the assigned working channel, confirmation signals including a a predetermined word sync code, and b. a confirmation message of 32 bits repeated three times including i. 4 bits identifying the type of message, ii. 1 bit identifying whether transmission trunked operation is in effect, iii. 1 bit identifying whether a group or individual call is being made, iv. 11 bits identifying the called group or individual unit, v. 14 bits identifying the calling unit, and vi. 12 bits of BCH error detection and correction data;
D. means for sending, over the assigned working channel, digital acknowledgement signals including a. a relatively longer string of dotting bits, followed by b. communications over the working channel to the called group or individual unit, and E. means for detecting on the assigned working channel, digital release signals including a. a predetermined word sync code, and b. two unit-keyed/unmute messages, each repeated three times and being 32 bit messages of the form i. 4 bits identifying the type of message, ii. 2 bits providing a block count, iii. 14 bits identifying the calling unit, and iv. 12 bits of BCH error detection and correction data.

66. Apparatus for achieving reliable and prompt communication within a trunked radio repeater system having a digital control channel and plural working channels, which working channels are assigned for temporary use of individual radio units specified by digital control signals on the control channel, said apparatus comprising:
transceiver means at a central site for transmitting and receiving radio frequency signals over said control channel and over said working channels; and means at said central site operatively coupled to said transceiver means for continually transmitting subaudible signals over an assigned working channel while the working channel remains

Claim 66 continued:

assigned said subaudible signals including a predetermined digital data field which is changed to a new value each time a new assignment of that working channel is made, said new value indicating that the working channel has been reassigned.