WO2002032011A1

WO2002032011A1 - Methods and arrangements relating to a radio communication system

Info

Publication number: WO2002032011A1
Application number: PCT/SE2001/002238
Authority: WO
Inventors: Gunnar Monell; Thomas ÖSTMAN; Jan Söderkvist; Peter WAHLSTRÖM
Original assignee: Telefonaktiebolaget Lm Ericsson (Publ)
Priority date: 2000-10-13
Filing date: 2001-10-12
Publication date: 2002-04-18
Also published as: AU2001296128A1; SE0003748D0

Abstract

In a TDMA based radio communication system different timeslot power control modes, including at least a normal mode timeslot power control mode and a synchronization power control mode, are applied for selectively controlling downlink transmission power levels of a plurality of timeslots on a downlink radio frequency carrier. Alternatively at least some digital symbols for synchronization in each burst transmitted in said plurality of timeslots on said downlink radio frequency carrier are transmitted at a transmission power level higher than or equal to the maximum transmission power level required by any mobile station currently assigned a timeslot on said radio frequency carrier.

Description

METHODS AND ARRANGEMENTS RELATING TO A RADIO COMMUNICATION SYSTEM

TECHNICAL FIELD OF THE INVENTION

The present invention generally concerns methods and arrangements relating to a radio communication system. Specifically, the present invention relates to power control methods for controlling downlink transmission power levels in a TDMA (time-division multiple-access) radio communication system. The invention also includes radio communication networks and radio communication systems implementing said methods .

DESCRIPTION OF RELATED ART

In a radio communication system, such as a cellular system, based on Time Division Multiple Access (TDMA) , the time domain is divided into time frames comprising a plurality of timeslots while the frequency domain is divided into a number of radio carrier frequencies . Each carrier frequency- timeslot combination constitutes a particular physical channel over which bursts of digital radio symbols can be transmitted. A plurality of channels is defined upon a single carrier and hence, separate communications'' can be effectuated with a plurality of mobile stations on a single carrier frequency. One example being the widely implemented TDMA system as described by the TIA/EIA-136 standard, (hereafter referred to as TDM (TIA/EIA-136) which uses three timeslots on each carrier, and hence each carrier can support communication with three mobile stations.

All Public Land Mobile Networks (PLMN) relies on a reuse of frequencies . The frequency spectrum assigned for a particular system is divided into frequency groups and the use of a frequency group is repeated with a geographical distance chosen so that the interference between the groups with the same frequencies are kept at an acceptable level. This is fundamental for a cellular communication system, often depicted as a hexagonal lattice. The distance between cells having the same group of frequencies is denoted the reuse distance. Another commonly used term is the reuse pattern. In e.g. TDMA(TIA/EIA-136) a typical reuse pattern is 7/21, where 7 refers to the reuse distance and 21 to the number of frequency groups. In GSM systems 4/12 is a common reuse pattern.

The growth of cellular communication has exceeded all expectations, both in the number of users and in the total communication time. This has put high demands on vendors of telecommunication equipment as well as operators of mobile systems to increase the capacity of the systems to be able to handle more calls and at the same time maintain the quality of the communication, especially the speech quality. The frequency spectrum is a scarce resource and to increase capacity by acquiring a larger part of the spectrum is in most instances not possible.

To meet the demand of more capacity in certain regions the cells are often split to smaller cells, but the principle cell pattern is kept. This is particularly common in downtown urban areas . Another effective way to increase capacity in a cellular system is to reduce the reuse distance, or alternatively worded, to tighten the reuse pattern. In a system compliant to TDMA (TIA/EIA-136) this could for example be to go from a 7/21 to a 5/15 or a 4/12 reuse pattern. When the distance between cells are reduced, either by tightened reused pattern and/or cell split the likeliness of one carrier in a first cell disturbing a carrier in a second cell using the same frequency, so called co-channel interference, is increased. Also adjacent-channel interference, the interference caused by overhearing carriers at nearby frequencies, deteriorate the signal. In areas of high uses of mobile phones, which often coincide with areas of complicated radio environment, typically experienced during busy hours in urban areas, the interference will be the limiting factor for the systems capacity. Such systems or parts of systems are often referred to as interference limited areas .

One effective way of controlling the interference in a mobile network is to not emit more energy than necessary neither from the mobile stations nor from the base stations. In all major PLMN systems, such as GSM and TDMA(TIA/EIA- 136) , care has been taken to limit the excess power used in the bursts sent from the mobile station to the basestation, the uplink communication. Typically the basestation measures the signal strength and signal quality of the uplink transmission and orders the mobile station to use an downlink transmit power level that gives sufficient signal for an acceptable communication quality. Similar principles are used for the downlink communication, the bursts sent from the basestation to the mobile stations, in some systems e.g. GSM. Other standards e.g. TDMA (TIA/EIA-136) were not originally designed for Downlink Power Control (DPC) , but with need for higher capacity it has become of high importance to also facilitate DPC in these systems. The most effective versions of DPC regulates the downlink transmit power per timeslot (TimeSlot Power Control, TSPC) i.e. different downlink transmit power can be used on the different timeslots on the same carrier frequency, depending on the need of the individual mobile stations. In this way the radiated energy from the basestation can be kept at a minimum.

In TDMA (TIA/EIA-136) systems much of today's used equipment, including both mobile stations and basestations, are constructed in compliance with older versions of the standard, for example the interim standard IS-136 rev. B, which do not include DPC. Many of the problems in introducing DPC in TDMA (TIA/EIA-136) arise from the fact that the standard did not originally provide for DPC and any new functionality introduced must not cause malfunction of older equipment, especially not the mobile stations.

IS-136 rev. B required the output power from the basestation to be close to constant on all carriers and timeslots. Many mobile stations still in use base e.g. Received Signal Strength Information (RSSI) and channel decoding on the assumption of nearly constant downlink output power. These problems are addressed in e.g. US patent application No. 09/625360 and US patent No. 6072792, respectively, which are incorporated by reference herein.

In TDMA(TIA/EIA-136) each timeslot consists of, among other parts, the data fields which hold the information payload and the synchronization field, which holds a known sequence of digital symbols, often referred to as the synchronization word. The use of the synchronization word is dual. During an established communication it is used for the equalization and at the establishment of a communication the mobile station uses the synchronization word to find the correct timeslot. At for example handoff, the mobile station is informed, by control signaling, of which carrier, i.e. what frequency, and which timeslot it is assigned to in the target cell. The mobile station tunes to the correct frequency and starts to search for any synchronization word, not just the synchronization word in its assigned timeslot. From information given in any of the transmitted synchronization words the mobile station can calculate the correct position in time of its assigned timeslot and the communication can continue. Accordingly, it would be highly desirable to provide a technique whereby the performance at handoff and call set-up is secured and still utilizes the decreased interference offered by timeslot downlink power control . SUMMARY OF THE INVENTION

The problem dealt with by the present invention is providing enhanced downlink power control in a radio communication system which eliminates or at least reduces the synchronization problems associated with downlink timeslot power control while at the same time being power efficient and not causing unnecessary interference on downlink radio frequencies .

The problem is solved by methods according to claims 1, 15 and 19, radio communication networks according to claims 24- 26 and a radio communication system according to claim 30.

One object of the invention is to provide an enhanced way of controlling downlink transmit power which eliminates or at least reduces synchronization problems associated with downlink timeslot power control while at the same time being power efficient and not causing unnecessary interference on downlink radio frequencies.

Yet another object of the invention is to reduce the muting time during handoff in a TDMA based system in which power levels in different timeslots on a carrier are selectively controlled.

Still another object is to increase the probability for successful handoffs and call set-ups.

An advantage afforded by the invention is that synchronization problems associated with downlink timeslot power control is eliminated or at least reduced while unnecessary interference on downlink radio frequencies are avoided.

Yet another advantage of the^' invention is that the muting time during handoff may be reduced in a TDMA based system in which power levels in different timeslots on a carrier are selectively controlled.

Still another advantage is that the probability for successful handoffs and call set-ups are increased.

A further advantage of the invention, when multiple carrier power amplifiers (MCPAs) are employed in the radio communication network, is that smaller and more cost efficient MCPAs can be used due to reduced likelihood of maximum power usage of the MCPAs .

Other objects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings and claims.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a radio communication system comprising a radio communication network and mobile stations .

FIG. 2a is a time-transmit power diagram illustrating use of a constant downlink transmission power level on a first downlink radio frequency carrier.

FIG. 2b-d are time-transmit power diagrams illustrating downlink transmission power levels, on a first downlink radio frequency carrier, on a second downlink radio frequency carrier, and on a third downlink radio frequency carrier before performing a handoff of one of the mobile stations in FIG. 1 from one cell to another cell.

FIG. 3a is time-transmit power diagram illustrating use of a constant downlink transmission power level on a third downlink radio frequency carrier. FIG. 3b-e are time-transmit power diagrams illustrating downlink transmission power levels, on a second downlink radio frequency carrier and a third downlink radio frequency, when a fourth mobile station is performing a handoff from a second cell to a third cell in FIG. 1.

FIG. 4a-c are time-transmit power diagrams illustrating, downlink transmission power levels, on a first downlink radio frequency carrier and a second downlink radio frequency, when a first mobile station is performing a handoff from a first cell to a second cell in FIG. 1.

FIG. 5 is a flow chart illustrating a downlink transmission power level control technique according to the invention.

FIG. 6 is a shematic block diagram illustrating one exemplary embodiment of a radio communication network according to the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 illustrates an exemplary radio communication system SYS1 comprising a radio communication network NET1 in the form of a cellular network and mobile stations MS1-MS5. In the exemplary radio communication system SYS1 illustrated in FIG. 1, communication between the cellular network and the mobile stations MS1-MS5 is based on the TDM (TIA/EIA-136) air interface specifications. The radio communication network NET1 comprises a mobile switching center MSC1 and base stations BS1-BS3 connected to the mobile switching center MSC1. The geographical area served by the mobile switching center MSC1 is divided into a number of cells including cells C1-C3. In each cell C1-C3 radio coverage is provided by one of the base stations BS1-BS3 respectively. The cell CI in which the first, second, and third mobile stations MS1-MS3 is currently located is denoted the serving cell and the corresponding base station BS1 is denoted the serving base station. In second cell C2 two mobile stations MS4-MS5 are situated and in the third cell C3 no mobile stations are situated at all for the moment. The mobile switching center MSC1 is responsible for switching calls to and from mobile stations located in the geographical area served by mobile switching center MSC1. It also controls major activities of the base stations BS1-BS3, such as call set-up and handoff of a mobile station from one cell to another. Note that in FIG. 1 only units necessary for illustrating the present invention are illustrated and that a typical cellular network may comprise a large number of mobile stations, several mobile switching centers, a greater number of base stations as well as other types of nodes such as home location registers.

FIG 2a-d illustrate power-time charts for communication in the downlink direction, i.e. from the radio communication network NET1 to the mobile stations MS1-MS5, on a first downlink radio frequency carrier Fl in the first cell Cl, on a second downlink radio frequency carrier F2 in the second cell C2 and on a third downlink radio frequency carrier F3 in the third cell C3. As shown, the time domain, for each radio frequency carrier F1-F3, is divided into TDMA frames, for example, TDMA frame TF. Each TDMA frame is then further divided into a number of timeslots, for example, three (3) timeslots TS11-TS13, TS21-TS23 and TS31-TS33. Accordingly, each downlink radio frequency carrier F1-F3, in FIG. 2a-d, is used to transmit bursts of digital radio symbols for upto three mobile stations. The first base station BS1 in cell Cl communicates with the mobile stations MS1-MS3 located in the cell Cl by transmitting the bursts of digital radio symbols on the first downlink radio frequency carrier Fl in the timeslots TS11-TS13. Thus, in FIG. 2a-b the first base station BS1 communicates with the first mobile station MSI in timeslot TS11, the second mobile station MS2 in timeslot TS12 and with the third mobile station MS3 in timeslot TS13. In FIG. 2c the second base station BS2 in cell C2 communicates on the second downlink radio frequency carrier F2 with the mobile stations MS4-MS5 e.g. second and third timeslots TS22 and TS23 respectively. FIG. 2d shows that no communication occurs in timeslots TS31-TS33 on the third radio frequency carrier F3 in the empty third cell C3.

It is also illustrated in FIG. 2a that according to the TDMA (TIA/EIA-136) standard the duration of each timeslot TS11-TS13 is approximately 6.67 msecs, during which, a burst of typically 162 radio symbols can be transmitted. Each transmitted burst includes a part used for synchronization SYNC11-SYNC13 consisting of 14 digital radio symbols. These symbols constitute a known sequence, often referred to as the synchronization word and used for various aspects of synchronization between the mobile station and the basestation. The duration of each synchronization word is typically approximately 0.576 msecs and according to the TDMA (TIA/EIA-136) standard the synchronization word is placed in beginning of each burst.

The today installed base of equipment in TDMA (TIA/EIA-136) does not utilize any form of power control in the downlink as previous versions of the standard, e.g. IS-136 rev. B did not support it. The downlink transmit power is typically set per cell and is supposed to be the same and constant for all frequency carriers and all timeslots in that cell. This is illustrated in FIG 2a with all bursts transmitted at the same constant power level a5. In order to meet the demand of increased capacity in the networks the superfluously emitted energy from the basestations need to be minimized. As previously stated a most effective method is to introduce downlink power control .

The exemplary radio communication network NETl utilizes so called downlink timeslot power control, TSPC . The transmit power level used for transmission of bursts to a mobile station in a certain timeslot is adapted to enable the mobile station to receive the bursts at a signal strength which is sufficient but not unnecessarily high, i.e. the transmit power level is adapted to match the power requirements of the mobile station. Thus, the power level used on a downlink radio frequency carrier typically varies from timeslot to timeslot.

For the purpose of regulating the output power, algorithms are implemented in the base stations which use parameters transmitted from the mobile station, e.g., the measured quality (e.g., Bit Error Rate BER) and the measured RSSI

(Received Signal Strength Information) of the downlink data, and radio network management parameters transmitted from the

MSC, e.g., acceptable speech quality, which is disclosed in e.g. WO patent PCT/SE00/00723 , which is incorporated by reference herein.

FIG. 6 illustrates more details of the radio communication network NETl introduced in FIG 1. The network NETl is equipped with at least one transmitter 606a-c for transmitting bursts of digital radio symbols to a plurality of mobile stations MS1-MS5 via a transmitter amplifier 607 and an antenna 608. The relative power level of each transmitted signal is determined by a power control unit 604. The power control unit 604 determines an power at which to transmit to a mobile station based, in part, on information reported by the mobile station to the base station, which information is received on antenna 601, amplified by amplifier 602, and processed by at least one receiver 603a-c. The power control unit 604 obtains received signal strength measurements from the at least one receiver relating to transmissions from a mobile station, as well as information relating to the transmit power used by the mobile station and the signal strength measured and reported by the mobile station. The elements 601-608 are typically colocated in a base station of radio communication network NETl.

FIG 1 together with FIG 2b provides an illustration of the effects of the TSPC method utilized by the radio communication network NETl. The second mobile station MS2 is shown to be located in a relatively remote region of the first cell Cl (i.e., at or near the border of the cell) . Hence, the downlink transmit power during the second timeslot TS12 is relatively high, as seen in FIG. 2b. The third mobile station MS3 is shown to be located closer to the base station BS1 than the second mobile station MS2 is. Hence, the downlink transmit power level during the third timeslot TS13 is lower than the downlink transmit power level during the second timeslot TS12. The first mobile station MSI, on the other hand, is shown in FIG. 1 as being located even closer to the base station BS1 and accordingly the downlink transmit power during the timeslot TSll is lower than both in timeslots TS12 and TS13. Thus, the first base station BS1 transmits a burst to the first mobile station MSI in the first timeslot TSll at downlink transmit power level al, increases the downlink transmit power level to a4 for transmission to the second mobile station MS2 in timeslot TS12 and decreases the downlink transmit power level to a2 for transmission of a burst to the third mobile station MS3 in timeslot TS13. This power control scheme will hereafter be refered to as normal mode TSPC. Methods and apparatuses to facilitate output power control in the above described manner are disclosed in e.g. US patent No. 6072792 and US patent application No. 09/399764, which are incorporated by reference herein.

The aspect of synchronization effecting the present invention relates to how the mobile station will find the correct timeslot on the assigned frequency carrier. This type of synchronization is necessary whenever a mobile station is assigned a new digital traffic channel, e.g. at call set-up or handoff. As an example, consider a situation where the fourth mobile station MS4 in FIG. 1, which is currently assigned to a digital traffic channel comprising the second timeslot TS22 on the second downlink radio frequency as illustrated by FIG. 2c, is ordered to perform handoff to a digital traffic channel comprising the second timeslot TS32 on the third downlink radio frequency channel F3, see FIG. 3a. The fourth mobile station MS4 then performs synchronization in two steps:

1. The fourth mobile station MS4 finds any synchronization word SYNC31-SYNC33 on the third downlink radio frequency carrier F3.

2. The fourth mobile station MS4 calculates where in time the assigned timeslot TS32 is positioned on the third downlink radio frequency carrier F3 using knowledge of which synchronization word was found and its time position in the frame when it was found.

The mobile station performs the first step by collecting data from the received downlink frequency carrier during a time period that is slightly longer than one timeslot. The received digital symbols are then searched for the known sequence of symbols constituting a synchronization word. The time period is chosen as to always contain at least one complete synchronization word, i.e. at least the length of one timeslot plus the length of one synchronization word.

The mobile station is typically not capable of doing the search for the synchronization word in real time, but has to first collect data during the complete said period and then analyzes the content in order to find the synchronization word. This is a time-consuming process, but it is normally completed within time-limit for a succesful handoff. If however, the mobile station is not capable of finding the synchronization word, it has to collect a new set of digital symbols and again search for the synchronization word. This will cause delay of the handoff, possibly resulting in a prolonged "muting" of the speech or in the worst case cause a handoff failure. The synchronization process at call setup is identical.

In systems deploying the interim TDMA standards such as IS- 136 rev. B, no downlink power control was used and the downlink transmit power level, e.g. at level a5 in FIG. 3a, was kept constant during all timeslots. Since all synchronization words as well as the rest of the timeslots were transmitted at the same high power level at all times, step 1. was in normal cases not a problem.

In a system using downlink power control, especially the TSPC as illustrated in FIG. 2b-d, it may be impossible or take unacceptable long time for the mobile station to perform the synchronization steps. This can be exemplified with the mobile station MS4 being ordered to handoff into cell C3 , e.g. to frequency carrier F3 and timeslot TS32, see FIG. 1 and FIG 3b. The TSPC algorithms will recognize the mobile stations power need and adjust the downlink transmit power accordingly, as seen in FIG. 3b. Typically for the handoff situation the mobile station MS4 will be very close to the cell border and the downlink transmit power level used will be close to maximum. No mobile stations were assigned to the third carrier F3 , i.e. for the third frequency carrier F3 all the timslots were idle, prior to the handoff and no signal will be transmitted in the neighboring timeslots TS31 and TS32. Since only 1 out of 3 synchronization words are transmitted the probability for the mobile station to quickly find a synchronization word according to step 1, is severely reduced. In the worst scenario the mobile station MS4 will repeatable sample data in a part of the TDMA frame where no synchronization word is transmitted and the handoff will fail. This could happen if the mobile station always sample the same part of the TDMA frame, e.g. starting in the beginning of timeslot TS33 and ending before the end of timeslot TS31. Even if the mobile station is able to vary where in the frame it will start its sampling period the average time needed to find a synchronization word would increase.

The problem would in many realistic cases remain even if the mobile station MS4 was assigned to a carrier transmitting on more than one timeslot . Illustrated as another exemplary scenario in FIG. 3c also timeslots TS31 and TS33 are in use. The power levels determined by the TSPC algorithms for said timeslots TS31-TS33 being al and a2, respectively. If the downlink transmit power levels used for timeslot TS31 and timeslot TS33 are lower than the power level needed for communication with the mobile station MS4, the mobile station MS4 may not be able to correctly receive and decode the data transmitted on TS31 and TS33. The mobile station MS4 will likely not recognize the synchronization words SYNC31 and SYNC33 of timeslots TS31 and TS33 and the same situation as in the case with an empty carrier (illustrated in FIG. 2) occurs.

One possible solution to the above-described problem with synchronization in a downlink power controlled TDMA system would be to temporarily inhibit the use of downlink power control on a downlink radio frequency carrier while a mobile station is attempting to synchronize to its newly assigned timeslot on the radio frequency carrier. Thus according to this solution, if the fourth mobile station is being handed off from the second timeslot TS22 on the second downlink radio frequency carrier F2 to the second timeslot TS32 on the third downlink radio frequency carrier F3 , the whole third downlink radio frequency carrier F3 is transmitted at full power (as illustrated in Fig. 3a) until the mobile station MS4 has found its assigned second timeslot TS32, and started transmitting data to the third base station BS3 in a corresponding uplink timeslot. After communication with the fourth mobile station MS4 on timeslot TS32 have been established, downlink power control would be resumed on the third downlink radio frequency carrier F3.

The solution described above produce unnecessary interference in the system since the entire TDMA frame is transmitted at a high power level during the period when the fourth mobile station MS4 is synchronizing. The idea of minimizing the interference made possible by introducing downlink power control is impaired.

The present invention provides enhanced downlink power control which eliminates or at least reduces the synchronization problems associated with downlink timeslot power control while at the same time being power efficient and not causing unnecessary interference on downlink radio frequencies.

A first exemplary scenario illustrating an examplary embodiment of the invention is illustrated in FIG. 3e. The synchronization words SYNC31-SYNC33 of timeslots TS31 and TS33 (and the incoming mobile station's timeslot TS32) are transmitted at a downlink power level high enough for the mobile station MS4 to correctly receive the transmitted digital symbols, e.g. power level a5. The remaining parts, i.e. the parts not containing synchronization words, of timeslots TS31 and TS33 are attenuated to a level appropriate for the mobile stations assigned to respective timeslot. In FIG. 3e no mobile stations are assigned to timeslot TS31 and TS33 and therefore the signal may be attenuated to zero in the parts of the timeslots not containing synchronization words.

According to the exemplary scenario illustrated in FIG. 3e, the second timeslot TS32 assigned for mobile station MS4 will during the synchronization process be transmitted at the power level a5 and also the synchronization words of the first and third timeslot TS31 and TS33, SYNC31 and SYNC33, will be transmitted at the same power level a5. When the synchronization is over, the downlink transmission power levels for the synchronization words are decreased to match power requirements of each respective mobile station, see FIG. 3b. In the present case of no mobile stations assigned to timeslots TS31 and TS33, downlink transmission power will be decreased to zero. In FIG. 3d is shown the power diagram for the second downlink radio frequency carrier F2 after handoff. After the handoff is completed only the fifth mobile station MS5 is located in the second cell C2. The third timeslot TS23 is assigned the fifth mobile station MS5 and in that timeslot a burst of radio symbols has a downlink transmit power level a2.

A second exemplary scenario illustrating the invention in a situation with more than one mobile station assigned to the frequency carrier is illustrated by FIG. 2c and FIG. 4a-c. The second mobile station MS2 is moving out of first cell Cl into second cell C2 and will be ordered to handoff to timeslot TS21 on frequency carrier F2. The carrier F2 also upholds communication with mobile stations MS4 and MS5 on timeslots TS22 and TS23, respectively, see FIG. 2c. Illustrated in FIG. 4a is the downlink transmission power situation during the synchronization process of mobile station MS2. Timeslot TS21 as well as the synchronization words of TS22 and TS23, SYNC22 and SYNC23, are transmitted at power level a5. The power level a5 is chosen to enable the second mobile station MS2 to correctly receive the transmitted digital symbols. The downlink transmit power levels, a3 and a2 , used for the remaining parts of timeslots TS22 and TS23 are determined by the TSPC algorithms in the same way as during normal mode TSPC. This refined TSPC scheme used during the synchronization process is denoted synchronization mode TSPC. On completion of the synchronization process, see FIG. 4b, the downlink transmission power used during the synchronizaton words SYNC22 and SYNC23 are attenuated to the same power level as the data carrying part, DATA22 and DATA23, of their respective timeslots TS22 and TS23. The normal mode TSPC algorithms do now determine the appropriate power levels used in all timeslots. FIG. 4c illustrates that after handoff timeslot TS12 on the first downlink radio frequency carrier Fl is idle, i.e. only timeslots TSll and TS13 are used for active communication on the carrier Fl .

The flowchart 500 of FIG. 5 illustrates an exemplary embodiment of the invention according to which a normal handoff routine is modified to incorporate the synchronization mode TSPC. In first step 501, the current serving base station BS1 sends a handoff request to the mobile switching center MSCl. In next step 502, the mobile switching center MSCl selects and seizes an idle (target) traffic channel e.g. second frequency carrier F2 and target timeslot TS21 (FIG. 2c and 4a-c) . Further in step 503, the mobile switching center MSCl orders the base station BS2 in the target cell C2 to begin downlink transmission on the seized traffic channel, i.e. in the target timeslot TS21 of the target downlink radio frequency carier F2. At the same time the target basestation BS2 is ordered to change downlink power control from the normal mode TSPC to Synchronization mode TSPC. In step 504, the target base station BS2 in the target cell C2 begin downlink transmission of bursts of digital radio symbols on the seized channel and changes downlink power control mode for the target downlink radio frequency carrier F2 to synchronization mode TSPC. According to this exemplary embodiment of the invention, while operating in synchronization mode TSPC, at least part of the synchronization words in at least one other timeslot, TS22 and/or TS23, on the target downlink radio frequency carrier F2 are transmitted using the same power level or a higher power level than the power level used for transmission in the target timeslot TS21. Preferrably all digital radio symbols for synchronization in all timeslots other than the target timeslot, i.e. both timeslots TS22 and TS23, on the target downlink radio frequency carrier F2 are transmitted using a transmission power level, or a plurality of transmission power levels, equal to or higher than the transmission power level used for transmission in the target timeslot TS21.The mobile switching center MSCl orders in step 505, the current serving base station BS1 to send a handoff order to the second mobile station MS2. The base station BS1 forwards the handoff order to the second mobile station MS2, in step 506 on the current traffic channel. Next, in step 507, the mobile switching center MSCl is informed by the current base station BS1 that the handoff order have been sent to the second mobile station MS2 and in step 508, the second mobile station MS2 acknowledges the receipt of the handoff order by sending a Mobile Ack Message on the return traffic channel (uplink transmission) . In step 509, the second mobile station MS2 tunes to the target downlink radio frequency carrier F2 and synchronizes according to steps 1-2 described previously, with the target timeslot TS21 on the target downlink radio frequency F2. The conversation is continued in step 510 by that the second mobile station MS2 begin sending bursts of digital radio symbols including data (uplink transmission) . Upon detecting uplink transmission from the mobile station MS2 in the correct uplink timeslot, the basestation BS2 will in step

511 change the downlink power control from synchronization mode TSPC back to normal mode TSPC. The synchronization words, SYNC22 and SYNC23, of timeslots TS22 and TS23 will again be transmitted at the same power level as the rest of the respective timeslot, see FIG. 4b. Continuing with step

512 the target cell C2 informs mobile switching center MSCl that the handoff was successful, and in step 513 the mobile switching center MSCl orders the first cell Cl to stop downlink transmission on timeslot TS12 on frequency carrier Fl, see FIG. 4c. The implementation of the invention will be very similar at call set-up or any other kind of accessing a new frequency carrier by a mobile station.

By introducing the above-described synchronization mode TSPC a mobile station synchronizing to a new frequency carrier will not experience the problem of not finding a synchronization word quickly since all synchronization words are transmitted at high enough downlink transmit power level. The probability of finding a synchronization word in a certain time period will be the same as if no downlink power control was used. At the same time, by only transmitting at a high power level when necessary, interference will be decreased compared to transmitting at a high constant level for all timeslot during synchronization.

The downlink transmit power level used for the target downlink timeslot as well as the power level for synchronization words of other timeslots of the target downlink radio frequency carrier, has for clarity been illustrated as being the maximum downlink transmit power level a5. However, one skilled in the art will recognize that a number of methods are taught for predetermining the needed initial downlink transmit power level. The predetermination can for example be based on measurements or based on apriori knowledge of the system configuration. Examples hereof are found in international patent application No. 99/34531 and US patent application WO 09/471128, which are incorporated by reference herein. One embodiment of the invention uses a predetermined power level for the downlink transmission of the target timeslot TS21, e.g. power level a4 in FIG. 4a. According to this embodiment, the same power level a4 will be used for the synchronization words SYNC22 and SYNC23.

In another embodiment of the invention it is recognized that not all synchronization words in all timeslots need to be transmitted at an increased power level in order for a mobile station to be able to synchronize. Depending on the mobile station characteristic it may be sufficient to e.g. transmit synchronization words SYNC21 and SYNC23, but not SYNCH22, at power level a5 to facilitate the synchronization. Alternatively which synchronization words to be transmitted at a high power level could be made to vary from one TDMA frame to another. E.g. in the first frame is SYNC21 transmitted at power level a5, in the second frame is SYNC21 and SYNC22 and in the third frame SYNC21 and SYNC23 are transmitted at power level a5.

In FIG. 2-4 all downlink transmit power attenuation has been illustrated as being instantaneous . In reality the attenuation rate will be limited both by the performance of the transceivers in the basestation and the dynamic abilities of the receiver in the mobile stations.

According to yet another exemplary embodiment of the invention, the power control unit 604 is adapted to control transmission power levels on a radio frequency carrier so that at least some, preferrably all, digital symbols for synchronization in each burst transmitted on a downlink radio frequency carrier are transmitted using a predetermined transmit power level equal to or higher than the maximum transmission power level required by any mobile station currently assigned a timeslot on said downlink radio frequency carrier. Said predetermined transmit power level may be set equal to the maximum power level defined for the base station or may alternatively be adapted to the maximum power level required by any mobile station currently assigned a timeslot on said downlink radio frequency carrier.

In radio communication systems where multiple carrier power amplifiers (MCPA) are employed, the usage of a high downlink transmission power level, during synchronization, on only parts of the radio frequency carrier instead of the whole carrier results in a smaller likelihood of maximum power usage of the ^*MCPA, and thus a smaller and more cost efficient MCPAs can be used for the same system.

As a person skilled in the art appreciates, application of the invention is in no way limited to only cellular radio communication networks conforming to the TDMA(EIA/TIA-136) specifications. Thus the invention is also applicable in other Time Division Multiple Access (TDMA) based radio communication systems, e.g. cellular networks adhering to the GSM- or PDC specifications, in which the downlink transmission power is controlled per timeslot (TimeSlot Power Control, TSPC) .

As will be recognized by those skilled in the art, the innovative concepts described in the present application can be modified and varied over a wide range of applications. Accordingly, the scope of patented subject matter should not be limited to any of the specific exemplary teachings discussed.

Claims

1. A power control method in a time division multiple access based radio communication system for selectively controlling power levels in a plurality of timeslots (TS21-TS23) of a radio frequency carrier (F2) for transmission of bursts from a base station (BS2) to at least one mobile station (MS2), wherein each burst includes digital symbols for synchronization (SYNC21- SYNC23) , c h a r a c t e r i s e d i n that said method comprises a step of: changing (504) downlink power control mode for said radio frequency carrier from a normal mode timeslot power control to a synchronization mode timeslot power control.

2. A method according to claim 1, wherein the method includes a further step of determining that a first mobile station (MS2) is about to synchronize to a first timeslot (TS21) on said radio frequency carrier (F2) and wherein said changing of downlink power control mode is performed upon said determining.

3. A method according to claim 2 , wherein during normal mode timeslot power control, transmission power levels of all timeslots (TS21-TS23) on said radio frequency carrier (F2) currently assigned for communication with mobile stations (MS4-MS5) are adjusted to power requirements of said mobile stations (MS4-MS5) respectively.

4. A method according to any one of claims 2-3, wherein during synchronization mode timeslot power control, a transmission power level adapted to power requirements of said first mobile station (MS2 ) is used for transmissions in said first timeslot (TS21) and at least some digital symbols (SYNC22) for synchronization in at least a second timeslot (TS22) on said radio frequency carrier (F2) are transmitted using at least one selected transmission power level equal to or higher than said adapted transmission power level .

5. A method according to claim 4, wherein during synchronization mode timeslot power control, all digital symbols (SYNC22) for synchronization in at least said second timeslot (TS22) are transmitted using said at least one selected transmission power level.

6. A method according to claim 4, wherein during synchronization mode timeslot power control at least some digital symbols for synchronization (SYNC21- SYNC23) in all timeslots (TS21-TS23) on said radio frequency carrier (F2) are transmitted using said at least one selected transmission power level.

7. A method according to claim 4, wherein during synchronization mode timeslot power control all digital symbols for synchronization (SYNC21-SYNC23) in all timeslots (TS21-TS23) on said radio frequency carrier are transmitted using said at least one selected transmission power level.

8. A method according to any one of claims 4-7, wherein during synchonization mode timeslot power control, at least some digital symbols for synchronization are transmitted in at least one idle timeslot (TS31, TS33) on said radio frequency carrier (F3) using said at least one selected transmission power level.

9. A method according to any one of claims 4-8, wherein said at least one selected transmission power level is equal to a maximum power level defined for said base station (BS2, BS3 ) .

10. A method according to any one of claims 2-9, wherein said method comprises a further step of: changing downlink power control mode for said radio frequency carrier (F2) from said synchronization mode timeslot power control to said normal mode timeslot power control upon detecting that said first mobile station (MS2) has successfully completed synchronization.

11. A method according to any one of claim 10, wherein said detection that said first mobile station (MS2) has successfully completed synchronization includes detecting transmissions of data in an uplink direction from said first mobile station (MS2) to said base station (BS2) in an uplink timeslot corresponding to said first timeslot (TS21) .

12. A method according to any one of claims 2-11, wherein said synchronization mode timeslot power control is initiated when a mobile switching center (MSCl) orders said base station (BS2) to start transmitting on said first timeslot (TS21) on said radio frequency carrier

(F2) .

13. A method according to any one of claims 1-12, wherein said synchronization mode timeslot power control is initiated due to a call set-up.

14. A method according to any one of claims 1-12, wherein said synchronization mode timeslot power control is initiated due to a handoff.

15. power control method in a time division multiple access based radio communication system for selectively controlling power levels in a plurality of timeslots (TS21-TS23) of a radio frequency carrier (F2) for transmission of bursts from a base station (BS2) to at least one mobile station (MS2) , wherein each burst includes digital symbols for synchronization (SYNC21- SYNC23) , c h a r a c t e r i s e d i n that said method comprises a step of: transmitting at least some of said digital symbols for synchronization in each burst using a predetermined transmit power level equal to or higher than the maximum transmission power level required by any mobile station (MS4-MS5) currently assigned a timeslot (TS22-TS23) on said radio frequency carrier (F2).

16. A method according to claim 15, wherein said predetermined transmit power level is equal to a maximum power level defined for said base station (BS2) .

17. method according to any one of claims 15-16, wherein all digital symbols (SYNC21-SYNC23) for synchronization in each burst are transmitted using said predetermined transmit power level .

18. A method according to any one of claims 15-17, wherein if reliable communication is possible with a mobile station (MS2), which is assigned a specific timeslot (TS21) on said radio frequency carrier (F2), using less than said predetermined transmit power level, part of each burst transmitted to said mobile station (MS2) is transmitted using a transmission power level matching the current power requirements of said mobile station (MS2) .

19. A method in a time division multiple access based radio communication system for selectively controlling power levels in a plurality of timeslots (TS21-TS23) of a radio frequency carrier (F2) for transmission of bursts from a base station (BS2) to at least one mobile station (MS2), wherein each burst includes digital symbols for synchronization, c h a r a c t e r i s e d i n that said method comprises steps of : determining that a first mobile station (MS2) is about to synchronize to a first timeslot (TS21) on said radio frequency carrier (F2); transmitting a first burst in said first timeslot (TS21) using a first power level; transmitting a second burst in a second timeslot (TS22) while using a second power level during transmission of at least some digital symbols (SYNC22) for synchronization in said second burst, wherein said second power level is equal to or higher than said first power level .

20. A method according to claim 19, wherein said synchronization is initiated due to a call set-up.

21. A method according to claim 19, wherein said synchronization is initiated due to a handoff .

22. A method according to claim 19-21, wherein said second power level is used for transmission of at least some digital symbols for synchronization in all timeslots (TS22-TS23) other than said first timeslot (TS21) .

23. A method according to claim 19-21, wherein said second power level is used for transmission of all digital symbols for synchronization in all timeslots (TS22-TS23) other than said first time slot (TS21) .

24. A time division multiple access based radio communication network (NETl) comprising at least a first power control unit (604), and at least a first transmitter (606a-c) , wherein said first transmitter is arranged for transmitting bursts in a plurality of timeslots of a radio frequency carrier (F1-F2), each burst including digital symbols for synchronization (SYNC11-SYNC13 , SYNC21- SYNC23), said radio communication network (NETl) c h a r a c t e r i s e d i n that said first power control unit (604) is adapted to control said first transmitter to apply a first power level for transmission of a first burst in a first timeslot (TS21) and to apply a second power level for transmission of at least some synchronization symbols (SYNC22) in a second burst transmitted in a second timeslot (TS22), wherein said second power level is equal to or higher than said first power level .

25. A time division multiple access based radio communication network (NETl) comprising at least a first power control unit (604), and at least a first transmitter

(606a-c) , wherein said first transmitter is arranged for transmitting bursts in a plurality of timeslots of a radio frequency carrier (F1-F2) , each burst including digital symbols for synchronization (SYNC11-SYNC13 , SYNC21- SYNC23), said radio communication network (NETl) c h a r a c t e r i s e d i n that said first power control unit (604) is adapted to control said first transmitter to apply a predetermined transmit power level for transmission of at least some of said digital symbols for synchronization in each burst using said predetermined transmit power level equal to or higher than the maximum transmission power level required by any mobile station (MS4-MS5) currently assigned a timeslot (TS22-TS23) on said radio frequency carrier (F2) .

26. A time division multiple access based radio communication network (NETl) comprising at least a first transmitter (606a-c) and at least a first power control unit (604), wherein said first transmitter (606a) is capable of transmitting bursts in a plurality of timeslots

(TS21-TS23) of a radio frequency carrier (F2) to mobile stations (MS1-MS5) in communication with said radio communication network (NETl) , each burst including digital symbols for synchronization (SYNC21-SYNC23) , said radio communication network (NETl) c h a r a c t e r i s e d i n that said first power control unit (604) is arranged for selectively controlling transmission power levels in said plurality of timeslots (TS21-TS23) on said radio frequency carrier (F2) while applying different timeslot power control modes including at least a normal mode timeslot power control mode and a synchronization mode timeslot power control mode.

27. A radio communication network (NETl) according to claim 26, wherein said radio communication network (NETl) further comprises means ^" for determining that a first mobile station (MS2) is about to synchronize to a first timeslot (TS21) on said radio frequency carrier (F2), and said first power control unit (604) is adapted to change (504) from said normal mode timeslot power control mode to said synchronization mode timeslot power control mode upon said determining .

28. A radio communication network (NETl) according to claim 27, wherein said first power control unit (604) is adapted to adjust transmission power levels of all timeslots (TS21- TS23) on said radio frequency carrier (F2) currently assigned for communication with mobile stations (MS2 , MS4- MS5) to power requirements of said mobile stations (MS2 , MS4-MS5) respectively while operating in said normal mode timeslot power control .

29. radio communication network (NETl) according to any one of claims 27-28, wherein said first power control unit (604) is adapted to adjust the transmission power level of said first timeslot (TS21) to match power requirements of said first mobile station (MS2) and to apply at least one selected transmission power level equal to or higher than said transmission power of said first timeslot (TS21) for transmission of at least some digital symbols for synchronization in at least a second timeslot (TS22) on said radio frequency carrier (F2) while operating in said synchronization mode timeslot power control.

30.A time division multiple access based radio communication system (SYS1) including a radio communication network (NETl) and at least a first mobile station (MS2), said radio communication network (NETl) comprising at least a first transmitter (606a-c) and at least a first power control unit (604) , wherein said first transmitter (606a) is capable of transmitting bursts in a plurality of timeslots^" (TS21-TS23) of a radio frequency carrier (F2) to mobile stations (MS1-MS5) in communication with said radio communication network (NETl), each burst including digital symbols for synchronization (SYNC21-SYNC23) , said radio communication system c h a r a c t e r i s e d i n that said first power control unit (604) is arranged for selectively controlling transmission power levels in said plurality of timeslots (TS21-TS23) on said radio frequency carrier (F2) while applying different timeslot power control modes including at least a normal mode timeslot power control mode and a synchronization mode timeslot power control mode.

31. A radio communication system according to claim 30, wherein said radio communication network (NETl) further comprises means for determining that said first mobile station (MS2) is about to synchronize to a first timeslot (TS21) on said radio frequency carrier (F2), and said first power control unit (604) is adapted to change (504) from said normal mode timeslot power control mode to said synchronization mode timeslot power control mode upon said determining.