WO1999023778A1

WO1999023778A1 - Method and apparatus for reducing the effect of a fading condition in a communications system

Info

Publication number: WO1999023778A1
Application number: PCT/US1998/022625
Authority: WO
Inventors: John Douglas Reed; John Stephen Ruppel
Original assignee: Motorola Inc.
Priority date: 1997-11-04
Filing date: 1998-10-27
Publication date: 1999-05-14
Also published as: TW406489B

Abstract

In a communication system (100) where a radio frequency signal is transmitted from a transmitting source (101) and received at a receiving unit (1022), a method and apparatus are disclosed for reducing an effect of a fading condition of the radio frequency signal at the receiving unit (1022). The method and apparatus include detecting the fading condition, monitoring the radio frequency signal for detecting a transmit power limiting condition occurring at the transmitting source (101), and changing an antenna characteristic in the communication system (100) when the transmit power limiting condition and the fading condition are present. In the monitoring step, the transmit power limiting condition is detected when a threshold associated with a cumulative number of the power increase commands is reached.

Description

Method and Apparatus for Reducing the Effect of A Fading Condition in a

Communication System

Field of The Invention The present invention generally relates to radio frequency communication systems, and more particularly, a cellular communication system.

Background of The Present Invention Radio Frequency (RF) communication systems very often adopt a power control mechanism to reduce the effects of problems associated with the RF signal propagation channel whereby improving the system performance. An RF signal experiences what is commonly known as fading as it propagates through an RF channel between a transmitting unit and a receiving unit. The effects of a fading condition in an RF channel result from summing the scattered rays at the antenna. The scattered rays are normally produced by one or more dominant rays that arrive at or near the receiving unit. A fade condition occurs when vector addition of rays at the antenna results in a reduction in the resulting signal quality compared to an average or some other threshold. The reduction in the signal quality is the result of a destructive condition of summing the scattered rays and the dominant rays that are received at the receiving antenna. Fading is furthermore characterized by the rate which the amplitude and phase of the resulting signal are changing. In a fast fade condition, the resulted RF signal amplitude and phase are rapidly changing. In a slow fade condition, the average amplitude and phase may remain substantially constant for a long period of time. Although both slow and fast fading may be present at any time, slow fades occur more often where transmit and receive units are stationary.

In cellular communication systems, a mobile unit does not experience the destructive effects of a slow fade condition for a sustained period since it is constantly moving. A slight change in the position of the antenna is sufficient to move away from a deep null in the RF signal amplitude that has been created by the destructive condition of summing the rays which is a characteristic of the slow fading condition. However, a recently proposed type of duplex radio telephone system called Fixed Wireless Terminal (FWT) requires that the terminals be at fixed locations. When a terminal is stationary, a slow fade is likely to happen, and may last for a long time. In general, to reduce the effect of a fading condition, the cellular system through its communication protocol standard commands the transmitting unit to increase the transmitting power in response to the fade condition occurring at the receiving unit. However, increasing power level at the transmitting unit is a limited solution since the transmitter has only a limited available power. The limited available power is due to the power amplifier saturating or reaching a pre-defined operating maximum limit. Moreover, on the forward link, the basestation has a system pre-defined maximum transmit power limit which is established due to practical system constraints. When the limited power level is reached, the communication link would severely degrade if additional signal power is needed to respond to a slow fade condition.

Therefore, there is a need for an improved method of reducing the effects of a fading condition in a communication system. Brief Description of Drawings

FIG. 1 depicts general components of a cellular communication system.

FIG. 2 depicts general block diagram of a transceiver according to one aspect of the present invention. FIG. 3 depicts a flow chart of a power control loop according to prior art.

FIG. 4 depicts a flow chart of a power control loop according to various aspects of the present invention.

FIG. 5 depicts general block diagram of a transceiver according to another aspect of the present invention. Detailed Description of the Preferred Embodiment(s)

In a communications system, according to the present invention, where a radio frequency signal is transmitted from a transmitting source and received at a receiving unit, a method of reducing an effect of a fading condition of the radio frequency signal at the receiver includes detecting the fading condition, monitoring the radio frequency signal for detecting a transmit power limiting condition occurring at the transmitting source, and changing a characteristic of an antenna of the communication system when the transmit power limiting condition and the fading condition are present. The change in the antenna characteristic includes changing the antenna directivity, gain, pattern shape, change of a null location or the antenna location of either the transmit antenna at the transmitter or a receiving antenna at the receiver or both. The occurrence of transmitter power limiting condition is reported by the unit performing the monitoring step. A quality of the radio frequency signal, such as bit error rate, determines the fading condition at the receiving unit. The transmit power limiting condition is characterized as the transmit power saturating condition or a pre-defined maximum transmit power limiting condition. In the monitoring step, the transmit power limiting condition is detected when the transmitting source fails to respond to a power increase command or reaches its maximum power setting. Furthermore, in the monitoring step the transmit power limiting condition is detected when a threshold associated with a cumulative number of the power increase commands is reached, or the threshold is reached over a period of time. The monitoring step is performed in either the transmitting source or the receiving unit or both. Therefore, as a result, when a fading condition is present, the present invention overcomes the problem associated with limited transmit power level.

With reference to FIG. 1, a cellular communication system 100 is conceptually shown to include a basestation 101, and a number of remote units 1021-1024. Although only four remote units are shown in FIG. 1, the number of remote units in system 100 may exceeds thousands depending on the capacity of basestation 101. Each remote unit has a two way communication link with the basestation 101. The direction of each communication link, in FIG. 1, is shown by the direction of arrow heads. Each of the communication links from the basestation to remote units is called a forward link, a communication link in the opposite direction is called a reverse link. Other names, such as uplink, for reverse, and down link, for forward, have been used in other communication systems. Each remote unit has at least one antenna. More than one antenna is very often used for the purpose of receive and transmit diversity combining. To provide diversity reception and transmission, the antennas are separated by a fix distance. In addition, the antennas may be set at a different polarization which reduces the required distant separation.

Most communication systems, such as Code Division Multiple Access (CDMA) and Group Special Mobile (GSM), have a transmit power control mechanism which is incorporated into its standard protocols.

With reference to FIG. 2, for example, the remote unit 1021 in response to a command by its controller 201 adjusts the transmitter 202 output power. As a result, the reverse link signal 105 may be received at a higher power level at the basestation 101. The command in 201 is generated in response to forward link 106 carrying a message that the signal received at the basestation 101 is degraded, and, therefore, requesting an increase of signal power. The degradation in signal is either due to far location of the remote unit 1021 from the transmitting unit which would cause large propagation loss or a fade condition in the RF channel. Since the transmitting and receiving units are at fixed locations, like in a FWT system, the degradation in the signal would be most likely due to a slow fade condition. If the transmitter 202 is in a saturating condition, or it has reached a pre-defined maximum limit, the command to increase power would not consequently have any effect on the transmit power level. The maximum power level may be set at any valid transmit power level as defined by the system requirements. According to present invention, a block 250 has been added to the antenna assembly to adjust the transmit antenna characteristic. The controller 201, upon detecting the fade condition and transmit limiting condition, adjusts phase and amplitude of one of the transmit signal before being transmitted. Although phase adjustment alone allows achieving the desired results, amplitude adjustment may also be included. The block 250 includes a splitter 261, and a phase and amplitude adjuster 260. The amplitude and phase of a signal may be adjusted by a variety of ways known in the art. A controllable attenuator may adjust the amplitude of the signal according to control signal 251; and a controllable delay circuit may delay the signal for the phase adjustment according to the control signal 251. The phase and amplitude adjustments allow enhancement of the total transmit antenna pattern observed from the receiving unit, thereby, eliminating the deep null from its former location close to the receiving unit to another location. The relocation of the deep null most likely is far enough from its former location that reduces the effect of the fade condition in the receiving unit.

The mechanics of a power control loop according to prior art is shown by way of a flow chart in FIG. 3. If the receiving signal carries a power control bit, the controller 201 detects that at a step 301. If the power control bit is at logic zero, in this example, it means that the transmitter should reduce its power level. However, if the power level is at the predetermined minimum level already, the transmitter should not reduce its transmit power level. If the power control bit is at logic 1, in this example, and the transmit power level is not at its maximum level, the transmit power is increased. The entire process is shown in FIG. 3 at steps 301-306.

To maintain a power control loop over the transmit power of a fix unit (FWT), or as may be called a subscriber unit, a continuous stream of power control bits is being communicated to the fix unit from the basestation based on CDMA protocols. The control bit stream includes step up and step down power control bits alternatively at an 800 Hz rate. If each step up is followed by a down step, then the power level will be toggling up and down by approximately a dB which then keeps the average power level at a constant level. If the signal power level is low, the basestation communicates power up commands at more intervals than power down commands. To maintain a power control over the basestation transmitter, the subscriber sends a request for more transmit power from the basestation transmitter. The subscriber is requesting power increase most likely due to detecting poor received frames signal quality. In response, the basestation transmits at a predetermined higher power level if it has not reached its maximum setting.

According to present invention, as shown in FIG. 4, the power control loop is modified to include a step 407 of initiating an antenna characteristic change when at step 305 is determined that the transmitter is in a saturating condition or has reached a pre-defined maximum limit and a fading condition is present at the receiving unit. It is common in the art to specify some level below saturation as a safe operating point, and this may be used to establish the maximum limit. This limited level may be as much as 10 to 20 dB below the transmit power saturating level to control the effect of interference or improve battery life among many other reasons. Also, it is common to set different power limits for the transmitting units which may depends on the location of the transmitting unit. The effect of signal degradation due to fading is closely tied to the transmit or receive antenna characteristics or both since even small percentage changes in the antenna characteristic will change the vector sum of rays at the receiver. Therefore, fading conditions are in general different at different antenna characteristics. A change in transmit antenna pattern in response to a power-up command when the transmitter is in a limiting condition most likely changes the fading effect occurring at the remote receiving unit. With reference to FIG 5, the unit 1022 is shown to include an antenna switch 550 which allows switching between the antennas 122 and 125. In this case according to present invention, each antenna has a pre- defined transmit pattern. To change the transmit antenna pattern, the controller 501 switches the transmit antenna. The control signal 551 from controller 501 would switch the transmitting antenna in response to a power-up command when the transmitter is in a limiting condition and a fading condition at the receiving unit is present. The available antennas, 122 and 125, are separated apart by some predetermined distance which each antenna would then provide different antenna characteristic. Additionally, the antennas 122 and 125 may be set to have different polarization to reduce the requirement of distant separation. Therefore, by switching from one to another antenna in response to a power level limiting condition at the transmitting unit and a fading condition at the receiving unit, the fading effect at the receiving unit is eliminated or substantially reduced. Other mechanisms are available to change the antenna patterns, such as, changing the direction of the transmitting beam by turning the antenna via a motor or a mechanical means.

Additionally according to present invention, the effect of fading condition in a receiver unit is reduced or eliminated when the receiving unit adjusts the antenna characteristics of the receive antennas. This is illustrated in 550 by the receiver switching its receiving antenna to another antenna which is physically located in a different location. Therefore, at step 407 with reference to FIG 4, the decision is made to modify the receiving antenna characteristic in response to fading condition and transmit limiting condition. The change in the receiving antenna characteristic is an alternative to changing the transmit antenna characteristic, however, a combination of changing the transmit and receiver antennas characteristics also achieve the results according to the present invention. The characteristics of the receiver and transmitter antennas can be modified in the same way which includes a change in antenna directivity, gain, pattern shape, change of a null location or the location of a antenna. There are a number of ways to determine in step 305 when a transmitter is in a saturating condition, or when the transmitter is at its pre-defined maximum limit. One method is to monitor the transmit power level when requesting transmit power increase. If the transmit power level does not change accordingly, the transmitter is considered to be in a saturating condition, or at its maximum limit. Another method is to characterize the transmitter power output characteristics before installation, and then during operation, the controller knowing this characteristic monitors the transmitter for finding the saturating condition or its pre-defined maximum limit.

If the step 305 decision is being made in the receiving unit, the receiving unit requests power increase from the transmitting unit repeatedly. When no improvement in the received signal quality is experienced at the receiving unit, the receiving unit can make the determination at step 305 that the transmitting unit is in a limiting condition. A message then will be send out to indicate the transmitting unit limiting condition. The signal quality generally improves if the transmitting unit is actually increasing the signal transmit power level. The quality of the signal is measured either by received errors or received signal strength indicator based on the received signal strength. In this case, the number of requests to increase power level made by the receiving unit should go above a predetermined threshold level to determine the occurrence of a transmit limiting condition. With reference to FIG. 4, the block 408 is shown to generate a threshold signal 409 whenever the condition to change the antenna characteristic according to present invention is satisfied. For example, if the number of power up commands exceeds a threshold number over a period of time, the condition to initiate an antenna characteristic change has been satisfied. If the power up command is for the basestation to increase its power level, the condition to change the antenna pattern is satisfied when the basestation has reached its predetermined maximum power level allocated to the subscriber unit after the basestation receives a number of power up requests repeatedly from the subscriber unit.

The command to change the transmit antenna pattern or receiver antenna location at step 407 can be executed from either the transmitting or receiving unit. Where the transmitter saturating condition or predefined maximum limit is detected by the transmitting unit, the same unit also initiates the process of changing the transmit antenna pattern or the location of the receiver antenna. If it is decided to change the receiving antenna location, the transmitter would transmit a message to the receiving unit accordingly. In another situation, the receiving unit which is experiencing the effect of the fading condition can initiate the process of changing the transmit antenna pattern or changing the location of its receiving antenna. To change the transmitter antenna pattern, the receiver transmits a message to the transmitting unit to change its transmit antenna pattern. In a more general approach where the system prefers changing transmit antenna pattern or receiver antenna location to be initiated from a unit other than the unit where the power limiting condition is being monitored or the unit where the fading condition is detected, a message is transmitted to the preferred unit. The message indicates that a power limiting condition has been detected in a transmitting unit and a fading condition in the receiving unit.

While the invention has been particularly shown and described with reference to a particular embodiment, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention.

The corresponding structures, materials, acts and equivalents of all means or step plus function elements in the claims below are intended to include any structure, material, or acts for performing the functions in combination with other claimed elements as specifically claimed. What is claimed is:

Claims

1. In a communications system where a radio frequency signal is transmitted from a transmitting source having a first antenna and received at a receiving unit having a second antenna, a method of reducing an effect of a fading condition of said radio frequency signal at said receiving unit, comprising the steps of: detecting said fading condition of said radio frequency signal; monitoring said radio frequency signal for a transmit power limiting condition occurring at said transmitting source; and changing an antenna characteristic of said communication system when said transmit power limiting condition and said fading condition are present.

2. The method as recited in claim 1 wherein said changing said antenna characteristic of said communication system is changing a characteristic of at least one of said first antenna and said second antenna.

3. The method as recited in claim 1 wherein said detecting step, a quality of said radio frequency signal at said receiving unit determines said fading condition.

4. The method as recited in claim 3 wherein said quality is a bit error rate of said radio frequency signal.

5. The method of claim 1 wherein said transmit power limiting condition is one of a transmit power saturating condition and a predefined maximum transmit power limiting condition.

6. The method as recited in claim 1 wherein said transmit power limiting condition is one of when said transmitting source fails to respond to a power increase command, when a threshold associated with a cumulative number of a power increase commands is reached, and wherein said threshold associated with said cumulative number of said power increase commands is reached over a period of time.

7. The method as recited in claim 1 wherein said monitoring step is performed in said receiving unit.

8. The method as recited in claim 7 further including transmitting a message indicating said transmit power limiting condition.

9. The method as recited in claim 1 wherein said monitoring step is performed in said transmitting source.

10. In a communication system, a controller for controlling power level of a transmit signal, comprising: a receiver for detecting a power control data bit; a transmitter for outputting said transmit signal at a power level in response to said power control data bit; and a threshold signal generator generating a control signal indicating change of an antenna characteristic of said communication system when said power level of said transmit signal is reached a limiting level and a fading condition of said transmit signal is present.

11. The controller as recited in claim 10 wherein said control signal is generated when said receiver detecting a plurality of said power control data bit, and said plurality of power control data bits includes a number of power up commands which exceeds a threshold in a time period.