Abstract:
In a wireless communications system having a transmitter that transmits a transmit diversity signal using multiple antennas, a channel quality metric is computed by measuring a first and second diversity branch signal quality for first and second diversity branches in the receiver. Thereafter, the channel quality metric is computed in response to a difference between the first and second diversity branch signal qualities. The first and second diversity branch signal quality measurements may be signal-to-noise measurements. In one embodiment, the channel quality metric is computed by taking a square root of a product of signal-to-noise ratios of the first and second diversity branches in the receiver.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The following application is a continuation of application Ser. No. 08/946,210 entitled “METHOD AND APPARATUS FOR GENERATING A POWER CONTROL COMMAND IN A WIRELESS COMMUNICATIONS SYSTEM”, filed on Oct. 7, 1997, which is hereby incorporated by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention is related in general to wireless communication systems, and more particularly to an improved method and system for generating a power control metric in an orthogonal transmit diversity communications system. 
     BACKGROUND OF THE INVENTION 
     In many wireless communications systems, and especially in cellular communication systems, it is important to control the transmitted power of a traffic channel in order to reduce cochannel interference. Cochannel interference is generated by other transmitters assigned to the same frequency band as the desired signal. And because all users transmit traffic on the same carrier frequency in a code division multiple access (CDMA) cellular system, reducing cochannel interference in CDMA systems is especially important because it directly impacts system capacity. If the cochannel interference is reduced, the CDMA system capacity may be increased. Therefore, it is a design goal to transmit a traffic signal with only an amount of power necessary to provide acceptable signal quality at the receiver, after it passes through the channel. 
     In this document, a “channel” may be defined as a path or paths of communication through a medium between a transmitter and a receiver. If the medium is air and communication takes place with radio frequency (RF) signals, such a channel is typically affected by fading. A “traffic channel” may be defined as a channel that carries data, whether representing voice or other information generated by the user, which the user intends to transmit via the channel. The traffic channel may be distinguished from other channels used by the communication system, such as channels that may be used to transmit timing, control, or other information supporting system operation. 
     Power control systems in cellular communication systems should compensate not only for signal strength variations due to the varying distance between the base station transceiver and the subscriber unit, but should also attempt to compensate for channel quality fluctuations typical of a wireless channel. These fluctuations are due to the changing propagation environment between the transmitter, or base station, and the receiver, or subscriber unit, as the user moves in the service area. 
     Existing power control systems used in CDMA cellular systems that operate according to J-STD-008, published by the Joint Technical Committee on Wireless Access, use the measurement and reporting of cyclic redundancy check (CRC) errors at the subscriber unit to control the power of the traffic channel at the base unit. This method of power control in response to CRC errors is used to implement a slow “ramping” power control scheme. The “ramping” occurs because the traffic channel power is increased by a relatively large amount when the subscriber unit reports CRC errors. After the large power increase, which often eliminates the CRC errors for some subsequent period, the power is reduced by a relatively small amount for each subsequent frame transmitted. Eventually, the power is reduced to a point where another CRC error occurs, and the power is once again increased by a relatively large amount. If channel quality remains constant, a graph of power transmitted in the traffic channel resembles a saw tooth, with large power increases followed by a series of small power decreases. 
     One problem with this method of power control is the delay encountered between the degradation of channel quality and the request for a power increase and the subsequent actual increase in power. The delay in requesting a power increase is caused by waiting for a frame to be received, and then waiting for frame decoding and the detection of a cyclic redundancy check error. Once the CRC error is detected, it must be reported to the base station, and the base station must respond by increasing traffic channel power. In current CDMA systems, it takes 20 milliseconds (mS) to receive a frame. Thus, the rate at which CRC reports or power control commands are sent to the transmitter is 50 Hz. This delay in the power control loop periodically causes the base to transmit too much power on the traffic channel, such as when a relatively large increase in power is requested and granted just as the channel quality has reached a minimum and starts to improve. If the traffic channel has too much power, cochannel interference increases and system capacity decreases. 
     With reference now to FIG. 1, there are depicted relevant portions of transceiver  20  that uses orthogonal transmit diversity (OTD). As illustrated, traffic channel data source  22  provides a stream of symbols, which may represent voice or data traffic of a plurality of users or channels. The rate that symbols are output from data source  22  is controlled by symbol clock  24 . 
     Symbols from traffic channel data source  22  are convolutionally encoded by convolutional encoder  26 . Convolutional encoder  26  encodes at a rate of one divided by “n”. This means that for every symbol entering convolutional encoder  26 , n encoded symbols are output. Clock multiplier  28  provides a clock for convolutional encoder  26  that is n times the rate of symbol clock  24 . 
     After traffic channel data symbols have been encoded, power control encoder  30  places uplink power control information into the stream of encoded symbols. In one proposed system, this is accomplished by inserting a power control bits in a predetermined bit locations in power control groups of a frame in the data stream. Thus, some traffic channel data bits are replaced, or punctured, by bits intended to direct the subscriber unit to raise or lower its transmit power level. The frequency at which power control bits are punctured remains at a predetermined frequency, which in a preferred system is 800 Hz. Additionally, the power level of the punctured power control bits are set at the full vocoder rate traffic power level. The power control bits are preferably evenly distributed among the transmit antennas. 
     After the power control bits are inserted in the data stream, commutator  32  distributes symbols among diversity branches of the orthogonal transmit diversity transmitter. As shown in transceiver  20 , there are two diversity branches defined by paths through spreaders  34  and  36 , which paths use different spreading codes to spread the symbols in each branch. 
     After the symbols are spread with multiple orthogonal spreading codes, the spread data outputs are amplified by amplifiers  38  and  40 . Amplifiers  38  and  40  are coupled to power controller  42  which controls the gain of amplifiers  38  and  40 . 
     The outputs of amplifiers  38  and  40  are each coupled to separate antennas  44  and  46 , which provide different signals that propagate through different paths r 1  and r 2 , before they may be received by a subscriber unit. Also note that one or both antennas, such as antenna  46 , may be used to receive power control information PC transmitted from a subscriber unit. This power control information is coupled to power controller  42  so that power controller  42  may set the gain of amplifiers  38  and  40 . 
     With reference now to FIG. 2 there is depicted selected portions of a subscriber unit  50  according to the prior art. As shown, antenna  52  receives signals through paths r 1  and r 2 , which carry traffic channel data and other control data. Antenna  52  is also used to transmit power control information PC to transceiver  20  shown in FIG.  1 . 
     Antenna  52  is coupled to down converter and demodulator  54 , which down converts and demodulates the received signals. 
     The output of downconverter and demodulator  54  is split to form diversity branches within subscriber unit  50 . These diversity branches correspond to the antennas and diversity branches within transceiver  20 . Thus, transceiver  20  in FIG. 1 is shown with two diversity branches, and subscriber unit  50  is also shown with two corresponding diversity branches. 
     The paths along diversity branches pass through despreaders  56  and  58 , respectively. These despreaders use despreading codes similar to the spreading codes used in transceiver  20 . 
     Clock recovery circuit  60  is also coupled to the output of downconverter and demodulator  54 . Clock recovery circuit  60  produces a symbol clock that is used by decommutator  62  to reassemble the symbol stream within subscriber unit  50 . The symbol clock is also used by power control group clock  63  to generate a clock having a frequency set at a power control group rate. Since in a preferred embodiment there are 16 power control groups in a 20 mS frame, the power control group rate may be 800 Hz. 
     The output of decommutator  62  is input into a decoder, such as Viterbi decoder  64 , for decoding the convolutionally encoded data. Following decoder  64  CRC circuit  66  performs a cyclic redundancy check on a frame of data to determine whether or not an error has occurred. The output of CRC circuit  66  is coupled to outer-loop threshold circuit  68 , which adjusts an outer-loop, or slower loop, threshold, which helps subscriber unit  50  maintain a selected frame error rate. 
     Subscriber unit  50  also uses fast power control, which is controlled by a faster inter-loop feedback mechanism which comprises signal-to-noise measurers  70  and  72  and an arithmetic mean calculator  74 . In a preferred embodiment, signal-to-noise measurers  70  and  72  are coupled to the diversity branches of subscriber unit  50  for measuring a channel quality, such as a diversity signal-to-noise ratio, for each diversity branch. The outputs of signal-to-noise ratio measurers  70  and  72  are coupled to a arithmetic mean calculator  74 , which calculates the arithmetic mean of the measured signal-to-noise ratios by adding them together and dividing by the number of signal-to-noise ratios. The arithmetic mean output by arithmetic mean calculator  74  is coupled to comparator  76 , which outputs a power control bit to instruct transceiver  20  to increase or decrease transmit power. This information is transmitted from antenna  52  as shown at signal PC, which is also received at antenna  46  in transceiver  20  (see FIG.  1 ). 
     Note that arithmetic mean calculator  74  operates at an inter-loop rate, which in a preferred embodiment is 800 Hz, while CRC circuit  66  and outer loop threshold circuit  68  operate at a frame rate, which in a preferred embodiment is 50 Hz. Clock divider  78  is used to divide the clock down and set the relative clock rates between the inter-loop and outer-loop. 
     The fast power control system shown in the orthogonal transmit diversity system that includes transceiver  20  and subscriber unit  50  malfunctions when one of radio frequency paths r 1  and r 2  is in a deep fade, and when the rate of convolutional encoder  26  is rate one-half. In this case, half the symbols, which are transmitted from one antenna, are lost due to the fade, and the symbols received from the other antenna are received with CRC errors, or frame errors, due to power control bits punctured by power control encoder  30 . Thus, every frame is received with an error in subscriber unit  50 , which causes outer loop threshold  68  to be adjusted rapidly so that it is soon requesting maximum transmit power from amplifiers  38  and  40  in transceiver  20 . When this occurs, the system assumes a malfunction and the call is dropped. Therefore, it should be apparent that a need exist for an improved method and system for generating a power control metric in an orthogonal transmit diversity communications system. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The novel features believed characteristic of the invention are set forth in the appended claims. The invention itself, however, as well as a preferred mode of use, further objects, and advantages thereof, will best be understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying drawings, wherein: 
     FIG. 1 depicts selected portions of a transceiver that uses transmit diversity according to the prior art; 
     FIG. 2 depicts selected portions of a subscriber unit that receives a transmit diversity signal according to the prior art; 
     FIG. 3 is a high-level block diagram of a subscriber unit that generates a channel quality metric in accordance with the method and system of the present invention; 
     FIG. 4 is a more detailed depiction of the channel quality metric computer shown in FIG. 3; 
     FIG. 5 is a more detailed illustration of the imbalance compensation computer shown in FIG. 4; 
     FIG. 6 is a transmit diversity reception simulator for simulating diversity branch imbalance conditions according to the method and system of the present invention; 
     FIG. 7 illustrates yet another embodiment of a channel quality metric computer in accordance with the method and system of the present invention; and 
     FIG. 8 is a high-level logic flowchart that illustrates the operation of the method and system of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     With reference now the figures, and in particular with reference to FIG. 3, there is depicted a high-level block diagram of a subscriber unit that generates a channel quality metric in accordance with the present invention. As shown, subscriber unit  90  contains many of the same functional components as subscriber unit  50  in FIG.  2 . However, according to an important aspect of the present invention, channel metric computer  92  includes a channel imbalance compensator that contributes to the computation of a power control command that is a function of an imbalance between the quality of signals received in different diversity branches in the subscriber unit. By compensating for the imbalance between the diversity branches, the lowest quality signal may be given a greater emphasis, or greater weight, as the power control command is computed. By giving the lower quality signal a greater weight, the power requested by the subscriber unit will increase sooner so that a frame error may be avoided. Avoiding this frame error is intended to prevent a request for a rapid power increase at the end of the frame wherein one of the diversity branches has entered a deep fade. 
     Referring now to FIG. 4, there is depicted the channel quality metric computer that is shown in FIG.  3 . As illustrated, channel quality metric computer  92  includes maximum selector  100 , imbalance computer  102 , imbalance compensation computer  104 , and imbalance compensator  108 . Maximum selector  100  receives channel quality measurements from each diversity branch in subscriber unit  90  and selects and outputs the maximum value. In a preferred embodiment, the channel quality measurements are signal-to-noise ratio measurements. 
     Imbalance computer  102  determines a magnitude of the difference between the channel quality measurements of the diversity branches. For example, in an embodiment with two diversity branches, imbalance computer  102  determines the difference between two channel quality measurements and outputs this difference to imbalance compensation computer  104 . This may be implemented by calculating the absolute differences in dB between the channel quality measurements of the diversity branches. 
     Imbalance compensation computer  104  determines a value of an imbalance compensation factor in response to input from imbalance computer  102 . In one embodiment, such as the embodiment shown in FIG. 5, imbalance compensation computer  104  may include table  108  that is used to look up a particular imbalance compensation  110  in response to a particular imbalance  112 , represented in dB. Therefore, an input from balance computer  102  is used to look up a value in table  108 , and such value is output by imbalance compensation computer  104 . 
     In order to compute values in table  108 , a simulator, such as simulator  120  shown in FIG. 6, may be used. In simulator  120 , reception simulator  122  is used to provide despread symbols that have been affected according to a simulated channel. For example, the outputs of reception simulator  122  are like the symbol streams input into spreaders  34  and  36  (see FIG.  1 ), respectively, as they would be affected by simulated channels r 1  and r 2 . 
     Within reception simulator  122  are variable gain amplifiers  124  and  126 , which may used by power controller  140  to set the gain of the diversity branches, as if the gain were set in a diversity transmitter. Also within reception simulator  122  are noise adders  125  and  127  that add independent noise samples to the generated signals to simulate the noise in channels r 1  and r 2 . 
     The outputs of reception simulator  122  are input into decommutator  130 , which reassembles the symbols in the proper order as they come from the simulated diversity branches through amplifiers  124  and  126 , respectively. 
     Following decommutator  130 , decoder  132  decodes the encoded symbol stream according to a convolutional coding rate set in rate register  134 . In a preferred embodiment, decoder  132  is a Viterbi decoder, and the convolutional coding rate is one-half. 
     The output of decoder  132  is coupled to frame error rate comparator  136 , which compares the frame error rate of decoder  132  to a target frame error rate stored in frame error rate register  138 . If the frame error rate from decoder  132  exceeds the target frame error rate in frame error rate register  138 , power control circuit  140  determines a power level increase that is needed to bring the measured frame error rate close to the target frame error rate. The output of power controller  140  is coupled to power level register  128 , which stores the current gain setting for amplifiers  124  and  126 . 
     The output of power controller  140  is also coupled to subtractor  142  which subtracts the current power level from the power level used to set a target frame error rate with no imbalance between diversity branches. Such a “no imbalance” power level is stored in register  144 . The value output by subtractor  142  is stored as imbalance compensation  110  in table  108 . In a preferred embodiment, imbalance compensation  110  is represented in dB. 
     The values stored in imbalance column  112  of table  108  come from imbalance setting  146 . Typically, this imbalance value would begin at 0, which allows a measurement of the “no imbalance” power level that is stored in register  144 , and increase up to a level that simulates a deep fade in a diversity branch. Setting an imbalance may be accomplished either by setting the relative amplitudes of simulated symbol streams, or by setting the relative amounts of simulated noise added to the symbol streams. 
     With reference again to FIG. 4, imbalance compensator  106  receives an imbalance compensation factor from imbalance compensation computer  104 . In a preferred embodiment, the output of maximum selector  100  is divided by the imbalance compensation factor to produce the channel quality metric. Thus, the maximum channel quality measurement is compensated earlier than in the prior art because compensation is based upon a detection of an imbalance between the diversity branches. In a prior art, arithmetic mean calculator  74  may indicate that the receiver has enough signal quality, and therefore not request a power increase until it is to late for the receiver to avoid several frames having errors. In contrast, the present invention asks for an increase in power soon enough to avoid a frame error in a current frame, thus avoiding several consecutive frames with errors. 
     Referring now to FIG. 7, there is depicted another embodiment of channel metric computer  92 . As a result of analyzing a graph of data points represented in table  108 , the mathematical approximation of such a graph is represented by multipliers  160  and  162 , and by adder  164 . In this embodiment channel quality for each diversity branch is input in dB into channel metric computer  92 . Then each channel quality measurement is multiplied by a constant, shown here as α and β. These products are then added in adder  164 , with the result being the channel metric. In a preferred embodiment, α and β are equal to 0.5, in which case the channel quality metric is the geometric mean of two diversity branch signal qualities, which may be expressed as:          Channel                 Quality                 Metric     =           (       E   b       N   t       )     1            (       E   b       N   t       )     2                                
     Therefore, in the case where (E b /N t ) 1  is the maximum diversity branch signal quality, it may be selected and multiplied by an imbalance compensation factor computed by the equation:          Imbalance                 Compensation                 Factor     =           (       E   b     Nt     )     2         (       E   b       N   t       )     1                                
     By using this imbalance compensation factor, the result is the same as using α and β=0.5 in the channel metric computer shown in FIG.  7 . Thus, it may be said that there is an implied imbalance compensation factor computed in channel metric computer  92  in the embodiment shown in FIG.  7 . 
     With reference now to FIG. 8, there is depicted a high-level flowchart that illustrates the operation of the method and system of the present invention. As shown, the process begins at block  200 , and thereafter passes to block  202  wherein the process measures a signal quality for each diversity branch in the receiver. In a preferred embodiment, the measured signal quality is a signal-to-noise ratio E b /N t , which is an energy per bit divided by noise power per propagation path. If there are multiple propagation paths received from a single diversity antenna, the E b /N t  measurements for each path (e.g., for each finger in a rake receiver) for a diversity branch are added to make a single signal quality measurement for that branch. 
     Next, the process calculates a difference, or imbalance, between the measured diversity branch signal qualities, as illustrated at block  204 . In a preferred embodiment, the imbalance in the signal-to-noise ratios relative to a selected diversity branch may be calculated. The selected diversity branch may be the diversity branch with the maximum signals-to- noise ratio. 
     After calculating an imbalance, the process determines an imbalance compensation factor, as depicted at block  206 . In one embodiment of the present invention, the imbalance compensation factor may be looked up in a table based upon the calculated imbalance. Values in the table may be determined experimentally in a simulator, such as the simulator shown in FIG.  6 . Alternatively, the imbalance compensation factor may be inherent in a mathematical computation such as shown in FIG.  7 . 
     After determining the imbalance compensation factor, the process divides a selected diversity branch signal quality by the imbalance compensation factor, as illustrated at block  208 . In a preferred embodiment, the maximum signal-to-noise ratio is divided by the imbalance compensation factor. By dividing the maximum diversity branch signal quality by the imbalance compensation factor that is proportional to the imbalance or difference between measured diversity branch signal qualities, the subscriber unit takes into account the signal imbalance between diversity branches and reduces the channel quality metric in what might be early in a frame time so that power at the transceiver might be increased over the remaining portion of the frame in order to avoid a frame error. 
     Finally, the process outputs the channel quality metric, as depicted at block  210 , and the process of computing a channel quality metric ends, as shown at block  212 . The channel quality metric output at block  210  is output by channel metric computer  92  in FIG. 3, and used by comparator  76  to compare it with the value of the outer loop threshold  68 . 
     According to an important aspect of the present invention, the channel quality metric computed by channel metric computer  92  measures an imbalance, or difference, between measured signal qualities in selected diversity branches of a receiver. By computing a channel quality metric in response to this imbalance in diversity branches, the receiver is able to incrementally request additional power at a time the imbalance is detected, rather than requesting a larger amount of power later, at the end of the frame, when it is determined that the frame has an error because one of the diversity branches has experienced a deep fade, and the puncturing of power control bits has caused the frame to be in error. 
     The present invention reduces the value of the channel quality metric upon detecting an imbalance so that power will be requested at the time the imbalance is detected, even though one of the diversity branches may apparently have enough signal quality to meet the target frame error rate. Additional power is needed because the one diversity branch having enough power may contain errors introduced by punctured power control bits. 
     The foregoing description of a preferred embodiment of the invention has been presented for the purpose of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Obvious modifications or variations are possible in light of the above teachings. The embodiment was chosen and described to provide the best illustration of the principles of the invention and its practical application, and to enable one of ordinary skill in the art to utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. All such modifications and variations are within the scope of the invention as determined by the appended claims when interpreted in accordance with the breadth to which they are fairly, legally, and equitably entitled.