Abstract:
A method ( 300 ) and an apparatus ( 202 ) for a digital diversity receiver having a first receiver branch ( 204 ) and a second receiver branch ( 206 ) for adjusting a receiver power control loop during a radio frame ( 100 ) of a known length are provide. The digital diversity receiver ( 202 ) receives a first signal ( 208 ) through the first receiver branch ( 204 ) during the radio frame ( 100 ), and receives a second signal ( 210 ) through the second receiver branch ( 206 ) during a portion of the radio frame ( 106, 110 ). The second signal ( 210 ) originates from a common original signal ( 212 ) as the first signal  208 . The digital diversity receiver ( 202 ) evaluates ( 308 ) a receiver power control parameter for the first interval ( 106 ) based upon the first signal ( 208 ) and the second signal ( 210 ), and compensates ( 310 ) the receiver power control parameter with an offset value for the first interval ( 106 ). The compensated receiver power control parameter is then applied ( 312 ) to the receiver power control loop during the first interval.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention generally relates to a power control mechanism for a communication device, and more specifically to a receiver power control mechanism in a communication device having a receive diversity capability.  
       BACKGROUND OF THE INVENTION  
       [0002]     Universal Mobile Telecommunications System-Frequency Division Duplex (“UMTS-FDD”) networks, such as Wideband Code Division Multiple Access (“WCDMA”) networks, use measurement intervals, usually in combination with compressed mode (“CM”), to permit a User Equipment (“UE”) such as a cellular mobile device to perform inter-frequency measurements on other UMTS cells, or on cells deployed using other supported Radio Access Technologies (“RAT&#39;s”), such as, but not limited to, Global System for Mobile Communications (“GSM”), General Packet Radio Service (“GPRS”) and Enhanced Data-rates for GSM Evolution (“EDGE”). By using a symbol puncturing or spreading factor reduction method, multiple vacant downlink timeslots are created either: a) in the approximate center of a 10 millisecond (“msec”) radio frame; or b) overlapping two adjacent 10 msec radio frames.  FIG. 1  illustrates an exemplary downlink radio frame  100  having a known length, which has a frame start  102  and a frame end  104 , segmented into three intervals: the first interval  106 ; second interval  108 , which is also referred to as a transmission gap; and the third interval  110 . The transmission gap  108  starts from a gap start  112 , which is also the end of the first interval  106 , and ends at a gap end  114 , which is also the beginning of the third interval  110 . Although the UE is not capable of simultaneously monitoring two frequencies, during the transmission gap  104 , the UE is able to change the frequency, and perform necessary inter-frequency measurements, and optionally inter-RAT measurements.  
         [0003]     In UMTS-FDD architectures supporting a receive diversity capability having a main receiver branch and a diversity receiver branch, it can be less costly to dedicate the main receive branch to UMTS-FDD and to permit the diversity receiver branch to be adaptable to either GSM or UMTS-FDD. This two-branch receiver approach provides reduced cost compared to, for example, a three-branch receiver system with two branches dedicated to UMTS-FDD and a third single branch dedicated to GSM. However, in the two-branch receiver, the UMTS-FDD signal in the diversity receiver branch is lost at the onset of an inter-frequency or inter-RAT measurement opportunity at the gap start  112  when the diversity receiver branch is switched to measure the GSM signal. This loss of the diversity receiver branch forces the two-branch receiver to revert to a single branch, or single antenna, using only the main receiver branch for the UMTS-FDD signal. When operating within a closed-loop power control scheme, such as in UMTS-FDD, however, this loss of the diversity receiver branch can be problematic. Because a signal-to-noise-ratio (“SNR”) of an observed signal at an output of a diversity combiner will experience an instantaneous loss of the SNR due to the loss of the diversity receiver branch, the quality of the soft decisions, such as log-likelihood ratios (“LLR&#39;s”), of the associated encoded symbol will be reduced and the probability of erasing the associated transmission time interval (TTI) frame will also be increased. For example, if an instantaneous channel impulse response observed at each antenna produces an SNR of a, which is identical for each antenna, the SNR at the diversity combiner output would be  2   a  just prior to the gap start  112  of the transmission gap  108 , and then would immediately fall by 3 dB to a after the diversity receiver branch is removed at the gap start  112 . If the main receiver branch had a lower SNR than the diversity receiver branch, then the reduction in SNR upon the loss of diversity branch signal input would be even greater.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0004]      FIG. 1  is an exemplary downlink radio frame having a transmission gap comprising seven consecutive slots suspended in the center of the downlink radio frame.  
         [0005]      FIG. 2  illustrates an exemplary environment in which a digital diversity receiver in accordance with at least one of the preferred embodiments may be practiced;  
         [0006]      FIG. 3  is an exemplary flowchart for adjusting a receiver power control loop in the digital diversity receiver during a radio frame in accordance with at least one of the preferred embodiments; and  
         [0007]      FIG. 4  is an exemplary block diagram of a wireless communication device having a digital diversity receiver in accordance with at least one of the preferred embodiments.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0008]     A digital diversity receiver, which is equipped with a first receiver branch and a second receiver branch, receives a first signal, which is a first version of a common original signal, in the first receiver branch and a second signal, which is a second version of the common original signal, in the second receiver branch during a radio frame. The radio frame may a data frame of a known data length or a time frame of a known time duration, and may comprise of a plurality of sub-frames. The radio frame is segmented into first, second, and third intervals, and the first receiver branch receives the first signal in all intervals. The second receiver branch, however, receives the second signal only during the first and third intervals, and receives a third signal, which is not related to the common original signal, during the second interval. The third signal may originate from a nearby base station compatible with the digital diversity receiver. During the first and third intervals, both the first and second signal are used to evaluate received signal parameters for the common original signal such as a receiver power control parameter and symbol log-likelihood ratios. Because during the second interval, the second receiver branch receives the third signal, which is unrelated to the common original signal, only the first signal is used to evaluate the received signal parameters for the common original signal. To avoid a sudden change in the signal parameters due to the absence of the second signal at the beginning of the second interval, the receiver power control parameter during the first interval is compensated.  
         [0009]      FIG. 2  illustrates an exemplary environment  200  in which a digital diversity receiver  202  in accordance with at least one of the preferred embodiments may be practiced. The digital diversity receiver  202  has a first receiver branch  204  and a second receiver branch  206 . The first receiver branch  204  receives a first signal  208  during the entire radio frame  100 , and the second branch  206  receives a second signal  210  during the first interval  106  and third interval  110 . The first signal  208  and the second signal  210  are different versions of a common original signal  212  originating from a common base station  214 , but are independently faded, respectively arriving at the first receiver branch  204  and the second receiver branch  206  after taking independent paths. The first signal  208  and the second signal  210  may also be correlated to each other based upon other characteristics such as their time of transmission and their signal frequencies. During the second interval  108 , the second receiver branch  206  receives a third signal  216 , which is not related to the common original signal  212 . The third signal  216  in this example is shown to originate from a separate base station  218 , however, the third signal  216  may be also be transmitted from the common base station  214  at a different signal frequency from the first signal  208  and the second signal  210 .  
         [0010]      FIG. 3  illustrates an exemplary flowchart  300  for adjusting a receiver power control loop in the digital diversity receiver  202  in accordance with at least one of the preferred embodiments. The process begins in block  302 , and the radio frame  100 , which has a known duration and a known number of time slots, is segmented into three consecutive intervals: first interval  106 , second interval  108 , and third interval  10  in block  304  as previously shown in  FIG. 1 , such that an appropriate routine can be applied for each interval. In block  306 , for the first interval  106 , the first receiver branch  204  receives the first signal  208  and the second receiver branch  206  receives the second signal  210 . As previously described, the first signal  208  and the second signal  210  are different versions of a common original signal  212  originating from a common base station  214 , but are independently faded, respectively arriving at the first receiver branch  204  and the second receiver branch  206  after taking independent paths. The first signal  208  and the second signal  210  may also be correlated to each other based upon other characteristics such as their time of transmission and their signal frequencies. In block  308 , a receiver power control parameter is evaluated based upon the first and second signals  208  and  210 . Symbol log-likelihood ratios (“LLRs”) of the common original signal  212  may also be evaluated in block  308  based upon the first signal  208  and the second signal  210  for the first interval  106 . Because the second signal  210  is known to become unavailable in the second interval  108 , the receiver power control parameter may alternatively be evaluated based only upon the first signal  208  in the first interval  106  or in the period prior to and including the first interval  106 . The digital diversity receiver  202  may then request an increase in the transmitted power of the common original signal  212  to compensate for the unavailability of the second signal  210 . Further, because the second signal  210  is known to become unavailable in the second interval  108 , the digital diversity receiver  202  may simply request an increase in the transmitted power of the common original signal  212  during the first interval  106  or during the period prior to and including the first interval  106 . In block  310 , the evaluated receiver power control parameter is compensated with an offset value, and in block  312 , the compensated receiver power control parameter is applied to the receiver power control loop. The offset value may be a constant value or a time varying value such as a ramp function linearly increasing from zero at the beginning of the first interval  106  to a predetermined value at the end of the first interval  106 . The compensated receiver power control parameter may then be compared against a target receiver power control parameter, and a request to change the transmitted power of the common original signal  212  may be made based upon the comparison.  
         [0011]     In block  314 , whether the end of the first interval  106  has been reached is checked. If the end of the first interval  106  has not been reached, the process returns to block  308 . If the end of the first interval  106  has been reached, then for the second interval  108 , the first receiver branch  204  continues to receive the first signal  208  and the second receiver branch  206  receives the third signal  216  in block  316 . In block  318 , the receiver power control parameter is re-evaluated based only upon the first signal  208 , and the symbol LLRs of the common original signal  212  may also be re-evaluated based only upon the first signal  208  for the second interval  108 . The re-evaluated receiver power control parameter is applied to the receiver power control loop in block  320 , which may include requesting an increase in the transmitted power of the common original signal  212  to compensate for the loss of the second signal  210 . The offset value and the requested increase in the transmitted power of the common original signal  212  may be calculated based upon a comparison between the evaluated receiver power control parameter of block  308 , which is based upon the first signal  208  and the second signal  210 , and the re-evaluated power control parameter of block  318 , which is based only upon the first signal  208 .  
         [0012]     In block  322 , whether the end of the second interval  108  has been reached is checked. If the end of the second interval  108  has not been reached, the process returns to block  316 . If the end of the second interval  108  has been reached, then for the third interval  110 , the first receiver branch  204  continues to receive the first signal  208  and the second receiver branch  206  again receives the second signal  210  in block  324 . Based upon the first and second signals, the receiver power control parameter is re-evaluated in block  326 , and the re-evaluated receiver power control parameter is applied to the receiver power control loop in block  328 , which may include comparing the re-evaluated receiver power control parameter against the target power control parameter, and requesting a change in the transmitted power of the common original signal  212  based upon the comparison. In block  330 , whether the end of the third interval  10  has been reached is checked. If the end of the third interval  110  has not been reached, the process returns to block  324 . If the end of the third interval  110  has been reached, then the process terminates in block  326 . Alternatively, the process  10  may loop back to block  306  for the next radio frame.  
         [0013]      FIG. 4  is an exemplary block diagram of the digital diversity receiver  202  in accordance with at least one of the preferred embodiments. The digital diversity receiver  202  comprises a processor  402 , which is configured to segment the radio frame  100  of a known length into the first interval  106 , the second interval  108 , and the third interval  110 , and is coupled to the first receiver branch  204  and to the second receiver branch  206 . The first receiver branch  204  is configured to receive the first signal  208  for the entire radio frame  100 , and the second receiver branch  206  is configured to receive the second signal  210  for the first interval  106  and the third interval  110 . As previously described, the first signal  208  and the second signal  210  are different versions of the common original signal  212  originating from the common base station  214 , but are independently faded, respectively arriving at the first receiver branch  204  and the second receiver branch  206  after taking independent paths. The first signal  208  and the second signal  210  may also be correlated to each other based upon other characteristics such as their time of transmission and their signal frequencies. A power control estimator  404  is coupled to the processor  402  and to both of the first receiver branch  204  and the second receiver branch  206 , and is configured to generate an estimate power control parameter. The processor  402  is further configured to direct the power control estimator  404  to generate the estimate power control parameter based upon the first signal  208  and the second signal  210  for the first interval  106 , based upon the first signal  208  only for the second interval  108 , and based upon the first signal  208  and the second signal  210  for the third interval  110 . Because the second signal  210  is known to become unavailable in the second interval  108 , the processor  402  may be alternatively configured to direct the power control estimator  404  to generate the estimate power control parameter based only upon the first signal  208  in the first interval  106  or in the period prior to and including the first interval  106 . An offset generator  406  is coupled to the processor  402  and to the power control estimator  404 , and is configured to generate an offset value and to generate an offset power control parameter based upon the estimate power control parameter and the offset value. The offset value may be a constant value or a time varying value such as a ramp function linearly increasing from zero at the beginning of the first interval  106  to a predetermined value at the end of the first interval  106 . The processor  402  may be further configured to direct the offset generator  406  to generate the offset value based upon a difference between the estimate power control parameter for the first interval  106  and the estimate power control parameter for the second interval. A power control parameter comparator  408  is coupled to the processor  402  and the offset generator  406 , and is configured to compare the offset power control parameter from the offset generator  406  and a target power control parameter for the first interval  106  and the third interval  110 . The processor  402  is further configured to direct the power control parameter comparator  408  to generate a request to vary a transmitted power of the common original signal  212  based upon the first signal  208  and the second signal  210  for the first interval  106  and the third interval  110 , and to increase the transmitted power of the common original signal  212  for the second interval  110 . Further, because the second signal  210  is known to become unavailable in the second interval  108 , the processor  402  may be simply configured to direct the power control parameter comparator  408  to generate a request to increase the transmitted power of the common original signal  212  during the first interval  106  or during the period prior to and including the first interval  106 . The digital diversity receiver  202  further includes a diversity combiner  410 , which is coupled to the processor  402  and both of the first receiver branch  204  and the second receiver branch  206 . The diversity combiner  410  is configured to generate a symbol log-likelihood ratio of the common original signal  212  for the first interval  106  and the third interval  110  based upon both of the first signal  208  and the second signal  210 . For the second interval  108 , the diversity combiner  410  is configured to generate the symbol log-likelihood ratio of the common original signal  212  based upon first signal  208 .  
         [0014]     While the preferred embodiments of the invention have been illustrated and described, it is to be understood that the invention is not so limited. Numerous modifications, changes, variations, substitutions and equivalents will occur to those skilled in the art without departing from the spirit and scope of the present invention as defined by the appended claims.