Interference Suppression for CDMA Systems

Abstract

Interference is cancelled from a baseband signal by synthesizing interference from estimated symbols in interfering subchannels. The estimated symbols are hard-coded, soft weighted, or zeroed, depending on the value of an estimated pre-processed signal-to-interference-and-noise ratio (SINR) in each subchannel in order to maximize a postprocessed SINR. The estimated pre-processed SINR is obtained from averages of estimated symbol energies and estimated noise variances, or from related statistical procedures.

1 . A CDMA receiver, comprising:
 a signal-to-interference-and-noise ratio (SINR)-estimation module in a receiver, the SINR estimation module configured for producing one or more SINR estimates corresponding to one or more subchannels of a received multipath signal;   a thresholding module configured for comparing each of the one or more SINR estimates to a threshold to determine whether to employ a weighted soft decision or a hard decision;   a companding module configured for generating the weighted soft decision or the hard decision determined by the thresholding module for each of the one or more SINR estimates; and   a suppressor configured for producing at least one interference-suppressed signal from one or more of the soft decisions or hard decisions generated by the companding module.   
     
     
         2 . The receiver recited in  claim 1 , wherein the SINR-estimation module is configured for producing the one or more SINR estimates by averaging symbol amplitudes over at least one of a set of parameters, the set of parameters including time and a plurality of subchannels. 
     
     
         3 . The receiver recited in  claim 1 , wherein the thresholding module is further configured for determining an estimated SINR below which weighted soft decisions provide better performance than hard decisions, and above which hard decisions provide better performance than weighted soft decisions. 
     
     
         4 . The receiver recited in  claim 1 , wherein the companding module generates a weighted soft decision for each of the one or more SINR estimates that is below the threshold and generates a hard decision for each of the one or more SINR estimates that is above the threshold. 
     
     
         5 . The receiver recited in  claim 1 , wherein the thresholding module is further configured for determining a cutoff SINR, below which corresponding symbol estimates are set to zero. 
     
     
         6 . The receiver recited in  claim 1 , wherein the thresholding module is configured for determining an estimation strategy based on whether subchannel constellations are known. 
     
     
         7 . The receiver recited in  claim 1 , further comprising a baseband receiver configured for producing one or more symbol estimates for one or more subchannels of the received multipath signal. 
     
     
         8 . The receiver recited in  claim 7 , wherein the thresholding module is further configured for determining a cutoff SINR, below which corresponding symbol estimates are set to zero. 
     
     
         9 . The receiver recited in  claim 7 , wherein the baseband receiver comprises a rake receiver. 
     
     
         10 . The receiver recited in  claim 1 , wherein the companding module is configured for producing at least one weight expressed by an estimate of 
       
         
           
             
               
                 
                   SINR 
                   k 
                   
                     ( 
                     1 
                     ) 
                   
                 
                 
                   
                     SINR 
                     k 
                     
                       ( 
                       1 
                       ) 
                     
                   
                   + 
                   1 
                 
               
               , 
             
           
         
       
       where SINR k   (1)  is a signal-to-interference-and-noise ratio for a user or subchannel k. 
     
     
         11 . The receiver recited in  claim 7 , wherein one or more of the SINR estimates are estimated via a function of the one or more symbol estimates. 
     
     
         12 . The receiver recited in  claim 1 , further comprising a synthesizer configured for producing a synthesized interference signal. 
     
     
         13 . The receiver recited in  claim 12 , wherein the synthesizer comprises a channel emulator configured for providing channel distortions to the synthesized interference signal. 
     
     
         14 . The receiver recited in  claim 1 , further comprising a channel emulator configured to perform channel emulation. 
     
     
         15 . The receiver recited in  claim 1 , wherein the suppressor is configured to perform scaling. 
     
     
         16 . The receiver recited in  claim 1 , wherein the suppressor is configured to perform at least one of subtractive suppression, weighted subtractive suppression, projective suppression, and weighted projective suppression. 
     
     
         17 . The receiver recited in  claim 1 , further comprising a rake combiner configured to perform maximal ratio combining across a plurality of rake fingers. 
     
     
         18 . The receiver recited in  claim 17 , wherein the SINR-estimation module is coupled to the Rake combiner. 
     
     
         19 . The receiver recited in  claim 1  configured to operate in at least one of a handset and a base station. 
     
     
         20 . A method of receiving a CDMA signal, comprising:
 producing in a receiver one or more SINR estimates corresponding to one or more subchannels of a received multipath signal;   comparing each of the one or more SINR estimates to a predetermined threshold for determining whether to employ a weighted soft decision or a hard decision;   generating the weighted soft decision or the hard decision determined for each of the one or more SINR estimates to produce at least one interference-symbol estimate; and   producing at least one interference-suppressed signal from a received baseband signal and the at least one interference-symbol estimate.   
     
     
         21 . The method recited in  claim 20 , wherein producing one or more SINR estimates comprises averaging symbol amplitudes over at least one of a set of parameters, the set of parameters including time and a plurality of subchannels. 
     
     
         22 . The method recited in  claim 20 , wherein comparing comprises determining an SINR below which weighted soft decisions provide better performance than hard decisions, and above which hard decisions provide better performance than weighted soft decisions. 
     
     
         23 . The method recited in  claim 20 , wherein comparing comprises determining a cutoff SINR, below which corresponding symbol estimates are set to zero. 
     
     
         24 . The method recited in  claim 20 , wherein comparing comprises determining an estimation strategy based on whether subchannel constellations are known. 
     
     
         25 . The method recited in  claim 20 , further comprising producing one or more symbol estimates for one or more subchannels of the received multipath signal. 
     
     
         26 . The method recited in  claim 20 , wherein generating comprises generating a weighted soft decision for each of the one or more SINR estimates that is below the threshold and generating a hard decision for each of the one or more SINR estimates that is above the threshold. 
     
     
         27 . The method recited in  claim 20 , wherein generating the weighted soft decision or the hard decision comprises producing at least one weight expressed by an estimate of 
       
         
           
             
               
                 
                   SINR 
                   k 
                   
                     ( 
                     1 
                     ) 
                   
                 
                 
                   
                     SINR 
                     k 
                     
                       ( 
                       1 
                       ) 
                     
                   
                   + 
                   1 
                 
               
               , 
             
           
         
       
       where SINR k   (1)  is a signal-to-interference-and-noise ratio for a user or subchannel k. 
     
     
         28 . The method recited in  claim 25 , wherein SINR k   (1)  is estimated via a function of one or more of the symbol estimates. 
     
     
         29 . The method recited in  claim 20 , further comprising producing a synthesized interference signal. 
     
     
         30 . The method recited in  claim 29 , further comprising emulating channel distortions in the synthesized interference signal. 
     
     
         31 . The method recited in  claim 20 , further comprising performing channel emulation. 
     
     
         32 . The method recited in  claim 20 , wherein producing at least one interference-suppressed signal includes scaling. 
     
     
         33 . The method recited in  claim 20 , wherein producing at least one interference-suppressed signal comprises performing at least one of subtractive suppression, weighted subtractive suppression, projective suppression, and weighted projective suppression. 
     
     
         34 . The method recited in  claim 20 , further comprising performing maximal ratio combining across a plurality of rake fingers. 
     
     
         35 . The method recited in  claim 25 , wherein producing one or more SINR estimates includes receiving the one or more symbol estimates from a Rake combiner. 
     
     
         36 . A chipset configured to perform the method recited in  claim 20 . 
     
     
         37 . A digital computer system programmed to perform the method recited in  claim 20 . 
     
     
         38 . A computer-readable medium storing a computer program implementing the method of  claim 20 . 
     
     
         39 . A base station configured to perform the method of  claim 20 . 
     
     
         40 . A handset configured to perform the method of  claim 20 . 
     
     
         41 . A CDMA receiver, comprising:
 a means for determining an SINR estimate for each of one or more subchannels of a received signal;   a means for determining whether to use weighted soft decisions or hard decisions for each of the one or more subchannel;   a means for generating the weighted soft decisions or hard decisions determined for each of the one or more SINR estimates; and   a means for producing an interference-suppressed signal from a received baseband signal and at least one interference-symbol estimate, wherein each of the at least one interference-symbol estimate comprises a hard decision or a weighted soft decision of a corresponding symbol estimate.   
     
     
         42 . The receiver recited in  claim 41 , further comprising a means for determining at least one SINR threshold. 
     
     
         43 . The receiver recited in  claim 42 , wherein the means for determining at least one SINR threshold is configured for determining an SINR below which weighted soft decisions provide better performance than hard decisions, and above which hard decisions provide better performance than weighted soft decisions. 
     
     
         44 . The receiver recited in  claim 43 , where the means for generating is configured for generating a weighted soft decision for each of the one or more SINR estimates that is below the SINR threshold and generating a hard decision for each of the one or more SINR estimates that is above the SINR threshold 
     
     
         45 . The receiver recited in  claim 42 , wherein the means for determining at least one SINR threshold includes a means for determining a cutoff SINR, below which the processed symbol estimates are set to zero. 
     
     
         46 . The receiver recited in  claim 41 , further comprising a means for generating at least one weight for weighting at least one of the plurality of soft symbol estimates. 
     
     
         47 . The receiver recited in  claim 46  wherein the at least one weight is expressed by an estimate of 
       
         
           
             
               
                 
                   SINR 
                   k 
                   
                     ( 
                     1 
                     ) 
                   
                 
                 
                   
                     SINR 
                     k 
                     
                       ( 
                       1 
                       ) 
                     
                   
                   + 
                   1 
                 
               
               , 
             
           
         
       
       where SINR k   (1)  is a signal-to-interference-and-noise ratio for a user or subchannel k. 
     
     
         48 . The receiver recited in  claim 41 , wherein the means for producing an interference-suppressed signal comprises a means for producing a synthesized interference signal from the at least one interference-symbol estimate. 
     
     
         49 . The receiver recited in  claim 48 , wherein the means for producing a synthesized interference signal includes a means for emulating channel distortions in the synthesized interference signal. 
     
     
         50 . The receiver recited in  claim 41 , wherein the means for determining an SINR estimate is configured for estimating an SINR in each of the one or more subchannels. 
     
     
         51 . The receiver recited in  claim 41 , wherein the means for producing an interference-suppressed signal is configured for performing scaling. 
     
     
         52 . The receiver recited in  claim 41 , wherein the means for producing an interference-suppressed signal is configured to perform at least one of subtractive suppression, weighted subtractive suppression, projective suppression, and weighted projective suppression. 
     
     
         53 . The receiver recited in  claim 41 , wherein the means for determining the SINR estimate is coupled to a means for producing one or more symbol estimates corresponding to a received baseband signal. 
     
     
         54 . The receiver recited in  claim 53 , wherein the means for determining the SINR estimate is configured for averaging amplitudes of the one or more symbol estimates over at least one of a set of parameters, the set of parameters including time and a plurality of subchannels. 
     
     
         55 . The receiver recited in  claim 41 , wherein the means for determining whether to use weighted soft decisions or hard decisions is further configured for determining an estimation strategy based on whether subchannel constellations are known. 
     
     
         56 . A CDMA receiver, comprising:
 a signal-to-interference-and-noise ratio (SINR)-estimation module in a receiver, the SINR estimation module configured for producing one or more SINR estimates corresponding to one or more subchannels of a received multipath signal;   a thresholding module configured for comparing each of the one or more SINR estimates to a threshold to determine whether to employ a weighted soft decision or a hard decision; and   a companding module configured for generating the weighted soft decision or the hard decision determined by the thresholding module for each of the one or more SINR estimates.