Method of receiving CDMA signals with parallel interference suppression, and corresponding stage and receiver

Abstract

A method of parallel suppression of interference, and corresponding stage and receiver is disclosed according to the invention, parallel suppression of interference is carried out starting from the signals selected by a maximum likelihood criterion based on the calculation of a metric and the search for the smallest possible metric.

1. A method of receiving CDMA signals with parallel interference suppression in which:
 a composite signal (r(t)) is received comprising a plurality of K signals corresponding to information symbols which have been spread in frequency by K different pseudo-random sequences,  
 these K signals are correlated using said K sequences, 
 the corresponding K symbols are estimated,  
 the K signals correlated in frequency are reconstructed by despreading  re- spreading  said estimated symbols through  using the corresponding pseudo-random sequences,  
 the contributions of the other  remaining (K−1) signals are subtracted from a despread  spread signal to provide K new signals, spread in frequency but cleared, at least in part of the interference,  
 
       this method being characterized in that:
 all the possible hypotheses on the signs of the NK correlated signals are formulated, where N is a whole number equal to 1 or to a few units  greater than zero,  
 for each hypothesis, one calculates the distance metric between the group of correlated signals undergoing processing and the corresponding signals before processing,  
 the hypothesis for which the metric is the smallest is retained, being the hypothesis which has a maximum likelihood,  
 only those signals corresponding to this maximum likelihood hypothesis are reconstructed.  
 
     
     
       2. A parallel interference suppression stage implementing the method according to  claim 1 , this stage comprising:
 K inputs receiving signals correlated in frequency,  
 K means of  for estimating (ES 1 , . . . , ES k , . . . , ES K ) K symbols corresponding to these K signals,  
 K means of  for reconstructing (R 1 , . . . , R k , . . . , R K ) signals respread in frequency using the corresponding pseudo-random sequences,  
 means of  for parallel interference suppression comprising K channels in parallel capable of subtracting from one despread  spread signal, the contributions of the other  remaining (K−1) despread  respread signals,  
 K outputs supplying K signals spread in frequency, cleared, at least in part of the interference,  
 
       this stage being characterized in that it comprises:
 means (M), placed between the estimation means (ES 1 , . . . , ES k , . . . , ES K ) and the reconstruction means (R 1 , . . . , R k , . . . , R K ,) and capable of  for formulating all the possible hypotheses on the signs of NK correlated signals, where N is a whole number equal to 1 or to a few units  greater than zero, and of  for calculating, for each hypothesis, the distance metric (M j ) between the group of  for correlated signals undergoing processing and the corresponding signals before processing, and of retaining the hypothesis (j) for which the metric (M j ) is the smallest, the hypothesis which offers a maximum likelihood.  
 
     
     
       3. Stage according to  claim 2 , in which the means of calculating the metric  placed between the estimating means and the reconstruction means comprise:
 means of  for formulating two hypotheses on the sign to be assigned to the amplitude of the signals supplied by the means of  for estimation,  
 means of  for calculating all the differences Z 0 (k)−Z i (k) j , where Z 0 (k) represents the signal at the output from the k th  matched filter of the input stage and Z i (k) j  the signal at the output from the k th  matched filter of the stage of row i, the signal being allocated the sign corresponding to each hypothesis j,  
 means of  for calculating the square of these differences, or (Z 0 (k)−Z i (k) j ) 2 , means of  for calculating the sum of these squares for all NK values of the signals, which leads, for each hypothesis (j), to the metric (M j ).  
 
     
     
       4. A receiver of CDMA signals that implements the method of  claim 1  and comprises:
 a general input (E) suitable for receiving a composite signal (r(t)) formed from a plurality of K signals corresponding to information symbols which have been spread in frequency by K different pseudo-random sequences,  
 an input stage with K channels in parallel each comprising filters (F 01 , . . . , F 0k , . . . , F 0K ) to correlate in frequency the composite signal (r(t)) through one of the K pseudo-random sequences, this stage supplying K signals correlated in frequency,  
 at least one parallel interference suppression stage (V 1 , V 2 , . . . ,),  
 filter stages positioned between the parallel interference suppression stages and comprising K filters (F 1k , F 2k , . . . ) matched to the pseudo-random sequences,  
 an output circuit (S) comprising K decision circuits (D 1 , . . . , D k , . . . , D K )  
 
       this receiver being characterized in that at least one of the parallel interference suppression stages is a stage according to claims  2  or  3 . 
     
     
       5. A receiver according to  claim 4 , in which the estimated signals have a certain reliability and in which the means (M) for formulating two hypotheses on the sign to be assigned to these signals  means placed between the estimating means and the reconstruction means only take into account the signals with a reliability below a fixed threshold, the other  remaining signals having a reliability above the threshold being used directly by the interference suppression  means for parallel interference suppression. 
     
     
       6. A receiver according to  claim 4 , in which the input stage with its filters and each stage of matched filtering comprise means of  weighting circuits (P 0k ) to weight the outputs from the filters (F 0k ), the output stage (S) comprising adders (AD k ) the inputs to which are connected to the weighting circuits (P 0k ) and the output from which is connected to the decision circuits (D k ). 
     
     
       7. A receiver according to  claim 4 , in which each stage of filtering is followed by a weighting circuit (P 0k , P 1k , P 2k , . . . ) arranged between the output from the filtering stage and the input to the interference suppression stage, the weighting depending on the reliability of the estimation made in the stage. 
     
     
       8. An apparatus, comprising:
   a receiver to receive a composite signal  ( r ( t ))  comprising a plurality of K signals corresponding to information symbols which have been spread in frequency by K different pseudo - random sequences,        said receiver further to correlate said K signals using said K sequences,        an estimation means to estimate the corresponding K symbols,        a reconstruction means to reconstruct the K signals correlated in frequency by re - spreading said estimated symbols using the corresponding pseudo - random sequences,        wherein the contributions of the remaining  ( K− 1   )  signals are to be subtracted from a spread signal to provide K new signals, spread in frequency but cleared, at least in part of the interference, wherein:        all the possible hypotheses on the signs of the NK correlated signals are to be formulated, where N is a whole number greater than zero,        for each hypothesis, the distance metric is to be calculated between the group of correlated signals undergoing processing and the corresponding signals before processing,        the hypothesis for which the metric is smallest is to be retained, being the hypothesis which has a maximum likelihood,        only those signals corresponding to this maximum likelihood hypothesis are to be reconstructed.     
     
     
       9. An apparatus, comprising:
   at least one spread - spectrum parallel interference suppression processing stage to receive a received signal comprising K spread - spectrum signals spread with K codes, to despread and demodulate the K spread - spectrum signals in parallel to obtain K extracted symbols, and, for each of the K extracted symbols, to generate a new spread - spectrum signal using the corresponding one of the K codes, and further to obtain an estimate of each of the K spread - spectrum signals by subtracting a quantity based on the remaining K− 1  new spread - spectrum signals from a previous input signal;        wherein said processing stage includes:        at least one estimator to provide amplitude estimates corresponding to the K extracted symbols; and        at least one metric calculator to calculate at least one metric based on the amplitude estimates and to choose a set of K maximum likelihood symbols based on the at least one metric, the set of K maximum likelihood symbols to be used as the K extracted symbols to generate the new spread - spectrum signals.     
     
     
       10. The apparatus according to  claim 9 , further comprising:
   a set of K parallel despreaders to receive the received signal and to provide K parallel inputs to a first one of said at least one processing stage.     
     
     
       11. The apparatus according to  claim 9 , wherein said at least one metric comprises a Euclidean distance metric. 
     
     
       12. The apparatus according to  claim 9 , further comprising:
   at least one reliability testing device to test of each of the K extracted symbols to determine that a number, Q, of the K extracted symbols are reliable, wherein said metric calculator is to calculate at least one metric only on the K - Q extracted symbols not determined to be reliable.     
     
     
       13. The apparatus according to  claim 9 , wherein said processing stage further comprises:
   at least one weighting circuit to weight at least one of the extracted symbols and a corresponding at least one of the set of K maximum likelihood symbols; and        at least one adder to add the resulting corresponding weighted symbols to obtain at least one of the K extracted symbols to be used to generate the new spread - spectrum signals.     
     
     
       14. A method, comprising:
   processing a received composite code - division multiple - access  ( CDMA )  signal, comprising K spread - spectrum signals spread using K codes, said processing comprising despreading and demodulating the K spread - spectrum signals in parallel to obtain K extracted symbols, and, for each of the K extracted symbols, generating a new spread - spectrum signal using the corresponding one of the K codes, and said processing further comprising obtaining an estimate of each of the K spread - spectrum signals by subtracting a quantity based on the remaining K− 1  new spread - spectrum signals from a previous input signal;        wherein said processing includes:        providing amplitude estimates corresponding to the K extracted symbols; and        calculating at least one metric based on the amplitude estimates and choosing a set of K maximum likelihood symbols based on the at least one metric, the set of K maximum likelihood symbols to be used as the K extracted symbols to generate the new spread - spectrum signals.     
     
     
       15. The method according to  claim 14 , wherein said at least one metric comprise a Euclidean distance metric. 
     
     
       16. The method according to  claim 14 , wherein said method comprises:
   performing said processing at least twice, each processing an initial processing utilizing results generated by the previous processing.     
     
     
       17. The method according to  claim 14 , wherein said method further comprises:
   determining a reliability of each of the K extracted symbols to determine that a number, Q, of the K extracted symbols are reliable, and calculating at least one metric only on the K - Q extracted symbols not determined to be reliable.     
     
     
       18. The method according to  claim 14 , wherein said processing further comprises:
   weighting at least one of the extracted symbols and a corresponding at least one of the set of K maximum likelihood symbols and adding the resulting weighted symbols to obtain at least one of the K extracted symbols to be used to generate the new spread - spectrum signals.