Method And Apparatus For Code Group Identification And Frame Synchronization By Use Of Reed-Solomon Decoder And Reliability Measurement For UMTS W-CDMA

Abstract

A method and apparatus having a modified Reed-Solomon decoder is used for finding a specific code group used by a base station and the frame timing synchronization with the base station. The modified Reed-Solomon decoder uses a standard Reed-Solomon decoder and some reliability measurements computed from the received code word symbols. If the reliability of a received symbol is too low, this symbol is considered as erasure. By selecting code word symbols with higher reliabilities and erasing code word symbols with lower reliabilities, the symbol error probability is reduced and the performance is improved. Several modified Reed-Solomon decoders and a few decoding strategies are introduced in order to decode the received code word sequences with a power- and memory-effective method.

1 . A method for generating a corrected code sequence, comprising:
 (a) receiving a frame of code word symbols;   (b) forming a sequence of hard decision symbols from said frame of code word symbols, each hard decision symbol having a corresponding reliability;   (c) sending said sequence of hard decision symbols and corresponding reliabilities to a modified Reed-Solomon decoder;   (d) generating a corrected code sequence using said modified Reed-Solomon decoder and ending said method if said modified Reed-Solomon decoder returns successfully;   (e) ending said method if total number of code word symbols received is greater than a limit;   (f) receiving a number of additional code word symbols;   (g) updating said sequence of hard decision symbols and corresponding reliabilities based on said additional code word symbols; and   (h) returning to step (c).   
   
   
       2 . The method for generating a corrected code sequence as claimed in  claim 1 , wherein an additional code word symbol with a receiving index m is used is said step (g) according to the steps of:
 (g1) determining a new hard decision symbol and a new corresponding reliability for said additional code word symbol;   (g2) identifying an associated hard decision symbol having a receiving index equal to the remainder of m divided by n, n being the number of symbols in said sequence of hard decision symbols;   (g3) increasing the corresponding reliability of said associated hard decision symbol if said new hard decision symbol is identical to said associated hard decision symbol;   (g4) decreasing the corresponding reliability of said associated hard decision symbol if said new hard decision symbol is not identical to said associated hard decision symbol and said new corresponding reliability is less than the corresponding reliability of said associated hard decision symbol; and   (g5) replacing said associated hard decision symbol and corresponding reliability with said new hard decision symbol and decreased new corresponding reliability if said new hard decision symbol is not identical to said associated hard decision symbol and said new corresponding reliability is greater than the corresponding reliability of said associated hard decision symbol.   
   
   
       3 . The method for generating a corrected code sequence as claimed in  claim 1 , wherein said modified Reed-Solomon decoder in said step (d) performs:
 (i) receiving a new symbol from said sequence of hard decision symbols;   (ii) declaring said new symbol as a valid symbol if the corresponding reliability of said new symbol is greater than a reliability threshold, or as an invalid symbol otherwise;   (iii) recording total number of valid symbols;   (iv) sending a code sequence based on valid symbols to a Reed-Solomon error-and-erasure decoder if said recorded total number of valid symbols is greater than a threshold of total number of valid symbols, or execute step (vi) otherwise;   (v) generating said corrected code sequence and ending said step (d) if said Reed-Solomon error-and-erasure decoder returns successfully; and   (vi) ending said step (d) if all symbols of said sequence of hard decision symbols have been received, or returning to step (i) otherwise.   
   
   
       4 . The method for generating a corrected code sequence as claimed in  claim 3 , wherein said sequence of hard decision symbols has at most 15 hard decision symbols, and said threshold of total number of valid symbols is an integer number between 3 and 15 and is a function of the number of hard decision symbols received. 
   
   
       5 . The method for generating a corrected code sequence as claimed in  claim 1 , wherein said modified Reed-Solomon decoder in said step (d) performs:
 (i) receiving a sequence of hard decision symbols including valid symbols and invalid symbols;   (ii) sending a code sequence based on valid symbols to a Reed-Solomon error-and-erasure decoder if total number of invalid symbols in said sequence of hard decision symbols is not greater than a threshold of total number of invalid symbols, or ending said step (d) otherwise;   (iii) generating said corrected code sequence and ending said step (d) if said Reed-Solomon error-and-erasure decoder returns successfully;   (iv) selecting k symbols from current valid symbols of said sequence of hard decision symbols, said k symbols having lowest k reliabilities among all current valid symbols of said sequence of hard decision symbols;   (v) declaring said k symbols invalid and determining total number of invalid symbols from said sequence of hard decision symbols; and   (vi) returning to step (ii) if total number of invalid symbols in said sequence of hard decision symbols is not greater than said threshold of total number of invalid symbols, or ending said step (d) otherwise.   
   
   
       6 . The method for generating a corrected code sequence as claimed in  claim 5 , wherein said sequence of hard decision symbols has at most 15 hard decision symbols, and said threshold of total number of invalid symbols is an integer number between 0 and 12. 
   
   
       7 . A method for generating a corrected code sequence, comprising:
 (a) receiving first and second frames of code word symbols;   (b) forming first and second sequences of hard decision symbols from said first and second frames of code word symbols;   (c) comparing each hard decision symbol in said first sequence with a corresponding hard decision symbol in said second sequence and declaring said hard decision symbol valid if corresponding hard decision symbols are identical and invalid if not identical;   (d) sending a code sequence based on valid hard decision symbols determined from step (c) to a Reed-Solomon error-and-erasure decoder if total number of invalid hard decision symbols is less than a threshold of total number of invalid hard decision symbols, or ending said method otherwise; and   (e) generating a corrected code sequence and ending said method if said Reed-Solomon error-and-erasure decoder returns successfully.   
   
   
       8 . A method for generating a corrected code sequence, comprising:
 (a) receiving a plurality of frames of code word symbols;   (b) forming a plurality of sequences of hard decision symbols, each sequence corresponding to one of all received frames of code word symbols;   (c) forming a new sequence of hard decision symbols, each hard decision symbol in said new sequence having a value determined by taking a majority vote of corresponding hard decision symbols in said plurality of sequences;   (d) sending said new sequence to a Reed-Solomon error-and-erasure decoder; and   (e) generating a corrected code sequence and ending said method if said Reed-Solomon error-and-erasure decoder returns successfully.   
   
   
       9 . The method for generating a corrected code sequence as claimed in  claim 8 , further comprising:
 (f) receiving a new frame of code word symbol if said Reed-Solomon error-and-erasure decoder fails to return a valid code sequence; and   (g) returning to step (b).