Soft Decision Decoding of Reed-Solomon Codes Using Sphere Decoding

A new soft decision decoding method for Reed-Solomon (RS) codes is proposed. This method uses sphere decoding in an effort to reduce the decoding complexity. With sphere decoding, instead of considering all of the possible transmitted codewords to determine the most probable one, we only consider the codewords whose distances from the received signal are smaller than a specific search radius. This results in a considerable reduction in the complexity. For an (N,K) RS code, we consider a set of K most reliable and independent positions of a codeword and for each of these positions, an ordered list of most probable transmitted symbols in decreasing order of probability is determined. We start from the hard-decision decoded codeword and we try to find more probable codewords. The search is started by selecting a tentative solution consisting of the K most reliable code symbols whose distance from the corresponding symbols in the received vector is less than the search radius. The acceptable values for each of these K code symbols are determined based of the ordered set of most probable transmitted symbols which means that for each code symbol, we start from the most probable one. We re-encode these K code symbols. If the resulting codeword is within the search radius, we add it to the list of the candidate transmitted codewords. The ordering that was discussed earlier will help finding the candidate codewords quickly. Our method results in considerable improvement of the performance of RS codes compared to hard decision decoding with a moderate increase in complexity.