Reduced-Complexity Soft-Output Viterbi Algorithm for Channels Characterized by Dominant Error Events

An algorithm to generate soft output or log-likelihood-ratios (LLR´´s) on channel bits for channels which can be characterized by dominant error events (DEE) is proposed. The magnetic recording and optical recording systems fall under such class of channels [5]. For these channels, a concatenated coding scheme consisting of an outer Reed-Solomon (RS) and an inner parity code with single bit parity or multiple parity bits is commonly used. The inner parity code is usually not decoded using a trellis to limit the computation overhead. Instead, the parity code is decoded by using a post-processor which comprises of a bank of filters. The reduced complexity post-processor using information reuse for decoding the inner parity code is first proposed. A variant of the post-processor is then developed to enable generate soft-output based on the DEE. The performance of the soft-output algorithm is close to the Soft-Output Viterbi Algorithm (SOVA), if the dominant error event list is appropriately chosen. The advantage is that the computational cost is much lesser compared to the SOVA. Conceptually, the proposed reduced-complexity SOVA does not consider all the paths in the trellis for computing the soft-output, rather it uses only on the DEE based paths to generate the soft output.