CRC-Aided Fixed Complexity Error Pattern Estimation Technique

Various techniques using cyclic redundancy check (CRC) codes for error correction have been proposed. In previous techniques, a small number of unreliable bits in a packet were toggled in order to change negative acknowledgement (NAK) into acknowledgement (ACK). The difficulty of using CRC code for the purpose of error correction is that the worst case complexity is high because the number of possible error patterns is $2^{N_{\mathrm {UR}}}$ where $N_{\mathrm {UR}}$ denotes the number of the unreliable bits. In this paper, we define the optimality of a set of error patterns using soft decisions when the set size as well as the number of unreliable bits is fixed. We generate optimal error patterns in an incremental manner based on the optimality definition. The worst case complexity of the proposed error pattern estimation technique is fixed at a low level, hence the name fixed complexity CRC-aided error pattern estimation (fcCRCEPE). We show that the optimal set of error patterns can be found with feasible computational complexity even when $N_{\mathrm {UR}}$ is in the order of hundreds. The proposed technique can be used alone and can also be used with all channel decoders that produce soft output.