Multi-layer data protection using N-Channel stop-and-wait Hybrid-ARQ in WiMAX

Many popular wireless networking protocols, e.g., WiMAX and UMTS, use Hybrid-ARQ for error control. HARQ combines the retransmission mechanism of ARQ protocol with the forward error correction techniques to give a better bit error rate performance under adverse channel conditions. WiMAX standard suggests a multichannel stop-and-wait HARQ. If the number of errors exceeds the error correction capability of the decoder, then a decoding failure is declared and a request for retransmission is made. In this paper, a method is proposed to extend the error correction capability at the receiver using the already available redundancy, i.e., the CRC. In case of a decoding failure, the proposed method uses the additional information available from the MAP decoder, as L-Values, to perform an iterative bit-flip decoding on the erroneous packets. The goal of the bit-flip decoding is to perform a certain number of trials, bounded by the threshold, to recover the transmitted packet. The CRCs of the packets transmitted on each channel are protected using Reed Solomon codes on a separate channel. It is shown, through simulation results that the number of retransmissions is significantly reduced with the proposed method and an improvement in the bit error rate and the packet error rate is obtained. It is also shown, that the probability of CRC collisions is reduced, thereby reducing the probability of decoding errors at the receiver.