Iterative Cross-layer Decoder for WiMAX Using Hybrid-ARQ

Hybrid Automatic Repeat Request is used as an error control and recovery mechanism in IEEE 802.16. Many different performance improvements over the standard error control mechanism have been proposed. One such improvement [1] considers the synergy between the MAC and PHY layers and proposes an adaptive fragmentation scheme, whereby the fragmentation is done only at the MAC layer at high bit error rates. This avoids duplicate fragmentations and enhances the throughput. In this paper, an enhancement over [1] is proposed by introducing a cross-layer decoding approach. In case of decoding failure at the PHY layer, the packets are not discarded rather passed on to the MAC layer along with the side information, i.e., the bit reliabilities, from the decoder at the PHY. MAC layer performs an additional decoding effort in the form of iterative decoding on such erroneous packets based on these bit reliabilities. Simulation results are presented showing a considerable coding gain at different values of E_b/N₀. The packet error rate is improved with this method and therefore the retransmission of packets is reduced.