A modified HARQ scheme associated with non-uniform constellation

Multilevel quadrature amplitude modulation (MQAM) is an attractive modulation technique to achieve high data rate transmission without increasing the wireless bandwidth. But each bit that makes up a MQAM symbol exhibits a different degree of BER performance for MQAM (M>4) constellation with Gray encoding. This will degrade the performance of the decoder with respect to having relatively equal BER performance. A modified HARQ scheme associated with nonuniform constellation is proposed to maximize the system throughput. It is proposed to use the nonuniform constellation together with the symbol bits rearrangement in the retransmissions to average and improve the reliability of each bit gradually by means of code combining in the receiver. Simulation results demonstrate that the proposed scheme can reduce the average transmission number and increase the throughput effectively, compared with those HARQ schemes using one constellation or constellation rearrangement. In particular, the higher MQAM modulation alphabet size and the more inhomogeneous bit reliabilities, the higher gain can be achieved by using the proposed scheme.