An Efficient Hybrid ARQ System Using Multilevel Coded Modulation with Reduced Constellation Size

In order to guarantee highly reliable communications over noisy channels, the use of retransmission techniques is necessary, and the hybrid auto repeat request (HARQ) technique has been widely adopted in packet-based data communication systems. A common approach is to retransmit the entire packet that contains uncorrectable errors. However, this approach is inefficient if the number of errors in those packets is small. In this paper, we propose a new HARQ system based on multilevel coded modulation and multi-stage decoding at the receiver. In this system, retransmission is performed based on each level of multilevel signaling with reduced constellation size. The major advantage of this approach is that not only retransmission is efficient, but also the receiver complexity for code combining is low. Moreover, this approach offers a flexibility in the retransmission patterns. We derive an optimal algorithm of retransmission based on the channel capacity rule in this framework. The simulation results demonstrate that the proposed HARQ can significantly outperform the HARQ system based on the bit-interleaved coded modulation (BICM).