Superposition Modulation Based Symmetric Relaying with Hybrid ARQ: Analysis and Optimization

We present a retransmission scheme based on superposition modulation for the symmetric relaying scenario when the number of retransmissions for a data packet is limited. We consider both diversity combining based as well as code combining based retransmission schemes. Under the assumption that the receiver implements a mechanism that can use all accumulated received mutual information when decoding the message, we derive the packet error probability (PEP) expressions for the proposed retransmission scheme for the case when only one retransmission is allowed. Based on the PEP expressions derived, we provide a closed-form solution for the optimal superposition ratio (the fraction of power used for the relaying operation). Simulation results show that the proposed retransmission scheme offers significant gains compared to a retransmission scheme based on classical decode-and-forward (DF) relaying.Funding agencies|Swedish Foundation for Strategic Research and the Excellence Center at Linkoping-Lund in Information Technology||Knut and Alice Wallenberg Foundation through a Royal Swedish Academy of Sciences|