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- and code-combining-based retransmission schemes. Assuming 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 with a retransmission scheme based on the classical decode-and-forward (DF) relaying.