Energy efficiency of HARQ-IR for two-way relay systems with network coding

For a two-way wireless relay system, in which two end nodes (EN) exchange their messages with the aid of a relay node (RN), the notion of network coding can be exploited to improve throughput. In this paper, the hybrid automatic repeat request with incremental redundancy (HARQ-IR) is applied to a two-way wireless relay system with network coding for reliable transmissions and its energy efficiency is studied. Two approaches of facilitating network coding are proposed including analog network coding (ANC) and physical-layer network coding (PNC). Using the notion of energy delay tradeoff (EDT) as a tool to evaluate the degrees of energy efficiency, we can show that the PNC- can be more energy efficient than the ANC-based scheme. Numerical results confirm the advantage of relay-aided transmissions with PNC in terms of energy efficiency.