Design and Outage Performance Analysis of Relay-Assisted Two-Way Wireless Communications

Two-way wireless communication has regained significant attention recently over relay channels and network coding techniques have been adopted to improve the spectral efficiency. In the literature, most works assumed knowledge of the channel state information at the transmitter (CSIT) and focused mainly on the three-step network coding (NetC) and two-step superposition coding (SupC) schemes. This work instead targets systems without CSIT and explores in detail the features and limitations of SupC and NetC under both the two-step and three-step frameworks. Equal-rate bi-directional traffic is considered and a comparative study of the schemes is provided. The maximum goodput and robustness to channel knowledge discrepancies are evaluated. Key results include theoretically derived outage regions and performance, and the analysis of the complementary role of NetC and SupC. Special features of the three-step framework and a simple yet efficient protocol, adaptive relay-assisted and direct transmission (ARDT), which smartly exploits the direct link in the absence of CSIT, will also be presented. Optimal power allocation at the relay is also derived for the three-step SupC scheme by exploiting the channel reciprocity (no CSIT required). A significant advantage of ARDT is demonstrated over the recently developed two-step SupC scheme. Numerical results together with extensive discussions are provided.