Multi-Relay Multi-User Asymmetric Two-Way Relaying Over Fading Channels: A Unified Outage Analysis and Location Optimization Study

We investigate the performance of a cellular two-way relay (TWR) network in which a multi-antenna base station (BS) communicates bidirectionally with a group of single-antenna users via single-antenna multiple relays using analog network coding (ANC). Using a user-relay selection scheme coupled with antenna selection at the BS, we try to maximize the end-to-end signal-to-noise ratios, and to combine the benefits of multi-antenna diversity, multi-relay diversity, and multi-user diversity. Specifically, we derive a simple, closed-form tight lower bound unified expression for the overall outage probability (OOP) under different traffic patterns for any fading channel for which the cumulative distribution function is available. To examine the generality of our derived expression, we consider Rayleigh, Nakagami-m, Rician, and Weibull fading channels. We also derive an asymptotic outage expression in the high signal-to-noise ratio (SNR) regime from which the impact of key system/channel parameters on the achievable diversity order are examined. In addition, we present an optimal relay location to minimize the OOP under various fading channels and traffic asymmetry. Finally, the theoretical findings are validated by simulations.