Signal Space Alignment for Asymmetric Multi-pair Two-way Relay X Channels

In this paper, we study the two-way relay channel (TWRC) with multi-user (MU), each of which exchanges information with its partners via a common access relay. All users are divided into two groups. Every user exchanges messages with each user in the other group. The two communication terminals use different modulation schemes. We term this system an asymmetric multi-pair two-way relay X channel (TWRXC), in which signal space alignment for network coding (SSA-NC) and network-coding-based interference nulling beamforming are employed. Considering modulation schemes of two communication terminals are different, we propose a novel reencode function at the relay and decoding rule at the user side to realize the reliable communication. The bit error rates (BERs) of the proposed system is derived. Monte Carlo simulations show that the proposed scheme can improve system performance significantly.