Reduced-Dimension Cooperative Precoding for MIMO Two-Way Relay Channels

We investigate efficient communications over MIMO two-way relay channels (TWRCs) of n_T <; n_R, where n_T denotes the number of antennas at each user and n_R denotes that at the relay. We propose a new reduced-dimension (RD) cooperative precoding scheme. In the proposed scheme, the two users cooperatively create n_T aligned eigen-modes, supporting n_T streams of physical-layer network coding. We investigate the design of the RD cooperative precoder and derive an asymptotically optimal solution. We analytically show that, in the worst case, the proposed scheme is within ½ bit per transmit antenna of the asymptotic sum-capacity of the MIMO TWRC. For fading MIMO TWRCs with i.i.d. Gaussian coefficients, we derive a closed-form expression of the average sum-rate of the proposed scheme using large system analysis. Our analytical result shows that, for a large system with n_T/n_R = ½, the proposed scheme is less than 0.16 bit per transmit antenna away from the capacity. Furthermore, this gap reduces as n_T/n_R increases, and vanishes as n_T/n_R tends to 1. It is demonstrated that the proposed scheme can significantly outperform other existing schemes in the literature.