Beamforming design for multi-user two-way relaying with MIMO amplify and forward relays

Relays represent a promising approach to extend the cell coverage, combat the strong shadowing effects as well as guarantee the QoS in dense networks. Among the numerous existing relaying techniques, two-way relaying uses the radio resource in a particular efficient manner. Moreover, amplify and forward (AF) relays cause less delays and require lower hardware complexity. Therefore, we consider multi-user two-way relaying with MIMO AF relays where a base station (BS) and multiple users (UT) exchange messages via the relay in this paper. We propose three sub-optimal algorithms for computing the transmit and receive beamforming matrices at the BS as well as the amplification matrix at the relay. Simulations show that block diagonalization (BD) combined with the algebraic norm-maximizing (ANOMAX) transmit strategy provides the best balance between complexity and performance, the zero-forcing dirty paper coding (ZFDPC) based design can perform well when the system is heavily loaded, and the channel inversion (CI) based design yields the lowest complexity. All three algorithms outperform a recently proposed technique from the literature.