Joint Relay Antenna Selection and Zero-Forcing Spatial Multiplexing for MIMO Two-Way Relay with Physical-Layer Network Coding

We consider a multiple-input multiple-output (MIMO) two-way relay network with two N-T-antenna (N-T >= 2) end nodes and one dedicated N-R-antenna (N-R > N-T) relay node. A physical-layer network coding (PNC) based joint relay antenna selection and zero-forcing (ZF) spatial multiplexing scheme is proposed to support N-T streams of bidirectional data exchanging with the help of N-T out of N-R selected antennas at the relay node. The optimum relay antennas are selected by a Max-Min criterion with respect to the post-processing SNR of the whole system and then the linear ZF based MIMO two-way relay transmission is achieved with the help of the selected relay antennas. The optimum relay antenna selection not only fulfills the ZF based scheme's requirement on the number of effective relay antennas but also provides an end-to-end diversity advantage to the N-T streams of bidirectional data exchanging with linear transceiver at each end node. The diversity order of the proposed scheme is analyzed. Explicit diversity order d = N-R - 1 is obtained theoretically for N-T = 2 streams of bidirectional data exchanging. For N-T >= 2 cases, a conjecture that d = N-R - N-T + 1 is obtained based on simulation results.