Degrees of Freedom for MIMO Two-Way X Relay Channel

Abstract

We study the degrees of freedom (DOF) of a multiple-input multiple-output\n(MIMO) two-way X relay channel, where there are two groups of source nodes and\none relay node, each equipped with multiple antennas, and each of the two\nsource nodes in one group exchanges independent messages with the two source\nnodes in the other group via the relay node. It is assumed that every source\nnode is equipped with M antennas while the relay is equipped with N antennas.\nWe first show that the upper bound on the total DOF for this network is\n2min{2M,N} and then focus on the case of N \\leq 2M so that the DOF is upper\nbounded by the number of antennas at the relay. By applying signal alignment\nfor network coding and joint transceiver design for interference cancellation,\nwe show that this upper bound can be achieved when N \\leq8M/5. We also show\nthat with signal alignment only but no joint transceiver design, the upper\nbound is achievable when N\\leq4M/3. Simulation results are provided to\ncorroborate the theoretical results and to demonstrate the performance of the\nproposed scheme in the finite signal-to-noise ratio regime.\n