Generalized Signal Alignment: On the Achievable DoF for Multi-User MIMO Two-Way Relay Channels.

This paper studies the achievable degrees of freedom (DoF) for multi-user multiple-input multiple-output (MIMO) two-way relay channels, where there are K source nodes, each equipped with M antennas, one relay node, equipped with N antennas, and each source node exchanges independent messages with an arbitrary set of other source nodes via the relay. By allowing an arbitrary information exchange pattern, the considered channel model is a unified one. It includes several existing channel models as special cases: 1) K -user MIMO Y channel; 2) multi-pair MIMO two-way relay channel; 3) generalized MIMO two-way X relay channel; and 4) L -cluster MIMO multiway relay channel. Previous studies mainly considered the achievability of the DoF cut-set bound 2N at the antenna configuration N<2M for the K -user MIMO Y channel; 2) \frac NM \in \big (0, 2+\frac 4K\big ] \cup \big [K-2, +\infty \big) for the multi-pair MIMO two-way relay channel; and 3) \frac NM \in \big (0, 2+\frac 8K^2\big ] \cup \big [K-2, +\infty \big) for the generalized MIMO two-way X relay channel. We also provide the antenna configuration regions for the general multi-user MIMO two-way relay channel to achieve the total DoF $KM$ . [ABSTRACT FROM AUTHOR]