Relay-aided interference alignment for the X channel with limited CSI

In this work, we investigate the impact of placing a half-duplex relay in the X channel in the presence of limited channel state information (CSI). Specifically, we consider the scenarios when the sources have no CSI, while the relay either has perfect CSI or delayed CSI. We develop transmission strategies for each scenario to facilitate interference alignment with the help of the relay. We show that for both scenarios, with single antenna at each source and destination, degree of freedom 4/3 can be achieved. This matches the degree of freedom established in previous work when perfect CSI is available at all nodes. For the case with perfect CSI at the relay, relay only needs to have 1 antenna, and can perform beamforming with the sources to facilitate interference alignment using the proposed scheme. For the case with delayed CSI at the relay, the relay can compensate the staleness of the CSI by adding an antenna. An important observation from this work is that relay can increase the degrees of freedom for the X channel when the sources have no CSI, in contrast with the case when perfect CSI is available at all nodes, where a relay cannot improve the degrees of freedom. The results are also extended to multi-antenna settings.