Guiding blind transmitters: Relay-aided interference alignment for the X channel

This paper investigates the impact of half-duplex relays on the degrees of freedom (DoF) of the K-user X channel when the transmitters are blind in the sense that no channel state information (CSI) at the transmitter (CSIT) is available. It is shown that adding relay nodes with CSI to the communication model is sufficient to recover the optimal DoF with CSIT. In particular, the relay nodes help steer the directions of the transmitted signals to facilitate interference alignment to achieve optimal DoF. It is shown that one relay node with K-1 antennas is sufficient to achieve the optimal DoF. When relays do not have multiple antennas, it is sufficient to have (K - 1)² relays to achieve the optimal DoF. The achievability is established using partial interference alignment and joint beamforming. DoF for the case when relays only have delayed CSI is also investigated.