An efficient method for channel state information feedback in the relay X-channel

The two-user relay-assisted X channel is considered when no channel state information (CSI) is available at the transmitter side. It is known that with a 2M-antenna relay and M-antenna users a DoF of is achieved in this channel if CSI is available at the relay. This is equal to the maximum achievable DoF in the ordinary X-channel (with no relay) with perfect CSI at the transmitters. An efficient CSI feedback method is proposed which reduces the amount of feedback necessary for the relay in order to maintain the maximum DoF. In addition, a quantization method is proposed for the case of limited feedback. The proposed method outperforms the conventional methods in terms of the number of feedback bits. Simulation results show performance improvement in the finite-SNR regime.