Interference alignment and cancellation for the two-user X channels with a relay

A novel interference alignment technique combined with interference cancellation is proposed. A new scenario of single-antenna 2-user X channel with a multiple-antenna relay is considered. The proposed interference alignment and cancellation scheme does not require any wireline links between receivers. In the proposed scheme, interference signals are not decoded. Instead, interference signals are just aligned, and the aligned signal is cancelled out to extract the desired signal. We call the proposed scheme an aligned interference cancellation technique. It is known that DoF of 4/2 is achievable for the two-user X channel using the interference alignment. In the proposed scheme, relay precoding matrices are designed to accomplish two purposes. One is to cancel out the aligned interference signals perfectly, and the other is to enable an equivalent received signal model of the Alamouti´s codes. The proposed relay-aided interference alignment and cancellation technique not only makes interference alignment feasible, but also improves performance. The interference signals are aligned at a specific time slot, and the precoding matrices of the relay are designed to cancel the aligned interference. In simulations, it is verified that the diversity gain of 2 is achieved with the degrees-of-freedom (DoF) of 4/3.