Retrospective Interference Alignment for the Two-Cell MIMO Interfering Multiple Access Channel

In this paper, we propose a new retrospective interference alignment for two-cell multiple-input multiple-output (MIMO) interfering multiple access channels (IMAC) with M receive antennas per base station (BS) and K users per cell each with N transmit antennas. The key idea of the retrospective IA lies in interference-purification during phase 1 and 2, and interference-repetition during phase 3. On the basis of the proposed method, we show that the achievable sum degrees of freedom (sum-DoF) is equal to 2M ([2M/N]+ 1) / (2 [2M/N] + 1) if K M <; NK and min{2KN, M} if NK ≤ M with completely delayed channel state information at the transmitters (CSIT). It is shown that having delayed CSIT can strictly increase the sum-DoF compared to the case with no CSIT for cellular networks, especially for uplink scenarios.