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

Abstract

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 $2M\left(\left\lceil\frac{2M}{N}\right\rceil+1\right)/\left(2\left\lceil\frac{2M}{N} \right\rceil+1\right)$ if $K\le M and $\min\{2KN,M\}$ if $NK \le M$ 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.