Retrospective Interference Alignment for Two-Cell Uplink MIMO Cellular Networks with Delayed CSIT

TL;DR

This paper introduces a retrospective interference alignment technique for two-cell MIMO uplink networks with delayed CSIT, demonstrating that outdated channel information can be exploited to improve sum-DoF beyond no-CSIT scenarios.

Contribution

It proposes a novel retrospective interference alignment method that leverages delayed CSIT to enhance sum-DoF in two-cell MIMO networks, achieving optimal bounds in certain cases.

Findings

Delayed CSIT increases sum-DoF compared to no CSIT.

Retrospective interference alignment aligns interference onto small subspaces.

Achieves optimal sum-DoF in two-cell two-user scenarios.

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 the 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 of no CSIT. The key idea is to align multiple interfering signals from adjacent cells onto a small dimensional subspace over time by fully exploiting the previously received signals as side information with outdated CSIT in a distributed manner. Remarkably, we show that the retrospective interference alignment can achieve the optimal sum-DoF in the context of two-cell two-user scenario by providing a new outer bound.