Achievable Sum DoF of the K-User MIMO Interference Channel with Delayed CSIT

TL;DR

This paper investigates the maximum sum degrees-of-freedom (DoF) achievable in a K-user MIMO interference channel with outdated channel state information, proposing new schemes that outperform previous methods especially when the number of transmit antennas exceeds receive antennas.

Contribution

The paper introduces two novel K-phase retrospective interference alignment schemes for MIMO IC with delayed CSIT, achieving the best known sum DoF performance.

Findings

Achieves asymptotic sum DoF of 64/15 N for large K when M/N ≥ K.

Proposes schemes that outperform previous methods in sum DoF performance.

Extends and generalizes existing RIA schemes for MIMO interference channels.

Abstract

This paper considers a $K$-user multiple-input-multiple-output (MIMO) interference channel (IC) where 1) the channel state information obtained by the transmitters (CSIT) is completely outdated, and 2) the number of transmit antennas at each transmitter, i.e., $M$, is greater than the number of receive antennas at each user, i.e., $N$. The usefulness of the delayed CSIT was firstly identified in a $K$-phase Retrospective Interference Alignment (RIA) scheme proposed by Maddah-Ali et al for the Multiple-Input-Single-Output Broadcast Channel, but the extension to the MIMO IC is a non-trivial step as each transmitter only has the message intended for the corresponding user. Recently, Abdoli et al focused on a Single-Input-Single-Output IC and solved such bottleneck by inventing a $K$-phase RIA with distributed overheard interference retransmission. In this paper, we propose two $K$-phase RIA schemes suitable for the MIMO IC by generalizing and integrating some key features of both Abdoli's and Maddah-Ali's works. The two schemes jointly yield the best known sum Degrees-of-Freedom (DoF) performance so far. For the case $\frac{M}{N}{\geq}K$, the achieved sum DoF is asymptotically given by $\frac{64}{15}N$ when $K{\to}\infty$.