Linear Degrees of Freedom of the MIMO X-Channel with Delayed CSIT

TL;DR

This paper analyzes the linear degrees of freedom in MIMO X-channels with delayed CSIT, providing new sum DoF and DoF region results using novel rank-ratio inequalities and a three-phase transmission strategy.

Contribution

It introduces the first linear sum DoF and DoF region characterizations for MIMO X-channels with delayed CSIT, using innovative rank-ratio inequalities and a structured transmission approach.

Findings

Derived the linear sum DoF for general antenna configurations.

Established the linear DoF region for symmetric antenna configurations.

Developed a novel rank-ratio inequality for received signal subspaces.

Abstract

We study the degrees of freedom (DoF) of the multiple-input multiple-output X-channel (MIMO XC) with delayed channel state information at the transmitters (delayed CSIT), assuming linear coding strategies at the transmitters. We present two results: 1) the linear sum DoF for MIMO XC with general antenna configurations, and 2) the linear DoF region for MIMO XC with symmetric antennas. The converse for each result is based on developing a novel rank-ratio inequality that characterizes the maximum ratio between the dimensions of received linear subspaces at the two multiple-antenna receivers. The achievability of the linear sum DoF is based on a three-phase strategy, in which during the first two phases only the transmitter with fewer antennas exploits delayed CSIT in order to minimize the dimension of its signal at the unintended receiver. During Phase 3, both transmitters use delayed CSIT to send linear combinations of past transmissions such that each receiver receives a superposition of desired message data and known interference, thus simultaneously serving both receivers. We also derive other linear DoF outer bounds for the MIMO XC that, in addition to the outer bounds from the sum DoF converse and the proposed transmission strategy, allow us to characterize the linear DoF region for symmetric antenna configurations.