Transmit Correlation Diversity: Generalization, New Techniques, and Improved Bounds

TL;DR

This paper explores how transmit correlation diversity in MIMO broadcast channels can be generalized beyond mutually exclusive eigenspaces, introducing new techniques to improve degrees of freedom and achievable rates, especially in massive MIMO systems.

Contribution

It extends transmit correlation diversity to overlapping eigenspaces, deriving new DoF and rate region bounds, and proposes techniques like product superposition and pre-beamforming for better performance.

Findings

Achievable DoF regions are tight in certain configurations with CSIR.

Generalized techniques improve sum rate in correlated massive MIMO systems.

Analysis of interference graph for multi-user eigenspace overlap.

Abstract

When the users in a MIMO broadcast channel experience different spatial transmit correlation matrices, a class of gains is produced that is denoted transmit correlation diversity. This idea was conceived for channels in which transmit correlation matrices have mutually exclusive eigenspaces, allowing non-interfering training and transmission. This paper broadens the scope of transmit correlation diversity to the case of partially and fully overlapping eigenspaces and introduces techniques to harvest these generalized gains. For the two-user MIMO broadcast channel, we derive achievable degrees of freedom (DoF) and achievable rate regions with/without channel state information at the receiver (CSIR). When CSIR is available, the proposed achievable DoF region is tight in some configurations of the number of receive antennas and the channel correlation ranks. We then extend the DoF results to the $K$-user case by analyzing the interference graph that characterizes the overlapping structure of the eigenspaces. Our achievability results employ a combination of product superposition in the common part of the eigenspaces, and pre-beamforming (rate splitting) to create multiple data streams in non-overlapping parts of the eigenspaces. Massive MIMO is a natural example in which spatially correlated link gains are likely to occur. We study the achievable downlink sum rate for a frequency-division duplex massive MIMO system under transmit correlation diversity.