On the Role of Transmit Correlation Diversity in Multiuser MIMO Systems

TL;DR

This paper introduces transmit correlation diversity in multiuser MIMO systems, showing it can significantly enhance capacity and multiplexing gains by exploiting nearly orthogonal large-scale channel eigen-directions among users.

Contribution

It proposes a novel concept of transmit correlation diversity and quantifies its capacity benefits in massive MIMO systems, considering training costs.

Findings

Transmit correlation diversity can increase system multiplexing gain up to loor{M/r}.

Properly accounting for training costs reveals significant capacity gains.

Correlation diversity acts as a new resource for improving multiuser MIMO performance.

Abstract

Correlation across transmit antennas, in multiple antenna systems (MIMO), has been studied in various scenarios and has been shown to be detrimental or provide benefits depending on the particular system and underlying assumptions. In this paper, we investigate the effect of transmit correlation on the capacity of the Gaussian MIMO broadcast channel (BC), with a particular interest in the large-scale array (or massive MIMO) regime. To this end, we introduce a new type of diversity, referred to as transmit correlation diversity, which captures the fact that the channel vectors of different users may have different, and often nearly mutually orthogonal, large-scale channel eigen-directions. In particular, when taking the cost of downlink training properly into account, transmit correlation diversity can yield significant capacity gains in all regimes of interest. Our analysis shows that the system multiplexing gain can be increased by a factor up to $\lfloor{M}/{r}\rfloor$, where $M$ is the number of antennas and $r\le M$ is the common rank of the users transmit correlation matrices, with respect to standard schemes that are agnostic of the transmit correlation and treat the channels as if they were isotropically distributed. Thus, this new form of diversity reveals itself as a valuable "new resource" in multiuser communications.