Cell-Free Massive MIMO with Low-Complexity Hybrid Beamforming

TL;DR

This paper proposes a low-complexity hybrid beamforming framework for Cell-Free Massive MIMO systems, reducing power consumption and complexity while maintaining high data rates through large-scale channel statistics.

Contribution

It introduces a hybrid beamforming design using limited RF chains and large-scale channel statistics, along with closed-form SINR expressions and a power optimization algorithm for uplink.

Findings

Random matrix approximations are highly accurate.

Trade-off identified between outage data rate and RF-chains.

Power optimization has limited impact in uplink with MMSE combiner.

Abstract

Cell-Free Massive Multiple-input Multiple-output (mMIMO) consists of many access points (APs) in a coverage area that jointly serve the users. These systems can significantly reduce the interference among the users compared to conventional MIMO networks and so enable higher data rates and a larger coverage area. However, Cell-Free mMIMO systems face multiple practical challenges such as the high complexity and power consumption of the APs' analog front-ends. Motivated by prior works, we address these issues by considering a low complexity hybrid beamforming framework at the APs in which each AP has a limited number of RF-chains to reduce power consumption, and the analog combiner is designed only using the large-scale statistics of the channel to reduce the system's complexity. We provide closed-form expressions for the signal to interference and noise ratio (SINR) of both uplink and downlink data transmission with accurate random matrix approximations. Also, based on the existing literature, we provide a power optimization algorithm that maximizes the minimum SINR of the users for uplink scenario. Through several simulations, we investigate the accuracy of the derived random matrix approximations, trade-off between the 95% outage data rate and the number of RF-chains, and the impact of power optimization. We observe that the derived approximations accurately follow the exact simulations and that in uplink scenario while using MMSE combiner, power optimization does not improve the performance much.