Max-Min Optimal Beamforming for Cell-Free Massive MIMO

TL;DR

This paper proposes an optimal beamforming method for cell-free massive MIMO systems that maximizes the minimum user SNR, providing a benchmark for practical designs.

Contribution

It formulates a max-min SNR optimization problem, proves its quasi-concavity, and derives an optimal solution using second-order cone programming.

Findings

The method achieves the best possible beamforming performance.

The problem is analytically shown to be quasi-concave.

The solution serves as a benchmark for low-complexity beamformers.

Abstract

This letter develops an optimum beamforming method for downlink transmissions in cell-free massive multiple-input multiple-output (MIMO) systems, which employ a massive number of distributed access points to provide concurrent services to multiple users. The optimum design is formulated as a max-min problem that maximizes the minimum signal-to-interference-plus-noise ratio of all users. It is shown analytically that the problem is quasi-concave, and the optimum solution is obtained with the second-order cone programming. The proposed method identifies the best achievable beamforming performance in cell-free massive MIMO systems. The results can be used as benchmarks for the design of practical low complexity beamformers.