Designing Multi-User MIMO for Energy Efficiency: When is Massive MIMO the Answer?

TL;DR

This paper derives optimal configurations for multi-user MIMO systems to maximize energy efficiency, revealing that higher transmit power and massive antenna arrays can be beneficial when using interference-suppressing precoding.

Contribution

It introduces a new model for energy efficiency in multi-user MIMO, providing closed-form solutions for optimal antenna count, user number, and transmit power, challenging common assumptions.

Findings

Optimal energy efficiency achieved with hundreds of antennas and many users.

Transmit power increases with the number of antennas, contrary to common belief.

Interference-suppressing precoding is essential for high SNR regimes.

Abstract

Assume that a multi-user multiple-input multiple-output (MIMO) communication system must be designed to cover a given area with maximal energy efficiency (bit/Joule). What are the optimal values for the number of antennas, active users, and transmit power? By using a new model that describes how these three parameters affect the total energy efficiency of the system, this work provides closed-form expressions for their optimal values and interactions. In sharp contrast to common belief, the transmit power is found to increase (not decrease) with the number of antennas. This implies that energy efficient systems can operate at high signal-to-noise ratio (SNR) regimes in which the use of interference-suppressing precoding schemes is essential. Numerical results show that the maximal energy efficiency is achieved by a massive MIMO setup wherein hundreds of antennas are deployed to serve relatively many users using interference-suppressing regularized zero-forcing precoding.