Optimal Design of Energy-Efficient Multi-User MIMO Systems: Is Massive MIMO the Answer?

TLDR

A multi‑user MIMO system is designed from scratch to uniformly cover a given area with maximal energy efficiency. The study seeks the optimal number of antennas, active users, and transmit power to maximize energy efficiency. The authors jointly analyze uplink and downlink with various base‑station processing schemes, introduce a realistic power‑consumption model, and derive closed‑form expressions for the EE‑optimal values of each parameter under zero‑forcing processing in single‑cell scenarios. The derived expressions reveal that transmit power increases with antenna count, enabling high‑SNR operation with interference‑suppressing processing, and numerical results show that a massive‑MIMO configuration with hundreds of antennas and ZF processing achieves the highest EE, a result that holds under imperfect CSI and in symmetric multi‑cell settings.

Abstract

Assume that a multi-user multiple-input multiple-output (MIMO) system is designed from scratch to uniformly cover a given area with maximal energy efficiency (EE). What are the optimal number of antennas, active users, and transmit power? The aim of this paper is to answer this fundamental question. We consider jointly the uplink and downlink with different processing schemes at the base station and propose a new realistic power consumption model that reveals how the above parameters affect the EE. Closed-form expressions for the EE-optimal value of each parameter, when the other two are fixed, are provided for zero-forcing (ZF) processing in single-cell scenarios. These expressions prove how the parameters interact. For example, in sharp contrast to common belief, the transmit power is found to increase (not to decrease) with the number of antennas. This implies that energy-efficient systems can operate in high signal-to-noise ratio regimes in which interference-suppressing signal processing is mandatory. Numerical and analytical results show that the maximal EE is achieved by a massive MIMO setup wherein hundreds of antennas are deployed to serve a relatively large number of users using ZF processing. The numerical results show the same behavior under imperfect channel state information and in symmetric multi-cell scenarios.