Energy and Spectral Efficiency of Very Large Multiuser MIMO Systems

TL;DR

This paper analyzes the spectral and energy efficiency of very large multiuser MIMO systems, demonstrating how large antenna arrays significantly enhance performance and reduce power requirements even with imperfect channel information.

Contribution

It derives capacity bounds for various detection schemes and quantifies the tradeoff between energy and spectral efficiency in large MIMO systems.

Findings

Large antenna arrays improve spectral and energy efficiency by orders of magnitude.

Power can be inversely proportional to the square root of the number of antennas with imperfect CSI.

Maximum-ratio combining becomes viable at low power levels due to reduced cross-talk.

Abstract

A multiplicity of autonomous terminals simultaneously transmits data streams to a compact array of antennas. The array uses imperfect channel-state information derived from transmitted pilots to extract the individual data streams. The power radiated by the terminals can be made inversely proportional to the square-root of the number of base station antennas with no reduction in performance. In contrast if perfect channel-state information were available the power could be made inversely proportional to the number of antennas. Lower capacity bounds for maximum-ratio combining (MRC), zero-forcing (ZF) and minimum mean-square error (MMSE) detection are derived. A MRC receiver normally performs worse than ZF and MMSE. However as power levels are reduced, the cross-talk introduced by the inferior maximum-ratio receiver eventually falls below the noise level and this simple receiver becomes a viable option. The tradeoff between the energy efficiency (as measured in bits/J) and spectral efficiency (as measured in bits/channel use/terminal) is quantified. It is shown that the use of moderately large antenna arrays can improve the spectral and energy efficiency with orders of magnitude compared to a single-antenna system.