Are mmWave Low-Complexity Beamforming Structures Energy-Efficient? Analysis of the Downlink MU-MIMO

TL;DR

This paper compares the spectral and energy efficiency of various low-complexity beamforming structures for mmWave MU-MIMO systems, revealing that fully-digital beamformers can sometimes outperform simpler solutions in energy efficiency.

Contribution

It provides a comparative assessment of suboptimal beamforming structures for mmWave MU-MIMO, considering recent data on hardware energy consumption and highlighting scenarios where fully-digital solutions are more efficient.

Findings

Fully-digital beamformers can outperform low-complexity solutions in energy efficiency.

Switch-based structures often perform poorly in realistic scenarios.

Complexity reduction does not always lead to energy savings.

Abstract

Future cellular systems based on the use of above-6 GHz frequencies, the so-called millimeter wave (mmWave) bandwidths, will heavily rely on the use of antenna arrays both at the transmitter and at the receiver, possibly with a large number of elements. For complexity reasons, fully digital precoding and postcoding structures may turn out to be unfeasible, and thus suboptimal structures, making use of simplified hardware and a limited number of RF chains, have been investigated. This paper considers and makes a comparative assessment, both from a spectral efficiency and energy efficiency point of view, of several suboptimal precoding and postcoding beamforming structures for the downlink of a cellular multiuser MIMO (MU-MIMO) system. Based on the most recently available data for the energy consumption of phase shifters and switches, we show that there are cases where fully-digital beamformers may achieve a larger energy efficiency than lower-complexity solutions, as well as that structures based on the exclusive use of switches achieve quite unsatisfactory performance in realistic scenarios.