On the Number of RF Chains and Phase Shifters, and Scheduling Design with Hybrid Analog-Digital Beamforming

TL;DR

This paper analyzes hybrid beamforming in multiuser massive MIMO systems, determining the minimal RF chains and phase shifters needed for optimal performance, and introduces a user scheduling method for limited RF chains.

Contribution

It provides a novel analysis of RF chain and phase shifter requirements for hybrid beamforming to match digital beamforming performance, along with a new user scheduling algorithm.

Findings

Hybrid beamforming can match digital beamforming with fewer RF chains and phase shifters.

A simple method reduces phase shifters with minimal performance loss.

Simulation confirms the effectiveness of the proposed design over existing methods.

Abstract

This paper considers hybrid beamforming (HB) for downlink multiuser massive multiple input multiple output (MIMO) systems with frequency selective channels. For this system, first we determine the required number of radio frequency (RF) chains and phase shifters (PSs) such that the proposed HB achieves the same performance as that of the digital beamforming (DB) which utilizes $N$ (number of transmitter antennas) RF chains. We show that the performance of the DB can be achieved with our HB just by utilizing $r_t$ RF chains and $2r_t(N-r_t + 1)$ PSs, where $r_t \leq N$ is the rank of the combined digital precoder matrices of all sub-carriers. Second, we provide a simple and novel approach to reduce the number of PSs with only a negligible performance degradation. Numerical results reveal that only $20-40$ PSs per RF chain are sufficient for practically relevant parameter settings. Finally, for the scenario where the deployed number of RF chains $(N_a)$ is less than $r_t$, we propose a simple user scheduling algorithm to select the best set of users in each sub-carrier. Simulation results validate theoretical expressions, and demonstrate the superiority of the proposed HB design over the existing HB designs in both flat fading and frequency selective channels.