Hybrid Digital and Microwave Linear Analog Computer (MiLAC)-aided Beamforming for Multiuser MIMO-OFDM Systems

2026-04-29

AI Summary

This work addresses the challenge in wideband multiuser MIMO-OFDM systems where conventional hybrid beamforming struggles to balance performance and radio frequency (RF) chain constraints. To overcome this limitation, the paper introduces, for the first time, a microwave linear analog computer (MiLAC) into wideband MU-MISO scenarios, proposing a digital–MiLAC hybrid architecture. Leveraging the weighted minimum mean square error (WMMSE) optimization framework, a low-complexity wideband beamforming algorithm is developed, and the minimum number of RF chains required to achieve near-full-digital performance under frequency-selective channels is analytically characterized. Simulation results demonstrate that, in highly frequency-selective channels, the proposed approach attains 89.93% of the sum rate achievable by a fully digital scheme using only 12.5% of the RF chains, substantially outperforming conventional hybrid architectures.

Abstract

Microwave linear analog computing (MiLAC) has recently emerged as a promising architecture for analog-domain beamforming. In particular, a hybrid digital-MiLAC architecture was proposed and was shown to achieve fully-digital beamforming flexibility in narrowband systems when the number of RF chains equals the number of data streams. However, its performance in wideband systems remains unexplored. This paper presents the first study of hybrid digital-MiLAC beamforming for wideband multi-user multiple-input single-output (MU-MISO) systems. We first characterize the minimum number of radio-frequency (RF) chains required for hybrid digital-MiLAC beamforming to realize an arbitrary set of fully-digital beamforming matrices across all subcarriers. It turns out that, unlike in the narrowband case, a larger number of RF chains is generally required in frequency-selective channels to achieve fully-digital beamforming flexibility, which may be unfavorable in practice. To study the performance of hybrid digital-MiLAC beamforming with a limited number of RF chains, we then formulate the average sum-rate maximization problem and develop an efficient weighted minimum mean-square error (WMMSE)-based algorithm for beamforming design. Simulation results show that hybrid digital-MiLAC beamforming consistently outperforms conventional hybrid digital-analog beamforming, and achieves $89.93\%$ of the fully-digital sum-rate while using only $12.5\%$ of the RF chains in highly frequency-selective channels.

Problem

• Hybrid beamforming
• Microwave linear analog computing
• Wideband MU-MISO
• RF chain constraint
• Frequency-selective channels

Innovation

• MiLAC
• hybrid beamforming
• wideband MIMO-OFDM
• RF chain reduction
• WMMSE