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arXiv · 2610.34603

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

Yiyang Peng·Zheyu Wu·Bruno Clerckx
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Abstract

Microwave linear analog computing (MiLAC) has recently emerged as a promising approach for reducing the hardware and digital-processing requirements of large-scale multiple-input multiple-output (MIMO) systems. In narrowband multiuser systems, hybrid digital-MiLAC beamforming can achieve digital beamforming flexibility with only as many radio-frequency (RF) chains as data streams. However, extending this advantage to wideband systems is nontrivial because practical MiLAC circuits exhibit frequency-dependent responses. In this paper, we investigate hybrid digital-MiLAC beamforming for multiuser MISO-OFDM systems with a frequency-dependent MiLAC circuit model. We characterize the frequency dependence of MiLAC by relating the susceptance of each tunable component across subcarriers to its center-frequency value. Under this frequency coupling, we show that a single MiLAC circuit generally cannot simultaneously match the desired beamforming directions across all subcarriers, highlighting the essential role of subcarrier-dependent digital precoding. We then develop a block coordinate descent (BCD) algorithm to jointly optimize the digital and MiLAC beamformers for average sum-rate maximization. In particular, an efficient alternating direction method of multipliers (ADMM) algorithm is custom-designed to handle the frequency-coupled circuit constraints in the MiLAC beamformer subproblem. Simulation results show that the proposed hybrid digital-MiLAC design outperforms conventional hybrid digital-analog beamforming, particularly when fewer RF chains are available and the channel is more frequency-selective. With $64$ transmit antennas and $4$ data streams, it attains $96.3\%$ of the digital sum-rate using only $8$ RF chains. Furthermore, under highly frequency-selective channels and using $4$ RF chains, it provides a $48\%$ sum-rate gain over the conventional hybrid design.

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Yiyang Peng, Zheyu Wu, Bruno Clerckx. 2026-10-25. Frequency-Dependent Hybrid Digital and Microwave Linear Analog Computer (MiLAC) Beamforming for Multiuser MIMO-OFDM Systems. https://arxiv.org/abs/2610.34603

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