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.

翻译：微波线性模拟计算（MiLAC）近年来作为一种有前景的模拟域波束成形架构而兴起。特别地，一种混合数字-MiLAC架构被提出，并证明在窄带系统中，当射频链路数量等于数据流数量时，能够实现全数字波束成形的灵活性。然而，其在宽带系统中的性能尚未被探索。本文首次研究了适用于宽带多用户多输入单输出（MU-MISO）系统的混合数字-MiLAC波束成形。我们首先刻画了混合数字-MiLAC波束成形在所有子载波上实现任意一组全数字波束成形矩阵所需的最少射频链路数量。结果表明，与窄带情况不同，在频率选择性信道中通常需要更多射频链路才能实现全数字波束成形的灵活性，这在实际应用中可能不具优势。为研究有限射频链路下混合数字-MiLAC波束成形的性能，我们随后构建了平均和速率最大化问题，并开发了一种基于加权最小均方误差（WMMSE）的高效波束成形设计算法。仿真结果表明，混合数字-MiLAC波束成形始终优于传统混合数字-模拟波束成形，并且在高度频率选择性信道中，仅使用12.5%的射频链路即可达到全数字和速率的89.93%。