Cell-free massive MIMO has matured into a key candidate technology for 6G and beyond, owing to its ability to provide nearly uniform service quality to many user equipments (UEs) over the same time-frequency resources. Unlike conventional cellular massive MIMO, the core idea is to distribute a large number of low-cost access points (APs) across the network and enable joint coherent transmission and reception. While early works largely assumed ideal hardware, hardware impairments become inevitable when APs are implemented with low-cost components. In this context, this paper investigates the adverse impact of low-resolution digital-to-analog converters (DACs) on the downlink performance of cell-free massive MIMO-OFDM systems. In contrast to prior studies that mainly quantify spectral-efficiency degradation under low-resolution DACs, we consider the design of quantized constant-envelope (CE) precoding, which additionally enables the use of highly power-efficient amplifiers. To the best of our knowledge, this is the first work on quantized CE precoding for cell-free massive MIMO-OFDM. Beyond adapting the classical maximum-antenna-power method, we propose a novel power-control strategy across APs that mitigates the detrimental effects of severely quantized transmitters by reducing the contribution of harmful APs. Simulation results demonstrate that the proposed power-control mechanism significantly improves the uncoded bit error rate performance.

翻译：无蜂窝大规模MIMO凭借其在相同时间-频率资源上为大量用户设备（UE）提供近乎均匀服务质量的能力，已成熟为6G及未来通信的关键候选技术。与传统蜂窝大规模MIMO不同，其核心思想是在网络中分布大量低成本接入点（AP），并实现联合相干传输与接收。早期研究多假设理想硬件，但当采用低成本组件实现AP时，硬件损伤不可避免。在此背景下，本文研究了低分辨率数模转换器（DAC）对无蜂窝大规模MIMO-OFDM系统下行性能的负面影响。与先前主要量化低分辨率DAC下频谱效率退化的研究不同，本文考虑了量化恒定包络（CE）预编码的设计，该设计还能支持高能效放大器的使用。据我们所知，这是针对无蜂窝大规模MIMO-OFDM系统的量化CE预编码的首项工作。除对经典最大天线功率方法进行改进外，我们提出了一种跨AP的新型功率控制策略，通过减少有害AP的贡献来缓解严重量化发射机带来的不利影响。仿真结果表明，所提出的功率控制机制显著提升了未编码误码率性能。