In the uplink of a cell-free massive MIMO system, quantization affects performance in two key domains: the time-domain distortion introduced by finite-resolution analog-to-digital converters (ADCs) at the access points (APs), and the fronthaul quantization of signals sent to the central processing unit (CPU). Although quantizing twice may seem redundant, the ADC quantization in orthogonal frequency-division duplex (OFDM) systems appears in the time domain, and one must then convert to the frequency domain, where quantization can be applied only to the signals at active subcarriers. This reduces fronthaul load and avoids unnecessary distortion, since the ADC output spans all OFDM samples while only a subset of subcarriers carries useful information. While both quantization effects have been extensively studied in narrowband systems, their joint impact in practical wideband OFDM-based cell-free massive MIMO remains largely unexplored. This paper addresses the gap by modeling the joint distortion and proposing a fronthaul strategy in which each AP processes the received signal to reduce quantization artifacts before transmission. We develop an efficient estimation algorithm that reconstructs the unquantized time-domain signal prior to fronthaul transmission and evaluate its effectiveness. The proposed design offers new insights for implementing efficient, quantization-aware uplink transmission in wideband cell-free architectures.

翻译：在无蜂窝大规模MIMO系统的上行链路中，量化在两大关键环节影响系统性能：接入点（APs）处有限分辨率模数转换器（ADCs）引入的时域失真，以及发送至中央处理单元（CPU）的信号在fronthaul链路上的量化。尽管两次量化看似冗余，但在正交频分复用（OFDM）系统中，ADC量化作用于时域，而信号需转换至频域后，方可仅对活跃子载波上的信号进行量化。这降低了fronthaul负载并避免了不必要的失真，因为ADC输出包含所有OFDM样本，而仅有部分子载波承载有效信息。虽然两种量化效应在窄带系统中已得到广泛研究，但在基于实际宽带OFDM的无蜂窝大规模MIMO系统中，二者的联合影响仍鲜有探讨。本文通过建立联合失真模型并提出一种fronthaul传输策略来填补这一空白，在该策略中，各AP在传输前对接收信号进行处理以降低量化伪影。我们开发了一种高效的估计算法，用于在fronthaul传输前重构未量化的时域信号，并评估其有效性。所提出的设计为在宽带无蜂窝架构中实现高效、量化感知的上行链路传输提供了新的思路。