Joint Impact of ADC and Fronthaul Quantization in Cell-Free Massive MIMO-OFDM Uplink
Özlem Tuğfe Demir, Emil Björnson
TL;DR
The paper tackles the joint effects of time-domain distortion from low-resolution ADCs and fronthaul quantization in the uplink of a wideband cell-free MIMO-OFDM system. It proposes a quantization-aware, ADMM-based scheme that reconstructs the clean time-domain signal at each AP before fronthaul transmission, leveraging OFDM structure and symbol constraints to mitigate double quantization. The method formulates a real-valued optimization with interval and constellation constraints and derives closed-form ADMM updates, enabling efficient estimation of the unquantized signal from ADC outputs and pilot information. Numerical results demonstrate substantial NMSE reductions compared to a quantization-unaware benchmark, across various ADC and fronthaul resolutions, highlighting the practical impact for efficient, quantization-aware uplink design in beyond-5G architectures.
Abstract
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.