Wideband Synchronized Bidirectional Distributed MU-MIMO With Sigma-Delta RoF: E2E Experimental Gains Compared to Collocated MIMO

Abstract

Cell-free (CF) massive MIMO (mMIMO), also referred to as distributed MIMO (dMIMO) in indoor environments, has emerged as a key enabling technology for beyond-5G wireless systems due to its macro-diversity, favorable propagation, and joint multi-access-point (AP) processing gains. While extensive theoretical studies have demonstrated its potential, experimental validation remains limited. In this work, we present an end-to-end, multi-user, multi-stream, wideband OFDM experimental dMIMO platform, consisting of a centralized processing unit connected via optical fiber fronthaul links to geographically distributed APs. Tight synchronization among APs is achieved using sigma-delta modulated radio-over-fiber (SDRoF), combining the synchronization advantages of analog RoF with the cost efficiency of digital RoF. Using this platform, we experimentally evaluate three CF processing architectures, centralized (L4), hybrid centralized–distributed (L3), and fully distributed (L2), benchmarked against a collocated cellular MIMO system, within a complete end-to-end physical-layer framework comprising packet structure design, pilot signaling, wideband OFDM processing, and spatial processing algorithms. We discuss the uplink (UL) and downlink (DL) timing and frequency synchronization methods, as well as packet demultiplexing, in detail. Experimental results demonstrate substantial macro-diversity and favorable propagation gains. For an 8×4 MIMO scenario with four collocated users and two APs separated by 3 m, centralized CF processing (L4) achieves a minimum per-user signal-to-interference-plus-noise-plus-distortion ratio (SINDR) of 24 dB, versus only 8 dB for the collocated cellular configuration ( λ/2 AP spacing), yielding a 16 dB gain. Under shadowing, the CF architecture maintains 30 dB minimum SINDR compared to 22 dB for the cellular system at K=2 users. Furthermore, we experimentally validate UL–DL duality and quantify the reciprocity mismatch impact, demonstrating a 10 dB DL degradation without hardware calibration in a single-user ( 8×1 ) configuration. To the best of our knowledge, this work represents one of the first comprehensive experimental validations of wideband dMIMO and quantifies its practical gains over traditional cellular architectures.