Differential Space-Time Block Coding for Phase-Unsynchronized Cell-Free MIMO Downlink

TL;DR

This paper introduces a differential space-time block coding scheme for cell-free MIMO systems that eliminates the need for phase synchronization among access points, maintaining high spectral efficiency despite phase misalignments.

Contribution

The paper proposes a novel DSTBC approach for CF-mMIMO that operates without phase synchronization, providing analytic bounds and a closed-form SINR expression.

Findings

DSTBC mitigates phase misalignment effects

Performance comparable to synchronized systems

Analytic bounds for spectral efficiency

Abstract

In the downlink of a cell-free massive multiple-input multiple-output (CF-mMIMO) system, spectral efficiency gains critically rely on joint coherent transmission, as all access points (APs) must align their transmitted signals in phase at the user equipment (UE). Achieving such phase alignment is technically challenging, as it requires tight synchronization among geographically distributed APs. In this paper, we address this issue by introducing a differential space-time block coding (DSTBC) approach that bypasses the need for AP phase synchronization. We first provide analytic bounds to the achievable spectral efficiency of CF-mMIMO with phase-unsynchronized APs. Then, we propose a DSTBC-based transmission scheme specifically tailored to CF-mMIMO, which operates without channel state information and does not require any form of phase synchronization among the APs. We derive a closed-form expression for the resulting signal-to-interference-plus-noise ratio (SINR), enabling quantitative comparisons among different DSTBC schemes. Numerical simulations confirm that phase misalignments can significantly impair system performance. In contrast, the proposed DSTBC scheme successfully mitigates these effects, achieving performance comparable to that of fully synchronized systems.