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

2025-11-09

AI Summary

In cell-free massive MIMO (CF-mMIMO) downlink systems, distributed access points (APs) often suffer from phase asynchrony, leading to severe degradation of coherent gain and system performance. Method: This paper proposes the first application of differential space–time block coding (DSTBC) to CF-mMIMO, enabling asynchronous transmission without requiring inter-AP phase synchronization or instantaneous channel state information (CSI). A rigorous theoretical model is developed to derive a closed-form expression for the signal-to-interference-plus-noise ratio (SINR), facilitating analytical performance bounds and quantitative comparison across diverse DSTBC schemes. Results: Simulation results demonstrate that the proposed scheme effectively mitigates performance loss induced by phase misalignment, achieving spectral efficiency close to that of the ideal fully synchronized case. Consequently, it significantly enhances the practical deployability of CF-mMIMO under realistic, non-ideal synchronization conditions.

Technology Category

• Constraint Satisfaction and Optimization: Distributed CSP/Optimization
• Cognitive Modeling & Cognitive Systems: Applications
• Planning, Routing, and Scheduling: Optimization of Spatio-temporal Systems

Application Category

• Systems and Infrastructure for Web, Mobile and WoT: Data management and stream processing for Web, mobile and wireless applications
• Security and Privacy: Large-scale security measurements
• User Modeling, Personalization and Recommendation: User privacy protection in personalized systems

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.

Problem

• Eliminates need for phase synchronization in cell-free MIMO downlink
• Addresses performance degradation from AP phase misalignments
• Enables coherent transmission without channel state information

Innovation

• Differential space-time block coding for phase-unsynchronized transmission
• Eliminates need for channel state information and phase synchronization
• Achieves performance comparable to fully synchronized systems