Performance Analysis of Cell-Free Massive MIMO Systems with Asynchronous Reception

TL;DR

This paper analyzes how asynchronous reception, including delays and phase mismatches, impacts the spectral efficiency of cell-free massive MIMO systems, highlighting the importance of accurate synchronization for optimal performance.

Contribution

It provides novel closed-form spectral efficiency expressions considering asynchronous effects and phase imperfections in cell-free massive MIMO systems.

Findings

Asynchronous reception degrades pilot orthogonality and coherent data transmission.

Accurate delay phase estimation is crucial for coherent gain.

UE oscillator phase has a larger impact on spectral efficiency than AP phase.

Abstract

Cell-free (CF) massive multiple-input multiple-output (MIMO) is considered as a promising technology for achieving the ultimate performance limit. However, due to its distributed architecture and low-cost access points (APs), the signals received at user equipments (UEs) are most likely asynchronous. In this paper, we investigate the performance of CF massive MIMO systems with asynchronous reception, including both effects of delay and oscillator phases. Taking into account the imperfect channel state information caused by phase asynchronization and pilot contamination, we obtain novel and closed-form downlink spectral efficiency (SE) expressions with coherent and non-coherent data transmission schemes, respectively. Simulation results show that asynchronous reception destroys the orthogonality of pilots and coherent transmission of data, and thus results in poor system performance. In addition, getting a highly accurate delay phase is substantial for CF massive MIMO systems to achieve coherent transmission gain. Moreover, the oscillator phase of UEs has a larger effect on SE than that of the APs, because the latter can be significantly reduced by increasing the number of antennas.