Impact of Mobility on Downlink Cell-Free Massive MIMO Systems

TL;DR

This paper evaluates how user mobility impacts the downlink spectral efficiency of cell-free massive MIMO systems, comparing downlink training and statistical CSI methods, and contrasting with cellular mMIMO performance under realistic conditions.

Contribution

It provides a comprehensive analysis of channel aging and pilot contamination effects in CF-mMIMO, highlighting the advantages of downlink training over statistical CSI in certain scenarios.

Findings

CF-mMIMO outperforms cellular mMIMO after accounting for mobility effects.

Downlink training is preferable over statistical CSI when data sequence length is optimized.

System parameters like user velocity and pilot lengths significantly influence spectral efficiency.

Abstract

In this paper, we analyze the achievable downlink spectral efficiency of cell-free massive multiple input multiple output (CF-mMIMO) systems, accounting for the effects of channel aging (caused by user mobility) and pilot contamination. We consider two cases, one where user equipments (UEs) rely on downlink pilots beamformed by the access points (APs) to estimate downlink channel, and another where UEs utilize statistical channel state information (CSI) for data decoding. For comparison, we also consider cellular mMIMO and derive its achievable spectral efficiency with channel aging and pilot contamination in the above two cases. Our results show that, in CF-mMIMO, downlink training is preferable over statistical CSI when the length of the data sequence is chosen optimally to maximize the spectral efficiency. In cellular mMIMO, however, either one of the two schemes may be better depending on whether user fairness or sum spectral efficiency is prioritized. Furthermore, the CF-mMIMO system generally outperforms cellular mMIMO even after accounting for the effects of channel aging and pilot contamination. Through numerical results, we illustrate the effect of various system parameters such as the maximum user velocity, uplink/downlink pilot lengths, data duration, network densification, and provide interesting insights into the key differences between cell-free and cellular mMIMO systems.