Mitigating User Communication Inconsistency Caused by Channel Aging in Scalable Cell-Free Massive MIMO Systems

In this paper, we propose the spectral efficiency standard deviation (SESD) as a new metric to evaluate the consistency of user communication experience in scalable cell-free massive multiple input multiple output (CF mMIMO) systems degraded by channel aging. Closed-form expressions of the downlink spectral efficiency (SE) and SESD are derived under channel aging conditions. To minimize SESD while satisfying a given minimum SE requirement associated with different services, we design a joint access point (AP) selection and power allocation scheme. This SESD minimization approach achieves approximately 6% reduction in SESD compared to the conventional sum-rate maximization and fixed allocation methods, effectively mitigating communication fluctuations. Furthermore, by mapping SE stability to the speech Mean Opinion Score (MOS), we show that SESD minimization also effectively improves the consistency of user-perceived quality of experience (QoE).