Uplink power control for cell-free massive MIMO URLLC systems over aging and spatially correlated channels

This paper investigates the impact of channel aging on the performance of cell-free (CF) massive multiple- input multiple-output (MIMO) systems for ultra-reliable low-latency communication (URLLC), considering spatial correlation and pilot contamination. Closed-form expressions are derived for the uplink achievable rate under finite block length (FBL) and maximum ratio combining (MRC). To maximize the weighted sum rate for all users, a maximum power optimization problem is formulated. The original non-convex problem is transformed into a series of geometric programming (GP) problems using successive convex optimization (SCO) and logarithmic methods, which are solved iteratively. The numerical results verify the correctness of our derived closed expression for the achievable rate under the FBL. Channel aging significantly degrades the performance of CF massive MIMO URLLC systems. Moreover, the proposed maximum power allocation strategy improves the 95%-likely per-user uplink spectral efficiency (SE) compared to systems without power control, effectively mitigating the adverse effects of channel aging.