Joint Transmission Power Allocation and User Data Rate Optimization for Downlink Cell-Free Massive-MIMO URLLC Networks

Abstract

Cell-free massive multiple-input multiple-output (MIMO) systems can improve transmission performance and spectral efficiency by supporting the nearby access points (APs). This paper addresses the allocation of transmission power and the optimization of user data rate for cell-free massive MIMO systems in the ultra-high reliability and low latency communication (URLLC) scenario, where all the APs simultaneously serve users over the same time/frequency resources. Considering the cell-free environment characteristics, we establish the downlink information transmission models. To guarantee the fairness among users, we aim to maximize the weighted sum rates of users by jointly user data rate optimization and transmission power allocation, subject to the constraints of the minimum data rate requirement, maximum transmission power and signal-to-interference-plus-noise ratio (SINR). Due to the complexity of the cell-free massive MIMO model, we propose a rate approximation method to find a lower bound of the weighted sum rate for the original problem. By the problem decomposition, the closed-form expression of lower bound is derived and the optimization problem is solved iteratively by using successive convex approximation and interior point methods. Simulation results confirm the effectiveness of the proposed method and show that URLLC can significantly improve transmission performance in cell-free massive MIMO networks.