Performance Analysis for URLLC in Cell-Free Massive MIMO Systems With Limited Fronthaul Capacity and Hardware Impairments

The limited-capacity fronthaul link between access points (APs) and central processing unit (CPU), along with signal distortions caused by hardware impairments at users and APs, lead to unreliable transmission, hindering the achievement of ultra-reliable low-latency communication (URLLC) in cell-free (CF) massive multiple-input-multiple-output (mMIMO) systems. In this paper, we investigate the performance of URLLC in CF mMIMO systems with limited fronthaul capacity and hardware impairments. First, a low-complexity fronthaul rate allocation scheme is proposed using rate-distortion theory in the considered system. Then, we derive the distribution of signal-to-interference-plus noise ratio (SINR) and analyze the delay violation probability (DVP) based on stochastic network calculus. Furthermore, a closed-form upper bound expression for DVP is also derived. To fulfill the requirements of next-generation URLLC, which demand additional key performance indicators, we further investigate effective energy efficiency and derive a closed-form lower bound, defined as the ratio of effective capacity to total power consumption. Extensive numerical results verify the accuracy of our derivations and indicate that a large deployment of APs can compensate for the reliability degradation caused by limited fronthaul capacity and hardware impairments. Additionally, the number of APs must be appropriately configured to ensure low latency and high reliability while avoiding energy waste.