Revisiting the Coverage Boundary of Multi-CPU Cell-Free Massive MIMO: CPU Cooperation Aspect

Cell-free massive MIMO (CF-mMIMO) provides flattened spectral efficiency throughout the network. Recently, multi-CPU CF-mMIMO (MC-CF-mMIMO) has been considered for addressing a scalability problem. However, MC-CF-mMIMO now suffers from a performance degradation problem in the boundary of the CPU overage, referred the CPU-edge. In this study, we analyze and improve the performance of MC-CF-mMIMO, focusing on the CPU-edge. First, we present a simple CPU cooperation scheme to mitigate the performance degradation problem on the CPU-edge. Then, we provide the closed-form expression of the downlink spectral efficiency according to the precoding schemes. Experimental results indicate that performance without CPU cooperation on the CPU-edge is degraded by 28.66% compared with CPU cooperation.