Analysis of CPU Placement of Cell-Free Massive MIMO for User-centric RAN

The authors have been studying a user-centric radio access network (RAN) towards the realization of homogeneous wireless quality "anywhere anytime" by introducing Cell-free massive MIMO (CF-mMIMO) technology. Assuming a double-star network, which is frequently seen in optical access networks, central processing units (CPUs) which process CF-mMIMO signals will be deployed at central and edge sites. Due to this configuration, user-centric RAN has a unique characteristic not shared by a conventional RAN; i.e., the wireless quality varies with CPU placement, because the number of available access points (APs), which affects the wireless quality, differs between the central and the edge sites. However, transport links between the central sites and edge sites will be subject to a heavy load when all APs are connected to the central site, because radio signals are transferred over them. That is, the placement of CPUs is an important problem to be solved to realize homogeneous wireless quality taking the load on the transport links into account. In this paper, we propose a CPU placement configuration where CPUs are deployed both at the central site and edge sites. Numerical simulation shows that the proposed placement configuration can flexibly adapt to the achieved wireless quality and throughput by changing vCPU placement and radio band width allocation to the central and edge sites.