Mobile Cell-Free Massive MIMO: A Practical O-RAN-Based Approach

Cell-Free Massive MIMO is a promising solution for next-generation radio access networks. Especially the user-centric variant, where users are served by a subset of access points, a cluster, has garnered significant interest. However, managing these clusters in case of user mobility remains a significant issue. Additionally, the practical implementation of such a network remains an open problem. This work introduces a realistic temporal channel model for Cell-Free Massive MIMO. Subsequently, we propose a framework to facilitate handover procedures in a distributed topology considering limited channel knowledge for cluster formation. Next, we develop two clustering and handover strategies: 1) a fixed clustering strategy where the central processing unit computes the ideal cluster when a cluster handover threshold is exceeded and 2) an opportunistic strategy where access points make autonomous decisions based on the local channel. Finally, we establish a significant connection between our findings and O-RAN, an emerging network architecture that offers open interfaces and network-wide control capabilities. Simulation results show that even low-complexity handover mechanisms can significantly improve spectral efficiency under user mobility compared to cellular networks.