Cell-Free Massive MIMO in O-RAN: Energy-Aware Joint Orchestration of Cloud, Fronthaul, and Radio Resources

The physical-layer foundations of cell-free massive MIMO are now well-established. The end-to-end network (from radio site to the cloud) analysis and energy-efficient operation are two critical factors to be considered for the deployment of cell-free massive MIMO functionality in a practical wireless network architecture. In line with the cloudification and virtualization in the open radio access networks (O-RAN), it is indisputable to envision prospective cell-free infrastructure on top of the O-RAN architecture. O-RAN is the key enabler for the end-to-end management of cell-free massive MIMO. In this paper, we explore the performance and power consumption of different radio technologies, i.e., cell-free massive MIMO and traditional small-cell systems, in the virtualized O-RAN architecture from an end-to-end perspective. We compare two different functional split options and different resource orchestration mechanisms, including the fully virtualized end-to-end, local cloud coordination-based, and radio-only resource allocation. In the end-to-end orchestration scheme, we aim to minimize the end-to-end power consumption by jointly allocating the radio, optical fronthaul, and virtualized cloud processing resources. We compare end-to-end orchestration with two other schemes: i)``radio-only'' where radio resources are optimized independently from the cloud and ii)``local cloud coordination'' where orchestration is only allowed among a local cluster of radio units. The spectral efficiency (SE) is either considered as an optimization constraint for each user equipment or as a sum SE in the objective function. We develop several algorithms to solve the respective end-to-end optimization problems.