Enabling Cell-Free Massive MIMO Systems With Wireless Millimeter Wave Fronthaul

Abstract

Cell-free massive MIMO systems have promising data rate and uniform coverage\ngains. These systems, however, typically rely on optical fiber based fronthaul\nfor the communication between the central processing unit (CPU) and the\ndistributed access points (APs), which increases the infrastructure cost, leads\nto high installation time, and limits the deployment flexibility and\nadaptability. To address these challenges, this paper proposes two\narchitectures for cell-free massive MIMO systems based on wireless fronthaul\nthat is operating at a \\textit{higher-band} compared to the access links: (i) A\nwireless-only fronthaul architecture where the CPU has a wireless fronthaul\nlink to each AP, and (ii) a mixed-fronthaul architecture where the CPU has a\nwireless link to each cluster of APs that are connected together via optical\nfibers. These dual-band architectures ensure high-data rate fronthaul and\nprovide high capability to synchronize the distributed APs. Further, the\nwireless fronthaul reduces the infrastructure cost and installation time, and\nenhances the flexibility, adaptability, and scalability of the network\ndeployment. To investigate the achievable data rates with the proposed\narchitectures, we formulate the end-to-end data rate optimization problem\naccounting for the various practical aspects of the fronthaul and access links.\nThen, we develop a low-complexity yet efficient joint beamforming and resource\nallocation solution for the proposed architectures based on user-centric AP\ngrouping. With this solution, we show that the proposed architectures can\nachieve comparable data rates to those obtained with optical fiber-based\nfronthaul under realistic assumptions on the fronthaul bandwidth, hardware\nconstraints, and deployment scenarios. This highlights a promising path for\nrealizing the cell-free massive MIMO gains in practice while reducing the\ninfrastructure and deployment overhead.\n