Front Haul Multi-Homing for Cell-Free Massive MIMO: A Low-Latency Approach With Distributed and Hierarchical CPU Deployment

In anticipation of the 6G system, it is crucial to address the challenges related to achieving high throughput and low latency over a wide area. This is particularly important for applications such as 4K 360-degree streaming, tactile communication, and autonomous driving. Cell-free massive MIMO (CF-mMIMO) has been proposed as a solution to enhance radio coverage by eliminating cell edges, where a central processing unit (CPU) orchestrates signal processing across distributed access points (APs). This paper explores the deployment of CF-mMIMO within a realistic hierarchical and distributed site architecture to meet the stringent latency requirements of 6G through mobile edge computing (MEC). However, the deployment of CF-mMIMO and utilization of MEC within a distributed and hierarchical architecture necessitates the placement of CPUs at edge sites. This arrangement may lead to radio quality degradation at the edges of distributed sites due to inter-site interference and AP selection limitations. To address this problem, we propose a multi-homing AP connection that allows APs to connect to multiple edge sites, thereby enabling signal processing across sites. Furthermore, we conduct an analysis of the fronthaul (FH) costs associated with multi-homing connections and propose a multi-homing configuration and multi-homing AP selection method that improves radio quality without incurring excessive FH costs. The proposed heuristic algorithm is introduced to optimize the AP connections for multi-homing, giving priority to APs near site edges to strike a balance between cost and performance. Simulation results demonstrate that our proposed method attains near-optimal throughput while preserving cost efficiency.