Multi-Stream Allocation in Semi-Coherent Cell-Free MU-MIMO Systems

Abstract

This paper presents a novel stream allocation algorithm for semi-coherent Cell-Free Multi-User Multiple-Input Multiple-Output (CF-MU-MIMO) networks. In such networks, groups of phase-coherent Access Points (APs) are organized into clusters that operate coherently inside the clusters, while inter-cluster phase coherence is not maintained, reflecting practical limitations in achieving network-wide synchronization. The proposed algorithm is tailored for multi-cluster service, where multiple clusters can simultaneously serve each User Equipment (UE). To support this flexibility, different stream sequences are constructed for each UE, and each of the sequences is initialized and dynamically constructed based on the strongest links across all serving clusters and the UEs. During the allocation process, both inter-cluster and inter-stream interference are mitigated using projection-based techniques, thereby improving spatial multiplexing efficiency. Compared to the greedy stream allocation which is commonly adopted in the literature as a low-complexity benchmark due to its near-optimal performance in downlink MIMO systems, the proposed method achieves higher performance with only a modest increase in complexity. Furthermore, a significant enhancement in the achievable sum rate is demonstrated relative to single-user-single-cluster allocation strategies in distributed cell-free systems.