Downlink Resource Allocation in Multiuser Cell-free MIMO Networks with User-centric Clustering

TL;DR

This paper develops an algorithm for optimizing user scheduling, power allocation, and beamforming in user-centric cell-free MIMO networks, demonstrating significant spectral efficiency gains over benchmarks and highlighting the importance of CSI and pilot assignment strategies.

Contribution

It introduces a novel joint optimization framework for user-centric clustering in cell-free MIMO, incorporating low-overhead pilot assignment and demonstrating superior performance.

Findings

Significant spectral efficiency improvements over benchmark schemes.

Coherent transmission outperforms non-coherent mode under various CSI conditions.

Effective pilot reuse strategies are crucial for network performance.

Abstract

In this paper, we optimize user scheduling, power allocation and beamforming in distributed multiple-input multiple-output (MIMO) networks implementing user-centric clustering. We study both the coherent and non-coherent transmission modes, formulating a weighted sum rate maximization problem for each; finding the optimal solution to these problems is known to be NP-hard. We use tools from fractional programming, block coordinate descent, and compressive sensing to construct an algorithm that optimizes the beamforming weights and user scheduling and converges in a smooth non-decreasing pattern. Channel state information (CSI) being crucial for optimization, we highlight the importance of employing a low-overhead pilot assignment policy for scheduling problems. In this regard, we use a variant of hierarchical agglomerative clustering, which provides a suboptimal, but feasible, pilot assignment scheme; for our cell-free case, we formulate an area-based pilot reuse factor. Our results show that our scheme provides large gains in the long-term network sum spectral efficiency compared to benchmark schemes such as zero-forcing and conjugate beamforming (with round-robin scheduling) respectively. Furthermore, the results show the superiority of coherent transmission compared to the non-coherent mode under ideal and imperfect CSI for the area-based pilot-reuse factors we consider.