Trajectory Optimization in User-Centric Distributed Massive MIMO Systems Enabled by UAV Swarms

User-centric (UC) distributed massive multiple-input multiple-output (D-mMIMO), commonly called cell-free mMIMO, is an important technology to ensure a more uniform coverage and higher spectral and energy efficiencies in next-generation communication systems. This paper investigates the performance of UC D-mMIMO systems enabled by a swarm of uncrewed aerial vehicles (UAVs). Specifically, it presents a comprehensive study on the deployment and trajectory optimization of UAVs as aerial transmission and reception points (TRPs) of D-mMIMO systems, considering systems composed solely of aerial TRPs and those implemented combining aerial and terrestrial TRPs. Moreover, user equipment (UE) mobility is modeled using a discrete-time Markov chain, and a novel approach to heuristically optimize the positions of aerial TRPs is proposed, considering the continuous movement of UEs in the coverage area. The proposed approach optimizes the three-dimensional location of each UAV under a time discretization framework, with the positioning of the UAVs being adjusted periodically, allowing for iterative trajectory optimization to improve the spectral efficiency (SE) performance of the UEs. Simulation results reveal that the proposed UAV trajectory optimization allows for significant SE improvement, especially for a low UE density scenario. Specifically, comparing the proposed method with a fixed position setup, an up to 47.84% increase in average SE is achieved.