Joint User Association and Beamforming for Dynamic Metasurface Antenna Based Cell-Free mmWave MIMO Systems

Cell-free millimeter wave (mmWave) multiple-input multiple-output (MIMO) systems have shown great potential for satisfying demands of extra-large capacity, ultra-massive connectivity and ultra-high reliability in future wireless networks, which however may suffer from significant energy consumption and hardware cost due to large-scale distributed antenna arrays operating at mmWave bands. To alleviate this issue, a novel dynamic metasurface antenna (DMA) array has been considered as an energy-efficient and cost-efficient alternative to the conventional fully-digital array deployed at each access point (AP). In this paper, we aim to maximize the weighted sum rate (WSR) in a DMA-based cell-free mmWave MIMO system by jointly optimizing user association, transmit beamformer, fronthaul compression and DMA weighting matrix, where both the limited fronthaul capacity and the near-field propagation effect are considered. To tackle this non-convex mixed-integer problem effectively, we propose a double-loop penalty dual decomposition (PDD) based algorithm with the aid of variable substitution and successive convex approximation (SCA) technique. In order to circumvent its high iteration complexity, we also develop a low-complexity alternating optimization (AO) based algorithm to find a high-quality suboptimal solution with a single-loop structure. Moreover, the extensions of our work to the partially-decentralized distributed implementation and the non-coherent transmission at distributed APs are discussed. Numerical results demonstrate that our proposed algorithms achieve superior performance over various benchmark schemes.