Array Configuration and Scaling Laws in Near-Field Uplink Multiuser MIMO

Future wireless networks must deliver high spectral efficiency in dense environments where many users require simultaneous high-rate communication. Higher carrier frequencies offer more bandwidth but suffer from shorter range and lower channel rank, making new capacity enhancing strategies essential. This paper studies joint antenna array design at the base station (BS) and user devices in line-of-sight (LoS) uplink multiuser multiple-input multiple-output (MU-MIMO) systems with the goal of exploiting radiative near-field effects to unlock full multiplexing gains. In particular, we establish a scaling law under which there exist uniform sparse array configurations whose sum rate gap to an interference-free upper bound decays with the number of BS antennas. For dense user deployments beyond this scaling regime, we propose a joint array design algorithm with provable performance guarantees, in which each user array is configured to maximize its single-user achievable rate conditioned on the BS array, followed by non-uniform BS array optimization using a submodular surrogate. Our simulation results verify that uniform sparse arrays achieve the interference-free regime predicted by the scaling law. Outside this regime, the proposed joint design closely tracks the interference-free upper bound, demonstrating the benefit of non-uniform BS array optimization.