Near-Field Communication with Massive Movable Antennas: A Functional Perspective

TL;DR

This paper develops a novel framework for optimizing antenna placement in near-field massive MIMO systems, using functional analysis to maximize data rates with movable antennas.

Contribution

It introduces a continuous antenna position and density function approach, deriving optimal configurations and proposing practical deployment strategies for near-field MIMO.

Findings

Optimal antenna density distribution enhances achievable rate.

Closed-form solution for LoS near-field scenarios highlights edge antenna importance.

Uniform circular arrays offer a good balance between performance and practicality.

Abstract

The advent of massive multiple-input multiple-output (MIMO) technology has provided new opportunities for capacity improvement via strategic antenna deployment, especially when the near-field effect is pronounced due to antenna proliferation. In this paper, we investigate the optimal antenna placement for maximizing the achievable rate of a point-to-point near-field channel, where the transmitter is deployed with massive movable antennas. First, we propose a novel design framework to explore the relationship between antenna positions and achievable data rate. By introducing the continuous antenna position function (APF) and antenna density function (ADF), we reformulate the antenna position design problem from the discrete to the continuous domain, which maximizes the achievable rate functional with respect to ADF. Leveraging functional analysis and variational methods, we derive the optimal ADF condition and propose a gradient-based algorithm for numerical solutions under general channel conditions. Furthermore, for the near-field line-of-sight (LoS) scenario, we present a closed-form solution for the optimal ADF, revealing the critical role of edge antenna density in enhancing the achievable rate. Finally, we propose a flexible antenna array-based deployment method that ensures practical implementation while mitigating mutual coupling issues. Simulation results demonstrate the effectiveness of the proposed framework, with uniform circular arrays emerging as a promising geometry for balancing performance and deployment feasibility in near-field communications.