Fluid Antenna Aided Orthogonal Beamspace Construction in Near-Field LoS-MIMO Channels

Abstract

This paper proposes a novel fluid antenna (FA) assisted orthogonal channel construction scheme for the near-field (NF) line-of-sight (LoS) multiple-input multiple-output (MIMO) system. Specifically, we consider a transmitter (Tx) equipped with a fixed-position uniform planar array (UPA) and a mobile receiver (Rx) equipped with a planar array consisting of FAs. We first introduce the concept of orthogonal mode (OM), which represents a set of specific positions of FAs for orthogonal channel construction. Under the NF spherical model, we show that these OMs can be effectively constructed in a 2-D lattice. To enhance system performance, we maximize the number of OMs within a predefined Rx region by rotating the Rx plane and carefully selecting the 2-D basis vectors. The channel orthogonality is maintained by dynamically moving the FAs to the OMs. We further propose an efficient analog beamforming method for spatial multiplexing and a beamspace modulation scheme to enhance capacity when the number of Tx radio frequency (RF) chains is limited. Simulation results validate that the proposed approach significantly improves NF LoS MIMO channel capacity in both single-user and multi-user communications systems.