Spatial Modulation for Tx-SIMO-FAS: Port Selection and Performance Analysis

2026-06-20

AI Summary

This work addresses the performance degradation in fluid antenna systems (FAS) caused by strong spatial correlation among densely packed ports, which diminishes port distinguishability and impairs spatial modulation efficacy. Focusing on a single RF-chain FAS transmitter paired with a multi-antenna SIMO receiver, the study proposes three correlation-aware port selection strategies: SF-EDAS enhances constellation separability, SOPS minimizes the condition number of the channel matrix, and CC-COAS jointly optimizes channel gain and decorrelation. A reliability analysis framework grounded in energy and extreme-value degrees of freedom is developed, complemented by maximum-likelihood detection and high-SNR diversity analysis. Experimental results demonstrate that the proposed schemes significantly outperform conventional spatial modulation and grouped benchmarks, with CC-COAS achieving the best trade-off between bit error rate performance and computational complexity.

Abstract

This paper considers a single-input multiple-output (SIMO) setup with a fluid antenna system (FAS) at the transmitter side and multiple fixed antennas at the receiver, which is referred to as a Tx-SIMO-FAS. We investigate the use of spatial modulation (SM) utilizing the FAS on a single radio-frequency (RF) chain while the receiver side performs maximum-likelihood detection. Unlike conventional antenna arrays, however, the large number of fluid antenna ports accommodated within a limited aperture introduces strong spatial correlation, which reduces the distinguishability of port indices and degrades the reliability of index detection. To address this challenge, three correlation-aware port-selection schemes are proposed: successive fluid Euclidean-distance-optimized selection (SF-EDAS), successive orthogonal port selection (SOPS), and correlation-constrained orthogonal array selection (CC-COAS). These schemes focus on enhancing received-constellation separation, improving channel-basis conditioning, and jointly optimizing channel gain and inter-port decorrelation, respectively. To understand the performance limits of FAS-SM, a reliability analysis is developed by decomposing the channel into an energy-based degree of freedom (DoF), and an extreme-value DoF. High signal-to-noise ratio (SNR) analysis reveals an effective diversity order determined by the number of selected ports, the number of receive antennas, and the energy-based spatial DoF. Furthermore, the aperture-limited array gain is characterized through a scalar equivalent independent-look approximation involving the Digamma function. Numerical results demonstrate that the proposed schemes significantly outperform conventional SM and grouping-based benchmarks. Among them, CC-COAS achieves the most favorable tradeoff between error performance and computational complexity.

Problem

• spatial modulation
• fluid antenna system
• spatial correlation
• port selection
• index detection

Innovation

• Fluid Antenna System
• Spatial Modulation
• Port Selection
• Spatial Correlation
• Diversity Order