Fluid Antenna Index Modulation for MIMO Systems: Robust Transmission and Low-Complexity Detection

TL;DR

This paper introduces a robust fluid antenna index modulation system with grouping and a low-complexity detector, significantly improving spectral efficiency and error performance in correlated MIMO channels.

Contribution

It proposes a novel FA grouping-based IM scheme and a structured AMP detector, enhancing robustness and reducing detection complexity in fluid antenna MIMO systems.

Findings

FAG-IM outperforms existing FA-IM under spatial correlation.

Structured AMP detector reduces complexity and improves BER.

System demonstrates practical feasibility with robust transmission.

Abstract

The fluid antenna (FA) index modulation (IM)-enabled multiple-input multiple-output (MIMO) system, referred to as FA-IM, significantly enhances spectral efficiency (SE) compared to the conventional FA-assisted MIMO system. To improve robustness against the high spatial correlation among multiple activated ports of the fluid antenna, this paper proposes an innovative FA grouping-based IM (FAG-IM) system. A block grouping scheme is employed based on the spatial correlation model and the distribution structure of the ports. Then, a closed-form expression for the average bit error probability (ABEP) upper bound of the FAG-IM system is derived. To reduce the complexity of the receiver, the message passing architecture is incorporated into the FAG-IM system. Building on this, an efficient approximate message passing (AMP) detector, named structured AMP (S-AMP) detector, is proposed by exploiting the structural characteristics of the transmitted signals. Simulation results confirm that the proposed FAG-IM system significantly outperforms the existing FA-IM system in the presence of spatial correlation, achieving more robust transmission. Furthermore, it is demonstrated that the proposed low-complexity S-AMP detector not only reduces time complexity to a linear scale but also substantially improves bit error rate (BER) performance compared to the minimum mean square error (MMSE) detector, thereby enhancing the practical feasibility of the FAG-IM system.