Summary
Wireless devices need ways to send information quickly and reliably, especially when antennas are small or limited. The paper explores a new method that uses fluid antennas—antennas that can change their position—to better use space and codes together to send data. The researchers combined code groups and antenna positions to carry information more efficiently and created methods to detect the information with low errors. They also analyzed how well these methods work under different signal conditions and showed that their approach reduces errors and increases throughput better than some existing methods. This could help improve future wireless communication systems.
fluid antenna systemsindex modulationspatial diversityorthogonal spreading codesmaximum-likelihood detectionbit error ratesingle-input multiple-output (SIMO)multiple-input multiple-output (MIMO)Rayleigh fadingmessage passing algorithms
Authors
Peng Zhang, Jian Dang, Yao Ge, Miaowen Wen, Ziyang Liu, Liang Wu, Zaichen Zhang, Yudong Yao
Abstract
Fluid antenna systems (FASs) provide reconfigurable spatial resources within compact apertures. In this paper, we introduce code-domain grouped index modulation (CGIM) and its spatial-code-domain extension, termed SCGIM, for FA-assisted transceivers. CGIM partitions the available orthogonal spreading codes into multiple subsets and jointly maps information onto their in-phase and quadrature indices and constellation symbols. In an Rx-FAS-assisted single-input multiple-output (SIMO) link, group-wise despreading separates the orthogonal code groups for parallel detection, while receive-port selection provides spatial diversity. SCGIM further associates interleaved Tx-FA port subsets with the code subsets, with the Tx-FAS conveying spatial-index information and the Rx-FAS providing selection diversity in a multiple-input multiple-output (MIMO) link. For SCGIM, we develop maximum-likelihood (ML), staged greedy (GD), and cross-domain index message-passing (CD-IMPD) detectors. CD-IMPD exchanges soft information over a cycle-free factor graph to account for the coupling between the spatial and code indices, requiring only one inward and one outward message pass. For CGIM, the BER is derived from the joint decision regions of the despread-domain observations and averaged over the Rx-FAS selected-gain distribution under Rayleigh, Nakagami-m, and additive white Gaussian noise channels. For SCGIM, an average-BER approximation is derived from a full-pair union bound using the selected-gain density ratio and exponentially tilted quadratic-form Laplace transforms. Simulation results validate the BER analysis and show that the proposed schemes achieve lower BER and higher throughput than the considered IM schemes, while CD-IMPD achieves near-ML BER performance with lower detection complexity.