radio system developers

Abstract: The Global Positioning System (GPS) provides critical positioning, navigation, and timing (PNT) services for numerous safety- and mission-critical applications, making it an attractive target for physical-layer attacks. While GPS spoofing and jamming have been extensively studied, GPS Signal Nullification (GSN)-a physical-layer attack that suppresses authentic satellite signals through destructive interference prior to acquisition-remains largely unexplored. Existing wireless signal nullification techniques target conventional communication systems and typically require victim-side information, such as channel estimates or synchronization parameters, making them impractical for GPS. Furthermore, many GPS spoofing defenses assume authentic satellite signals remain available as trusted references, an assumption invalidated by GSN. To address this gap, we present GhostLock, a learning-assisted blind GSN framework requiring only an approximate target location estimate. GhostLock integrates a Blind Adaptive Filter (BAF) with a deep learning-based synchronization module to generate and align an anti-signal using only the attacker's locally received GPS signal, eliminating the need for victim-side observations. We implement GhostLock on a real-time software-defined radio (SDR) platform using GPS-SDR-SIM and GNU Radio. Experimental results demonstrate successful suppression of GPS signal acquisition, establishing the feasibility of blind GSN and motivating new detection and mitigation techniques for this emerging GPS threat model.

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.