PHY Authentication for Ambient IoTs in Near-Field XL-MIMO Backscatter Communication Systems

This letter exploits near-field (NF) spherical wavefronts to derive geometry-dependent physical signatures for backscatter device (BD) authentication in extremely large-scale multiple-input multiple-output (XL-MIMO) backscatter communication systems. We propose a binary element-wise delay shift keying (EW-DSK) modulation scheme based on a multi-antenna BD architecture. The proposed scheme introduces a deterministic, bit-dependent delay that shifts the power delay profile (PDP) while preserving the aperture-dependent curvature induced by NF propagation. Using the dominant PDP component extracted across the receive antennas, we develop a compact delay-curvature signature extractor and a corresponding authentication test to mitigate BD impersonation attacks. Simulation results evaluate the proposed scheme in terms of curvature extraction accuracy, authentication accuracy, and spoofing success probability under different spoofer distances, signal-to-noise ratios, and effective bandwidths.

Scalable Multi-BD Access for OFDM-Based Symbiotic Backscatter Communications

We propose an interference-free multi-backscatter device (BD) access scheme for orthogonal frequency division multiplexing (OFDM)-based symbiotic backscatter communication (SBC). Specifically, we introduce an orthogonal frequency-code spread (OFC) scheme in which the transmitter reserves empty subcarriers for BDs to incorporate their information through frequency shifting, which effectively suppresses direct-link interference (DLI). Since enabling simultaneous access over shared subcarriers introduces inter-backscatter device interference (IBDI), a blockwise orthogonal code-spread is applied to assign each BD to a unique codeword, thereby mitigating IBDI with orthogonal transmissions of all BD signals in the code domain. We develop a non-coherent energy detector whose performance is analyzed in terms of average probability of missed detection (PMD) and sum-rate under Rayleigh fading. The simulation results validate the OFC scheme and demonstrate low PMD values and high spectral efficiency compared to conventional code-assisted multiple-BD access schemes.