Delay Alignment Modulation for Secure ISAC Systems

2026-02-24

AI Summary

This work addresses the dual challenges of degraded sensing performance and vulnerability to eavesdropping caused by multipath propagation in multi-user downlink integrated sensing and communication (ISAC) systems. To this end, it introduces delay-aligned modulation (DAM)—a novel approach applied for the first time to secure ISAC—by imposing controllable path delays at the transmitter. This strategy enables coherent superposition of multipath signals at legitimate users to enhance received power, while simultaneously inducing delay mismatches at eavesdroppers to strengthen physical-layer security. By integrating a two-stage angle-delay sensing protocol with path-based zero-forcing precoding, the proposed framework jointly optimizes secrecy spectral efficiency and the Cramér-Rao bound under sensing accuracy constraints. Simulation results demonstrate that the scheme significantly outperforms the strongest-path baseline, achieving substantial improvements in both communication security and sensing performance.

Technology Category

• Planning, Routing, and Scheduling: Optimization of Spatio-temporal Systems
• Intelligent Robots: Multimodal Perception & Sensor Fusion
• Computer Vision: Multi-modal Vision

Application Category

• Security and Privacy: Large-scale security measurements
• Search and Retrieval-Augmented AI: Retrieval-Augmented Generation (RAG) and multi-modal RAG
• Systems and Infrastructure for Web, Mobile and WoT: Data management and stream processing for Web, mobile and wireless applications

Abstract

This paper introduces delay-alignment modulation (DAM) for secure integrated sensing and communication (ISAC). Due to the broadcast nature of multi-user downlinks, communications are vulnerable to eavesdropping. DAM applies controlled per-path symbol delays at the transmitter to coherently align the multipath components at the intended user, enhancing the received signal power, while simultaneously creating delay misalignment at the eavesdropper (Eve). To mitigate sensing degradation caused by multipath propagation, we propose a two-stage protocol that first estimates the angle and then the delay of the line-of-sight (LoS) path after suppressing multipath interference. We derive the secrecy spectral efficiency (SSE) and the Cramer-Rao (CRB) of the target delay. Finally, we develop a path-based zero-forcing (ZF) precoding framework and formulate a max-min SSE design under CRB and power constraints. Simulation results show DAM significantly outperforms the strongest-path (SP) benchmark in terms of SSE, while meeting sensing requirements, since intentional delay alignment at legitimate users degrades reception at Eve.

Problem

• secure ISAC
• eavesdropping
• multipath propagation
• secrecy spectral efficiency
• sensing degradation

Innovation

• Delay-Alignment Modulation
• Secure ISAC
• Secrecy Spectral Efficiency
• Multipath Interference Suppression
• Path-based Zero-Forcing Precoding