Delay Alignment Modulation for Secure ISAC Systems

TL;DR

This paper proposes delay-alignment modulation (DAM) to enhance secure communication in ISAC systems by coherently aligning signals at legitimate users and creating delay misalignment at eavesdroppers, improving secrecy and sensing performance.

Contribution

The paper introduces DAM for secure ISAC, including a two-stage protocol for multipath estimation, a theoretical analysis of secrecy and sensing metrics, and a zero-forcing precoding framework for optimal security.

Findings

DAM outperforms the strongest-path benchmark in secrecy spectral efficiency.

DAM maintains sensing performance while degrading eavesdropper reception.

The proposed framework effectively balances security and sensing in multipath environments.

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.