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Delay Alignment Modulation for Secure ISAC Systems

Tianyu Lu, Jiajun He, Mohammadali Mohammadi, Michail Matthaiou

TL;DR

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.

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.

Delay Alignment Modulation for Secure ISAC Systems

TL;DR

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.

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.
Paper Structure (25 sections, 1 theorem, 30 equations, 2 figures)

This paper contains 25 sections, 1 theorem, 30 equations, 2 figures.

Table of Contents

  1. Introduction
  2. System Model
  3. Propagation Environment
  4. Sensing Channel
  5. Communication Channel
  6. Signal Model
  7. Sensing Signal
  8. Communication Signal
  9. Target Sensing
  10. Angle Sensing
  11. Discrete-Time Received Signal
  12. Slot Representation
  13. Subframe Representation
  14. Frame Representation
  15. Delay Sensing
  16. ...and 10 more sections

Key Result

Proposition 1

The CRB for the delay parameter $\tau_{s,1}$, $\text{CRB}(\tau_{s,1}) = [\mathbf{J}^{-1}]_{11}$ is expressed as where $\zeta\! =\! \frac{2}{\sigma_a^2}|\mathbf{a}^{H}(\varphi_{s,1})\mathbf{f}_{s}|^2$, $\mathbf{R}_J \triangleq \mathbf{S}_d\mathbf{S}_d^{H}\!\in\!\mathbb{C}^{M\times M}$, and $\widetilde{\mathbf{p}}_{\tau_{s,1}} \!= \!\frac{\partial \mathbf{p}(\tau_{s,1})}{\partial \tau_{s,1}} \!=\!

Figures (2)

  • Figure 1: SSE comparison between DAM and SP for the worst-case UE.
  • Figure 2: CRB versus the transmit power.

Theorems & Definitions (1)

  • Proposition 1