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Secure Spatial Signal Design for ISAC in a Cell-Free MIMO Network

Steven Rivetti, Emil Bjornson, Mikael Skoglund

TL;DR

The paper addresses secure integrated sensing and communication (ISAC) in a cell-free MIMO network by jointly designing transmitted waveforms with artificial noise (AN) to degrade an eavesdropper (Eve) while meeting UE SINR requirements. It formulates a CRB-based objective to minimize the Fisher information on target angles, and solves the resulting nonconvex problem via semidefinite relaxation (SDR). The analysis shows that the optimal AN covariance is rank-1 and directed toward Eve, and simulations reveal a pronounced trade-off between sensing accuracy and communication performance, including how Eve proximity and SINR constraints influence the CRB. Overall, the approach yields a tight SDR solution and provides practical insights into secure ISAC design in distributed, spectrum-sharing networks with physical-layer security considerations.

Abstract

In this paper, we study a cell-free multiple-input multiple-output network equipped with integrated sensing and communication (ISAC) access points (APs). The distributed APs are used to jointly serve the communication needs of user equipments (UEs) while sensing a target, assumed to be an eavesdropper (Eve). To increase the system's robustness towards said Eve, we develop an ISAC waveform model that includes artificial noise (AN) aimed at degrading the Eve channel quality. The central processing unit receives the observations from each AP and calculates the optimal precoding and AN covariance matrices by solving a semi-definite relaxation of a constrained Cramer-Rao bound (CRB) minimization problem. Simulation results highlight an underlying trade-off between sensing and communication performances: in particular, the UEs signal-to-noise and interference ratio and the maximum Eve's signal to noise ratio are directly proportional to the CRB. Furthermore, the optimal AN covariance matrix is rank-1 and has a peak in the eve's direction, leading to a surprising inverse-proportionality between the UEs-Eve distance and optimal-CRB magnitude.

Secure Spatial Signal Design for ISAC in a Cell-Free MIMO Network

TL;DR

The paper addresses secure integrated sensing and communication (ISAC) in a cell-free MIMO network by jointly designing transmitted waveforms with artificial noise (AN) to degrade an eavesdropper (Eve) while meeting UE SINR requirements. It formulates a CRB-based objective to minimize the Fisher information on target angles, and solves the resulting nonconvex problem via semidefinite relaxation (SDR). The analysis shows that the optimal AN covariance is rank-1 and directed toward Eve, and simulations reveal a pronounced trade-off between sensing accuracy and communication performance, including how Eve proximity and SINR constraints influence the CRB. Overall, the approach yields a tight SDR solution and provides practical insights into secure ISAC design in distributed, spectrum-sharing networks with physical-layer security considerations.

Abstract

In this paper, we study a cell-free multiple-input multiple-output network equipped with integrated sensing and communication (ISAC) access points (APs). The distributed APs are used to jointly serve the communication needs of user equipments (UEs) while sensing a target, assumed to be an eavesdropper (Eve). To increase the system's robustness towards said Eve, we develop an ISAC waveform model that includes artificial noise (AN) aimed at degrading the Eve channel quality. The central processing unit receives the observations from each AP and calculates the optimal precoding and AN covariance matrices by solving a semi-definite relaxation of a constrained Cramer-Rao bound (CRB) minimization problem. Simulation results highlight an underlying trade-off between sensing and communication performances: in particular, the UEs signal-to-noise and interference ratio and the maximum Eve's signal to noise ratio are directly proportional to the CRB. Furthermore, the optimal AN covariance matrix is rank-1 and has a peak in the eve's direction, leading to a surprising inverse-proportionality between the UEs-Eve distance and optimal-CRB magnitude.
Paper Structure (17 sections, 19 equations, 7 figures)

This paper contains 17 sections, 19 equations, 7 figures.

Table of Contents

  1. Introduction
  2. Related work
  3. Contributions
  4. System model
  5. Communication observation
  6. Multistatic sensing
  7. Transmitted waveform joint design
  8. CRB derivation
  9. Optimal transmit waveform
  10. Semidefinite reformulation
  11. Simulation results
  12. CRB dependency by $\psi$
  13. Relaxation's tightness
  14. Artificial noise characterization
  15. CRB vs SINR and proximity
  16. ...and 2 more sections

Figures (7)

  • Figure 1: illustration of the Cell-free MIMO network where ISAC APs serve communication users while sensing an Eve
  • Figure 2: simulated APs and UEs layout, the UEs are randomly placed withing the green box
  • Figure 3: AP $1$ (top) and AP $2$ (bottom) CRB on $\theta_m$ for different $\psi$
  • Figure 4: achieved eve's SNR vs $\gamma$ for different values of $\psi$
  • Figure 5: optimal AN beampattern obtained for $\gamma=2$
  • ...and 2 more figures