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RIS-Assisted Secure ISAC: Fundamentals, Applications, and Future Directions

Zhendong Li, Jinyuan Huang, Weichun Zhao, Zhou Su, Enyu Shi, Wen Chen

arXiv:2610.00216v1eess.SP

TL;DR

ISAC’s broadcast transmissions expose user data and sensing information to eavesdroppers, especially when direct links are blocked. This article surveys RIS-assisted secure ISAC and joint active-passive beamforming. Its case study reports improved secrecy performance and spatial nulling under fully blocked direct links.

  • Problem

    ISAC’s broadcast physical-layer transmissions are vulnerable to eavesdropping, exposing both legitimate-user data and sensing information.

    Source: §I · p. 1

  • Method

    The article surveys RIS-assisted secure ISAC and describes joint BS beamforming and RIS phase control for spatial energy allocation.

    Source: §I · p. 1, B. Architectures of RIS-Assisted… · p. 2, B. Active-Passive Beamforming · p. 5

  • Results

    The case study demonstrates that joint active and passive beamforming can substantially improve secrecy performance and enable effective spatial nulling under fully blocked direct links.

    Source: §V · p. 6, §VII · p. 7

  • Takeaways & Limitations

    RIS-assisted secure ISAC extends RIS use from link enhancement toward security-aware and sensing-aware control of the electromagnetic environment.

    Source: §VII · p. 7

    Practical deployment requires progress in hardware-efficient architectures, scalable optimization, real-time channel and environment acquisition, and intelligent reconfiguration under dynamic conditions.

    Source: §VII · p. 7

Abstract

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Integrated sensing and communication (ISAC) has emerged as a key enabler for sixth-generation (6G) networks, yet its broadcast nature at the physical layer leaves transmissions vulnerable to malicious eavesdropping, particularly when physical blockages degrade direct links. Reconfigurable intelligent surface (RIS) offers a promising defense paradigm at the physical layer to address these issues, by reconstructing virtual line-of-sight links and dynamically reshaping the spatial propagation environment. In this article, we provide a comprehensive overview of RIS-assisted secure ISAC systems, covering its fundamentals, practical applications, and open future research directions. We first outline cooperative deployment topologies and basic electromagnetic operating principles. We then analyze the technical advantages of passive RIS integration in mitigating bottlenecks in resource competition across communication secrecy, sensing precision, and hardware sustainability. We also present representative applications, including urban vehicular networks, monitoring with unmanned aerial vehicle support, industrial Internet-of-thing, and privacy in smart homes. A case study quantifies gains in secrecy rate and spatial nulling capabilities achieved through joint active and passive beamforming under full blockage of direct links. Finally, we identify key open challenges and future research directions to enable practical, scalable, and intelligent deployment of RIS-assisted secure ISAC systems.

I. INTRODUCTION

ISAC’s broadcast transmissions expose both user data and sensing information to eavesdroppers, while blockage can undermine conventional BS-side active defenses. The article surveys RIS-assisted secure ISAC, its joint active-passive design rationale, and a blocked-link case study.

I. INTRODUCTION

  • ISAC’s broadcast transmissions expose legitimate-user data and environmental topology and target-state information to malicious eavesdroppers.Existing BS-side active beamforming and artificial-noise schemes can balance secrecy and sensing under favorable LoS conditions, but blockage and hostile NLoS channels degrade their performance.Source: §I · p. 1
  • RIS phase, amplitude, and polarization control can create cascaded paths among the BS, users, and targets when direct propagation is blocked.The overview also identifies joint active-passive beamforming and a case study of secrecy-rate and spatial-nulling gains under direct-link blockage.Source: §I · pp. 1–2
  • Joint BS beamforming and passive RIS phase shifts provide spatial degrees of freedom for enhancing legitimate signals, nulling eavesdroppers, and easing secrecy-sensing-hardware trade-offs.The article reports that representative alternating-optimization designs have delivered radar SNR and secrecy-rate gains.Source: §I · p. 1
  • The article surveys RIS-assisted secure ISAC fundamentals, applications, and future directions, with sections on enabling technologies and a numerical case study.It identifies cascaded channel estimation, joint beamforming, sensing-driven handovers, and open challenges as topics covered.Source: §I · pp. 1–2

II. RIS-ASSISTED SECURE ISAC SYSTEMS

The section describes quasi-passive RIS hardware and a cooperative architecture that reconstructs cascaded paths when direct links are obstructed. It also explains how reflection control can support communication security and sensing in NLoS settings.

A. RIS Hardware and Electromagnetic Principles

  • RIS hardware uses programmable subwavelength elements whose controlled impedance changes the phase shift of incident signals.The article focuses on quasi-passive reflection-type RIS and assumes ideal hardware unless stated otherwise.Source: A. RIS Hardware and Electromagnetic… · p. 2
  • Quasi-passive RIS reflection requires no complex RF-chain processing, adds no thermal noise, and avoids self-interference that could impair weak sensing echoes.A phase-shift mask can deflect or focus electromagnetic waves to control channels and bypass obstacles.Source: A. RIS Hardware and Electromagnetic… · p. 2

B. Architectures of RIS-Assisted Secure ISAC

  • The cooperative architecture comprises a BS, RIS, legitimate users, and sensing targets, with the RIS reconfiguring channels to sustain secure ISAC operation.The figure depicts this architecture in a blocked environment.Source: B. Architectures of RIS-Assisted… · p. 2, Figure 1 · p. 2
  • When obstacles disrupt the direct BS-target path, RIS reflection vectors form virtual LoS links that bypass obstructions and direct energy toward legitimate directions.The architecture uses RIS location identified through sensing and pre-optimized reflection vectors; directional beams can spatially isolate eavesdropping paths.Source: B. Architectures of RIS-Assisted… · p. 2
  • RIS beam focusing can counter double-path echo loss and provide additional NLoS observation angles for estimating target range, velocity, and azimuth.The passage also states that this can help maintain high-resolution perception under partial target occlusion.Source: B. Architectures of RIS-Assisted… · p. 2

C. Deployment Schemes

RIS deployment can be fixed or mobile and centralized or distributed, with the choice trading control overhead and broad coverage against mobility and resilience to obstruction. These schemes support dynamic links and varied network applications.

C. Deployment Schemes

  • Fixed RISs on facades, tunnel entrances, or street lamps use offline placement planning and quasi-static channels to cover persistent blind spots with low CSI acquisition overhead.The passage characterizes this scheme as energy efficient and capable of sustaining cascaded-link stability.Source: C. Deployment Schemes · p. 2
  • Mobile RIS units on UAVs or ground robots can adjust their poses along target trajectories and build secure virtual LoS links in volatile environments.The passage describes deep reinforcement learning as one scheduling approach for adaptive three-dimensional positioning.Source: C. Deployment Schemes · pp. 2–3
  • Centralized large RIS arrays simplify BS-RIS control and provide broad secure coverage through high-gain narrow beams, whereas distributed units improve resilience to dense-obstruction shadow fading.Distributed RISs form multipath reflective links from different angles, mitigating single-point blockage and supporting tracking accuracy and domain-wide protection.Source: C. Deployment Schemes · p. 3
  • The deployment approach shifts from static patching to adaptive relaying and from single-array gain to multipath spatial diversity.The article names vehicle coordination, UAV defense tracking, and IIoT asset monitoring as representative scenarios.Source: C. Deployment Schemes · p. 3

III. ADVANTAGES AND TYPICAL APPLICATIONS

RIS spatial control can reshape blocked links to support secrecy and sensing, while phase coordination offers a mechanism for separating legitimate and eavesdropping directions. The section also describes sensing benefits from focused energy and additional spatial observations.

III. ADVANTAGES AND TYPICAL APPLICATIONS

  • The architecture’s strategic value is attributed to physical-topology reconfiguration, with gains in communication secrecy and sensing robustness and quasi-passive hardware described as green and cost-effective.These are presented as advantages aligned with future wireless-network requirements.Source: §III · p. 3

A. Secrecy and Sensing Synergetic Gains

  • RIS deployment flexibility and real-time reflection-vector optimization can construct virtual LoS paths when obstructions sever communication or detection links.The passage says this can restore connectivity, secure transmission, and target sensing, and turn blocking environments into programmable security barriers.Source: A. Secrecy and Sensing Synergetic Gains · p. 3
  • Phase coordination across RIS elements forms directional beams that can weaken legitimate-eavesdropper channel correlation and isolate eavesdropping paths toward high secrecy capacity.RIS-assisted covert-communication schemes against unauthorized detectors have also been explored.Source: A. Secrecy and Sensing Synergetic Gains · p. 3
  • RIS energy focusing can partly compensate for double-propagation loss in long-range sensing echoes and provide multidimensional observations for localization and environmental modeling.The passage connects these capabilities to complex dynamic scenarios and the absence of a direct LoS path.Source: A. Secrecy and Sensing Synergetic Gains · p. 3

B. Economic and Hardware Advantages

Passive RIS avoids active RF-chain costs and noise amplification, while flexible deployment supports physical-layer security and perception across 6G vertical industries.

B. Economic and Hardware Advantages

  • Passive RIS requires no costly RF chains, power amplifiers, or digital baseband units, and adds no active thermal noise amplification or self-interference.These properties prevent weak sensing echoes from being overwhelmed by noise generated by active components.Source: B. Economic and Hardware Advantages · p. 3
  • Low power consumption and flexible provisioning can reduce capital and long-term operational expenditures while supporting security and perception in energy-limited or remote environments.The passage frames this deployment model as balancing network evolution with economic sustainability.Source: B. Economic and Hardware Advantages · p. 3

C. Application Scenarios

1) Urban V2X Coverage:

RIS deployments address blocked propagation across urban, emergency-monitoring, and industrial settings, using fixed, mobile, or distributed configurations to support secure links and sensing.

1) Urban V2X Coverage:

  • Building-facade RISs use real-time vehicle poses to form cascaded virtual LoS links around urban-canyon blind spots while maintaining secrecy rates and target-detection accuracy.Large centralized arrays serve major intersections, while smaller distributed units cover minor intersections and route signals around corners.Source: 1) Urban V2X Coverage · p. 4

2) UAV-Aided Monitoring:

  • UAV-mounted mobile RISs tune reflection geometry along target trajectories to mitigate interception risks and maintain tracking when terrain or vegetation blocks direct links.The scenario covers search-and-rescue and low-altitude emergency monitoring involving vehicles or helicopters and ground terminals.Source: 2) UAV-Aided Monitoring · p. 4

3) Industrial IoT:

  • In factories, passive RIS networks bypass barriers created by metallic equipment and moving robotic arms, reducing privacy-leakage risks while supporting instruction transmission and precise robot positioning.Small RIS elements mounted on workshop surfaces create multiple propagation paths described as programmable security walls.Source: 3) Industrial IoT · p. 4

4) Smart Home Privacy:

RISs can protect smart-home monitoring across blocked indoor links, while secure ISAC depends on reliable cascaded channel estimates that capture both communication channels and sensing parameters.

4) Smart Home Privacy:

  • Tiny RIS units embedded in walls or furniture bypass indoor partitions, while physical-layer obfuscation and directional beams protect monitoring echoes and private data from eavesdropping.The setting includes blocked links between a gateway and smart devices or residents.Source: 4) Smart Home Privacy · p. 4

IV. KEY ENABLING TECHNOLOGIES

  • Practical RIS security requires accurate environmental parameters, joint resource optimization, and sensing-assisted dynamic defense.These are identified as technical bottlenecks in translating RIS physical gains into system-level security.Source: §IV · p. 4

A. Cascaded Channel Estimation

  • Cascaded channel estimation addresses high dimensionality and low received SNR, using BS pilots and RIS spatial sparsity to reduce training overhead while extracting range, angle, and Doppler information.Sensing echoes support inference of relative positions for obstacles, legitimate users, and potential eavesdroppers, informing controllable virtual LoS links.Source: A. Cascaded Channel Estimation · pp. 4–5

B. Active-Passive Beamforming

Joint BS and RIS beamforming allocates spatial energy to balance secrecy and sensing, while sensing-driven prediction can trigger beam handover before dynamic obstacles disrupt a virtual LoS link.

B. Active-Passive Beamforming

  • Jointly coordinating BS precoding and RIS phase shifts balances user QoS, target illumination, and eavesdropper suppression through spatial energy allocation.Destructive phase interference creates spatial nulls at eavesdropper locations, while sensing design minimizes detection errors toward the Cramér-Rao bound.Source: B. Active-Passive Beamforming · p. 5

C. Dynamic Tracking and Handover

  • Radar echoes provide obstacle velocity, azimuth, and Doppler parameters for reconstructing motion trajectories and controlled virtual LoS links over an extrapolated time axis.The passage names multi-target tracking algorithms or long short-term memory networks as possible trajectory-reconstruction approaches.Source: C. Dynamic Tracking and Handover · p. 5
  • When a geometric collision model predicts imminent blockage, coordinated RIS scheduling or proactive beam handover can occur before link disruption, reducing outage-related privacy risks.The figure contrasts this sensing-driven predictive scheme with reactive handover; seamless transitions are stated to depend on favorable conditions.Source: C. Dynamic Tracking and Handover · p. 5, Figure 3 · p. 5

V. CASE STUDY

Under completely blocked direct paths, the case study evaluates joint active and passive beamforming to maximize secure communication while meeting user QoS and radar echo requirements. The results show higher secure rates across RIS sizes and spatial separation of legitimate-user and eavesdropper directions.

V. CASE STUDY

  • The simulation maximizes total secure communication rate subject to BS power limits, legitimate-user QoS, and radar echo-power thresholds, using alternating optimization with semidefinite relaxation and successive convex approximation.Source: §V · p. 5
  • The proposed joint optimization scheme outperforms all benchmarks across RIS array sizes, while total secure communication rate increases monotonically with the number of RIS elements.Fig. 4 presents total secure communication rate versus RIS-element count under different benchmarks.Source: §V · p. 6, Figure 4 · p. 6
  • Identical downlink-communication and uplink-sensing RIS coefficients or random uplink sensing phases limit performance, while disabling artificial noise or BS sensing constraints causes substantial rate drops.Source: §V · p. 6
  • The RIS beampattern concentrates gain toward legitimate users, nulls the eavesdropper direction, and maintains adequate echo power toward the sensing target.The reported spatial allocation supports effective decoupling between legitimate data delivery and interception.Source: §V · p. 6, Figure 5 · p. 6
  • RIS spatial degrees of freedom and adaptive channel reshaping alleviate communication-sensing resource competition under severe blockage.Source: §V · p. 6

VI. OPEN CHALLENGES AND FUTURE DIRECTIONS

Passive RIS operation and blocked, dynamic channels make synchronization difficult, while hardware imperfections can distort beams and degrade sensing and security. Robust synchronization and impairment-compensating beamforming remain open bottlenecks.

VI. OPEN CHALLENGES AND FUTURE DIRECTIONS

  • A passive RIS lacks RF chains and baseband processing, preventing direct estimation of its clock offsets and phase noise; double-path fading can also leave synchronization pilots at very low SNR.Source: A. Synchronization and Hardware Limits · p. 6
  • Phase quantization errors, amplitude-phase coupling, and thermal-aging bias can distort reflection beams, reduce radar range and angular resolution, and leak confidential energy toward unintended directions.Source: A. Synchronization and Hardware Limits · p. 6
  • Robust synchronization under fully passive constraints and flexible beamforming that compensates for hardware impairments remain critical bottlenecks.Source: A. Synchronization and Hardware Limits · p. 6

B. Hidden Eavesdropper Defense

Hidden, non-cooperative eavesdroppers require sensing-driven detection and beam-nulling decisions, while dynamic deployments demand faster, more robust security management. The article concludes that practical RIS-assisted secure ISAC also depends on advances in hardware, optimization, environment acquisition, and reconfiguration.

B. Hidden Eavesdropper Defense

  • Hidden eavesdroppers may emit no signals, so secure ISAC must use radar sensing to detect and identify them amid clutter and NLoS reflections before making beam-nulling decisions.Source: B. Hidden Eavesdropper Defense · p. 7

C. AI-Driven Security Management

  • Millisecond-level response, high-dimensional optimization, out-of-distribution blockages, and complex action spaces motivate lightweight, explainable, physics-driven AI for adaptive closed-loop security orchestration.Source: C. AI-Driven Security Management · p. 7

VII. CONCLUSION

  • Practical deployment requires progress in hardware-efficient RIS architectures, scalable optimization, real-time channel and environment acquisition, and intelligent reconfiguration under dynamic conditions.Source: §VII · p. 7
  • The case study demonstrated improved secrecy performance and effective spatial nulling from joint active and passive beamforming under fully blocked direct links.Source: §VII · p. 7

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