Integrated Sensing & Communications (ISAC)

• ISAC is a dual-function technology that integrates radar sensing and communication using shared spectrum and adaptive waveforms for efficient 6G networks.
• It employs waveform design strategies—sensing-centric, communication-centric, and joint designs—to balance SINR for users and SCNR for sensing targets.
• ISAC systems leverage physical-layer security measures, including secure beamforming and adaptive techniques, to mitigate eavesdropping and enhance overall performance.

Integrated Sensing and Communications (ISAC) schemes enable wireless systems to simultaneously perform communication and sensing tasks using shared spectrum, hardware, and waveforms. In contrast to traditional systems where communication and radar/sensing are decoupled, ISAC explicitly merges these two functionalities to achieve spectrum, energy, and infrastructure efficiency—key criteria for forthcoming 6G wireless networks. Core architectural, algorithmic, and security paradigms distinguish ISAC from legacy paradigms, raising both design challenges and new opportunities, notably in the context of secure transmission, waveform optimization, and practical hardware implementations.

1. Waveform Design Strategies for ISAC

ISAC waveforms must satisfy divergent requirements: maximizing signal-to-interference-plus-noise ratio (SINR) at legitimate communication users, while ensuring strong signal-to-clutter-plus-noise ratio (SCNR) at echo receivers for reliable target sensing. Three principal waveform design strategies are outlined:

• Sensing-Centric Design: Communication information is embedded into classical radar waveforms, e.g., using pulse interval modulation or index modulation. Typically, the main beam serves sensing, while communication is carried in sidelobes. This approach is simple but the data rate is limited by the radar’s pulse repetition frequency due to tight coupling between symbol generation and radar pulses.
• Communication-Centric Design: Standard communication waveforms (e.g., those from 802.11 protocols) are reused for sensing. For example, pilot signals, preamble sequences, and their favorable auto-correlation properties assist estimation of target parameters. Although this supports high data rates, the sensing function can be suboptimal because such waveforms are not intrinsically tailored for radar operation.
• Joint Sensing-Communications Design: The most advanced philosophy jointly optimizes waveforms for both SCNR and SINR, subject to power constraints and beampattern requirements. The optimization often takes the form

$$\text{Maximize:} \quad \text{Secrecy Rate} \triangleq \log_2(1+\Gamma_{\text{LU}}) - \log_2(1+\Gamma_E)$$

subject to beampattern/SCNR requirements and total power constraints.

Joint design problems involve fractionally structured constraints (ratios of power) and are commonly relaxed (e.g., by dropping rank-one constraints) for tractability, enabling the use of convex optimization tools like semidefinite programming (SDP).

2. Security Trade-offs and Physical Layer Solutions

ISAC transmission inherently elevates security concerns. The necessity of high-power illumination of sensing targets directly conflicts with the goal of minimizing information leakage, since targets may act as eavesdroppers:

• Dilemma: Boosting power toward a target increases SCNR for sensing, but also raises the eavesdropper’s SINR (𝛤_E), potentially enabling unauthorized decoding of confidential information.
• Physical-layer Security Tools:
  • Secure Beamforming: Design beams to maximize 𝛤_LU for legitimate users while minimizing 𝛤_E for targets, ensuring the secrecy rate

  $$R_{\text{secrecy}} = \left[\log_2(1+\Gamma_{\text{LU}}) - \log_2(1+\Gamma_E)\right]^+$$ - Artificial Noise/Jamming: Inject tailored noise toward the eavesdropper/target direction while preserving signal quality for users. - Proactive Security (Sensing-aided): Use real-time knowledge of the environment gleaned from the ISAC system’s own sensing to predict the eavesdropper channel and adapt beamforming or waveform design accordingly.

3. Sensing-Assisted PHY Security

The ISAC transmitter’s intrinsic access to environmental information (via its own sensing function) can be leveraged to enhance security:

• Channel Uncertainty Mitigation: If the eavesdropper’s (target's) location is known only within a given angle interval, transmit beamforming can be made robust by optimizing over this uncertainty region, effectively minimizing the maximum possible 𝛤_E.
• Adaptive Beamforming: By continuously updating its estimate of the target's direction and adaptively designing the transmit spatial pattern, the ISAC system can ensure that the legitimate users' channels are always stronger than potential eavesdropper channels, even under localization uncertainty.
• Numerical Evidence: Simulations in the source demonstrate higher achieved secrecy rates when the system actively leverages real-time environmental sensing as compared to baseline PHY-security schemes.

4. Low-Cost Hardware Architectures for Secure ISAC

Efficient, scalable ISAC deployment must account for hardware constraints:

• Hybrid (Analog-Digital) MIMO: Uses a reduced number of digital RF chains mapped to a large antenna array via analog phase shifters, reducing complexity and energy costs.
• Directional Modulation (DM): Modulation is performed directly in the radio domain at the antenna units, often exploiting parasitic elements or analog phase shifters. This allows the system to “push” the received symbols at the legitimate user into constructive interference regions and, using sensing-derived knowledge of eavesdropper angles, “push” symbols at the eavesdropper towards destructive regions, thereby both saving hardware and increasing security.
• Symbol-level Precoding: For modulation schemes like QPSK or 8PSK, the transmitter jointly designs RF- and digital-domain coefficients so the received vector at the user ($\mathbf{s}_{\text{LU}}$) is strictly within the correct detection region, while the vector at the target/eavesdropper ($\mathbf{s}_{E}$) falls outside.

These architectures enable cost-effective, secure ISAC solutions scaled for dense network and edge deployments.

5. Current Open Problems and Research Directions

The integration of security into ISAC introduces numerous unresolved challenges:

• Radar Location/Identity Privacy: When radar and communication share beamformers, information about the radar system can unintentionally be revealed. New privacy-preserving information exchange protocols are needed.
• Standardization and Compatibility: Integrating secure ISAC into current 5G NR physical-layer waveforms (e.g., CP-OFDM, filter-OFDM, UF-OFDM) is unresolved; solutions must consider spectral and regulatory compliance.
• Network-Level Security and Performance Analysis: Most research focuses on single-link scenarios; rigorous network-scale evaluations (possibly leveraging stochastic geometry) accounting for SINR and latency distributions are not yet mature.
• Robustness to Imperfect Info: Real deployments feature sensing and channel estimation errors. Designing robust secure waveform and beamforming strategies with both bounded and random uncertainty models is a critical open topic.
• Satisfying URLLC and Massive Access: When applications demand ultra-reliable, low-latency communications (URLLC) and massive connectivity, integrating these demands with the dual goals of robust sensing and security entails multi-objective optimization across time, spectrum, and power resources.

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This comprehensive perspective highlights that ISAC unifies communication and sensing at the radio interface, but creates new design trade-offs at the intersection of waveform design, optimization, security, and hardware constraints. By actively leveraging the sensing function to aid PHY security, and through careful waveform/beamforming algorithm and hardware design, future 6G networks can simultaneously achieve high-performance communication, accurate environmental sensing, and guaranteed information security (Wei et al., 2021).