Bistatic Integrated Sensing and Communication in the Presence of a Disco Reconfigurable Intelligent Surface: Disruption, Enhancement, or Both?

Abstract: Integrated sensing and communication (ISAC) is widely regarded as one of the key enabling technologies for future sixth-generation (6G) wireless communication systems. In this work, we investigate a bistatic ISAC system in the presence of a disco reconfigurable intelligent surface (DRIS), whose random and time-varying reflection coefficients emulate a "disco ball." The introduction of the DRIS breaks the underlying assumption in existing ISAC systems that the sensing and communication channels remain static or quasi-static within the channel coherence time. We first develop a bistatic system model incorporating the DRIS and characterize all involved wireless channels. Then, an ISAC waveform design that balances sensing and communication performance is proposed by formulating a Pareto optimization problem, where the trade-off is controlled through a tunable factor. Communication and sensing performance in the bistatic ISAC system are quantified by the signal-to-interference-plus-noise ratio (SINR) and the Cramer-Rao lower bound (CRLB), respectively. To quantify the impact of the DRIS on the bistatic ISAC system, we derive the statistical characteristics of DRIS-induced active channel aging (ACA) channels for communications and the cascaded DRIS-based sensing channel. Then, we establish a theoretical lower bound on the SINR and closed-form CRLB expressions in the presence of a DRIS. The analysis reveals several distinctive properties of the DRIS in bistatic ISAC systems. In particular, the DRIS degrades communication performance significantly due to the introduction of ACA interference. In contrast, with respect to sensing performance, the DRIS decreases the estimation accuracy of the angle of departure (AoD) while concurrently enhancing that of the angle of arrival (AoA). Numerical results validate the derived theoretical analysis and confirm these DRIS-induced behaviors.

• The paper shows that DRIS-induced active channel aging converges to an i.i.d. complex Gaussian distribution, with variance scaling by DRIS size and cascaded path loss.
• It formulates an ISAC waveform co-design as a Pareto optimization to balance multi-user interference suppression for communications and covariance shaping for sensing.
• The analysis reveals anisotropic sensing impacts: AoD estimation degrades while AoA accuracy improves, exposing vulnerabilities in future ISAC systems.

Bistatic ISAC under Disco RIS: Analytical Characterization of Disruption and Sensing Anisotropy

System and Threat Model

The paper introduces a bistatic integrated sensing and communication (ISAC) architecture incorporating a disco reconfigurable intelligent surface (DRIS), which operates as a randomized, rapidly time-varying reflector, akin to a "disco ball." This DRIS is configured independently of the ISAC system and rotates its phase and amplitude coefficients stochastically, inducing unpredictable channel variation well within the conventional channel coherence time.

The system comprises a multi-antenna ISAC base station (BS), several single-antenna communication users, a sensing receiver with its own array, and a DRIS with a massive number of discrete-state reflecting elements. The bistatic radar scenario enables simultaneous communications with end users and radar-based sensing (target detection), prototypical of 6G ISAC visions.

Figure 1: A bistatic ISAC topology illustrating the location of the DRIS and its randomized element-wise reflections, introducing active channel aging (ACA) effects.

DRIS-Induced Channel Aging: Statistical Characterization

Classical ISAC and RIS-assisted systems rely on quasi-static channel models, with CSI acquisition in a pilot phase, exploited throughout subsequent data transmission. The DRIS, with its randomized, rapid phase variation, nullifies this assumption, creating actively aged channels even during a single coherence interval. The paper models both the direct and DRIS-reflected links, leveraging near-field and far-field models appropriately, and explicitly formulates the statistical characteristics of the DRIS-induced active channel aging (ACA) effect.

Key analytical results include:

• The ACA channels—differences in channel states seen in pilot versus data phases—are shown to converge to i.i.d. complex Gaussian distributions as the number of DRIS elements increases, with variance scaling proportionally to both DRIS size and the cascaded path loss.
• DRIS-induced ACA interference is fundamentally different from that due to passive RIS or propagation-induced channel aging—it cannot be mitigated by increased transmit power and is uncorrelated with system choices.

ISAC Waveform Co-Design: Pareto Optimization

The paper poses ISAC waveform optimization as a Pareto problem, balancing multi-user interference suppression for communications and covariance shaping for radar sensing. The formulation introduces a tunable trade-off factor, $\kappa$, dictating performance focus.

• The optimal waveform for sensing strictly enforces a transmit covariance constraint matching that of a desired radar codebook, at the expense of strong multi-user interference.
• The ISAC Pareto design interpolates between communications and sensing optima, and achieves this via an SDP that is shown to admit globally optimal, rank-one solutions.

Theoretical Bounds: Communication and Sensing Performance Degradation/Enhancement

SINR Degradation: Analytical Lower Bound

The main communication metric is the sum-rate over all users, with SINR at each user analyzed in closed-form. Under DRIS-induced ACA, the expected SINR lower bound at user $k$ is

$${\gamma_k} \geq \frac{{\mathbb{E}[|s_{k,l}|^2]}}{\mathbb{E}[|\text{MU interferers}|^2] + P_0\mathscr{L}_\mathrm{cas,k}N_\mathrm{D}\overline{\mu} + \sigma^2}$$

where the ACA penalty term $$P_0\mathscr{L}_\mathrm{cas,k}N_\mathrm{D}\overline{\mu}$$ reflects both transmit power and DRIS element count, demonstrating that increased system power exacerbates, rather than mitigates, DRIS impact.

Figure 2: Sum-rate vs. transmit power; the presence of DRIS leads to significant degradation, and the analytical lower bound closely matches simulation.

Figure 3: Sum-rate degradation increases with the number of DRIS elements, in agreement with the scaling predicted by the lower bound.

Figure 4: Communication degradation diminishes as the DRIS is placed farther from the ISAC BS, showing the path loss dependence of the ACA effect.

Anisotropic Sensing Impact: CRLB Analysis for AoD and AoA

For sensing performance, the paper derives closed-form Cramér-Rao lower bounds (CRLB) for angle-of-departure (AoD, $\theta_1$) and angle-of-arrival (AoA, $\theta_2$) estimation. Notably, the DRIS exerts a direction-dependent effect:

• AoD Estimation Degradation: The CRLB for AoD increases significantly, with the error floor rising as either the number of DRIS elements grows or the DRIS approaches the transmitter.
• AoA Estimation Enhancement: Contrarily, the CRLB for AoA estimation decreases with larger/more proximal DRIS, improving angular resolution at the sensing receiver.

Figure 5: (a) MSE of AoD estimation increases, (b) MSE of AoA estimation decreases, with higher power, showing the opposite trends induced by DRIS presence.

Figure 6: AoD estimation error monotonically worsens and AoA error monotonically improves as DRIS size increases.

Figure 7: AoD error climbs and AoA error diminishes as DRIS is positioned closer to the ISAC BS, quantifying the anisotropic sensing consequence.

Practical and Theoretical Implications

Communication Security and Robustness

The results imply that passive, energy-free adversarial DRIS structures can fundamentally challenge ISAC networks, rendering standard countermeasures (e.g., power control) ineffective. DRIS-induced ACA interference is independent of ISAC system parameters or user locations.

Sensing Masking and Information Leakage

The anisotropic property—the selective degradation of AoD but not AoA estimation—has both attack and system design implications. An adversary can mask transmitter location/fingerprint while leaving receiver-based localization intact. This rather counterintuitive property could be leveraged for privacy protection or exploited for directionally selective jamming.

Scalability and Infrastructure Attacks

The linear scaling of disruption with DRIS element count predicts severe vulnerability in dense or large-area deployments. Because the DRIS can be camouflaged and requires no energy input or knowledge of system operation, physical-layer security must devote increased attention to such “environmental” jamming modes.

Prospects and Future Directions

This analytical framework calls for rethinking ISAC waveform and resource allocation strategies under severe, environment-induced channel aging. Potential research threads include:

• Joint DRIS detection and mitigation techniques leveraging environment mapping and non-stationarity detection.
• DRIS-resilient ISAC waveform designs decoupling communication and sensing tasks or leveraging diversity orthogonal to DRIS effects.
• Active monitoring for environmental anomalies in the electromagnetic landscape as part of ISAC security.

Conclusion

The paper delivers quantitative, closed-form analysis of the impact of a randomly modulated DRIS on bistatic ISAC performance. It is rigorously shown that DRIS-induced channel aging severely degrades communication reliability, scales with DRIS size and proximity, and cannot be overcome by increased transmission power. In sensing, the DRIS selectively impairs AoD estimation but enhances AoA accuracy, producing a strongly anisotropic disruption and enhancement profile. These results expose substantial new vulnerabilities and system design trade-offs for future ISAC architectures in adversarial environments.