Integrated Sensing and Covert Communication In Low-Altitude Networks: A Smart Radio Environment Perspective

Technical Summary: Integrated Sensing and Covert Communication in Low-Altitude Networks: A Smart Radio Environment Perspective

Problem Statement
The rapid evolution of low-altitude economies and the advent of 6G have expanded the deployment of Unmanned Aerial Vehicles (UAVs) for applications such as logistics, autonomous driving, and smart transportation. However, the broadcast nature of wireless channels in Low-Altitude Networks (LANs) exposes these systems to significant security risks, including signal interception and malicious attacks. While Integrated Sensing and Communication (ISAC) offers a promising solution by enabling UAVs to sense targets and communicate simultaneously, complex urban environments present critical challenges. Specifically, the coexistence of large-scale and small-scale fading in these environments degrades both covert transmission performance and sensing precision. Furthermore, existing research often relies on simplistic Line-of-Sight (LoS) channel models that fail to capture the realities of urban operations, and practical issues such as UAV flight jitter can impair the performance of emerging technologies like Reconfigurable Intelligent Surfaces (RIS). There is an urgent need for robust solutions that can suppress covert channel fading and counteract sensing precision loss without relying solely on traditional security measures.

Methodology
This paper proposes a framework that integrates Smart Radio Environment (SRE) technologies into Integrated Sensing and Covert Communication (ISACC) to address the aforementioned challenges. The methodology is structured as follows:

1. SRE Taxonomy and Integration: The authors introduce the concept of SRE, where wireless channels are treated as optimization variables. They categorize SRE enabling techniques into four dimensions:

   • Electromagnetic: Utilizing Reconfigurable Intelligent Surfaces (RIS) and their derivatives (e.g., STAR-RIS, BD-RIS) to manipulate signal phase and amplitude.
   • Spatial: Employing Movable Antennas (MA) to flexibly adjust antenna element spacing and exploit spatial Degrees of Freedom (DoFs).
   • Frequency: Applying Frequency Diverse Arrays (FDA) and Movable Signals (MS) to introduce frequency offsets for beam pattern manipulation.
   • Waveform: Using Intelligent Programmable Waveforms (IPW) to embed signals into background noise.
     The paper explores how these dimensions can be jointly applied to four specific ISACC modes: Cooperative and Non-cooperative ISAC for both Aerial Base Stations (ABS) and Ground Base Stations (GBS).

2. Case Study Formulation: To demonstrate the efficacy of SRE, the authors conduct a case study focusing on Movable Antenna (MA)-enabled ISACC. The scenario involves a UAV performing covert data collection from multiple Ground Base Stations (GBSs) while a malicious "warden" attempts to monitor the transmission.

   • System Model: The system models Air-to-Ground (A2G) links using Rician fading to account for both LoS and non-LoS components. The GBSs are equipped with MAs, and the UAV operates in full-duplex mode, simultaneously receiving data and transmitting jamming signals to the warden.
   • Optimization Problem: The objective is to maximize the minimum covert rate across all GBSs. This requires the joint optimization of five variables: UAV flight trajectory, jamming power, MA positions, bandwidth allocation, and communication/sensing beamforming vectors.
   • Algorithm: Recognizing the high coupling of variables and the complexity of the objective function, the problem is modeled as a Partially Observable Markov Decision Process (PO-MDP). The authors propose a Heterogeneous Multi-Agent Twin Delayed Deep Deterministic Policy Gradient (HMATD3) algorithm. This approach utilizes a Centralized Training and Decentralized Execution (CTDE) framework, where users, MAs, and the UAV act as distinct agents.

Key Contributions

• Conceptual Framework: The paper is the first to systematically introduce SRE technologies into the domain of ISACC in LANs, providing a comprehensive survey of how SRE can mitigate channel fading and enhance sensing precision.
• Dimensional Analysis: It elaborates on four distinct dimensions of SRE (electromagnetic, frequency, spatial, and waveform) and maps them to specific ISACC application scenarios, offering a structured approach for future system design.
• Novel Optimization Scheme: The authors propose a novel joint optimization scheme for MA-enabled ISACC. By leveraging the spatial DoFs of movable antennas, the scheme aims to improve covert throughput while maintaining sensing capabilities.
• Algorithmic Innovation: The development of the HMATD3 algorithm addresses the computational challenges of multi-variable optimization in dynamic environments, demonstrating how learning-based methods can achieve stable execution strategies for covert data collection.

Results
Simulation results from the case study validate the proposed approach:

• Convergence: The HMATD3 algorithm's reward curve achieves asymptotic convergence after approximately 6,000 iterations, indicating that the agents successfully learn a stable strategy to maximize the minimum user covert rate.
• Trajectory and Strategy: The optimal UAV trajectory involves flying toward the centroid of the GBSs for data collection, then maneuvering closer to the warden to execute effective jamming, before returning to the destination. This path balances data collection rates with jamming effectiveness.
• Performance Gain: The integration of MAs provides significant gains in covert communication performance, particularly under relaxed covertness constraints. The results show that increasing the number of MAs expands the feasible region for selection, thereby increasing spatial DoFs and further improving covert throughput compared to benchmark schemes.

Significance
The paper claims that integrating SRE with ISACC offers a novel and promising pathway to enhance security in complex low-altitude urban environments. By actively manipulating the wireless channel, SRE technologies can suppress covert channel fading and improve sensing precision, which are critical for the reliability of future 6G-enabled LANs. The authors emphasize that while these technical means improve transmission security, they must be deployed within applicable legal and regulatory frameworks to prevent misuse. The work identifies several open challenges, including the need for accurate channel correlation models for movable elements, hybrid-dimension joint designs for heterogeneous networks, and the quantification of channel estimation errors on covertness. Ultimately, the paper positions SRE-enabled ISACC as a critical research direction for achieving dual-security (covertness and secrecy) in next-generation low-altitude networks.