Relay Selection for Covert Communication

Relay Selection for Covert Communication with an Active Warden

We consider covert communication with multiple relays and an active warden who not only sends jamming signals but also aims to detect the covert transmission. In the relay system with the active warden, the most critical factor is the channel between the relay and the warden, as the warden leverages this channel to transmit jamming signals while trying to detect the presence of covert communication. To mitigate the impact of the active warden, we propose a relay selection scheme that selects the relay with the minimum channel gain to the warden.

We analyze the performance of the proposed scheme and demonstrate how increasing the number of relays leads to performance improvements based on analytical results. Numerical results show that the analytical predictions closely match the simulations, and our proposed scheme effectively increases the covert rate while minimizing the threat posed by the active warden.

In two-phase DF relay-assisted covert communication, we proposed the relay selection where Willie sends jamming signals to interfere with the covert transmission. In order to minimize the performance degradation caused by Willie, the relay that has the minimum channel gain to Willie is selected to forward the covert message. In the analytical results, we demonstrated that the proposed scheme effectively increases the detection error probability at Willie in the second phase, allowing the relay to use more power to forward the covert message while still satisfying the covertness constraint. 

We also showed that the received SINR of the relay increases asymptotically with the number of relays. By optimizing the transmit power at Alice and the relay, the proposed scheme can increase the covert rate, especially in scenarios with a strong channel to Willie. The numerical results confirmed that the analytical results align closely and showed that the proposed scheme can achieve the higher covert rate compared to reference schemes.

This work has considered a simplified fixed-power jammer model to facilitate initial analysis and design. However, in practical scenarios, jammers may adapt their power dynamically to disrupt communications more effectively. In future work, investigating relay selection schemes under more sophisticated and adaptive jamming strategies will be important to enhance system robustness and applicability in realistic environments. Furthermore, the current model treats the covert message as a scalar value transmitted within a single communication interval with fixed channel uses. In reality, covert messages consist of multiple symbols transmitted over varying durations, and the message length critically impacts detection performance. 

Incorporating explicit temporal dynamics and variable message lengths into the covert communication model will significantly increase analysis complexity and is therefore an important direction for future research. Additionally, this work assumes that the adversary Willie is an untrusted node within the network who participates in the pilot exchange process. This enables channel estimation and simplifies analysis; however, it may not capture more adversarial scenarios where Willie remains silent or avoids pilot transmission. Future research should address cases with incomplete or unavailable CSI regarding the adversary, potentially employing blind or indirect estimation techniques.

temperature sensor, pressure sensor, motion sensor, proximity sensor, light sensor, gas sensor, humidity sensor, infrared sensor, touch sensor, accelerometer, gyroscope, ultrasonic sensor, biosensor, chemical sensor, optical sensor, magnetic sensor, RFID sensor, flow sensor, vibration sensor, IoT sensor

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