Covert Cooperative Backscatter Communications with Hardware Impairments

Most of the existing works in the area of covert backscatter communication (BackCom) neglect residual hardware impairments (RHIs) that are unavoidable in practical radio frequency (RF) transceivers, and may have an impact on the communication covertness and reliability. Motivated by this fact, in this paper, we investigate a covert cooperative BackCom network under RHIs. To characterize the communication covertness, we derive an analytical expression for the eavesdropper's detection error probability by assuming the availability of statistical channel state information (CSI). To assess the communication reliability, a closed-form expression for the system outage probability is derived via a high transmit signal-to-noise (SNR) approximation. Using the above results, we prove that RHIs can enhance the communication covertness but significantly degrade outage performance. To further improve the covert performance, we propose to maximize the minimum detection error probability by jointly optimizing the power reflection coefficient and the detection threshold while satisfying the communication reliability requirement. As the formulated problem is non-convex, we use a block coordinated descent (BCD) method to decompose it into two sub-problems, one of which is shown to be convex and the other one is a decreasing function with respect to the power reflection coefficient in the high transmit SNR regime. Then, we develop a BCD-based iterative algorithm to obtain its locally optimal solutions. Simulation results are provided to verify the correctness of our analytical results and the superior performance of the proposed iterative algorithm compared with the baseline schemes in terms of covertness under the same communication-reliability constraint.