부산대학교 도서관

This paper considers a multi-jammer-assisted covert communication scenario, where a transmitter communicates with a receiver without being detected by a warden under the assistance of multiple friendly jammers. The transmission rate maximization problem subjecting to the covertness constraint by jointly optimizing the jammer selection strategy and power control of the transmitter is studied under the following three channel models: 1) Additive white Gaussian noise (AWGN) channel; 2) Block fading channel without channel state information (CSI) acquisition cost; 3) Block fading channel with CSI acquisition cost. Under the AWGN channel, considering achievable schemes of the transmitter, relative entropy is adopted to lower bound the detection error probability at the warden. We decouple the joint optimization problem into two sub-problems and prove that selecting one jammer with the maximum ratio of the jamming power at the warden to that at the receiver is optimal with proper transmit power control in an interference-limited environment. Motivated by this key analytical conclusion, we then study the second channel model's optimal single jammer selection strategy. Considering the worst case that the warden employs the optimal detector, we analytically show that the optimal selection criterion in this channel model is choosing the jammer with the maximum ratio of the average jamming power at the warden to the instantaneous jamming power at the receiver. Given this selection criterion, we then study the problem of how many jammers should be explored when the acquisition cost of jammers' CSI cannot be ignored. We formulate the optimization problem as an optimal stopping problem and propose two optimal strategies for two cases that whether the previously explored jammers are allowed to select. The numerical results evaluate the theoretical results and show the superiority of the proposed jammer selection strategies. Furthermore, we also present the impact of some key parameters on the covert rate, such as the covertness constraint, the number of jammers, and the CSI acquisition time of one jammer, which provide more valuable insights.