Joint User Selection and Power Allocation Scheme in Secure Communications Assisted by Multiple Friendly Users

In this paper, we study a secure communication scheme assisted by multiple distributed friendly users over fading channels. To defeat a malicious user, friendly users transmit artificial noise continuously regardless of whether the transmitter is sending signals or not. The idle nodes in wireless networks can be selected as friendly users to assist in achieving secure communications. Due to the different positions of friendly users relative to the malicious user and receiver, the assisting abilities of friendly users in secure communications are different. To explore the potential of friendly users, a metric factor is proposed in this paper to evaluate the assisting abilities of friendly users. Based on the evaluating results, an optimum user selection and power allocation scheme is designed to maximize the secure throughput under the power limit and secure constraint. Compared with the benchmark scheme, the proposed secure communication scheme is easy to implement. Moreover, according to the numerical results, the proposed scheme can improve the secure throughput by more than 16% compared with the benchmark scheme when the secure constraint is larger than 0.5.