Secure Resource Allocation for Cooperative OFDMA System With Untrusted AF Relaying

In this paper, we consider a cooperative orthogonal frequency division multiple access (OFDMA) system, where a source communicates with multiple users with the help of an untrusted relay. Since we assume no direct links between source and users because of the shadowing effect, the positive secrecy rate of the system cannot be obtained directly. Addressing this issue, we employ the user-aided cooperative jamming method to improve the secrecy performance. With the aim of maximizing the weighted sum secrecy rate, we formulate a joint resource allocation problem of power allocation, subcarrier assignment and subcarrier pairing. By using the alternative optimization and the Lagrange dual method, we solve this non-convex problem efficiently. Furthermore, to reduce the calculation complexity, we propose two suboptimal algorithms. Numerical results are presented to demonstrate the advantage of the proposed algorithms compared with the benchmarks on the secrecy performance. Moreover, we show that our proposed algorithms are more sensitive to user priority weights than benchmarks.