Secure OFDM Transmission in Wireless Networks with Untrusted Relays

Physical layer security is becoming a promising technology to face the security challenges of diversified and rapidly-growing wireless applications. In this paper, we propose a three-hop secure Orthogonal Frequency Division Multiplexing (OFDM) transmission protocol for the wireless networks with untrusted relays. In the proposed protocol, instead of exploiting the degree of freedom over spatial domain in multi-antenna systems, the transmission is secured through single-antenna small wireless devices by the use of multiple sub-carriers in frequency. With the objective to maximize the security rate, we formulate the optimal problem for sub-carrier power distribution and time-domain artificial noise design. Since this problem is non-convex, we further approximate and reformulate it into a train of convex problems through relaxation. By solving the relaxed problem, we design an iterative power distribution and time-domain artificial noise scheme for the proposed three-hop protocol. Simulation results demonstrate that the proposed protocol is robust to untrusted relays and can achieves higher secrecy rate compared with the existing two-hop OFDM secure transmission protocols.