Enhancing physical layer security in AF relay-assisted multicarrier wireless transmission

In this paper, we study the physical layer security problem in the dual-hop orthogonal frequency division multiplexing-based wireless communication system. First, we consider a single-user single-relay system and study a joint power optimization problem at the source and relay subject to individual power constraint at the 2 nodes. The aim is to maximize the end-to-end secrecy rate with optimal power allocation over different subcarriers. Later, we consider a more general multiuser multirelay scenario. Under high signal-to-noise ratio approximation for end-to-end secrecy rate, an optimization problem is formulated to jointly optimize power allocation at the base station, the relay selection, subcarrier assignment to users, and the power loading at each of the relaying node. The target is to maximize the overall security of the system subject to independent power budget limits at each transmitting node and the orthogonal frequency division multiple access-based exclusive subcarrier allocation constraints. A joint optimization solution is obtained through the duality theory. Dual decomposition allows to exploit convex optimization techniques to find the power loading at the source and relay nodes. Furthermore, an optimization for power loading at relaying nodes along with relay selection and subcarrier assignment for the fixed power allocation at the base station is also studied. A suboptimal scheme that explores joint power allocation at all transmitting nodes for the fixed subcarrier allocation and relay assignment is investigated. Finally, simulation results are presented to validate the performance of the proposed schemes.