  In this paper, we consider the Physical Layer Security(PLS) problem in
orthogonal frequency division multiple access (OFDMA) based dual-hop system
which consists of multiple users, multiple amplify and forward relays, and an
eavesdropper. The aim is to enhance PLS of the entire system by maximizing sum
secrecy rate of secret users through optimal resource allocation under various
practical constraints. Specifically, the sub-carrier allocation to different
users, the relay assignments, and the power loading over different sub-carriers
at transmitting nodes are optimized. The joint optimization problem is modeled
as a mixed binary integer programming problem subject to exclusive sub-carrier
allocation and separate power budget constraints at each node. A joint
optimization solution is obtained through Lagrangian dual decomposition where
KKT conditions are exploited to find the optimal power allocation at base
station. Further, to reduce the complexity, a sub-optimal scheme is presented
where the optimal power allocation is derived under fixed sub-carrier-relay
assignment. Simulation results are also provided to validate the performance of
proposed schemes.
