Resource Allocation in Secure Multicarrier AF Relay System under Individual Power Constraints

We consider a dual-hop amplify-And-forward (AF) relay-Aided multicarrier system between a source and destination, in which there is an external eavesdropper which taps the signals in both hops. We consider individual power constraints at source and relay. The relay pairs the carriers in the second hop and enhances the system sum rate through multicarrier diversity. The sum rate maximization problem belongs to the class of mixed-integer nonlinear program (MINLP) due to the variables corresponding to subcarrier (SC) pairing being binary and its joint optimization with source and relay powers. We propose a solution based on alternate maximization and show that it converges to a local maximum. We also derive a computationally less complex and optimally computable solution based on transforming the problem to a single power variable which, ultimately, is a lower bound to sum rate. We analyze both the solutions when either the source power or relay power, or both, are very high. In the process, we also derive asymptotically achievable upper bound to the sum rate. We also show that either full relay power or full source power, or both, are used at the optimal point. Our numerical results demonstrate an improved sum rate over a scenario of no pairing at relay.