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Joint resource allocation involving optimization of subcarrier allocation,subcarrier pairing (SCP), and power allocation in a cooperative secureorthogonal frequency division multiple access (OFDMA) communication system withuntrusted users is considered. Both amplify and forward (AF), and decode andforward (DF) modes of operations are considered with individual power budgetconstraints for source and relay. After finding optimal subcarrier allocationfor an AF relayed system, we prove the joint power allocation as a generalizedconvex problem, and solve it optimally. Compared to the conventional channelgain matching view, the optimal SCP is emphasized as a novel concept of channelgain tailoring. We prove that the optimal SCP pairs subcarriers such that thevariance among the effective channel gains is minimized. For a DF relayedsystem, we show that depending on the power budgets of source and relay, SCPcan either be in a subordinate role where it improves the energy efficiency, orin a main role where it improves the spectral efficiency of the system. In anAF relayed system we confirm that SCP plays a crucial role, and improves thespectral efficiency of the system. The channel gain tailoring property of SCP,various roles of SCP in improving the spectral and the energy efficiency of acooperative communication system are validated with the help of simulationresults.

Subcarrier Pairing as Channel Gain Tailoring: Joint Resource Allocation for Relay-Assisted Secure OFDMA with Untrusted Users