Utility Regions for DF Relay in OFDMA-Based Secure Communication with Untrusted Users

This letter investigates the utility of a trusted decode-And-forward relay in OFDMA-based secure communication system with untrusted users. For deciding on the use of relay, we first present optimal subcarrier allocation policies for direct communication (DC) and relayed communication (RC). Next, we identify exclusive RC mode, exclusive DC mode, and mixed (RDC) mode subcarriers which can support both the modes. In RDC mode, we present optimal mode selection policy and a suboptimal strategy independent of power allocation which is asymptotically optimal at both low and high SNRs. Via numerical results we present insights on relay utility regions.