  This paper investigates the utility of a trusted decode-and-forward relay in
OFDMA-based secure communication system with untrusted users. For deciding
whether to use the relay or not, 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. For 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. Finally, via
numerical results we present insights on relay utility regions.
