  Providing physical-layer security for mobile users in future broadband
wireless networks is of both theoretical and practical importance. In this
paper, we formulate an analytical framework for resource allocation in a
downlink OFDMA-based broadband network with coexistence of secure users (SU)
and normal users (NU). The SU's require secure data transmission at the
physical layer while the NU's are served with conventional best-effort data
traffic. The problem is formulated as joint power and subcarrier allocation
with the objective of maximizing average aggregate information rate of all NU's
while maintaining an average secrecy rate for each individual SU under a total
transmit power constraint for the base station. We solve this problem in an
asymptotically optimal manner using dual decomposition. Our analysis shows that
an SU becomes a candidate competing for a subcarrier only if its channel gain
on this subcarrier is the largest among all and exceeds the second largest by a
certain threshold. Furthermore, while the power allocation for NU's follows the
conventional water-filling principle, the power allocation for SU's depends on
both its own channel gain and the largest channel gain among others. We also
develop a suboptimal algorithm to reduce the computational cost. Numerical
studies are conducted to evaluate the performance of the proposed algorithms in
terms of the achievable pair of information rate for NU and secrecy rate for SU
at different power consumptions.
