  In this letter we consider resource allocation for OFDMA-based secure
cooperative communication by employing a trusted Decode and Forward (DF) relay
among the untrusted users. We formulate two optimization problems, namely, (i)
sum rate maximization subject to individual power constraints on source and
relay, and (ii) sum power minimization subject to a fairness constraint in
terms of per-user minimum support secure rate requirement. The optimization
problems are solved utilizing the optimality of KKT conditions for pseudolinear
functions.
