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, 1) sum rate maximization subject to individual power constraints on source and relay, and 2) 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. Index Terms-DF cooperative communication, pseudolinear optimization, secure OFDMA, resource allocation. R ELAYING along with OFDMA is being considered as a promising technology for providing high data rate connectivity anywhere, anytime [1] . Physical layer security aspects in relay-assisted communication has recently gathered considerable attention in the research community [2] . Based on the relaying strategy, e.g., amplify-and-forward (AF) or decodeand-forward (DF), resource allocation problems are formulated differently and are thus investigated separately. Broadly, there exist two kinds of wire tapping scenarios: single eavesdropper with trusted users [3]-[5] and untrusted users [6] . The study in [3] considered subcarrier and power allocation problems in an AF relay-assisted OFDM system with single eavesdropper. Assuming availability of direct path, [4] considered sum rate maximization problem under total system power constraint in DF relay-assisted secure cooperative communication (DFSCC) for a single source-destination pair with a single eavesdropper. Multiuser resource allocation problem in OFDMA-based DFSCC with single eavesdropper was solved in [5] . Recently, resource allocation problems for improving secure capacity and system fairness in OFDMA system with untrusted users and single friendly jammer have been considered in [6] . To the best of our knowledge, OFDMA-based DFSCC with multiple untrusted users has not yet been considered in the literature. We consider two resource allocation problems. First, sum secure rate maximization is studied subject to individual power constraints on source and relay, due to their geographically apart locations. Second, sum power minimization is solved subject to the fairness constraint in terms of per-user minimum support secure rate requirement. The key contributions are as follows: (i) We derive secure rate positivity constraints for each Manuscript
Recently, REF proposed two different power optimization schemes at source and relay under individual power constraint, one achieves sum secrecy rate maximization in the presence of untrusted users while the other achieves fairness for minimum requirement of secrecy per user.
OFDMA-Based DF Secure Cooperative Communication With Untrusted Users
IEEE Communications Letters
IEEE Communications Letters
Mathematics Computer Science