Throughput maximization in multiantenna OFDMA downlink under secrecy rate constraints

This paper deals with the problem of sum rate maximization for a multiuser orthogonal frequency-division multiple access channel with secrecy rate constraints. We consider the case of a multiple-antenna base station (BS) and several single-antenna downlink receivers; a single secure user, a single eavesdropper and several normal users (without secrecy requirements). The eavesdropper intends to wiretap the message of the secure user and the BS aims to protect its transmission by appropriately scheduling normal users and enforcing spatial multiplexing between them and the secure user. A frequency (subchannel) and power allocation problem that aims to maximize the sum rate of the normal users, while a secrecy rate constraint is ensured for the secure user, is formulated. The resulting resource allocation problem is non-convex. Based on the dual problem and some well-defined transformations, we provide an iterative resource allocation algorithm with linear complexity with respect to the number of normal users and subchannels. In addition, two low-complexity solutions that are based on the decoupling of the subchannel and the power allocation subproblems, are investigated. Numerical results are provided to illustrate the performance of all the proposed solutions.