Outage optimal subcarrier allocation for downlink secure OFDMA systems

Security and privacy issues are becoming more and more important in orthogonal frequency division multiple access (OFDMA) based 4G wireless communication systems. Recently, the security issue was treated from the perspective of physical layer, which mainly focused on improving the average secrecy rate in wireless fading channels. In this paper, the subcarrier allocation problem is considered for secure OFDMA systems. The objective is to minimize the outage probability of each user while guaranteeing the target secrecy rate. Although the induced optimization problem is a probabilistic integral programming, a novel random bipartite graph (RBG) approach will still be proposed to solve this problem. The random rotation based Hopcroft-Karp (R²HK) algorithm, which enjoys a logarithm-polynomial complexity, will then be proposed to minimize the secrecy outage probability with fairness assurance. By analyzing the properties of the RBG based R²HK algorithm, an approximation for the secrecy outage probability will be obtained in closed-form formulas. Simulation results will not only illustrate the accuracy of the theoretical results in this paper, but also show the performance gain of the proposed algorithm.