QoS-Based Binary Signature Design for Secure CDMA Systems

Physical layer security, aiming at preventing the confidential information from eavesdroppers, has drawn significant attentions in recent years. In this paper, we address the problem of securing downlink single-user and multiuser transmissions in code division multiple-access systems. Particularly, we propose a novel binary signature design method to minimize the reception of the confidential messages at the eavesdropper. The popular signal-to-interference-plus-noise ratio (SINR) metric is adopted as a pragmatic security performance measurement. Specifically, for securing the single-user transmission, we aim to design a binary signature to minimize the eavesdropper's SINR while maintain the legitimate user's SINR at a desired level. However, the optimal exhaustive searching method has complexity growing exponentially with the signature length. Thus, we propose two suboptimal low-complexity semi-definite relaxation (SDR)-based and Rayleigh-quotient-based binary signature design algorithms, which have near optimal performance with much lower computational complexity. Furthermore, we extend the findings in the single-user case to multiuser scenario. Particularly, we propose to design a set of quasi-orthogonal signatures based on eavesdropping channel and select the conditionally optimal binary signature for each user iteratively until convergence. Simulation studies confirm our analytical performance predictions and demonstrate the benefits of the proposed secure signature design algorithms.