Physical Layer Secure Binary Signature Design for Wiretap CDMA Systems

Due to the open access nature of wireless communication environment, physical layer security has brought increasingly high attention in recent years, attempting to protect the confidential communication away from eavesdroppers. In contrast to the current secrecy designs, which generally exploit the spatial Degree-of-Freedom (DoF) (e.g. MIMO and cooperative relay approaches) or the temporal DoF (e.g. waveform-based schemes), we propose in this paper a novel binary signature design for the secure transmission of downlink code-division multiple-access (CDMA) systems by exploring the DoF in the code domain. In particular, we exploit the characteristics of CDMA communications and the diversity of multipath Rayleigh fading channels. We propose to elaborately design the spreading code to minimize the output signal-to-interference-plus-noise ratio (SINR) of the eavesdropper while ensuring the legitimate users' quality of service (QoS)/SINR. The proposed algorithms can provide a near optimal performance compared with the exhausting searching approach, but with a much lower computational complexity. Simulation studies confirm our analytical performance predictions and demonstrate the benefits of the proposed secure signature design algorithms.