Secure degrees of freedom in cooperative K-helper MIMO cognitive radio systems

We study the achievable secure degrees of freedom (DoF) in a cooperative MIMO cognitive radio system comprised of one primary source-destination pair, multiple secondary source-destination pairs and an eavesdropper against whom the primary user intends to secure its data. In this system, multiple secondary user pairs help to secure primary user's data against eavesdropping. In return, these secondary users are allowed to access primary user's spectrum. All users, including primary user pair, K≥2 secondary user pairs and the eavesdropper are equipped with M antennas. We investigate the secure DoF without the knowledge of eavesdropper's channel state information (CSI), using the interference alignment concept combined with a zero inter-user interference constraint. A beamforming design is proposed to achieve secure DoF d for the primary user and DoF d for all secondary users if Kd≤M. The case of untrusted secondary users is also investigated where the secondary users may potentially eavesdrop. Simulation examples corroborating the theoretical results are presented.