Secure Degrees of Freedom in MIMO Cognitive Radio Systems

We study the secure degrees of freedom (DoF) in a cognitive radio system. A cognitive radio user helps to secure primary user's transmission against an eavesdropper and in return, primary users allows secondary user to access its licensed spectrum. All users are equipped with multiple antennas. We investigate the secure DoF using interference alignment concept with partial channel state information (CSI). When eavesdropper is interested only in primary user's data, it is shown that if all users, including primary transmitter-receiver pair, secondary transmitter-receiver pair and eavesdropper, have M antennas, achievable secure DoF d(p) and DoF d(s) of primary and secondary users, respectively, satisfy 2d(p) <= M and d(s) <= M-2d(p).