On Degrees of Freedom of Cognitive Networks with User Cooperation

In this letter, the degrees of freedom (DoF) outer bound is characterized for cognitive networks. We have devised an interference alignment scheme with appropriate cooperation between primary users and secondary users to achieve the DoF outer bound. Moreover, to encourage the primary users to participate in the proposed cooperation, we also design two aided interference alignment schemes, where both the primary users and the secondary users can tap more DoF out of the cognitive channels than the existing systems and thereby benefit from the cooperation.