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In a cooperative cognitive radio network, secondary users are able to negotiate with primary users for dedicated transmission opportunities through providing tangible service. Two novel and simple user cooperation frameworks are introduced to improve the spectral efficiency and utilization. To this end, an orthogonal signaling-based cooperation technology for leveraging the degrees of freedom in two-dimensional modulation is discussed. In the proposed framework, anSU is enabled to simultaneously transmit its own data and relay another user's packets in two orthogonal channels. Specifically, in the framework of user cooperation between PUs and SUs, a cross-layer design for cooperative communications between an active PU and an SU is first investigated. In the framework of cooperative spectrum leasing by a group of SUs, a cross-layer design for cooperation among SUs is then presented. Cooperation …