| Summary: | Aiming at realizing ubiquitous spectrum access and improving the spectrum efficiency in a software-defined network, in this paper, we adopt the concept of cognitive radio and propose a joint subcarrier and power allocation (JSPA) scheme for reciprocally benefited spectrum access with secondary users (SUs) cooperating with primary users (PUs). In our proposed JSPA scheme, SUs adopt the decode-and-forward relaying protocol to help PUs in a two-stage way. Meanwhile, a fraction of unallocated licensed spectrum is allocated for the secondary transmission in every stage. However, if the two-stage cooperation still cannot satisfy the PUs' outage quality-of-service (QoS) requirement, SUs then switch to the access mode and entirely capture the licensed spectrum. Our proposed scheme is to maximize the average transmission rate of SUs through joint optimal subcarriers and power allocation under the constraint of PUs' outage QoS requirement. The closed-form expressions about the outage probability and average transmission of both PUs and SUs are derived, and we prove the optimality of our scheme. Simulation results show that, compared with the conventional cognitive cooperation scheme, the average transmission rate of SUs is improved.
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