It has become a research hotspot that the secondary user (SU) utilizes the frequency bands from primary users (PUs) while assisting PUs with data transmission in cognitive radio networks (CRNs). In this paper, a new cooperation paradigm is studied in cognitive multicast networks, in which one SU assists the data communication of two PUs simultaneously to harvest multiple spectrum access opportunities. Meanwhile, network coding is also employed to compress two PUs' transmission data and improve the resource utilization. Specially, the design of the network coding-based cooperation transmission is formulated as a joint power and spectrum allocation problem. The problem aims at maximizing SU's achievable transmission rate and guaranteeing the minimum transmission rate requirements of both PUs. To solve the problem efficiently, a finely-matched algorithm is proposed by exploiting the Karush-Kuhn-Tucker (KKT) conditions. Simulation results show the proposed network-coding-based primary-secondary cooperation can improve significantly the secondary transmission rate.
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