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Based on the partially observable Markov decision process, this paper investigates spectrum access in cognitive networks and proposes an optimal spectrum access algorithm to maximize the throughput. The suboptimal solution to the optimal spectrum access strategy is given by use of the greedy algorithm. In order to achieve the optimal channel access, the channel access state of authorized users is estimated according to the historical information of spectrum sensing. Simulation results show that the optimal spectrum access algorithm would make the secondary users achieve a higher throughput and the networks have the higher spectrum utilization. Compared with the random access and the optimal cognitive MAC protocol, the optimal spectrum access algorithm would improve the throughput by 25–40% and 8–12%, respectively.

We develop a secure random linear network coding scheme on wiretap networks where a wiretapper can only eavesdrop on a limited number of channels. On one hand, by refining Lima's "locked coefficien ...