This paper addresses the design of opportunistic spectrum access (OSA) where secondary users are allowed to sense and access multiple channels in the spectrum without causing unacceptable interference to primary users. Integrated in OSA design are a spectrum sensor at the physical (PHY) layer and a sensing and an access strategy at the MAC layer. Within the framework of partially observable Markov decision process, we develop a separation principle for the joint OSA design, leading to an explicit optimal design of the spectrum sensor and a closed-form optimal access strategy when spectrum sensor and access strategy are designed independently across channels. We also propose two heuristic approaches that exploit the correlation among channel occupancies, one at the PHY layer and the other at the MAC layer. Simulation results indicate that the exploitation of channel correlation at the PHY layer is more effective than that at the MAC layer, and that the detection capability of the spectrum sensor can be improved by exploiting MAC layer information.
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