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We consider a cognitive radio system with one secondary user (SU) accessing multiple channels via periodic sensing and spectrum handoff. We propose an optimal spectrum sensing and access mechanism such that the average energy cost of the SU, which includes the energy consumed by spectrum sensing, channel switching, and data transmission, is minimized, whereas multiple constraints on the reliability of sensing, the throughput, and the delay of the secondary transmission are satisfied. Optimality is achieved by jointly considering two fundamental tradeoffs involved in energy minimization, i.e., the sensing/transmission tradeoff and the wait/switch tradeoff. An efficient convex optimization procedure is developed to solve for the optimal values of the sensing slot duration and the channel switching probability. The advantages of the proposed spectrum sensing and access mechanism are shown through …