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In wireless ad hoc networks, all nodes have equal right to access the medium. Hence, the performance of this mode is mostly limited by traffic congestion. To alleviate such a problem, Cognitive Radio (CR) technology can be used. Recent researches show that a large amount of licensed spectrum remains unused at a specific time. The contribution of this paper is to investigate a CR-based Medium Access Control (MAC) layer for wireless ad hoc networks. We aim to focus on a Cognitive MAC protocols for a unlicensed user which can be enabled to access the large amount of unused spectrum allocated for a licensed user, in an intelligent way, without causing any harmful interference. We propose a cognitive MAC protocols based on the theory of the Partially Observed Markov Decision Process (POMDP), which sense the radio spectrum, detect the occupancy state of different primary channels, and then opportunistically communicate over unused channels (spectrum holes). The objective is to make efficient decisions on which channels to sense and access, that ensure maximization of the throughput of the secondary user.