**3. MDP applications in cognitive radio**

MDP has been frequently applied in the design of MAC protocols in cognitive radio. They can be classified into two classes: decentralized and centralized access protocols. In the decentralized case, each unlicensed user is responsible of performing spectrum sensing and spectrum access, in general with limited, and sometimes unreliable, information about the spectrum usage. In consequence, it is usual to find partially observed MDP (POMDP) formulations of the problem, which easily become intractable when the dimension of the problem increases. The access of secondary users to the spectrum should have the less possible impact on licensed users. When including these restrictions on the formulation the resulting problem is a constrained POMDP. In the centralized case, a central device, generally referred to as spectrum broker, performs spectrum management, controlling the access of secondary users to idle spectrum channels. It is usually assumed that the spectrum broker has perfect information about the spectrum usage, therefore the problem is formulated as an MDP, or as a CMDP if constraints are included.
