I. Introduction
Today, as wireless communication witnesses explosive growth in the number of customers, the efficiency of spectrum usage has become a major concern. Recent measurements show that in the conventional networks, where spectrum is allocated statically, a large percentage of licensed bands remain unused over 90% of time [1]. To address this spectrum under-utilization problem, the concept of Cognitive Radio (CR) network [2] is introduced. CR network provides higher spectrum efficiency via a heterogeneous wireless structure and dynamic spectrum access techniques. Specifically, in a CR network, a secondary network is overlaid with a primary network. Secondary users detect and reuse temporarily unused frequency bands of primary users without causing unacceptable interference to primary users.