I. Introduction
Since spectrum scarcity and under utilization are two factors inhibiting the growth of wireless networks, more dynamic and agile ways to utilize wireless spectrum are necessary [2]. Thus, in cognitive radio (CR), a node can automatically detect which spectrum slots are in use by licensed (primary) users and which are not, and it can opportunistically transmit over vacant channels. This mode of operation clearly optimizes the spectrum usage while minimizing interference to licensed users [2]. Such nodes are referred to as cognitive or secondary nodes. The standardization of cognitive networks has already begun with IEEE 802.22 (television white spaces), ECMA 392, IEEE 802.11af, and DySPAN 1900.7 coming into the fray [3]. Cognitive concepts may also feature in the development of fifth generation cellular networks [4].