Access to radio spectrum is largely based on the principle of fixed resource allocation. With the proliferation of wireless applications/services in the last couple of decades, in many countries, most of the available spectrum has been allocated. This results in the radio spectrum scarcity which poses a serious problem for the future development of the wireless communications industry. On the other hand, careful studies of the usage pattern reveal that most of the allocated spectrum experiences low utilization. This motivates the concept of dynamic spectrum access via cognitive radio technologies, which allows secondary networks to use unused or under-utilized radio spectrum from primary licensed networks.