I. Introduction
With the rapid development of wireless communication technology, the demand for spectrum resources is increasing. however, the resources are limited and non-renewable. The time and space statistics given by the US Federal Communications Commission (FCC) Spectrum Policy Task Report showed that the utilization of long-term fixed spectrum resource allocated by the spectrum management organization for the authorization system and services is from 15% to 85%[1]. Some frequency bands are overloaded, such as the mobile phone network, but quite a few bands are not fully utilized, such as amateur radio. It is clear that the current utilization of spectrum resources is not scientific, which causes the waste of the limited resources. Therefore, it is important to find out more efficient techniques of spectrum allocation to obtain better utilization of spectrums. In this context, cognitive radio technology[2] come into our mind.