说明：  In the last few years, there has been enormous improvement in mobile communication services. Due to this rapid growth in the field of communications, the demand for wireless spectrum has increased rapidly. In the early days in order to remove the interference problem, the spectrum was assigned statically. To assign the spectrum statically, the fixed spectrum assignment policy was used. Due to the fixed spectrum assignment policy, the spectrum was not efficiently utilized and remained vacant most of the time. The study by Federal Communications Commission (FCC) shows that the spectrum is available but its usage is not efficient. Such kind of spectrum inefficiency can be overcome by the use of Cognitive Radio.