Cognitive radio networks enabling techniques

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It appears that with the introduction of each new service at highly desirable frequency bands (e.g. 1 to 3 GHz), bandwidth scarcity becomes more acute. For decades, most of the demand for efficient utilization of spectrum has been addressed by improvements in modulation, coding and multiple access techniques. Further efficiency improvements by these conventional approaches seem to be limited. More importantly, it has been determined that often there is no real scarcity of spectrum to meet the increased demand. Scarcity is due to static and permanent assignment of the bands only to license holders. Considering space-time distribution of spectrum, the desirable frequency bands are severely underutilized at many locations for most of the time. Once the conventional model is modified to allow unlicensed users to share the spectrum, the scarcity can disappear. Hence the critical issue is to find ways to dynamically utilize the available bandwidth. Cognitive Radio (CR) allows unlicensed secondary users to share the spectrum that is licensed to primary users. This sharing may take place if secondary users transmit when primary user is not active, or by transmitting at a level not harmful to the license holder.