Spectrum awareness: Techniques and challenges for active spectrum sensing

Radio spectrum is not efficiently used, mainly due to the prevailing rigid frequency allocation policy. Only some bands of the spectrum - such as those bands used by cellular base stations - are heavily used. Many bands are not used at all or are used only part of the time. Radios using cognitive radio technology are aware of their frequency environment. They can improve spectral efficiency by sensing the environment and then filling the discovered gaps of unused licensed spectrum with their own transmissions. In this review, different spectrum awareness techniques for a cognitive radio system are classified and discussed. Advantages and challenges for each technique are presented. Awareness techniques are classified into passive and active techniques and definitions for these terms are suggested. The focus in this review is on active techniques. In addition, classifications based on the response time and topology are discussed. Primary challenges for active spectrum sensing are introduced and the need for cooperation between users is considered. Finally, emerging spectrum awareness techniques are outlined.