Spectrum scarcity is one of the biggest challenges that modern world is facing. Efficient use of available licensed spectrum is becoming more and more critical with increasing demand and usage of radio spectrum. Researches show that the usage is not uniform throughout licensed spectrum rather it is heavy in certain parts and has portions that are utilized inefficiently. Some researchers even claim that more than 70% of licensed frequency band is not in use, most of the time. Different researches are in progress and ways are being found to efficiently utilize the available licensed...