Introduction

The growing need for spectrum for telecommunications is sustained by two possible main streams: the increase in spectrum efficiency and the increased capability to fill the temporary spectrum empty spaces. While the first is aimed at improving radio access technologies, making the signal waveforms more complex and thus more demanding in terms of transmitter linearity, the second approach requires transceiver flexibility in order to adapt the parameters according to channel occupancy in an opportunistic way. Opportunity is shown by on-field measurements which, in the below 6 GHz band-width, have shown that actual utilization at the time, place and direction is 0.25% in the 3–4 GHz band, and drops to 0.13% for the 4–5 GHz band, while does not exceed 5% in 5–6 GHz. Even in the most congested bands, such as below 2 GHz, utilization is less than 50%. It is clear that allocations are not all being utilized by their expected users: the priority users (PU). Chapter 1 provides a comprehensive discussion of the topic across the world. The availability of white spaces is taken by the cognitive radio (CR) approach, provided that the spectrum sensing and transceiver reconfigurability are performed over the widest spectrum band to give the highest effectiveness in the use of unused spectra [1].