I. INTRODUCTION
Studies show that 100% and 58.6% of Internet traffic
generated by smartphones and PCs, respectively, is carried
over wireless interfaces. 69% and 57% accounts for WiFi,
which operates in unlicensed spectrum, and 31% and 1.6%,
respectively, accounts for cellular interfaces operating in licensed
spectrum [1]. These numbers show that licence-exempt
(or unlicensed) spectrum already plays a vital role in meeting
the capacity challenge related to the mobile data crunch. The
amount of transmitted mobile data will continue to grow, at
an estimated compound annual growth rate (CAGR) of 78%
from 2011 to 2016 [2]. To support this demand we need
even more pervasive WiFi deployments, which are, however,
limited by interference stemming from unlicensed operation.
Another possible solution to meet this demand is to increase
the cellular network’s density, which comes in the form of
small cells (e.g. femtocells) that operate in a licensed spectrum
underlay to macrocells. However, there is an alternative at
hand: whitespace spectrum and cognitive radio technologies.