  Traditionally, two different policies to access the radio spectrum have
coexisted: licensed regulation, whereby the rights to use specific spectral
bands are granted in exclusivity to an individual operator; or unlicensed
regulation, according to which certain spectral bands are declared open for
free use by any operator or individual following specific rules. While these
paradigms have allowed the wireless communications sector to blossom in the
past, in recent years they have evidenced shortcomings and given signs of
exhaustion. For instance, it is quite usual to encounter fully overloaded
mobile communication systems coexisting with unused contiguous spectral bands.
This clearly advocates for a more flexible and dynamic allocation of the
spectrum resources which can only be achieved with the advent of the so-called
cognitive radios and networks. This whitepaper provides an accurate description
of priority research activities and open challenges related to the different
functionalities of cognitive radios and networks. First, we outline the main
open problems related to the theoretical characterization of cognitive radios,
spectrum sensing techniques as well as the optimization of physical layer
functionalities in these networks. Second, we provide a description of the main
research challenges that arise from a system point of view: MAC protocol
optimization, traffic modelling, RRM strategies, routing paradigms or security
issues. Next, we point out other problems related to the practical hardware
implementation of cognitive radios, giving especial emphasis to sensing
capabilities, reconfigurability and cognitive control and management. Finally,
we succinctly report on a number of current activities related to the
standardization of cognitive radio systems.
