  Cognitive Radio (CR) networks presents a paradigm shift aiming to alleviate
the spectrum scarcity problem exasperated by the increasing demand on this
limited resource. It promotes dynamic spectrum access, cooperation among
heterogeneous devices, and spectrum sharing. Spectrum sensing is a key
cognitive radio functionality, which entails scanning the RF spectrum to unveil
underutilised spectral bands for opportunistic use. To achieve higher data
rates while maintaining high quality of service QoS, effective wideband
spectrum sensing routines are crucial due to their capability of achieving
spectral awareness over wide frequency range(s)\ and efficiently harnessing the
available opportunities. However, implementing wideband sensing under stringent
size, weight, power and cost requirements (e.g., for portable devices) brings
formidable design challenges such as addressing potential prohibitively high
complexity and data acquisition rates. This article gives a survey of various
wideband spectrum sensing approaches outlining their advantages and
limitations; special attention is paid to approaches that utilise sub-Nyquist
sampling techniques. Other aspects of CR such as cooperative sensing and
performance requirements are briefly addressed. Comparison between sub-Nyquist
sensing approaches is also presented.
