Today's wireless networks are characterized by a fixed spectrum assignment policy. However, a large portion of the assigned spectrum is used sporadically and geographical variations in the utilization of assigned spectrum ranges from 15% to 85% with a high variance in time. The limited available spectrum and the inefficiency in the spectrum usage necessitate a new communication paradigm to exploit the existing wireless spectrum opportunistically. In contrast to current systems where the spectrum allocation is static, future cognitive radio devices will be able to seek and use in a dynamic way the frequencies for network access; this will be done by autonomous detection of vacant bands in the radio spectrum. This new networking paradigm is referred to as NeXt Generation Networks or Dynamic Spectrum Access (DSA) or cognitive radio networks. In this paper the novel functionalities and current research challenges of the cognitive networks are explained in detail. More specifically, a brief overview of the cognitive radio technology is provided and the cognitive network architecture is introduced. Moreover, the cognitive network functions such as spectrum management, spectrum mobility and spectrum sharing are explained in detail. Open research issues in these areas are also outlined.
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