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Digital dividend has created much of free spectrum in TV bands from 470- 790 MHz This free spectrum is called TV white Space and it can be used to solve spectrum scarcity problem. The frequency spectrum is allocated to licensed users by the government authorities of that particular country. International Telecommunication Union-Radio communications have started allowing unlicensed users to use the white spaces. But there are regulations on using these white spaces. This paper focuses on various regulating authorities, standards used for opportunistically accessing the spectrum. The basic requirement is there should not be any harmful interference to the incumbent user. The prospective of Cognitive radio improves spectral efficiency, by making secondary users opportunistically access spectrum gaps, left by primary users. In this paper different methods to identify spectrum gaps and use it efficiently, are discussed. Coexistence issues of multiple SUs are also discussed in brief.
Radio spectrum is very costly and scarce natural resource. By 2020 there will be requirement for more than 1720 MHz  of RF spectrum [ITU-R] [M-2078], as there is increase in communication devices. The increase in wireless devices is exponential whereas the RF spectrum available will be fixed. Thus there will be spectrum crunch which is mainly due to static spectrum allocation. This static allocation leads to unutilized or underutilized spectrum. Spectrum is allotted statically to the licensed users, so it cannot be used by unlicensed users. But it has been reported from various studies that spectrum is underutilized in almost all frequency bands at a given time and location. This unutilized spectrum is called “White space” or “Spectrum hole” or “Spectrum opportunity”
As many government authorities have released the TV white space for secondary users, the problem of spectrum scarcity has been solved. Many regulations have been framed for Primary and secondary user coexistence as well as secondary and secondary user coexistence.
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