National governments currently allocate bands of spectrum (sometimes based on guidelines from the ITU) for use by anyone so long as they respect certain technical limits, most notably, a limit on total transmission power. Unlicensed spectrum is decentralized: there is no license payments or central control for users.
Traditional users of unlicensed spectrum include cordless telephones, and baby monitors. A collection of new technologies are taking advantage of unlicensed spectrum including Wi-Fi, Ultra Wideband, spread spectrum, software defined radio, cognitive radio, and mesh networks.
" class="external">http://www.acmqueue.org/issue3/openspec1.cfm?client_no=NEW
" class="external">http://openict.net/projects/openspectrum/
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