Use of radio frequency bands of the electromagnetic spectrum is regulated by governments in most countries, by allocating specific frequency bands to particular types of uses, such as licensed bands for commercial radio and television broadcasting, cellular telephony, mobile networks such as CDMA2000, WCDMA, HSPA, LTE, and IMT, maritime radio, police, fire, and public safety radio, GPS, radio astronomy, earth stations for satellite communications, and many other uses. Governments also allocate unlicensed bands, for example, for Wireless Regional Area Network (WRAN) broadband access for rural areas and wireless local area networks (WLAN) and wireless personal area networks (WPAN), such as the industrial, scientific, and medical (ISM) band.
In the United States, the Federal Communications Commission (FCC) regulates use of the radio spectrum, including radio and television broadcasting. Frequencies are allocated according to a bandplan in which guard bands are assigned between the allocated radio bands to avoid interference between adjacent signals. There are also unassigned frequency bands in the spectrum that either have never been used or have become free as a result of changes in technology. Unassigned or un-used frequencies also appear locally inside the frequency bands, which are otherwise allocated in other locations. The unassigned frequency bands and guard bands are referred to as white spaces.
TV white space may be broadly defined as broadcast television spectrum that is unused by licensed services. There are at least two categories of TV white space: [1] Dedicated TV white space is a portion of the spectrum that the FCC has reallocated to unlicensed use from previously analog broadcast usage, and [2] Locally unused spectrum by licensed TV broadcasters in a geographic area.
[1] Dedicated TV white space: In the United States, the FCC has dedicated approximately 400 MHz of white spaces for unlicensed use that became unused after a federally mandated transformation of analog TV broadcasting to digital TV broadcasting. However, the FCC has prohibited unlicensed use of white spaces from interfering with existing licensed uses, including digital TV stations, low power TV stations, cable TV headends, and sites where low power wireless microphones are used. Various proposals have been made for unlicensed use of the white spaces left by the termination of analog TV, for example rural broadband deployment, auxiliary public safety communications, educational and enterprise video conferencing, personal consumer applications, mesh networks, security applications, municipal broadband access, enhanced local coverage and communications, fixed backhaul, and sensor aggregation for smart grid meter reading.
[2] Locally unused spectrum by licensed TV broadcasters: The FCC has adopted rules to allow unlicensed radio transmitters to operate in the broadcast television spectrum at locations where that spectrum is not being used by licensed broadcasters. The FCC proposes two mechanisms to enable the unlicensed transmitter to discover the available channels: geo-location and database based approach, and spectrum sensing. The use of one of the mechanisms is required for the unlicensed transmitter. The FCC proposed the use of geolocation to establish the location of the unlicensed transmitter and a database of TV bands use by licensed broadcasters organized by their geographic coverage areas, to enable the unlicensed transmitter to know where local TV band white spaces may be available. The FCC proposed the use of spectrum sensors in the unlicensed transmitter to detect the presence of the incumbent, primary TV broadcaster's signal in the local TV band to enable the unlicensed transmitter to immediately relinquish using the band. A primary user in such a local TV band would be an incumbent TV broadcaster licensed to operate in that band, but in those geographic areas where there are no licensed incumbent TV broadcasters in operation, other unlicensed secondary users may make use of that band. There may be also other incumbent users in the TV band, which the secondary users should avoid, such as program making and special events (PMSE) systems.
In addition to the United States, other countries are also considering to enable unlicensed, secondary operation in TV band white spaces. The requirements may slightly differ in different countries, e.g. in the United States the maximum transmit power for unlicensed device is defined based on the device type, whereas in Europe location specific maximum transmission power has been considered. In that case the maximum allowed transmission power for an unlicensed device would depend on the device geo-location, i.e. the distance from the primary users. The device characteristics, such as emission mask/ACLR (adjacent channel leakage ratio) may affect the maximum allowed transmission power.
Other RF spectrum white spaces may be defined as RF spectrum that is locally unused in certain geographic areas, such as for example frequency allocations from maritime radio in landlocked areas remote from the sea. A primary user in such a maritime radio band would be a maritime radio licensed to operate in that band, but in those geographic areas where there are no licensed maritime radios in operation, other unlicensed secondary users may make use of that band. Similarly, locally unused spectrum white spaces may be present in certain geographic locations, such as the frequency allocations from 2.025 GHz to 2.110 GHz for earth stations to transmit to communications satellites, in areas remote from such earth stations. A primary user in such a satellite earth station radio band would be a satellite earth station licensed to operate in that band, but in those geographic areas where there are no satellite earth stations in operation, other unlicensed secondary users may make use of that band. Further, other schemes of secondary use of spectrum, other than unlicensed schemes may exist, such as licensing, regulator defined policies, cognitive principles, or authorized shared access.