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In telecommunications, white spaces refer to frequencies allocated to a broadcasting service but not used locally. National and international bodies assign different frequencies for specific uses, and in most cases license the rights to broadcast over these frequencies. This frequency allocation process creates a bandplan, which for technical reasons assigns white space between used radio bands or channels to avoid interference. In this case, while the frequencies are unused, they have been specifically assigned for a purpose, such as a guard band. Most commonly however, these white spaces exist naturally between used channels, since assigning nearby transmissions to immediately adjacent channels will cause destructive interference to both. In addition to white space assigned for technical reasons, there is also unused radio spectrum which has either never been used, or is becoming free as a result of technical changes. In particular, the switchover to digital television frees up large areas between about 50 MHz and 700 MHz. This is because digital transmissions can be packed into adjacent channels, while analog ones cannot. This means that the band can be "compressed" into fewer channels, while still allowing for more transmissions. In the United States, the abandoned television frequencies are primarily in the upper UHF "700-megahertz" band, covering TV channels 52 to 69 (698 to 806 MHz). U.S. television and its white spaces will continue to exist in UHF frequencies, as well as VHF frequencies for which mobile users and white-space devices require larger antennas. In the rest of the world, the abandoned television channels are VHF, and the resulting large VHF white spaces are being reallocated for the worldwide (except the U.S.) digital radio standard DAB and DAB+, and DMB. ==White spaces devices== Various proposals, including IEEE 802.11af, IEEE 802.22〔(【引用サイトリンク】title=IEEE 802 LAN/MAN Standards Committee 802.22 WG on WRANs (Wireless Regional Area Networks) )〕 and those from the White Spaces Coalition, have advocated using white spaces left by the termination of analog TV to provide wireless broadband Internet access. A device intended to use these available channels is a "white-spaces device" (WSD). Such devices are designed to detect the presence of existing but unused areas of airwaves, such as those reserved for analog television, and utilize these unused airwaves to transmit signals for Internet connectivity. Such technology is predicted to improve the availability of broadband Internet and Wi-Fi in rural areas. Early ideas proposed including GNSS receivers and programming each WSD with a database of all TV stations in an area, however this would not have avoided other non-stationary or unlicensed users in the area, or any stations licensed or altered after the device was made. Additionally, these efforts may impact wireless microphones, medical telemetry, and other technologies that have historically relied on these open frequencies. Professional wireless microphones have used white space "for decades" previous to so-called white space devices.〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「White spaces (radio)」の詳細全文を読む スポンサード リンク
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