In telecommunications, white spaces refer to frequencies allocated to a broadcasting service but not used locally.[1]
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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.
Various proposals, including IEEE 802.11af, IEEE 802.22[2][3] 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. However, these efforts may impact wireless microphones, medical telemetry, and other technologies that have historically relied on these open frequencies.
A device intended to use these available channels is a "white-spaces device" (WSD). These are designed to detect the presence of existing signals, such as TV stations and other wireless users, and to then avoid the use of these channels.[4] Early ideas proposed including GPS 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.
Full power analog television broadcasts, which operated between the 54 MHz and 806 MHz (54-72,76-88,174-216,470-608,and 614-806)[5] television frequencies (Channels 2-69), ceased operating on June 12, 2009 per a United States digital switchover mandate. At that time, full power TV stations were required to switch to digital transmission and operate only between 54-698 MHz. This is also the timetable that the white space coalition has set to begin offering wireless broadband services to consumers. The delay allows time for the United States Federal Communications Commission (FCC) to test the technology and make sure that it does not interfere with existing television broadcasts. Similar technologies could be used worldwide as much of the core technology is already in place.[6]
Theatrical producers and sports franchises hoped to derail or delay the decision, arguing that their own transmissions — whether from television signals or from wireless microphones used in live music performances — could face interference from new devices that use the white spaces. However, the FCC rejected their arguments, saying enough testing has been done, and through new regulations, possible interference will be minimized.
More of the broadcast spectrum was needed for wireless broadband Internet access, and in March 2009, Massachusetts Senator John Kerry introduced a bill requiring a study of efficient use of the spectrum.[7] Academics have studied the matter and have promoted the idea of using computing technology to capture the benefits of the white space.[8]
The White Spaces Coalition consists of eight large technology companies that planned to deliver high speed internet access beginning in February 2009 to United States consumers via existing "white space" in unused television frequencies between 54-698 MHz (TV Channels 2-51). The coalition expected speeds of 80 Mbps and above, and 400 to 800 Mbit/s for white space short-range networking.[9] The group includes Microsoft, Google, Dell, HP, Intel, Philips, Earthlink, and Samsung Electro-Mechanics.
The White Spaces Coalition was formed after the Senate set the date to cease analog broadcasts. It doesn't appear to have a website, but many of the companies involved in the White Spaces Coalition are also involved in the Wireless Innovation Alliance.
Google sponsored a campaign named Free the Airwaves with the purpose of switching over the white spaces that will be cleared up in 2009 by the DTV conversion process by the FCC and converted to an un-licensed spectrum that can be used by Wi-Fi devices.[10] The National Association of Broadcasters disapproves of the project because they claim that it would reduce the broadcast quality of their TV signals.[11]
The Federal Communications Commission's Office of Engineering and Technology released a report dated July 31, 2007 with results from its investigation of two preliminary devices submitted. The report concluded that the devices did not reliably sense the presence of television transmissions or other incumbent users, hence are not acceptable for use in their current state and no further testing was deemed necessary.[12]
However, on August 13, 2007 Microsoft filed a document with the FCC in which it described a meeting that its engineers had with FCC engineers from the Office of Engineering and Technology on August 9 and 10. At this meeting the Microsoft engineers showed results from their testing done with identical prototype devices and using identical testing methods that "detected DTV signals at a threshold of -114 dBm in laboratory bench testing with 100 percent accuracy, performing exactly as expected." In the presence of FCC engineers, the Microsoft engineers took apart the device that the FCC had tested to find the cause of the poor performance. They found that "the scanner in the device had been damaged and operated at a severely degraded level" which explained the FCC unit's inability to detect when channels were occupied. It was also pointed out that the FCC was in possession of an identical backup prototype that was in perfect operating condition that they had not tested.[13]
TV broadcasters and other incumbent users of this spectrum (both licensed and unlicensed, including makers of wireless audio systems) feared that their systems would no longer function properly if unlicensed devices were to operate in the same spectrum. However, the FCC's Office of Engineering and Technology released a report dated October 15, 2008, which evaluated prototype TV-band white spaces devices submitted by Adaptrum, The Institute for Infocomm Research, Motorola and Philips. The report concluded that these devices had met the burden of "proof of concept" in their ability to detect and avoid legacy transmissions,[14] although none of the tested devices adequately detected wireless microphone signals in the presence of a digital TV transmitter on an adjacent channel.
On November 4, 2008, the FCC voted 5-0 to approve the unlicensed use of white space,[15] thereby silencing opposition from broadcasters. The actual Second Report and Order was released ten days later and contains some serious obstacles for the development and use of TV Band Devices as they are called by FCC. Devices must both consult an FCC-mandated database to determine which channels are available for use at a given location, and must also monitor the spectrum locally once every minute to confirm that no legacy wireless microphones, video assist devices or other emitters are present. If a single transmission is detected, the device may not transmit anywhere within the entire 6 MHz channel in which the transmission was received.[16] It is hoped that, within a year, this new access will lead to more reliable WiFi and other technologies.
On September 23, 2010 the FCC released a Memorandum Opinion and Order that determined the final rules for the use of white space for unlicensed wireless devices.[17] The new rules removed mandatory sensing requirements which greatly facilitates the use of the spectrum with geolocation based channel allocation. The final rules adopt a proposal from the White Spaces Coalition[18] for very strict emission rules that prevent the direct use of IEEE 802.11 (Wi-Fi) in a single channel effectively making the new spectrum unusable for Wi-Fi technologies.
On February 27, 2009, the National Association of Broadcasters (NAB) and the Association for Maximum Service Television, Inc. (MSTV) asked a Federal court to shut down the FCC's authorization of white space wireless devices. The plaintiffs allege that portable, unlicensed personal devices operating in the same band as TV broadcasts have been "proven" to cause interference despite FCC tests to the contrary. The lawsuit was filed in a United States Court of Appeals for the District of Columbia Circuit. The Petition for Review states that the FCC's decision to allow white space personal devices "will have a direct adverse impact" on MSTV's and NAB's members, and that the Commission's decision is "arbitrary, capricious, and otherwise not in accordance with law.".[19] A Motion to Govern the case is due to be considered on February 7, 2011.[20]
On February 24, 2010, officials in Wilmington, North Carolina, which was the test market for the DTV transition, unveiled a new municipal wireless network, after a month of testing. The network will use the white spaces made available by the end of analog TV. Spectrum Bridge will work to make sure TV stations in the market do not receive interference ("no interference issues" have been reported), and the company hopes to do the same if similar service becomes nationwide. The "smart city" network will not compete with cell phone companies but will instead be used for "national purposes", including government and energy monitoring. TV Band Service, made up of private investors, has put up cameras in parks, and along highways to show traffic. Other uses include water level and quality, turning off lights in ball parks, and public Wi-Fi in certain areas. TV Band has an 18-month experimental license.[21]
On June 29, 2011, One of the largest commercial tests of white space Wi-Fi was conducted in Cambridge, England. The trial was conducted by Microsoft using technology developed by Adaptrum and backed by a consortium of ISP's and tech companies including Nokia, BSkyB, the BBC, and BT, with the actual network hardware being provided by Neul. In the demonstration, the Adaptrum whitespace system provided the broadband IP connectivity allowing a client-side Microsoft Xbox to stream live HD videos from the Internet. Also as part of the demo, a live Xbox/Kinect video chat was established between two Xbox/Kinect units connected through the same TV whitespace connection. These applications were demonstrated under a highly challenging radio propagation environment with more than 120 dB link loss through buildings, foliage, walls, furniture, people etc. and with severe multipath effects.[22]
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