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The unused portions of the UHF spectrum, often called "white spaces," represent an encouraging new opportunity for wireless networks, promising substantial bandwidth and long transmission ranges through the exploitation of relatively unused spectrum.
Available UHF white space includes 180 MHz of unused bandwidth from TV channel 21 at 512 MHz to channel 51 at 698 MHz, excluding channel 37. This white space became available for broadband network use when the FCC issued a ruling on November 4, 2008 that permitted unlicensed devices to reuse this TV channel spectrum. This landmark ruling was based on successful tests performed by the FCC on white space equipment prototypes submitted by Adaptrum, Microsoft, Philips, and Motorola. Testing with these prototype devices demonstrated that solutions existed for accurate and agile sensing of incumbent signals and subsequent prevention of interference with incumbent devices, namely television broadcasts and wireless microphone transmissions, which was a key part of the FCC’s white space enabling ruling.
Networking over UHF white spaces is fundamentally different from conventional WiFi networking along three axes: spatial variation, temporal variation, and fragmentation of the UHF spectrum. You need to understand the differences between UHF white space spectrum and the unlicensed ISM (industrial, scientific, and medical) bands to appreciate the white space networking problem.
First, protocols for both bands exploit spatial variation in the spectrum, but spectrum use is more complex in the UHF band, because the FCC ruling requires non-interference wit the preexisting wireless transmissions of incumbent devices.
Second, devices using the UHF band for broadband networking must be designed to handle spectrum fragmentation, because incumbents can be found operating in any portion of the UHF white space. As a consequence, each fragment of available UHF spectrum can have a different bandwidth unlike WiFi bands for example. A UHF TV channel is relatively narrow (6 MHz in the US), and aggregating contiguous UHF channels improves white space networking throughput. Consequently, any white space network implementation must support variable width channels.
Third, RF transmissions in white spaces are subject to temporal variations because wireless microphones can become active at any time without warning. Experiments by Microsoft and Harvard researchers show that even a single white space packet transmission can cause audible interference with wireless microphone. As a result, a white space network access point AP and its networking clients must quickly disconnect and then rapidly reconnect using a different available channel when interference with an incumbent wireless device is detected.
The first two hurdles do not seem particularly insurmountable, but the third – the need to coexist in a white space band without interfering with existing primary use devices – seems nearly impossible. Simple solutions to this problem do not appear feasible. For example, one simple approach is to simply avoid using channels where wireless microphones might be used. However, blacklisting known wireless microphone channels is overly conservative and leads to inefficient spectrum use, because wireless microphones are used intermittently for limited durations and can be found on any UHF white space channel. Consequently, avoiding wireless microphone channels essentially means abandoning the entire UHF white space spectrum.
A more sophisticated approach would build a historical database of wireless microphone usage patterns that white space network access points would query to determine the channels being used at any given time. However, the measurements made by the Microsoft and Harvard researchers show that wireless microphone use is unpredictable, making such a database impractical.
Instead, researchers have developed ways to quickly detect transmission from incumbent devices and to switch network transmissions to a backup channel when interference is imminent. The detection scheme is called SIFT (Signal Interpretation before Fourier Transform). For a given center frequency F, the SIFT algorithm samples a bandwidth of 1 MHz around F at 1 millions samples/sec. Variations of the SIFT algorithm allow a white space network access point to quickly detect activity in a channel.
The second critical part of the scheme is to have a backup channel ready to use when a packet cannot be sent over a channel because of the sudden onset of incumbent device channel use. The access point advertises the existence and location of this backup channel in its beacon channel so that clients to that access point know where to switch when commanded to do so. Finally, the access point must monitor communications in the backup channel and must pick an alternate backup channel if necessary.
Although the result is somewhat more complex bandwidth use algorithm than employed by existing Wi-Fi access points, the SIFT-based algorithm only requires the use of some additional processing power to make white channel reuse as broadband networking spectrum a reality. In the world with crowded RF spectrums, you can expect to see more ingenious methods for spectrum reclamation to appear in the future, and it may not be long before off-the-shelf white space RF modems become available for broadband data communications.
About the Author
Steve Leibson is a freelance content creator and marketing/lead-generation consultant specializing in high-tech companies. Leibson is also a former vice president of content for Reed Business and former editor in chief for three publications, including EDN.
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