Identifying and using vacant "white space" spectrum for Wi-Fi may get simpler, more efficient, and cheaper thanks to a new project by Microsoft and a team of Chinese researchers.
The project focused specifically on analysing indoor white spaces, a name for vacant VHF and UHF TV channels. Then the researchers created algorithms and software, used with RF sensors, to create a system to identify and track this indoor spectrum. The system, called White-space Indoor Spectrum EnhanceR or WISER, is able to identify 30% to 50% more white space spectrum than alternative methods, most of which have been designed for outdoors.
A paper outlining the project and the prototype system is available in on online PDF file. The authors are Ranveer Chandra, with Microsoft Research, and five colleagues with the Chinese University of Hong Kong: Xuhang Ying, Jincheng Zhang, Lichao Yan, Guanglin Zhang, and Minghau Chen.
A cluster of RF sensors in a building sample the airwaves to identify and assess indoor white spaces. That data, along with the locations of wireless access points and of self-reporting clients, is stored in geo-location database. Some of the WISER algorithms deal with profiling the building, others with where to place the sensors, according to Chandra.
For the prototype sensors, the researchers used Universal Software Radio Peripheral (USRP) devices, which are computer-hosted software radios, but they could have been "any spectrum analyser with a low noise floor -- there are a few available in the market," Chandra says. In this case, the sensors ran simple software to talk via HTTP with the geo-location database.
One key advance with WISER is that a wireless client device doesn't have to do the white space spectrum sensing itself. It simply determines its location using any indoor location technique, via Wi-Fi or Bluetooth for example, and then reports its position to the geo-location database. In response, the database returns the set of white space channels available at that location, and the client uses one of them to connect to the Internet, Chandra says.
WISER itself can be embedded in future Wi-Fi access points, "which would make it very simple to deploy," Chandra says. Alternatively, a building owner could deploy small WISER sensors that could be plugged into wall outlets, scan the white space spectrum, and report their findings to the database over the interior electrical wiring by Powerline or via Wi-Fi.
Interest in white spaces has exploded since the 2008 FCC ruling that allowed unlicensed devices to use locally vacant TV channels. Regulators in other countries have followed suit. These devices, like Wi-Fi ones today in some 5-GHz channels, have to detect that the channel is vacant before taking it over. [See also "White spaces: Technology overview" from 2010.]
These unused channels, if they can be efficiently and accurately identified, promise a big boost in spectrum for Wi-Fi use. The biggest demand for that spectrum is indoors, according to Chandra. A six-month analysis of white space spectrum in Hong Kong locations found that there is about 40% more white space spectrum available indoors than outdoors.
The problem, according to Chandra, is that "most trials and studies of white spaces done before have focused on outdoor scenarios."
Microsoft Research has been working with CUHK academics since 2010 on these issues, including research aimed at showing how to use this extra spectrum.
The FCC allows two methods for detecting free channels: spectrum sensing, by the device radio, or querying an Internet-based geo-location Web service. According to the WISER research paper, the most commonly used method is the geo-location database, in part because spectrum sensing is costly; and at low thresholds, it's difficult to do accurately with off-the-shelf hardware.
The geo-location method doesn't need hardware and it's easier to deploy. But it has "inherent inefficiency," according to the researchers. That's because it uses propagation modeling rather than direct measurements to identify available spectrum "and hence, is very conservative in the channels it returns for a given location."
In July 2013, the FCC approved for use a white spaces database created by Google, designed to track vacant TV spectrum.
In essence, WISER is designed to draw on the strengths of both approaches and sidestep their weaknesses. Low-cost spectrum sensors make for accurate identification of indoor channels and to do so cost-effectively; the use of a local geo-location database relieves the clients from having to do their own scanning.
WISER was designed to minimise the cost of the spectrum sensors, and to improve the accuracy of identifying usable white space channels, without causing interference itself. A prototype WISER network was deployed at Ho Sin Hang Engineering building on the CUHK campus.
Among other conclusions, the prototype deployment found that "it suffices to identify strong channels via long-time sensing and then focus resources to track the slow-varying white space availability of weak-to-normal channels." The study also found that for a given white space channel there is a "strong correlation in signal strengths and white space availability across different locations. This suggests that we can infer the channel vacancies of multiple correlated locations from those of one or a few representative locations."
The research also found that the "indoor white spaces have different characteristics from the outdoor ones. For example, there are more contiguous unutilised TV channels indoors, which are able to support high bandwidth communication."
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