Wireless Trasmission

WISER – New Technology that effortlessly identifies ‘White Space’ spectrum indoors’ Radio frequency spectrum, the airwaves over which wireless devices communicate, is in increasing demand throughout the world. Access to spectrum is currently regulated by strict licensing systems that limit the users and the applications of any given set of frequencies.

The unfortunate result is that a significant amount of spectrum goes unused at any given time or place. In most places, radio spectrum is allocated by strict licensing to different wireless applications in a way that is highly inefficient. Some spectrum frequencies are highly congested while some are idle. A growing number of countries have thus begun to allow users to get access to certain spectrum frequencies on a license-exempt basis, which is similar to Wi-Fi.

Given the rapid growth of worldwide demand for radio spectrum in wireless communication, TV white spaces, the unused portions of TV spectrum, offer the first and promising opportunity to provide additional spectrum for users. Microsoft Research published a new paper in partnership with The Chinese University of Hong Kong proposing a new system for improving indoor use of wireless technologies in cities.

Based on measurements taken from more than 30 diverse locations in a typical city, the study found that more than 50 to 70 percent of spectrum in the TV band alone goes unused. These vacant frequencies are called TV white spaces, or just white spaces. White spaces are the first frontier for forward-thinking regulators throughout the world, who are increasingly recognizing just how inefficient our current methods of spectrum allocation are, and moving toward new approaches.

In order to make use of unlicensed access to white spaces, wireless devices must somehow detect what frequencies are available for use in their location using dynamic spectrum access (DSA) technologies. While spectrum sensing technology is available, it can be expensive and difficult to implement. It also has to tread a very fine line between identifying as much unused spectrum as possible and avoiding interference with other users.

Therefore, most regulation on white spaces has turned to geo-location databases to direct traffic in license-exempt bands of spectrum While they work well to limit interference and don’t pose the same cost issues as sensors, geo-location databases provide a very conservative appraisal of which frequencies are unused. Particularly in cities where the wireless environment is complex, there are often far more white spaces available for use than a geo-location database might indicate. There is also the added complication of indoor versus outdoor use.

Even though 70 percent of demand for spectrum is in indoor environments and significantly more spectrum is left unused indoors, the majority of trials and studies of white space technology have focused on outdoor applications. What’s more, the study found that white spaces are often available in bigger chunks indoors, making them easier to use for high-bandwidth communication. “TV white spaces have the potential to provide a significant amount of additional spectrum that is needed for wireless applications,” said Ranveer Chandra, senior researcher at Microsoft.

“Although 70 per cent of the demand for spectrum comes from indoor environment and significantly more TV band spectrum is left unused indoors than outdoors, most trials and studies of white spaces done before have focused on outdoor scenarios.”

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. Here comes in WISER- (White-space Indoor Spectrum Enhancer). WISER uses spectrum sensing technology but – unlike previous approaches – optimizes the position of a limited number of sensors. This control costs and maximizes effectiveness, without losing out on accuracy. WISER improves indoor access to radio frequency spectrum that can improve reception performance of wireless technologies.

WISER was developed by researchers at the Chinese University of Hong Kong (CUHK) and Microsoft Corp. The researchers claim WISER can identify 30 percent to 50 percent more white space spectrum than alternative methods, most of which have been designed for outdoor use. The project focused on analyzing indoor white spaces, a phrase for vacant VHF and UHF TV channels. They used algorithms and software, coupled with RF sensors, to create a system to identify and track this indoor spectrum. Some of that spectrum is congested, while some frequencies are idle.

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.”

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. According to the research, some of the WISER algorithms deal with profiling the building, others with where to place the sensors. Although 70 percent of the demand for spectrum comes from the indoor environment and significantly more TV band spectrum is left unused indoors than outdoors, most trials and studies of white spaces done before have focused on outdoor scenarios.”

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 analyzer with a low noise floor — there are a few available in the market,” Chandra, one of the authors of the research paper 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, Ranveer Chandra, says. WISER itself could be embedded in future Wi-Fi access points, “which would make it very simple to deploy,” Chandra added. 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.

While there are several interesting directions that could be taken in future research on WISER, this development is an important step forward toward better spectrum sensing techniques and, ultimately, greater spectrum abundance and improved wireless communication. With a combination of technical innovation such as the development of WISER, and the regulatory progress on dynamic spectrum access that we’re seeing throughout the world, it’s a bright future for wireless technology. Microsoft Research has been working with CUHK academics since 2010 on these issues, including research aimed at showing how to use this extra spectrum.

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 unutilized TV channels indoors, which are able to support high bandwidth communication.” 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.