White-Space Networking Goes Live
by George Lawton
Under an experimental license from the US Federal Communications Commission (FCC), Spectrum Bridge launched the first US wireless broadband network using empty airwaves ("white spaces") in the broadcast television spectrum, which emerged in the transition to digital TV. The white-space network is providing broadband Internet services to the unincorporated community of Claudville, Virginia, and could pave the way for broader adoption of the technology — across empty TV spectrum initially and a much wider range eventually.
The TDF Foundation funded the network, which uses software and webcams from Microsoft, PCs from Dell, and Spectrum Bridge networking equipment. It was designed to show the superior range and penetration of TV's lower frequencies compared to Wi-Fi, noted Peter Stanforth, Spectrum Bridge CTO. Wi-Fi operates in the much higher ISM (industry, science, medicine) radio spectrum (2400–2450 MHz). TV's lower frequency (76–600 MHz) offers substantially better range and penetration, which Stanforth said makes it ideal for providing last-mile Internet connectivity in rural areas. It also offers good penetration between walls and floors for local networking.
TDF had previously built a community networking center in Claudville with a high-speed Internet connection. In the white-space deployment, Spectrum Bridge provided the radios to link three remote base stations, each with local Wi-Fi access.
Unlicensing White Space
Michael Calabrese, vice president of the New American Foundation public policy institute, first proposed the idea of white space for unlicensed communication in 2001. Proponents say it will help open the airwaves for better broadband connectivity and provide services that would compete with those of cable TV and telephone companies, thereby lowering pricing and spurring innovation.
Opponents — including the National Associations of Broadcasters (NAB) and wireless-microphone vendor Shure — are concerned that white-space networking could impair broadcasters and wireless microphone users that are already using this spectrum.
NAB spokesperson Kristopher Jones said, "The NAB has had concerns about white-space networking from the outset. Our business is based on delivering an interference-free pristine picture. If a consumer is not getting a good picture, they will change the channel." Jones said the difficulty is that the white-space devices would be unlicensed. "When you have a device that's unlicensed and unmonitored," he explained, "if it does cause problems, it's difficult if not impossible to track down the offending devices."
The FCC is in the final stages of drafting plans to manage the use of white space. It's looking at two components for eliminating interference: sensing chips that automatically cut off when they detect a TV or microphone transmission and a database for looking up empty channels in an area before transmitting.
The FCC is proposing to limit the maximum power of white space devices to 4 watts, which is equivalent to Wi-Fi. But the propagation characteristics of the lower frequencies provides about four times the range of the ISM band, said Stanforth.
Each TV channel occupies 6 MHz of spectrum, but the channel space is not contiguous. There are many large blocks of channels with substantial gaps between them. For example, channels 5 and 6 occupy the 76–88 MHz range, followed by a 66 MHz gap before channels 7–13 in the 174–216 MHz range.
Rural areas have many vacancies, whereas cities have only a few. In total, TV has far less available white space than the ISM band, particularly in crowded markets.
Dawn of the Frequency API
One challenge to white-space networking has been determining who's using what spectrum in a particular location. The FCC maintains a database but not in a form that makes it easy to find information in real time.
Spectrum Bridge developed database technology that adds a metadata layer to the FCC database to simplify the search for available spectrum in an area. The company has also developed an API that lets a device automatically query the database. This combination can help a receiver determine whether operating in a particular frequency range is liable to interrupt a local TV station.
Under the FCC's proposal, when a radio is installed, it must automatically determine its location using GPS or have a technician do so. The white-space device would use the radio location and database to identify free channels. The devices would check the database at least once every 24 hours. In some cases, the devices could be forced to recheck the database whenever another permitted user begins to use the frequency.
"Once you build this kind of white-space infrastructure," Stanforth said, "and you have a radio with a dynamic range that can query a database, there's no reason that you would be limited to TV white space. There's quite a lot of other spectrum defined as secondary market spectrum, some of which is free today."
A bill introduced in the US Congress in July, the Radio Spectrum Inventory Act, would open white space across the entire unused spectrum for unlicensed operation.
George Lawton is a freelance technology writer based in Monte Rio, California. Contact him at glawton@glawton.com.
