TV White-Space Networks Get Smart

27 July 2011—Many technorati still remember what happened to Steve Jobs a year ago. During a demo of the then new iPhone 4, the usually smooth showman was thwarted on stage by his company’s Wi-Fi network, which was clogged by the data demands of the reporters and bloggers in the room.

The failed demo was one of the more visible examples of the need for more wireless bandwidth—a sort of "super Wi-Fi," or as Google founder Larry Page likes to call it, "Wi-Fi on steroids." Such an offering could arrive as a result of the U.S. Federal Communications Commission’s (FCC) release of rules for commercial use of unlicensed television spectrum last year. Several telecom regulatory agencies in other countries plan to follow suit: Brazilian telecommunications regulator Anatel; the Telecom Regulatory Authority of India (TRAI); the State Administration of Radio, Film, and Television in China; and the Infocomm Development Authority of Singapore have all shown interest in freeing up their TV spectrum for use in wireless broadband services.

As television broadcasting moves from analog to digital, it frees up a lot of spectrum. In analog transmission, each TV channel uses a bandwidth slice of 6 megahertz. But in digital transmission, this very slice can pack up to four channels. The freed-up spectrum, along with other unused spectrum in the 50–700 MHz television band, is called digital TV white space. Use of the 50–700 MHz band could not only decongest the existing 3G and new 4G networks but also provide better coverage in rural areas, because this spectrum has superior propagation properties to bands such as Wi-Fi’s 2.4 gigahertz and 5 GHz.

But how do you keep the TV space from getting just as congested as the 2.4-GHz radio band does now? By being smart about how you use the bandwidth.

At Alcatel-Lucent’s Bell Labs (particularly at its facilities in Bangalore, India; New Jersey; and Stuttgart, Germany), researchers have developed a white-space network-access device that would work in the enterprise setting (for example, in an auditorium at Apple). They have designed a wireless local-area network (LAN) system, in which a central controller in the network efficiently allocates spectrum based on the demands at specific access points.

The reason the network collapsed when Jobs was showing off his gadget is that the Wi-Fi network architecture does not take into consideration the fact that many devices on a network operate at different data rates. Given the current explosion in wireless broadband demand, says Bell Labs India researcher Vikram Srinivasan, the MAC (Media Access Control) protocol of a network, which decides who gets how much bandwidth, has to be intelligent and agile enough to provide fair access to receivers regardless of their distance from the transmitter.

Srinivasan’s group has designed a multiple-radio network architecture, in which the base-station radios have the ability to adjust their own bandwidth, tune their center frequency (lower frequencies propagate much farther than higher frequencies), and throttle their power. In the traditional system, these elements were determined by the communications standards.

The unique nature of the spectrum makes designing networking algorithms and protocols a challenge. There are several digital TV white spaces in the band, and the demands of access points vary over time. "Since in a TV band you don’t get continuous contiguous spectrum—a typical white-space chunk has 6 MHz—you need at least two radios to tune to different bandwidths. Our algorithms decide which radio will choose which bandwidth," says Srinivasan.

In a demo session in Bangalore, as Big Buck Bunny starts streaming on a computer, many others are allowed to log in to the network. Soon, the movie starts blurring. Four people each plug in a "white box," as they’re calling it, and the distortion disappears. In its final form, the 20- by 20-centimeter white box will be shrunk to either a USB stick or a network card, say Bell Labs researchers.

Bell Labs India head Viswanath Poosala says the product is ready for commercialization, and the company is looking at it in two ways: as a networking product and as an enabler of networked "virtualized services," in which overburdened 3G or LTE networks could offload their traffic to white-space spectrum.

"Rural Internet connectivity is a big application in India," says Poosala, so there it’s likely to be used as an independent network. But in developed countries, the second application would be used "as a backhaul to the cellular network," he says.

The hardware has also proved challenging. A big problem is that some power leaks into the parts of the spectrum adjacent to the channel, causing interference. Researchers say the new FCC mandate is "sharp," meaning that little power can be allowed to leak. "It can be addressed by, say, using power amplifiers, which are traditionally used in cellular base stations, but it adds to the cost," says Bell Labs researcher Supratim Deb.

Meeting the emission requirement is challenging but doable, says Neeraj Srivastava, a vice president at Spectrum Bridge, in Lake Mary, Fla. In January the FCC gave Spectrum Bridge the task of ensuring that white-space networks do not cause interference with licensed television broadcasts and other protected users. "There will be some added cost, but costs are highly dependent on achieving economies of scale," Srivastava says.

Spectrum Bridge, along with Microsoft, Dell, Google, and others, have deployed operational networks using this spectrum in some U.S. cities, mainly for research and for smart-grid applications, the latter of which have low-bandwidth demands.

In the first residential trial of a white-space network, Rice University, in Houston, began testing Bell Labs’ white-box hardware at its campus in April. Edward Knightly, a professor of electrical and computer engineering at Rice, is in charge of the $1.8 million effort. The Houston network uses its own lab’s software with Bell Labs’ frequency translators. According to Knightly, the group is getting ready to deploy its second residential hotspot. "We’re just weeks away from deployment of ’blue scale’ to a home with white-space access," he says.

Knightly says the current state of digital TV white-space technology is similar to that of 2.4-GHz technology during the early 1990s, when that band opened up. "There’s certainly a sense that things would take off. Whether it’ll be a niche product or will take off at a large scale, where every device will have [white-space access capability], is not quite clear yet," he says.

Such white-space networks are the first manifestation of "opportunistic" and "cognitive networking," says Victor Bahl, director of the Mobile Computing Research Center at Microsoft Research, in Redmond, Wash. "If we get this right, we can begin looking at additional frequency bands and push further on developing the concept of primary and secondary users. We can move towards a world where spectrum scarcity is no longer an issue."