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.