This article is reproduced with the permission of New Scientist for exclusive use by Nova users.

Long-range Wi-Fi threat to satellite communications
15 March 2008
From New Scientist Print Edition.
Jeff Hecht

WiMAX, a long-range version of Wi-Fi, has been billed as one of the best ways to bring broadband internet connections to rural areas. But the system may have a critical flaw. According to tests by a trade body representing the satellite industry, the radio frequencies WiMAX uses will interfere with satellite communications in some of the very locations where it could bring the most benefit.

WiMAX uses a static base-station antenna to distribute digital data at speeds of up to 40 megabits per second. This is especially useful in areas where cable, DSL broadband and even dial-up might not exist, like parts of the developing world.

But last week the Florida-based Satellite Users Interference Reduction Group (SUIRG) revealed test results showing that WiMAX antennas can disrupt satellite reception up to 20 kilometres away from a base station. They say they have demonstrated conclusively that WiMAX poses a significant interference threat to satellite signals.

At issue is the use of microwave frequencies between 3.4 and 3.6 gigahertz, known as the "extended C band". Long used to transmit signals from geosynchronous satellites to Earth stations, these frequencies are also used for data and voice communications on the ground.

The allocation of frequencies takes place every four years at the World Radiocommunications Conference (WRC). WiMAX was allowed to use the extended C band at the 2007 conference in Geneva.

Although WiMAX's use can still be vetoed at a national level, the WRC decision was ill-informed, says Bob Ames of the SUIRG. "The big concern we have is for South America," he adds. While developed countries have moved their satellite systems to frequencies above 10 gigahertz, tropical and developing countries still use the extended C band because it can penetrate heavy tropical rain, and because the equipment is cheap. Lacking cable networks, they rely on satellites for phone calls, internet access and TV reception.

The WiMAX Forum, the trade body promoting WiMAX, says satellite users can avoid interference by encircling dishes with metal grilles.

Ames says the satellite industry will lobby individual countries to get them to block WiMAX or reduce its impact. They want to avoid incidents like that in Bolivia, when engineers began testing WiMAX during the 2006 soccer World Cup.

"They turned it on and about 80 per cent of Bolivians stopped getting World Cup TV pictures, so they had to stop the test," he says.

From issue 2647 of New Scientist magazine, 15 March 2008, page 28

For the latest from New Scientiist visit www.newscientist.com