xMax sparks low power wireless revolution
The kimono opens, but the black box is still under wraps...
By Peter Judge | Published: 01:39, 04 November 2005
A little-known start-up has demonstrated wireless broadband 1000 times more efficient than WiMax – and claims the technique could also make wireless LANs that will run for years on watch batteries.
xG Technology, based in Sarasota, Florida, used a transmitter not unlike a cordless phone base station, operating in the unlicensed - and crowded - 900MHz band, to send a 3.7Mbit/s data signal to a radius of 18 miles across the suburbs of Miami, using 50mW and an omnidirectional antenna.
The data rate and specifications don't exactly match xG's original plan, which we reported in July, as the demo uses a narrower band and a lower data rate. However, with an omnidirectional antenna, the demonstration appears to have transmitted a signal at a power density other schemes would find impossible. The company's xMax technology uses a novel modulation scheme, described at xG's own site.
Our analysis of the problems xMax may still face, and our attempts to kick the tyres of the demo are here.
"xMax is unconventional," said Stuart Schwartz, professor of electrical engineering at Princeton Universithy, who has scrutinised xG's demonstration set-up, speaking at the xMax demonstration. "It is clever and innovative, but it is not magic. It uses single cycle modulation, and needs much less power than other technologies."
Single-cycle modulation is the invention of xG's chief technology officer Joe Bobier, with backing from Mooers Branton, a merchant bank, whose founder Rick Mooers also serves as xG's chief executive. The modulation scheme alters the frequency of individual cycles of the carrier wave, which has the effect of introducing very low power side-bands to the signal.
One would expect effects this small to be very hard to measure, but Bobier has invented a receiving antenna, which is cheap to build but (presumably using some kind of subtraction) filters out all signals that don't use the xMax modulation. At this stage, with patents pending (the patents are here), the technology behind this is very much under wraps, and was literally present at the demonstration in a "black box".
At the demonstration with other reporters, we were able to verify that the signals were being sent wirelessly, and checked the distance by GPS, but had to take the 50mW base station - and its omnidirectional antenna - on trust, since it was at the top of an 850ft mast. The demonstration will be repeated for the US press next week.
The system carried 7.4 Mbit/s per MHz per Watt, said Professor Schwartz. By comparison, GSM would have around 0.0058, and CDMA/EV-DO about 0.0085 Mbit/s per MHz per Watt.
xG has promised to have commercial systems ready by the second half of next year, which can be sold off-the-shelf. "We're in discussion with channel partners, to make a shrinkwrapped package that would allow anyone to set up as a wireless ISP," said Mooers."In the longer term we see it rolling out to chips that will be in millions of different units."
The same technology could also make licensed broadband services more efficient, said Professor Schwartz: "WiMax would require 90 base stations to cover the same area."
As a modulation technique, xMax can be used in many different ways, but xG has seized on wireless broadband, because of the current level of activity there. "What Joe built is so broad, we had to focus on one thing," said Mooers.
It can be applied to different parts of the radio spectrum, and can be used with or without a synchronisation beacon. For the highest bandwidth it could be used across a broad band of spectrum but using power levels below the signal level at which a licence would be required. Alternatively, it could be restricted - as with this example, to a narrower band, where no licence is required.
"I have also seen it demonstrated as a personal-area network, giving 2Mbit/s over 40ft, using 3 nanoWatts," said Schwartz. With that level of power, a Wi-Fi like network could operate for years on small batteries, he said.
Beyond that, it could even modulate signals carried over copper. Bobier suggested that this might even allow DSL technologies that give much higher data rates and greater range.
Before any of this happens, more demonstrations are needed, to show the system is robust against interference and multipath, and can operate in an area more crowded than 18 miles of swamp. It will also need to be approved by the FCC and other regulators round the world.
