Breaking on through to 4G
Once every two or three years, a new company appears and makes extraordinary claims about fundamentally new technology. Often, these prove to be more hype than substance; sometimes, however, an idea has the potential to change the game.
This years' contender is xG Technology, a Florida company with a wireless data system called xMax. This will outperform any existing radio technology, says the company, providing bandwidth and range that substantially outclass 3G, 802.11x and ultrawideband while using far less power and causing far less interference. So far, the company has been reticent to describe exactly how this system works, but ZDNet UK has spent some time in conversation with Chris Whiteley, the programme manager for xMax, and Joe Bobier, president and chief executive of xG Technology and the inventor behind the idea. Although we'll have to wait for three more months before the company goes properly public, Whiteley and Bobier discussed the basic idea behind xMax and lent some credence to the possibility that it may indeed work as advertised.
xMax's basic advance lies in improving the link between transmitter and receiver, and to understand that it's important to return to the basics of wireless. One of the things that everyone knows about radio technology is that if you increase the power of the transmitter, you get more range. Greater range also equates to greater bandwidth — if you're listening to a weak station on FM the signal is mono, noisy and unsatisfactory. A much stronger signal comes over as high quality stereo with little or no noise. Likewise, as you walk away from a Wi-Fi hotspot, you don't lose connectivity instantly — you get a slower, more uneven connection first.
Like most things that everyone knows, this is wrong. Or rather, it disguises the true physics of radio. The amount of data you can get through a channel doesn't depend on the power as such: it actually depends on the amount of signal compared to the amount of noise. The signal-to-noise ratio (SNR) is the absolute mathematical limit to data transfer rates. Push more signal through the channel by upping the power — or having a more focussed antenna, or widening the bandwidth — and you can send more data. Reducing the noise will also increase the amount of data that can be sent, without having to boost the signal in any way. "We have nine patents," said Whitelely, "The demodulator is the key technology. All radio systems have to have gain. All others are brute force, we're finesse."
There are two main sorts of noise which exercise the minds of radio engineers. The first is thermal noise, which is caused by electrons moving about randomly in antennas and electronic components inside the receiver. This is what you see if you unplug the antenna from your television: each speckle on the screen is caused by the coincidental judder of a small group of electrons. The hotter the system, the more the electrons move and the higher the noise level — which is why radio astronomers dunk their receivers in liquid nitrogen and other cryogenic coolants. In general, though, there's not much you can do about thermal noise beyond designing with low-noise components.
The second and more interesting source of noise is everybody else. Every transmitter produces noise outside its specified signal. Regulations limit how much noise leaks out and how far away from the central frequency it is, but cannot stop it happening altogether. As a result, in urban environments the noise floor — the signal level without any particular transmission — can be much higher than the thermal noise. This substantially limits the amount of data transferable for a given signal power.
The key to xG's claim lies in their receiver technology, which will reject the noise produced by other transmitters — anything that looks like an ordinary carrier is nulled out — while detecting the pulsed wavelets that xG uses to communicate. "We're taking signals that are not single-cycle modulated and throwing them away," said Whiteley, "and we're operating sub noise-floor. We won't interfere with anyone, nor they with us. The SNR is to our advantage." Any signal that isn't a single cycle long is removed: in other words, most of the artificial noise in the channel no longer exists. xG says that this can give their system as much as 49dB advantage — the equivalent of being able to detect a signal tens of thousands of times weaker than other systems can manage. xG isn't saying how this aspect of the receiver works, only that it's simple to implement with standard analogue radio techniques and that in practical tests it lives up to the mathematical models they've produced.
Assuming this advantage is real, it can be spent in a number of ways: it means the system can transmit weak signals that don't interfere with existing users, that a network can be set up with far fewer transmitters per square kilometre than existing technologies require, that a handset can be set up to use much lower power transmissions than before...
...or that much higher bandwidths per-Watt can be achieved. Alternatively, aspects of all of the above can be mixed and matched. In tests over the air in Florida, xG says that a 900MHz transmitter running at 150mW gave equivalent performance to a standard system at 50W, achieving a range of around 50km from an antenna at 260m.
The transmission side of xG's system takes ideas from a number of places. It encodes information onto tiny pulses of radio energy in a similar fashion to some kinds of Ultrawideband (UWB), but includes a timing signal on a separate frequency that makes the receiver's job a lot easier. This and other differences means that xG encodes one bit per pulse, whereas UWB encodes one bit across hundreds or thousands of pulses, giving the new system a much greater data density per MHz of bandwidth used.
In this mode, the central timing signal is strong enough to be picked up by conventional radios but is narrow enough to fit into a 5 or 10 kHz channel previously used for voice or paging signals. The data carrying component of the signal is much wider — potentially tens or hundreds of megahertz — but is below the noise floor as seen by ordinary radios. This underlay principle means that an xG transmission can co-exist in a busy band with many other services without interference. Multiple xG transmissions can share the same frequency by having different timing signals, which corresponds to having different spreading functions in spread spectrum systems such as CDMA. Optionally, the xG system can modify one or both the timing and data carrying components to better work alongside existing systems.
One of the biggest challenges facing inventors of novel radio systems is that of regulation. National and international regulators have already comprehensively divided out the band to existing systems run by incumbents — leaving little room for innovation. Newcomers must also demonstrate that they won't cause interference — particularly with very-wide-band systems that make use of large blocks of frequencies in use by others. Because of this, xG is limiting its activities to the US for the foreseeable future: it thinks that its system is already legal under a creative interpretation of the American rules but is in talks with the FCC to get an explicit blessing. And Bobier says that xMax has an advantage over UWB: "One of the potential issues of UWB as a personal area network is that you can't control the transmitter density. You could have hundreds or thousands of transmitters in a square kilometre, possibly interfering with legacy systems even though each individual transmitter is low power. Ours transmit on much lower power levels and use time-division multiple access (TDMA). One user device transmits at a time, and one base station. Much more controlled, much less noisy."
xG says that although it was expecting fixed radio networks to be the first to use its technology, it has seen surprisingly strong interest from mobile operators and equipment manufacturers. Indeed, the company claims that it is the only serious contender for 4G. In the words of Joe Bobier: "Mobile carriers are looking for non-standard solutions, because of the perceived threat of fixed carriers getting mobile before mobile gets high data rates. We told them that we have a way to get an increased service, battery friendly, lower-cost network by increasing range and reducing equipment required. There's nothing else on the horizon that leads to the 4G path. "
The formal unveiling of the system is planned for the beginning of November, when the company is also going to announce a partnership with an American telco, and the first products are expected in mid-2006. xG doesn't plan to make much itself — it will build demonstration units and work with partners to integrate its physical-layer intellectual property with their higher-level protocols. Until that unveiling, it's not possible to say how well the claims match reality nor how the technology will adapt to real world operating conditions — two caveats that have sunk many likely contenders in the past. But xG is making all the right noises: it remains to be seen if their innovative receiver technology can pull off the same feat.