Breaking on through to 4G

...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.