New technology promises broadband in sub-GHz

A new technology holds the promise of cheaper broadband wireless base stations and of phones with talk-time measured in days, not hours. xG Technologies says that its xMax, which will be publicaly demonstrated in September, will deliver 40 Mbps over 15 miles, using less than one watt of power at sub-GHz frequencies. The key: Such low frequencies have so far been considered unsuitable for broadband. "Our unique benefit is we can use narrow channels [at low frequencies] where you get better propagation characteristics," says Joe Bobier, the technology's inventor and president of xG Technologies. "If you can use these frequencies, yet still be broadband, then you compare favourably with fixed wireless technologies that are doing things in the microwave area where you have line of sight issues."

The main advantage of sub-GHz frequencies is that they penetrate obstacles well (thus obviating the line-of-sight requirement of higher frequencies). The disadvantage of these low frequencies is that governing bodies have sliced these frequecnies into such small narrowband channels that it is not possible to cobble  sufficient spectrum to transmit broadband data in this spectrum by conventional means. Shannon's Law requires a wider band of spectrum to carry more data.

So what does xMax do? It uses one narrowband channel, but sends more data than could possibly be fitted into that channel. It does so without violating the laws of physics by making sure that the channel does not carry the payload, but is instead used only to coordinate and synchronise the sender and receiver. The information is then transmitted in a wideband signal (or Flash Signal, as xG calls it). The data is thus sent out-of-band but at power levels which are lower than those permitted for unintentional out-of-band signals by normal transmitters. This is the beauty in xMax's scheme: The levels of out-of-band emissions are highly regulated, but xMax's emissions are even lower than the allowed emissions.

The company chose the name "xMax" for a reason -- its intention is for the technology to be cosidered a "beyond-WiMax" technology. xG hopes to propose it as a new PHY for WiMax. The company says that in September it will have a base station between Fort Lauderdale and Miami covering a radius of 15 miles with a shared bandwidth of 40 Mbps, all provided by less than one watt, using the unlicensed 900MHz ISM band. The first demonstration will be omnidirectional, but greater throughout would be possible with a sectorised station or using a wider noise-level band. xG will be formally launched in November and then presented at the 3GSM show in Barcelona next Spring. I cannot wait for September. I cannot wait for Barcelona.

For more on xMax technology:
- see Peter Judge's Techworld's discussion
- read Jay Lyman's Technewsworld report 
- check out this P2Pnet story

BACKGROUND: Shannon's Law, fromulated by mathemetician Claude Shannon, defines the theoretical maximum rate at which error-free digits can be transmitted over a bandwidth-limited channel in the presence of noise. It is usually expressed in the form C = W log2(1+S/N), where C is the channel capacity in bits per second, W is the bandwidth in hertz, and S/N is the signal-to-noise ratio. Note that error-correction codes may improve the communications performance relative to uncoded transmission, but no practical error correction coding system yet exists which can closely approach the theoretical performance limit given by Shannon's Law. Some systems, using sophisticated encoding and decoding, can approach half of the so-called Shannon limit for a channel having fixed bandwidth and signal-to-noise ratio.

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