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February 04, 2006

Comments

a friend

I have been following this technology for months in many forums and webs, it is a clever idea to achieve a communication channel putting low level spreaded spectrum signals around a big carrier. But nobody say the real truth, that is, if you want to carry out another not interfering channel, the distance between carriers will be at least the used bandwidth not the carrier bandwidth, so reducing the efficiency of a feasible cellular system which needs, at least, three frequencies to be operative. In fact, thinking about spectral efficiency (Bits divided by the real ocupied bandwidth, not the 3dB carrier bandwidth) it is just like any other spread spectrum system, very few bits/s/Hz. Moreover, They say a DSL application should improve actual systems. I suspect that it will be imposible because this system should maintain power transmission masks (at most -40 dBm/Hz) in the carrier and then the spreaded spectrum information should be so power lowered and distorted by the cable channel that the system would not improve actual systems at all. Nevertheless I think there are many applications for it.

brough

Dear friend, Thanks for the comment. I don't know if xG is for real or not, but the idea of spread spectrum is real and a little different than comes across in your comment. Forget about xG's carrier for a moment and just talk spread spectrum, e.g. CDMA or UWB.

It's certainly true with Frequency Division Multiple Access (FDMA) that adjacent carriers for separate communications channels can interfere. However with Code Division Multiple Access (CDMA) and Ultra-Wide Band (UWB) all of the communications channels share the same spectrum. They are distinguished by coded modulation while sharing the same spectrum.

The metric "bps per Hz" is appropriate for FDMA where communications channels are separated by frequency, i.e. in Hz. It's not a useful metric for spread spectrum where communications channels share the same frequencies.

I was speculating that xG transmits the useful information using spread spectrum and that their carrier is only present to simplify clock-recovery at the receiver (thus reducing the cost of the receiver). IF this is true, i.e. if they are using Ultra Wide Band or another form of spread spectrum modulation across the entire spectrum below 1 GHz, then they have at least one advantage over FCC-approved UWB schemes -- radio waves below 1GHz carry much further than those above 3 GHz. The question then would be how much total power (across many, many MHz)?

I don't know if xG is for real, but I like the idea that by licensing 5 KHz of spectrum, they get formal permission to spread low-level signals through the rest of the spectrum. That beats the FCC-approved UWB which is only allowed to spread it's low-level signals between 3 GHz and 10 GHz.

Thanks,
- rbt

saal

Sir, Pls give me an explanation about "single cycle modultion
technique" which is used in xMax technology.

brough

The term single-cycle modulation appears to have been coined by Joe Bobier of xG Technologies. As I understand it, from reading his patent applications, he is manipulating individual cycles of his carrier signal. For example, one patent application talks about flipping the polarity of a single cycle of the carrier signal, i.e. flipping the phase by 180 degrees. This is done to individual, perhaps widely spaced, cycles of the carrier wave in response to changes in the modulating signal, i.e. the information signal.

Dan Plesse

As far as I know xG Technologies, Inc. is traded on the London Stock Exchange! xG Technologies, Inc. is traded on the London Stock Exchange's Alternative Investment Market (AIM), a global market for growing companies. The stock is listed under the symbol XGT.L I am hoping to report news about them via opticalalert.com my newsletter. XGT.L is the ticker for yahoo.

http://finance.yahoo.com/q?s=XGT.L

brough

Yes, they went public on the London AIM on November 20, 2006, i.e. subsequent to my original post, but possibly with a change in business strategy as well. I haven't followed them in detail, especially as there's been nothing new about their single cycle modulation technology, but people interested in their history might find these URLs relevant:
http://disruptivewireless.blogspot.com/2006/03/vowlan-vo3g-yesterdays-news-try-new.html
http://eurotelcoblog.blogspot.com/2006/09/xg-ipo-cojones-de-acero_27.html

an engineer

There is no free lunch. Transmitting a pilot tone in a band far away in frequency from the modulation isn't very useful, because the pilot will not be phase coherent with the modulation - fading channels won't allow it (it's beyond the "coherence bandwidth"). While you might be able to make it work when the terminals are fixed, it sounds impossible in a mobile link. In OFDM, you use pilot tones for precisely the same purpose, and they are much more useful because they are embedded in the same channel with the data tones - which is how WiMax works. Also, in a bi-directional link, the receiver will also have to transmit a pilot, because the transmitter won't be able to determine the phase impairments to it's own pilot over the path.
Sending data using xMax sounds like an ok system for one-way data broadcast to a fixed termial, but won't be so great for bi-directional or mobile. It may have some uses, but isn't going to become "4G."

Dan Plesse of Opticalalert News

xG posted a few more data items
January 25, 2007 xG Signs Agreement with UK-based Telecommunications Company
January 26, 2007 xG Confirms Positive Lab Beta Trials Prior to Imminent Field Testing

Phil Karn

I've written up some initial analyses of xG's claims on my website. Summary: the inventor clearly does not understand the most basic principles of communications theory. Investors should be *very* wary. See:

www.ka9q.net/xmax.html
www.ka9q.net/tristate.html

mike burenstein

xgt it sounds like a good deal to me.

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