'Whispering' Wireless Internet

Zondar writes "MSNBC is reporting about a new radio filtering technology allows an ISP to use already-occupied frequencies to transmit and receive data. From the article: 'xMax, the latest innovation in broadband communications, is a very quiet radio system that uses radio channels already filled up with noisy pager or TV signals ...' and 'xMax is trespassing radio frequencies, although trespassing is not the right word, because we're allowed to transmit a signal if it doesn't interfere with other, stronger signals...' Too good to be true? Sounds like it would just raise the noise floor, to me."

Comments

Spread-spectrum

[DrLex]

This is just some kind of spread-spectrum technology, nothing new... The signal consists of pseudo-noise. If the receiver knows the key to this pseudo-noise and can synchronize to it, he can decipher the message. This idea and this technology have been around for years.

Re:Spread-spectrum

[pete-classic]

This is pretty much off topic, but the inventor of spred-spectrum was hot.

-Peter

Re:FCC

[bluGill]

Or will the TV stations roll out internet service themselves, since they have the license?

Will we become invisible to ET SETI searchers?

[G4from128k]

If everyone switches to wideband, low-power, densely-coded, mesh-network transmissions, then I suspect that the Earth will become virtually invisible to extraterrestrials who try to use SETI-style, pattern-in-RF methods. With nobody broadcasting at high power on a simple-coded narrow-band carrier, the RF emissions of the planet will become indistinguishable from noise.

I wonder if each civilization goes through a short RF-detectability phase before they so densely pack the spectrum with so many emitters that they become invisible, too.

Re:Will we become invisible to ET SETI searchers?

[wowbagger]

The idea that the Earth's RF emissions are detectable from any distance whatsoever is WRONG.

I've heard people say "But the Earth radiates as much RF as a star" - BULLSHIT. The Earth doesn't even radiate as much as Jupiter. The only thing is that the Earth's radiation is in narrower bandwidths and thus more detectable.

However, ignoring losses due to the inter-[planetary|stellar] medium, the signal strength of ANY signal goes down as the square of the distance (even highly collimated signals still diverge, and thus quadruple their area as distance doubles once you get out of the near-field effects).

Do the math: Assume a gigawatt transmitter. Assume that this transmitter is collimated to the point that at 100,000,000 kilometers the beam is 1 kilometer wide, and treat the transmitter as a point source. (BTW - that is an power density of just under 1.3 kilowatts per square meter - about the same as the total solar power at the Earth's surface).

At just ONE light-year the signal is just over nine billonths as strong - call it 10 microwatts to keep it to about 2 significant digits. At 4 light years, it is down to less than a microwatt per square meter. At 100 light years, it is one nanowatt per square meter.

And remember, we started with an INCREDIBLY collimated, INCREDIBLY powerful emission - normal transmissions are a thousandth this powerful, and a million times more diffuse.

The SETI project is NOT looking for alien TV or broadcast radio. SETI is looking for a Mount Arecibo class radio telescope transmitting a narrow bandwidth high power signal designed especially for a SETI system to see.

Re:I'm surprised

[Phil Karn]

It certainly seems obvious, but receiver sensing doesn't really work. You can't rely on the absence of a signal at the transmitter to ensure that you won't interfere with someone if you transmit, and conversely you aren't guaranteed to interfere with a signal you can hear. This is the problem with plain CSMA on radio channels.

A better approach is to have each receiver (not transmitter) indicate where and when it is listening so that other transmitters can avoid interfering with it. Busy Tone Multiple Access (BTMA), proposed way back in the 1970s, is probably the earliest such scheme. The MACA (Multiple Access with Collision Avoidance) scheme I invented for amateur packet radio circa 1990 that found its way (with enhancements by others) into 802.11 is basically time-division BTMA on a single channel.

A few years after I proposed MACA, I also suggested a more general purpose dynamic frequency coordination scheme for the amateur service based on packet radio. It was inspired by the backlash to the proposals to broaden the use of spread spectrum on the ham bands. You'd have a coordination channel on which receivers would broadcast the frequencies and times that they were listening so that nearby transmitters could avoid interfering with them. You could get fancy and have each transmitter send a test transmission to see if a receiver is bothered by it, and if not then that transmitter would not have to defer to that receiver.

Naturally this never went anywhere because the vast majority of hams are not really interested in any kind of technical innovation. They didn't want to have to do anything new just to continue using the frequencies they've always used, which they tend to treat as their own personal property. The spread spectrum proposal was eviscerated, and I let the idea drop. I wouldn't be surprised if the xG guys are now trying to patent my ideas. Wouldn't be the first time companies have tried to claim innovations placed into the public domain by hams as their own.

Re:FCC

[dingleberrie]

No, it's not UWB. Unless PCs and radios are the same because they both have displays, make sound, and draw power.

Typical transmissions use a center or carrier frequency and have what's called sideband noise, which is a fairly strong signal around the carrier frequency. This sideband is information needed as part of the primary transmission, but it is noise to its neighboring frequencies. This makes your 96.6 FM station really have an allocation of 96.5 to 96.7 MHz. The tuner locks into the carrier frequency and then gathers the information from the sidebands.

Ultra wide band distributes all of its information across several frequencies (generally near 1 GHZ of bandwidth with center frequencies varying from 3 to 10 GHz) without providing any RF power above the FCC limits for stray radiation, even at the center frequency.

xMax, however, is designed for sub-GHz channels. It places a significant amount of power on the center/carrier frequency like traditional transmissions. In contrast to traditional transmissions, however, xMax spreads the sideband information over a large bandwidth and thus the power amplitude per frequency is below the FCC mandated power limits for stray radiation (like UWB).

The net effect for xMax is that the primary signal it is so narrow that it can slip in between the existing allocated channels without emitting sideband information into neighboring, already channels. This makes it attractive for a way to cram more information into limited spectrum.