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'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."
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.
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.
www.eFax.com are spammers
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.