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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.
from the article:
The first xMax network is currently being built in Miami and Fort Lauderdale where one base station can deliver broadband Internet over a 40 square mile area.
But with that much area, you need to start worrying about capacity. What good is it to cover 40 sqmi when you can't get a packet through:
The capacity of that wireless network is not bigger than any other wireless technology, which means that more base stations need to be added if a certain number of people are using the network -- typically several hundreds to a 1,000 users.
"Scientists don't change their minds, they just die." -- Max Planck
I didn't see any mention of the FCC in this article at all, something that may be indicative of a lack of approval from the relevant bodies. It's all very well the inventors/creators saying that this is technically ok, but when the people who are allocated the frequency range this technology operates in have problems with the raised noise or extra signals, or even just object to something else intruding on their licenced spectrum, I wonder what will happen.
Business Voyeur
I'm surprised that no-one's actually tried something like this before. What with the prevalence of radios that can adjust themselves to noise conditions, it seems that it would be fairly obvious to build one that could listen to the frequency (or frequencies) it wanted to transmit on and intelligently avoid stomping on other, old-fashioned signals in the vicinity. It's interesting, 'cause I just got done reading about something like this in this rather weird, but oddly compelling book.
Or will the TV stations roll out internet service themselves, since they have the license?
An area of 40 square miles is a circle of radius 3.6 miles (5.8 km). Is that really more than a digital phone tower can manage, for example?
I’m old enough to remember 16K of memory being described as “whopping”
See this article for an explanation of some of the technical details of the system.
Mea navis aericumbens anguillis abundat
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.
Two wrongs don't make a right, but three lefts do.
UWB (Ultra Wide Band) by any other name... It already got shot down once. Now it's rebranded and trying again. It shall get shot down again.
Sendmail is like emacs: A nice operating system, but missing an editor and a MTA.
this einstein quote seems appriopriate
"You see, wire telegraph is a kind of a very, very long cat. You pull his tail in New York and his head is meowing in Los Angeles. Do you understand this? And radio operates exactly the same way: you send signals here, they receive them there. The only difference is that there is no cat.
If you know the characteristics of a signal exactly, you can recover it from below quite a bit of noise. One of the experiments I have my students do is to recover a signal 20 dB below the noise. It is trivially easy to do. The amount of data you can send, on the other hand, is approximately zilch.
Shannon's law describes the amount of data you can send as a function of the signal to noise ratio. As long as you are willing to put up with low bit rates it is no problem to use a signal below the noise floor.
Several of the posters have assumed that these guys have re-invented cdma. That's not necessarily the case (although it might be).
http://www.wirelessnetdesignline.com/howto/uwb/163 103775
An xMax-enabled system has several advantages of over a UWB network. Primarily, whereas UWB emissions require several gigahertz of spectrum, the "narrowband" version of xMax only requires sidebands on the order of several megahertz. The carrier synchronous nature of xMax also bests UWB, which uses thousands of pulses to represent one symbol.
Paradoxically, UWB is often designed as a PAN technology for use in the 3.1- to 10.6- GHz range and other limited uses in higher bands (24 GHz), leading to potentially high transmitter density. Given the amount of power emitted into adjacent bands, the cumulative likelihood of interference is high. In contrast, xMax is designed as a WAN technology, leading to a low transmitter density and lower interference potential. FCC rules also prohibit UWB applications from using spectrum below the 3.1-GHz band, whereas xMax is designed for sub-GHz use.
Lastly, xMax is a more efficient, agile system that requires as little as 6 MHz for broadband data transmission and can frequency-hop to vacant spectrum. As stated, the xMax signal is carrier-synchronous, making detection easier. UWB, on the other hand, doesn't use a carrier; timing must be embedded in the information, requiring large contiguous swaths of spectrum. Note that UWB requires higher signal power when measured using equivalent resolution bandwidth.
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.
although trespassing is not the right word, because we're allowed to transmit a signal if it doesn't interfere with other, stronger signals...
Damn right it's not the right word, and it wouldn't be even if it weren't legal to transmit on those same freqs. You can't trespass on frequencies because frequencies are not anyone's property. We gotta shake off this relentless trend of treating rights and licenses as property. To use a more familiar example, nobody "owns" music, not even the composer. Rights holders don't own anything at all, they merely control the rights to do specific things for a limited time.
The distinction isn't semantic nitpicking, it's very important because treating rights as property gives the copyright control industry an unfair advantage in any public discussions about rights issues. They like to play the part of the plucky little old lady chasing down a purse snatcher, or the outraged homeowner defending his castle against burglars and government goons. They get away with it because the public has been taught to overlay the simple and familiar concept of property on much more complicated issues. Treat rights as what they are -- temporary conditions set by the government -- and various rights and DRM issues suddenly require a lot more thought, which they should.