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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
It's a crazy tech, supposedly outside the FCC regulations. More info available here
"Our technology uses a narrowband channel, and places a carrier there for an extremely precise clock in the receiver," says Bobier. "The transmitter also transmits information in side bands, at levels lower than ultra-wideband. We are able to get performance comparable to a wideband licensed trasmission."
The low-power channel it uses can overlap with other users, because it is below the noise floor, creating "dual use" for the radio spectrum, claims Mooers.
"Scientists don't change their minds, they just die." -- Max Planck
This uses a wider frequency band than a TV station. Plus, since digital television is coming RSN, they won't be able to use another tech to make a broadband internet style transmission anyway.
"Scientists don't change their minds, they just die." -- Max Planck
See this article for an explanation of some of the technical details of the system.
Mea navis aericumbens anguillis abundat
It isn't CDMA. It appears to be a combination of a narrowband pilot carrier and a wideband PPM signal that transports the data.
Mea navis aericumbens anguillis abundat
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.
The technical details are sparse but here are two links.
6 3700624 that has a little more information.
In the faq http://www.xgtechnology.com/faq.htm there is a brief description. Note that the spectrum plot shown is basically worthless because it does not show any signal details.
Here is a magazine article http://www.mwee.com/showArticle.jhtml?articleID=1
Note the following: In the first is the statement that Shannon's theorem is not violated but no justification is given. In the second it says that most power is put in the carrier. Both of these statements should raise red flags. Be skeptical!
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.
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.
Here's an important clue, from their FAQ: "The narrowband channel allocation that xMax uses to coordinate reception of its wideband xG Flash Signal is not the system's information-bearing bandwidth."
So, it's a very narrowband pilot signal plus low level wideband signal with some new filtering/shaping tricks and maybe frequency agility on the wideband part.
The pilot is strong, easy to find, on a known frequency, shaped to occupy minimum bandwidth, and carries low-bitrate control info - like where and when to find the "flash" information-bearing carrier. It also may be a system clock reference (why not?). Being a clock reference would allow for more fancy demodulation techniques (yielding better BER performance) to be used on the other signal, because the lack of need to do clock recovery from the weak "flash" carrier.
The problem they had was that the regulators looked in the regulation books, and discovered that the only category that really fitted was spark gap transmitters, which were banned outright (spark gap transmitters transmit across all the wavelengths simultaneously and can cause enormous interference).
However it seemed a bit ridiculous, because the powers intended to be used for spread spectrum are really minute, and unlikely to cause interference, nevertheless 'rules are rules'.
Recently the argument was made that hairdryers often produce sparks from the brushes in the electric motors, and these don't produce significant interference, and these aren't banned; hairdryers are basically spark gap transmitters; and spreadspectrum produces much less interference than hairdryers.
The regulators hmmed and hahed, and it's looking like spreadspectrum is being permitted at very low powers in America.
Other countries like the UK have followed suit.
Incidentally, the noise floor often isn't affected measurably by this stuff, except at very close range. Noise doesn't add linearly and many of these systems are well below the noise floor. Also see Shannon Hartley theorem.
-WolfWithoutAClause
"Gravity is only a theory, not a fact!"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.
This of course is the source of the free wifi "organiser" http://nocat.net/ URL.
from a random wireless advisor post : from an article on Yale's website "The Physics of Cellphones" (but dated 2003) : from a MIT mailing-list : from a zoning petition : most of the other links that i saw agreed pretty closely with the 2.5 mile radius mark, the Yale paper that is notably different might be a typo? the zoning petition is even older than the Yale paper, so i don't think it is a technology issue
FYI Phil Karn ^^^ (a.k.a. KA9Q) is destined to be one of those people who gets written into the history books. IANAHistorian but AFAIK Phil is at least somewhat responsible for a significant proportion of Qualcomm's rather impressive success with CDMA (a.k.a. the cellphone technology that kicks the ass of all others).
;-)
...Thanks Phil!
Not only that; I most remember using his early dos-based TCP/IP stack and tools (repackaged by Demon Internet in the UK, BackInTheOldDays) which, while minimally friendly, were damn fine.
Apparantly rumour has it that there existed a system of sending information without wires before 802.11 that he was an amateur dabbler in..
My metaphorical hat goes off to him for decades of hard, smart, cheerful, pioneering work.
[FrLz]