Using Visible Light for Data Transfer 351
James Evans writes "Wired has an article about a New Zealand company which has developed a technology to transmit data at speeds up to 400Mbps up to 4km. They are working to have it more resistant to changes in weather, as well as increasing the distance. It has a number of advantages, including lack of federal regulation of the spectrum, as it is of course, visible light."
In related terrestrial networking news, waytoomuchcoffee writes "Science Blog reports that the backbone for the World's Fastest Network is up and running. It's a fiber optic 40 gigabit per second connection between Chicago and LA. Teragrid is a project by the National Science Foundation designed to link up supercomputer centers."
Federal Regulation (Score:5, Interesting)
Re:Federal Regulation (Score:2, Funny)
Re:Federal Regulation (Score:5, Interesting)
Reason?
People will use headlights for another 50 years...Lights will be integral part of cities... Unless they ban flashing of lights.... this cant be outlawed
Consider this, when you flash your lights to an oncoming vehical, you are conveying information, or atleast acknowleding its presense, the tech was already there, its the 400 MBPs that is wow!
But I wonder how robust or secure is this.... can an airplane with flashing lights bring down your server.....?
Re:Federal Regulation (Score:5, Insightful)
Re:Federal Regulation (Score:4, Informative)
Sounds awfully like the old telecom's monopolies in Europe (hanging an ethernet cable out of your window and into your neighbor's for a LAN party was illegal, because only the state operated telecoms had the right to establish communications across property lines), and has AFAIK nothing to do with usage of spectrum. Encroaching on a frequency that is not you are not licensed to use is illegal, even if both (intended) endpoints of communication are on the same property.
Re:Federal Regulation (Score:5, Informative)
If you want to have your computer/network equipment fried, go ahead and drop what ever you like out the window.
To be on the safe side, drop wireless or optical out the window as optical cables doesn't transmit electricity...
Re:Federal Regulation (Score:4, Informative)
Re:Federal Regulation (Score:5, Funny)
Consider this, when you flash your tits to an oncoming vehicle, you are also conveying information.
Yet there are sometimes laws against it.
Re:Federal Regulation (Score:3, Insightful)
Re:Federal Regulation (Score:3)
Re:Federal Regulation (Score:3, Interesting)
Re:Federal Regulation (Score:2)
You are advised (in the Highway Code) not to flash your lights to other motorists, simply because they may misinterpret your signals. For instance, you are waiting to turn right, I flash to say "thanks for waiting", you think I mean "go ahead" - result is a big mess. Same applies to signalling to pedestrians that you will wait for them to cross - a bad idea in case another motorist doesn't wait.
FYI The Highway Code is a set of advisory rules - not legislation - which define good driving practise. You cannot be prosecuted for breaking any of those rules, but it is often used as a definition of safe driving. For instance when you take your driving test it is those rules which you are being measured against. You can make a certain number of minor mistakes and still pass however.
Re:Federal Regulation (Score:3, Informative)
Are you aware that the link you provided says the exact opposite of what you claim?
Federal Regulation Exists already (Score:2)
Tune your cars headlights so that the point directly to the oncoming driver's eyes, and you will get fined. AFAIK lighting around airports is regulated too, so that the airport landing lights are more visible to pilots. I think however that the airport case only affects people designing anythin bigger, like lighting highways/stadiums.
However, the leds used in the article propably don't endanger anyones life, and light is not a scarce resource, so hard to believe using leds for data transmission will be regulated
Unless ofcourse someone decides it endangers existing profit infrastructures... DMCA wasn't the first law done to protect profit..
Re:Federal Regulation (Score:2, Interesting)
Re:Federal Regulation (Score:4, Informative)
The astronomers push for using sodium vapor lamps that exhibit a very narrow emission spectrum. Very nearly all of the light is concentrated in two very close together spectral lines [gsu.edu] - hence they can easily filter out all of the streetlights with a simple filter.
The more common mercury vapor lights have a much broader emission spectrum [gsu.edu], therefore making filtering out their light much more difficult.
Pringles Can Replaced by Bedroom Mirror (Score:4, Funny)
It's already banned in Britain. (Score:3, Interesting)
I don't know about the US. (The FCC has been moving to open, rather than close, bands for some time now.) But it's already banned in Britain.
You probably already know that radio broadcasting in Britain is (or was a few years back - just in case they've changed their mind) a government monopoly. People tried to work around that in various ways.
One of them was a company that did a cute hack: They shined an infrared laser straight up, and modulated it with an entire FM band full of radio stations (similar to the way you can put a private FM band on a cable TV wire). Anybody who wanted to could mount a photocell or infrared-sensing diode (in a little telescope) on their window sill, point it at the invisible pillar of light, and couple it to a radio to receive the new band. Business model was to rent the stations out as commercial broadcast stations with all of London as target market.
The agency in charge of the British radio monopoly (British Post Office?) complained. And parlement extended the top end of their jurisdiction from whatever the previous legal end of the microwave spectrum was to infinity.
So in Britain, if it's electromagnetic energy (even gamma rays) and you can use it to beamcast or broadcast information, you need a license.
Hmmmm.... (Score:5, Funny)
Visible light regulation (Score:5, Interesting)
Re:Visible light regulation (Score:5, Funny)
Re:Visible light regulation (Score:2)
Re:Visible light regulation (Score:2, Interesting)
Re:Visible light regulation (Score:5, Informative)
Usually "light pollution" is considered to be lighting up the sky.
Also, will it come with a warning, such as "Do not look at transmitter with remaining good eye"?
The system uses LEDs not lasers.
Re:Visible light regulation (Score:2)
Re:Visible light regulation (Score:2)
Beachfront properties with nesting turtles have blackout restrictions where they aren't allowed to have any lights that would confuse the turtles.
Granted, that has nothing to do with the article but it's an example of governmental control of lights.
And as far as it being a LED, not a laser... stare into one of those keychain fob thingies with the ultra-bright LEDs and tell me your vision isn't fubar.... from a couple hundred feet away it's nothing but from a dozen or so feet it takes a while for that spot to go away.
flashlight (Score:5, Funny)
http://hksoul.myftp.org/
First Light? (Score:4, Funny)
Re:First Light? (Score:2, Insightful)
It is not even rare (atleast with biggest finnish cable provider [www.htv.fi]). They are receiveing the signal via satellite most likely and when there's heavy snowstorm or rains, picture quality is really bad in some cases. So, anyone with cabledish themselves can verify this also ;)
Lack of regulation (Score:4, Interesting)
It sounds like a VERY nice system for short-range, non-critical communictaions, but personally, I can't think of any points I would want to communicate to where I have line-of-sight... If I could get an inexpensive device that could communicate for about 10 miles, I would certainly get several.
Re:Lack of regulation (Score:5, Insightful)
They give an example in the article. Where you need to communicate across a public road. (N.B. in New Zealand "motorway" means any surfaced road.)
Indeed any case where you need to communicate between several buildings fairly close together. Digging a cable trench is very expensive.
If I could get an inexpensive device that could communicate for about 10 miles, I would certainly get several.
They estimate that it can do up to 11km. With a single repeater 16km sounds plausable.
Re:Lack of regulation (Score:3, Informative)
As another FYI, when I was last there, there was only one Motorway in the whole of NZ
Re:Lack of regulation (Score:3, Funny)
yeah, but when will it transmit miles?
Re:Lack of regulation (Score:5, Interesting)
A few years ago I designed and built from scratch an infrared based automatic timing and scoring system for racing cars. The advantage over the current radio frequency transponder systems was that it required no modifications to the physical plant ( such as having to bury a cable under the track surface). You could set it up anywhere, at any time.
The limitations because of the line of sight requirement proved intractable in practice though. While I still use my system for track testing, and find it superiour to rf systems for such under "standard conditions" ( especially with an IR laser as the light source) I have had to abandon the project as impracticable for real world application in actually scoring races.
Obviously network systems based on similar technologies will face the same, or similar, limitations.
"Yeah, the network went down. Flock of pigeons again."
KFG
Handy (Score:2)
This might be quite good if it's low cost.
Re:Lack of regulation (Score:3, Informative)
Well off the top of my head I'd say that if you made a device like a colimater which is basicaly t telescope the light is shined thru backwards it would greatly increse the range. All of the light would be packed into a beam, Mirrors up to 10 inches in diameter and accurate to 1/10 wave are available. This would send a beam 10 inches in dia so any obstructions from snow would be much less likely. Amature astromomers make their own mirror, some times as large as 18 inches in dia. after that its a matter of just pumping enough power thru the thing.
The thing wouldn't be completely reliable, but I'd bet that when the system goes down the weather would be so bad that not many people would be at work anyways.
Fiber Optics? (Score:5, Interesting)
I seem to remember this being done a long time ago. I've got an electronics book with a schematic for a serial 28k transmitter using visible light.
--Quentin
Re:Fiber Optics? (Score:5, Interesting)
There is one critical difference: ease of installation. Installing a fiber optic line is really cumbersome, since it involves lots and lots of digging.
This could really be something for high speed communications infrastructures. Take cities: digging is hard, and radiowaves pletiful, even so much as to people being afraid of them.
Pigeons could be a problem though
Re:Fiber Optics? (Score:2)
It's also expensive, since you are likely to have to dig up paved surfaces and repave them after. (As well as avoiding all the other services which might be down there.) That's combined with having to get permission from several land owners to do the digging in the first place.
Re:Fiber Optics? (Score:5, Funny)
Not if you use sufficiently powerful lasers >:)
Re:Fiber Optics? (Score:2, Funny)
Actually, this becomes a feature: feed the homeless, reduce the pigeon population and communicate data. What more could you ask?
I see a patent here someplace.
Re:Fiber Optics? (Score:2, Funny)
Re:Fiber Optics? (Score:3, Interesting)
Then again you probably will have more attenuation, fibre channels being solid and these beams having to pass through air (containing dust etc.)
Red Light Destrict (Score:5, Funny)
Re:Red Light Destrict (Score:2, Funny)
Re:Red Light Destrict (Score:2)
Fiber Optics (Score:3, Insightful)
We have been streaming voice data over fiber-optic lines for a while now, and even digital data signals for networking. This sounds like fiber-optic transmission without the actual fiber-optic line! Very cool, indeed.
Perhaps this is the future of truly wireless computing?
One thing the article states is that the current range is about 11 km. This seems a little short. However, considering this is a line-of-sight type of thing, that does make sense. Give 'em time, and they'll get it down to hundres of miles with good reliability, and then I think we'd see a bit move towards it for WAN technology and business usage.
Re:Fiber Optics (Score:3, Informative)
Re:Fiber Optics (Score:2, Informative)
Well, we just shut the laser on and off and did not modulate, anyway speeds of > 1Gbit should be possible with this simple technology. The problems of this design are the adjustment, free line of sight (weather, insects, birds), eye safety, other light sources (sun!) and a very sensitive receiver circuit.
The maximum range is infinite but it can be limited by the protocol (e.g. Ethernet) due to collision windows.
sending signals with flags and flashlights (Score:2, Funny)
So we can cause a denial of service with a flash light or one of those extremely irritating laser pointers, or how about the police speed radar targeting system...
on off on off on off on off on OFF
on off on off on off on OFF
on off on OFF
ON OFF
or maybe they could try carrier pigeon or paper wrapped around arrow shafts to cross the street
NZ's problem is Switzerland nicked their best sailors.
Re:sending signals with flags and flashlights (Score:2)
Cool (Score:4, Insightful)
I would have some security concerns though since it makes it a lot easier for those of malicious intent to intercept the signal as its basically being broadcast in the open. The technology would seem to lend itself naturally to strong encryption though.
I think they could be onto something big here.
Similar stuff... (Score:5, Informative)
Re:Similar stuff... (Score:2, Informative)
might have to give it a try.
Anything using optics (Score:2)
Bandwidth stealing... (Score:2, Funny)
They wouldn't even know you where there!
Gives a whole new meaning to the phrase (Score:2, Interesting)
already available and widely used (Score:3)
(here here)...Re:already available and widely used (Score:5, Informative)
It I am not sure how this is article bestows very interesting or novel information. Granted, the article mentions the wavelengths used are "visible", and "red". My guess is that they are emitting somewhere between 600 and 800 nm (typical visibly range is from 400 nm (purpleish) to 700 nm (red) however this is not a strict cut off, and if bright enough, even above 830 nm is visiblish).
Most telecom takes place at about 1550 nm, well into the infrared, but this is primarily because the typical fiber has nice properties in this range (absorption and dispersion). Therefore I am not sure there is much fundamental difference between infrared light telecom and visible telecom. Indeed they use very similar laser material (GaAs-based or InP-based diodes), are modulated the same way, etc.
Possibly this is neat because it is free-space optical stuff. However this (as pointed out previously) is not new [free-space-optics.org]. There are companies that are in [fsona.com] place [airfiber.com] as [lightpointe.com] we [mrv.com] speek [aeiwireless.com]. Maybe deregulation may be of interest, but if the light it kept at the same wavelength as in fiber, then there is no need for an electronic klugey transceiver (detect the light in the fiber at 1550nm and drive a laser to re-emit the same signal at 6xx nm). Instead, an add-drop filter could be slapped on to the end, pick off the right wavelength, and feed that to a fiber which could be collimated as the source. This collimated beam then could travel over kilometers with no trouble. An all optical solution has a much
just a thought
One of several FSO plays... (Score:3, Interesting)
I work for a company that is currently developing a LED-based FSO system -- Omnilux [omnilux.net].
The big push now in the FSO market is to find the right balance between performance and cost. Too many companies were trying too hard to push data longer distance, then faster, costs be damned.
nostalgic (Score:3, Interesting)
I mean, same thing except, well, faster...
The cool part, though, is that now the router's status LEDs are actually good for something. You can theoretically face two routers toward eachother and that's IT! done! until some idiot walks between them. ha!
but really though, The thing with radio we seem to not be able to do with light yet is frequency modulation. If we can do that, I think we can push some very serious bandwidth through this spectrum.
The data-hiding possibilities are immense. you can technically send humongous amouts of data through a TV set, even, if it was made of as many LEDs as there are pixels, and by varying the each LED just ever-so-slighly. You can be watching the TV for pictures, and your Aibo would be sitting beside you, downloading zillions of bytes of data, and gaining consciousness (sorry just watched the animatrix, heh).
Lighwave pollution? (Score:3, Interesting)
But after reading the article, and seeing how they'd use LEDs (they don't say how big though), and the bandwidths involved, the lights would seem to be constanly on, do you think?
That'd mean no real lightwave pollution (it's all line-of-sight) and little visual pollution or distractions due to thousands of flashing lights?
Of course, I still have to wonder about the effects of different weather. I see it'll still work with a hand moving in front of it, but what about heavy smog days, or blizzards? Would torrential rain make problems with light refraction??
I guess it beats training swallows to carry coconuts engraved with data packets from rooftop to rooftop (they could grip 'em by the 'usks)
Hats off to the Kiwis for this one though, it sounds pretty exciting :)
Woohoo! I can't wait to see my IT Manager scaling our office building to deal with pigeons nesting on the transmitter!
"Nature will find a way..."
IR is safer than visable! (Also IR not regulated) (Score:5, Interesting)
A very important point is that Infra Red light is absorbed by the cornia (outside) of the eye and dosnt penatrate to the retina where it can cause real damage. Visable light does penetrate (obviously) to the retina and WILL fuck your eyes up. I've worked with IR lasers for a few years, they are much safer than visable light devices.
Also saying use of visable light avoids licencing isues is a bit misleading.
As to my knowlage, no country regulates visable, IR or even UV unless in lasers (or other sources) where they may get to the powers likey to cause physical danger (not very relavent here, less so with IR rather than visable light).
Put your hand up if you need a licence for your IR TV remote controal!
Anyway, a practical solution would be to use lasers of differnet wavelengths and swich to the correct one depending on weatehr conditions. EG fog attenuates some wavlengths strongly, rain scatters a differnt set of wavelengths more readily, etc (As a crude example, consider the different wavelenghs reaching your eyes from the sun in these different weather conditions)
This technique of swithing to the most aproprate wavelength for the conditions is used in army laser range finders.
Re:IR is safer than visable! (Also IR not regulate (Score:4, Informative)
Infra red eye safety (Score:5, Informative)
A lot of infrared (IR) lasers (common examples are Nd:YAG or Ti:sapphire) operate in the near infrared
Yep, you are right. Some near IR wavelengths will be let through the cornea, and you wont have the blink reflex to protect your eye. However, this is slightly misleeding as the vast majority of IR (at wavelenghts a little further from the visable) is safe. Especially at the power levels discussed here.
The only time it decomes dangerous is when the IR light is strong enough to heat the cornea!
For example, at 1.55 microns (wavelength most suited to optical fibre) the British Standard guidelines state the maximum permisable exposure to the eye at this wavelength is the same as skin. In simple laymans terms, it has to be strong enough to burn flesh (skin or eye) before it will damage the eye!
Of couse, the real bastard lasers are UV. A fairly dangerous wavelength (suntan anyone) that you cant see. Not good for your eyes either!
UV laser danger (Score:4, Informative)
UV lasers can be bad, but they don't do retinal damage at short enough wavelengths. In fact, UV is used in "Laser Vision Correction", because it ablates the cornea nicely without penetrating at all into the retina.
For really severe retinal damage, visible and near IR are the worst.
400Mb? 1 Gb is old news. (Score:5, Informative)
I investigated this for networking a couple of buildings my company had near together. Pretty cool stuff. You could get a gigabit connection over a few km of thin air. Cheaper units did 155Mb and for dirt cheap you could get 10Mb. Short range units used LEDs. Longer range ones used lasers.
I've been wondering why consumer ISP's haven't taken to this yet. It's a great last mile solution.
--Keepiru
--slashsuckATvegaDOTfurDOTcom
Re:400Mb? 1 Gb is old news. (Score:2)
2 words: Atmospheric conditions, fog, heavy rains, etc can and do impact on the bandwidth.
AH HA!! (Score:2, Funny)
Already done at Xerox PARC (Score:5, Insightful)
The beam went over a public highway, and after one woman went into a ditch after it startled her one foggy morning, they coarsened the beam to make it invisible.
this isn't a new thing (Score:2, Interesting)
Safety and Reliability (Score:2, Interesting)
Re:Safety and Reliability (Score:2, Interesting)
Interesting anecdote (Score:5, Informative)
They tried it in a conference. They wanted to telecast conference proceedings in a building some distance away using this method. They set up this equipment, tested everything the night before the opening day, works perfectly.
First day of conference. No signal. The receiver didn't see the transmitter at all. Total flop.
So they checked it thoroughly again that night. Everything was still working fine.
Next morning: same story. No signal.
This repeated on all 3 days of the conference.
Organizers were left scratching their heads. Funny part is, it worked at night and failed at day without their touching anything. Sabotage? The devil??
Later they found it was because the light beam was getting bent in daytime due the temperature gradient (same way that mirages occur). Poof.
Of course, these are just problems that will inevitably occur when a technology is in its nascent phase, I'm sure it'll get ironed out as it goes commercial.
The article talks about rain and fog, but is silent on the sunlight issue.
What about right of way? (Score:2, Insightful)
I wasn't involved directly in that project, so I don't know if it was needed because it was microwaves, or just in general.
I wouldn't rush to think this is some sort of easy method to solve problems, though.
Hope they think of this... (Score:2, Insightful)
next-steep on networking? I don't think so (Score:2, Funny)
Neighborhood network? perhaps. Just imagine a lanparty on my neighborhood, and every tv/vhs/dvd/thing-with-a-infrared-remote-control getting weird
drmad.
Breakthrough technology! (Score:2)
free space optical is nothing new (Score:3, Informative)
there are significant limitations on this tech however. cheif among them is reliability in various weather conditions. rain, fog, snow, and passing birds tend to cause havok with a laser beam. setting a laser up to point to a target 1 or 2 kilometers away is no small feat, and even harder is making sure it stays on target months and years later.
there's a reason why most wireless shorthaul links use microwaves, as the laser technology really doesn't work very well.
A company in Ottawa Canada manufactures these (Score:3, Informative)
Check out http://www.plaintree.com - they use eye safe LEDs for transmission, with speeds up to 155 MBPS, or T1/E1 at ranges to 3KM. They are using this at the Ottawa airport. They have been in business since 1988.
Big whoop (Score:3, Funny)
Since the laser's light is coherent, you can use this interference to reconstruct subtle changes in the distance from the laser to the reflective surface. In other words, you can eavesdrop on someone by looking at how the windows in the room vibrate! Supposedly this was once used to find out what people were saying in an embassy.
At short distances you can use a grapefruit instead of a window, but talking into a grapefruit is just weird. :)
Terabeam got me on the way to work today. (Score:3, Informative)
I was driving to work North on Willows Rd in Redmond, WA today and I saw Terabeam's laser flashing away. It is pointed almost directly along the road to their satellite building to the South.
I haven't noticed it before, so I wondered if they had some beam dispersement issues. I got a little concerned that the laser was damaging my eyes. Hopefully the power on the lazer diode is turned down. You never know what is going on in development hardware ;)
The light looks like your run of the mill strobe light. The pattern looks like the LEDs on your hub. Not sure why because that flash speed is not consistent with the marketing info on www.terabeam.com [terabeam.com].
Unfortunately the fog here in the valley tends to prohibit their use of the system.
Federal regulation (Score:3, Funny)
Oh... you mean here?
Re:is anyone else thinking... (Score:5, Funny)
The problem is really tired arms.
Yeah, but more like ultra high speed morse code (Score:5, Funny)
On the other hand, bad weather, or anything that might block the light's path, can cause slowdowns or power failures.
"File transfer failed: Code 75(flock of seagulls)"
*honk*
Re:Yeah, but more like ultra high speed morse code (Score:3, Funny)
Actually, what about interference from RFC 1149 [linux.no] network (a physical layer network run over trained pidgeons)?
Re:Yeah, but more like ultra high speed morse code (Score:3, Funny)
Re:Disadvantages... (Score:2)
Re:Disadvantages... (Score:2, Insightful)
Re:Disadvantages... (Score:3, Insightful)
That was the first thing that occured to me when I read the post. It will probably be like everything else. First comes the technology, then years later everyone slaps themselves on the forhead for not thinking of making it secure from the beginning. Like Telnet, FTP, POP, 802.11, IM's, etc.....
Not security, but reliability... (Score:2, Insightful)
I think the realiability is bigger issue. What if someone wants to cut your operations. Big piece of carton or huge van to front of emitter could quite efectively cut it. How's that for denial of service
Well, i admit that i havent read the article so i dont know how it actually operates but what about "light noise" from other sources.. Or other co-existing "light hubs" in the area. How do they effect the data and its reliability. Only way to prevent this (which i can think off) is using laser as transport medium light and thats not so new anymore is it. And DoS'n laser is even more simpler since the lightbean is really narrow usually and doesnt spread as "normal light".
Re:Not security, but reliability... (Score:3, Insightful)
The cool thing about an optical line-of-sight transmitter: by hauling yourself to the roof with a pair of binoculars, you can look directly down the link and SEE the NSA/FBI/CIA/local cops' prism tap.
With WiFi, you never know if a tap is sitting in a van down the street, or the local 7-11 has an NSA tapping station behind the storeroom.
But optical, ah, you can SEE the buggers listening in on your conversations.
Snicker. Wouldn't it be cool if you could jack the power on the laser, and melt the bastard's tapping opticals? Priceless.
Re:Disadvantages... (Score:2)
Here's [slashdot.org] what I wrote in another thread:
If I actually care about something being secure, it's either done through SSH (or scp to copy files), or I use SSL encryption on my instant messaging [jabber.org], or PGP encrypted e-mails. I don't care if someone's able to tell who the recipient is and what the subject is. My ISP probably logs that anyway. Wireless networking as it is now supports all that and more. What's wrong with it then?
It can be secure... (Score:3, Informative)
As for the service interruption concern, it would seem prudent to use several redundant beams spaced at irregular vertical and horizontal intervals - wide enough so that a small flock of pigeons doesn't interrupt service.
Re:Disadvantages... (Score:2)
Er, in other words, no.
Re:A great idea (Score:3, Informative)
Re:A great idea (Score:3, Interesting)
Consider this, most cellphones around the world operate at 1500MHZ and so have a seemingly impressive maximum THEORECTICAL data transfer rate of 750Mbits/sec. Unfortunately due to physical contraints on modulation systems a good rule of thumb is that the actual data rate provided is about 1/2000 of this and so we end up with around 375 Kbits/sec that is just coming out with 3G systems.
You should really read up on communications theory, especially the basic stuff done by Shannon, Nyqvist and all the others.
The theoretical limit for a digital signal modulated in a perfect noiseless analog channel is infinity for any frequency! Where you get your numbers from I have no idea but they are totally incorrect!
And if you hava a noisy channel the theoretical maximum is dependent on the bandwidth and the noise, nothing else. And just because a signal hase a basefrequency of 1.5 GHz doesn't mean that it has a bandwidth of 1.5 GHz. Go check, all mobile phones have a much, much smaller bandwidth. We are talking orders of magnitude here.
Re:A great idea (Score:5, Interesting)
So what if they operate at 1500MHz? It is the bandwidth that is important. Example: Radio stations operate at 90-110MHz range. But each one has a bandwidth of around 30Khz.
So, first of all, the cellphones have a bandwidth allocated within a frequency range around 1.5Ghz. Let's take GSM, which is the most widely used standard:
In case you don't understand, it is simple. If you have a single signal at 1.5Ghz frequency, you could get a data rate equalling half the frequency. However when you transmit data you basically cause side-frequencies to appear in your spectrum. Do not assume that just because the system transmits at the base frequency of 1.5Ghz that the signal spectrum will be just a point at 1.5Ghz and 0 everywhere else. The spectrum will spread. If you use up all your possible bandwidth the spectrum will take up all the frequencies from 0 to 1.5Ghz.
Furthermore, consider the fact that there are many cellphones, sharing infrastructure. The protocol does both time-division and frequency-division multiplexing. While a *single* cellphone could perhaps work with a station at .75Gbit, this ceases to be the case when you add a few thousand cellphones. The band is subdivided to a pre-specified number of sub-bands - not only that, but there is also some time-division multiplexing going on, with each cellphone only doing rx/tx at a fraction of the total time.
Of course, the same is true for all electromagnetic wave devices.
Re:A great idea (Score:2, Informative)
Not exactly, if you (theoretically) modulate a 1.5 gig signal with a signal at half that frequency, two sidebands appear in this case from 0.75 gig to 2.25 gig is the resulting signal spread. So if you use all your possible bandwidth (and again this is theoretical, I know of no system that 'coud' do this) then a bandwidth of 0-3gig will result. Interestingly, in this scenario (ie FM or AM modulation) the bandwidth consumed is double what the maximum modulating frequency is. So one sideband is 'wasted' as it carries a mirror of the others information. Also the carrier frequency itself is redundant, it carries no useful information. Given these facts, Single Side Band (SSB) is much more efficient in terms of bandwidth & power required (power saved in the transmission of carrier & one sideband can be used to increase the remaining sideband)
Re:light in NZ very precious (Score:2)