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Optical Cellphones
foondog writes "Here is a story over at News.com about optical cellphones. It seems that the Department of Defense has given a grant to the University of California to develop optical cellphones that are faster and more secure. This sounds a little strange to me since you would need a line of site with no obstacles in the way to use this. The article doesn't explain how this might work."
It's not necessarily true that you need a line of sight with no obstacles in the way for an optical communications device to work. There are parts of the EM (optical) spectrum which pass through ordinary objects. Infrared, for example, can "bleed through" most walls, allowing infrared photography of the sort sometimes used by law enforcement to see behind closed doors. On the other hand, gamma rays and x-rays, which are very high frequency, are stopped by few things besides lead.
Actually, current cellphones are, in a way, optical, since they use RF. Radio waves are a kind of light of much lower frequency than the visible spectrum, and they easily leak through all kinds of solid objects. I would assume that this new research project aims at using *higher frequency* optical communications, possibly using a laser for focused rather than diffused (RF-style) transmission. Only transmitting on a direct line of sight has obvious utility for security, and that line of sight doesn't necessarily have to be onobstructed.
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Lasers are out of the question for this. Lasers produce a very narrow collumnated beam. No good for cellphones. Probably something more along the line of bright LEDs would be better.
Big lasers, with lots of power. Could be dangerous.
It wouldn't need to be high power at all. Hobbyists have been experimenting with optical wireless communications for several years. It's not dangerous. Although the hobbyists use fixed points with either lasers (milliwatt power) or focused LEDs to transmit light. This DoD thing seems pretty crackpot to me. Why not just use high frequency microwaves? (Probably around 500 ghz to 1 thz) You have all the bandwidth you could ever use for cellphones in that range, and you wouldn't need fancy optical devices like super-sensitive photodetectors.
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Not an actual cell phone, but a point-to-point intercom involving binoculars and infrared transmissions. The voice was converted to (analog) IR light and transmitted through optics that created a very narrow beam. At the other end, the IR receiver was mounted in the eye piece of the binoculars and converted the light back to sound. The two devices had to be aimed very accurately at each other. That way a spy in the west could communicate with his pimp in the east across the border with very low probability of interception. They actually had this on the History Channel a few years back.
From what I hear, every soldier has a mirror. On a sunny day, you can use the mirror to signal aircraft for miles.
The mirror has the advantage of not needing batteries, being resistant to shock, etc.
Of course it doesn't work in clouds or dark, and bandwidth, well... leaves something to be desired.
So if they can do this with infrared and talk through it, that seems perfectly reasonable to me. One advantage of LOS is that you have to get in the way of the thing to jam it. Of course the receiver has to be intelligent enough to ignore signals from the wrong part of town, or signals that don't carry the right code, but it's a solveable problem.
Of course, any signal, especially an IR laser, gives away your position if the enemy can see it.
For all intensive purposes, "whom" is no longer a word. That begs the question, "who cares"?
People are posting about "why?"
Consider the havoc that nuclear explosions play with radio frequencies.
Consider having a method of secure remote communications which does not rely on radio frequencies of any type in such a situation.
Kinda makes you stop and think about things.
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Wouldn't a better solution be cellphones which support heavy encryption?
No, because at some point or another, the encryption will be cracked, and there may be recordings of the signal, which can be decoded later.
Unless, of course, you use a one-time-pad system. (But then you have to worry about the entropy level of your key)
What's this Submit thingy do?
Lasers produce a very narrow collumnated beam
Oh I wish....
Hi powered gas lasers, pumped lasers etc come with a very narrow collumnated output.
Diode lasers, as used in your DVD, CD player, laser pointer etc, come with a highly divergant beam. Say +/-15 degrees in plane of substrate, +/-5 degrees perpendicular. Optics are then used to focus or collumnate the beam. Unfortunately, this is often expensive in small quantities (as much or more than the cost of the laser)
wot no sig
No matter how you scramble and encode the communication the human voice will always have certain keys that can be easily discerned in a conversation.
An enemy can easily grab and record a radio signal. Then the digitally recorded file can be feed in a Beowulf cluster of cheap computers. That data can within a few minutes can decode your voice and thus get your tactical information.
Actually, no. Any broadcasted data can be encrypted because it can simply be treated as a block of digital information. Once encrypted, the data, voice or otherwise, is transformed into a stream of pseudo random noise. If the encryption is strong, as it is in many military applications, then it will take far more than a few minutes for a Beowulf cluster to decrypt it!
But what you said about radio broadcasts being big bomb targets is true, however, and is probably the main impetus behind this optical phone research.
"Directional signalling means much less interference, and therefore much less consumption of precious spectrum, and less need for those pesky and expensive cell towers." Yea, instead you will have those small, cheap orbital satellites! Seriously, one of the most significant benefits of low frequency radio communications in battlefield is that radios work through foliage, small terrain obstacles etc.. I wouldn't want to be the "optical radio" guy trying to get a clear shot at some satellite when in a dense forest. With current equipment it is impossible to pinpoint well placed & planned radio positions (perhaps using directional antennas) with means of radio intelligence. The main use of radio intelligence is to get an approximate idea of enemy formations etc., not to send artillery shells on the poor fellow pushing transmit button. In near future (20+ years) it will probably be possible for _US_ to have enough accurate systems to pinpoint enemy transmitters, but even then there won't be enough artillery to put out all those grunts when the action begins and hundreds of radios will start screaming on different frequencies. Using homing missiles for killing grunts with radios would be horrible overkill (and an expensive one), and probably it wouldn't even work as radio transmission are kept as short as possible. Not to mention the deflections of radio waves etc. that would probably send the homing missile of course at some stage. All in all, I don't believe optical communications using satellites will be a viable option in near future battlefield communications. Perhaps when Raczak's Roughnecks get to the field... (Recommended reading: Starship Troopers by Heinlein ;))
The problem you describe is not "radio broadcast vs optical narrow beam", it is "broadcast vs narrow beam". Once I've decided to go narrow beam for these reasons, why would I go optical rather than microwave?
(The beam divergence is inversely proportional to the number of wavelengths wide your transmitter/reflector is, which means that smaller wavelength requires a smaller transmitter apperature to achieve a given beam divergence, but surely microwaves are good enough, and have much better penetration.)
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You misunderstand what happens in a good encryption system - ENTROPY is added to the information, so any patterns (even encoding the same information multiple times) are not discernable in reasonable amount of time by hacker. And voice will have small variations from one recording to the next, which will result in great chaotic variations in output, so I'll take a well-encrypted VOICE pattern over a TEXT one if I knew text or message was very likely to be similar from one transmission to the next.