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Sharing the Airwaves: Spread-Spectrum Broadcasting
NaDrew writes "From the SFGate: Hal Plotkin writes about how Spread-Spectrum broadcasting technology could revolutionize the way we listen to the radio, and make it incredibly easy for neo-broadcasters to start their own services. Sadly, he writes, the "often technically inept U.S. Congress has complicated the situation in recent years by shortsightedly instructing the FCC to sell or lease additional bands of spectrum that had been reserved for other uses." Not to mention "the media conglomerates that own most of the nation's TV and radio stations have a vested interest in the status quo and won't easily give up their hammerlock on what, in the end, are public airwaves." A fascinating article that also includes some history of Spread-Spectrum technology (did you know it was patented by Hedy Lamarr?)." A good primer to spread spectrum if you're new to the issue.
(did you know it was patented by Hedy Lamarr?)
That's Hedly!
*** Where are we going? And what's with this handbasket?
Sure if you are not tuned in, it looks just like noise.
But if people are not careful what will happen is the noise floor is raised higher and higher till it drowns out signals.
Yah that isn't close to happening yet, but the way it's being hyped as if everyone and anyone can broadcast at the same time, it might happen sooner than expected.
Using spread spectrum technology greatly increases bandwidth available. We are figuring this out 75 years after the invention of radio, so wouldn't any one else out there be doing the same thing? Are we wasting time looking for a strong signal from space when a spread spectrum signal would look just like increased static coming from a planet.
Free cell phone tracking
No! You don't say! What an obscure piece of historical trivia. Wherever did you discover that?
I suppose next you're going to bust out with the news that Milton Berle was hung like a horse.
Edith Keeler Must Die
RIAA: What, a new way to distribute music? Kill it quick, before we figure out that it could make us a boatload of money!!
US Military: That's our bandwidth! We need all the spectrum we can get to bomb an Afghan hut!
Steve Case: Did the military say bandwidth?? We should buy that up and meter it.
Um. Radio is light is electromagnetic radiation. But radio has a benefit, because it is not absorbed by air. Why? Because it's long-wavelength. Basically any gaseous atmosphere is going to be transparent to radio waves, so they are probably using that - probably a liquid environment will also be transparent to it.
You're also wrong that any form of EM radiation travels well in space - dust really sucks, and it preferentially absorbs higher wavelengths (because the dust can absorb them). Radio travels well in space, light does not.
Besides, SETI is looking at an 'intelligent' portion of the spectrum (I believe... they may have switched) - the 21 cm line of hydrogen. We can't look at any large portion of the spectrum - that's really friggin' difficult. So we assume that if they're trying to contact us, they're using an intelligent wavelength.
SETI isn't looking for stray communications, in any case. They're looking for a signal intended for us to notice. If SETI fails, that doesn't mean that there isn't anything out there - it simply means they aren't trying to contact us in the way we think they are, OR they aren't trying to contact us at all.
The FCC isn't interested in increasing the number of radio stations out there. The RIAA doesn't want more broadcasters competing with them. What the government and big business want is as few channels and companies as possible so that they can have tighter and tighter control. For example, if you set up a pirate radio station (ie. you don't pay the FCC's fees), then the FCC will come and shut you down ASAP. This isn't because they're "protecting the airwaves", it's because you aren't paying them. It's been affordable to set up your own FM station for a long time, but the FCC's licenses are extremely high in order to keep normal people off the air.
I'm not sure about the actual numbers, but I think it's in the hundreds of thousands of dollars a year to keep an FM music station on the air.
Travis
Therin lies the real problem with spread spectrum vs. conventional broadcasting channels. The limitations of conventional communications techniques is part of what stimulated the development of these more efficient algorithms. And since different groups are confined to their own frequencies, there was little stepping on toes.
But with spread spectrum, there is no pressure to be efficient, because there are no direct limitations on how much you can broadcast. The only problem is background "noise" from other broadcasters, and the easiest way to overcome that is with a more powerful broadcast. The obvious result is an escalation of more people talking louder.
It's like being in a nightclub, where everybody has to scream and repeat themselves to make themselves heard, but communication is near impossible anyway. The only one that can really be heard are the super-amplified guys on stage.
What we need is a more efficient and publicly-accessible use of the airwaves. Deregulating them will give the folks with the big antennas more control, not less.
---If you can't trust a nerd, who can you trust?
Using spread spectrum technology greatly increases bandwidth available
This just isn't true, any more than weaving around increases the width of a roadway. Sure, it might let you dodge some obstructions, but in the process you become an obstruction to other trafic. The total amount of information that can be carried doesn't increase. Just like in the weaving-around-on-the-road analogy, you have to ask, what would happen if everyone did this? The answer, of course, is that to a good first aproximation all those other SS broadcasters would look like noise to you; so the ambient noise level goes up and the S/N ratio falls, meaning less information gets through.
How much less? well, in an ideal world, if you do everything right you only lose exactly as much as you thought you'd gained. TNSTAAFL.
Each photon you reveive can only tell you so much. You can't beat the uncertanty principle with hand waving.
-- MarkusQ
Spread Spectrum is no magic bullet.
,or.. that other way (how does one describe DSSS?).
Instead of taking a section of spectrum, and, say, dividing it into a hundred equal slices frequency-wise and assigning a slice to each of a hundred users, you take that same spectrum and allow thos ehundred people and divide the spectrum up time-wise,
The point is.. it's not a magic bullet. There is still limited spectrum, and hence, limited bandwidth.
Whether it's common spread spectrum (DSSS, FHSS) or the new UWB thing everyone talks about every six months... it's still limited.
The benefit of spread spectrum over other methods is simply that radios can all be equal, and the 'sharing' can be accomplished algorithmically, rather than by physical frequency boundaries... which should make things more flexbile.
Actually, electrical engineers are quite aware of compression. They just do it in hardware.
Signals sent using RF cannot be completely compressed. You need some extra sync information and redundancy in the data to actually demodulate the signal and to confirm that a valid packet was received. The power in RF signals is like 1/1000000 of a normal digital signal. There is always noise masking your signal. When you try to amplify the signal to make it easier to process, you are adding more noise in the process.
I hate when comp sci guys talk about RF. RF is hard enough for electrical engineers. You are not simply pushing bits into a register when you transmit signals - you have to deal with InterSymbol Interference, fading, and lots of other fun effects.
What do you want to do - go back to the good ol days of AM and FM? There are only a few blocks of spectrum left after the parcels that have been sold off or reserved for military. The only reason that 2.4GHz is available for Bluetooth and 802.11 is because it was considered "undersirable property" because H2O has a resonance there - so 1000W microwave ovens are working that frequency over.
Finally, it sucks that amateur bands and ham radio keeps getting spectrum stolen.
No, I don't trust in god. He'll have to pay up front, like everybody else.
2) Inter-Modulation Distortion. This is a general class of problems resulting from non-linearity in amplifiers. It manfests itself on a spread spectrum link as noise --just like the near/far problem. Your options for getting around this problem in a spread spectrum receiver are few: Basically all you can do is build a higher power front end amplifier (consume more power). With narrowband systems you can take advantage of resonant circuits as well.
3) Sequency management. Someone has to coordinate these things somehow. Many are embracing spread spectrum as a way to get rid of the FCC. That's unfortunate. Yes, they're doing their job quite poorly and yes, it's been this way for a long time. That doesn't mean anarchy is better, or that the FCC's mission is irrelevant.
4) Data transmission != spread spectrum. Efficient use of spectrum is laudable. That doesn't mean that you must spread to be efficient, however. There are plenty of very well known modulation techniques which can be used for data transmission. The discussion of data broadcast or point to point data transmission has no bearing on whether one ought to use spread spectrum or not.
5) Making a transition from narrowband communications to spread spectrum communications systems is too expensive, difficult, and impractical to consider. You simply can't change every aircraft radio and air traffic control facility overnight. You can't just shut off all broadcast stations and tell everybody to buy new radios. You'd have a major riot on your hands.
6) Current Broadcast programming sucks. Did adding all those channels to Cable TV improve regular TV programming? Does anyone think XM radio will do good things for FM radio? Get real.
Most of the discussion on spread spectrum right now is more about the disadvatages of narrowband when scaled up and the advantages of spread spectrum on a small scale. However spread spectrum doesn't scale up any better than narrowband communications has. The technologies and limits are still the same. This is not a magic solution. This is merely one method out of several for signal multiplexing.
I can imagine a day when spread spectrum systems will be more common. However it does very little to solve issues such as re-engineering the FCC to be less flaccid and useless, sending high speed data over the airwaves, or how to improve broadcast programming. It's just a technology, it's not a way of doing business.
Nearly fifty percent of all graduates come from the bottom half of the class!