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Twisted Radio Beams Could Untangle the Airwaves
Urchin writes "The radio frequency spectrum available for wireless communication is becoming increasingly crowded thanks to new wireless technology. A solution to the shrinking space might be to put a spin on radio beams during their transmission, to produce a twisted beam, according to Swedish physicists. In theory, huge amounts of data could be sent in the pitch of the twist, which is distinct from the amplitude and frequency of radio waves — the features used at the moment to send information."
The article suggests the technique only works really for point-to-point transmission. Regular amplitude/phase modulation (QAM) is still the best generally I'd imagine.
The article appears to be referring to right or left circular polarization, as opposed to horizontal or vertical polarization. A horizontally-oriented dipole transmitting near a vertically-oriented dipole will be heard much more faintly - 20db+ quieter. Similarly, a left-polarized antenna won't interfere with a right-polarized antenna. But a circularly-polarized antenna will still interfere with a horizontally or vertically polarized antenna - it'll only be 3db weaker.
There is a reason that FM is polarized in the direction it is: any other direction is relative.
FM is vertically polarized because that means that a car needs only have a vertical antenna to catch the signal, if they polarize it horizontally then the antenna on the car needs to rotate every time the car turns.
At least this is what I was told in my RF/microwave design class.
AFAIK (yes, I did RTFA), this is tantamount to adding another method of data transmission using more of the envelope. You still have the frequency being used and still have a portion of the carrier plus sideband transmitted, no matter what type or method of transmission is used. This may be used to embed something similar to a sub-carrier, or a unique identifier. More directivity and narrower beamwidth during point-to-point transmissions will do wonders to keep the RF floor down.
Don't spend your life lamenting your life.
You are aware that digital radio techniques all use amplitude, frequency or phase modulation, right? The difference is that the modulation is digital (or thereabouts) rather than analog.
Yes, you are, and no, they aren't.
This is about modulating the orbital angular momentum of photons, a property that wasn't even discovered until 1992.
Each photon can have an integer quantity of orbital angular momentum (0, 1, 2, 3...) without obvious limit (or in the opposite direction, -1, -2, -3...). In principle, and increasingly in experiment, it is possible to encode information by modulating the orbital angular momentum carried. This provides and entirely separate channel with its own bandwidth in addition to traditionally understood modulation. They're right to be excited about it; it has the potential of being just as big in scope as was the invention of radio.
See http://www.physics.gla.ac.uk/Optics/play/photonOAM/
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See http://www.physics.gla.ac.uk/Optics/play/photonOAM/
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They're using physics that wasn't even discovered until 1992.
See http://www.physics.gla.ac.uk/Optics/play/photonOAM/
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This is slightly different than simple polarization, see here: http://www.newscientist.com/article/mg18224515.000 -- full article requires log-in. Or here: http://www.physics.gla.ac.uk/Optics/play/photonOAM/ The point here is that a "pulse" can now encode more than just an "on/off" state. Instead, a pulse now encodes a "twistiness" level of states (can be 1, 2, 3, or up to 250 as in the NS article.) So, a 2GHz signal can now carries, let's say, 2x8 = 16 Gb/s. The trouble, it seems, is to construct a receiver capable of correctly identifying the pulses.
Check out the wikipedia page for OFDM: http://en.wikipedia.org/wiki/OFDM
First paragraph: "Each sub-carrier is modulated with a conventional modulation scheme (such as quadrature amplitude modulation or phase shift keying)."
You're right, analog transmissions are generally less efficient for transmitting data. Your AM radio vs. digital radio example isn't quite fair, since the digital radio is compressed - that is, it isn't actually transmitting as much information as the analog channel.
Your overall point is flawed. It is not the type of modulation (AM or FM) that is at fault, but rather the analog nature of the transmission. This new technique actually sounds like it would NOT lend itself naturally to analog transmissions - it would be far more likely to be used with digital encoding.
In fact, you could probably use OFDM if you want to, just like it is commonly used with amplitude modulation.
It won't take but a second for you to stop guessing that it's about polarization once you see their clear explanation that it's different.
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This article has a good explanation of the difference between Orbital Angular Momentum and Polarization of EM waves.
If you look at the cross section of a "normal" polarized EM beam, the electric field amplitude and direction at every point of the cross section are in the same phase - although that direction may be up, down, or rotate over time depending on the polarization.
In an EM beam with orbital angular momentum, the electric field amplitude at different points on the cross section are in different phases - although it is my understanding they are usually all in the same polarization.