'Twisted' Waves Could Boost Capacity of Wireless Spectrum

New submitter Ogi_UnixNut writes "In Venice, Italy, physicists have shown that it is possible to use two beams of incoherent radio waves, transmitted on the same frequency but encoded in two different orbital angular momentum states, to simultaneously transmit two independent radio channels. In principle this allows the implementation of an infinite number of channels in a given, fixed bandwidth, even without using polarization, multiport or dense coding techniques. It's potentially a boon for congested spectrum problems, although at the moment I suspect it would only work for directional links."

Comments

Not really new

[scsirob]

This has been used for ages by HAM radio operators. Horizontal and vertical polarization antenna's can be used independently, or even together to create circular polarization. See: http://www.astronwireless.com/topic-archives-antennas-polarization.asp and http://en.wikipedia.org/wiki/Antenna_(radio)

BULLSH!T

[AB3A]

Any time someone starts talking of infinite channel capacity, you know they're going to be full of crap. Shannon's limit is a Mathematical principle. There is no such thing as "infinite" bandwidth/channel capacity.

What they're actually discussing is the spatial equivalent of spread spectrum. In other words, they have their own custom reflector with its own unique shape that can be reversed so that a coherent signal with minimal inter-symbol interference would be present. It is not a bad idea, except that you would need a line of sight path with very little exposure to the first Fresnel zones. Reflections would be a bitch to deal with.

Also note this method reduced point source noise, but it doesn't eliminate it. Likewise, a spread spectrum signal is still detectable as increased noise in a narrow-band radio.

Re:BULLSH!T

[Angst Badger]

Shannon's limit is a Mathematical principle.

Unfortunately, most people have next to no understanding of mathematics beyond some rote memorization from school. This is just another example of people confusing analog signals with magic. To be fair, the actual researchers involved probably understand this quite well, but the scientifically uneducated class from which science and technology journalists are drawn is another matter.

The non-mathematical version, for those interested, is that yes, analog signals are continuous and so can occupy an infinite number of states. The reason you can't get infinite bandwidth out of that is because both the transmitters and receivers have limited precision, and because there is always noise, which is another manifestation of the Second Law. For example, there are an infinite number of real numbers between 0 and 1. If you could actually use all of that space, you could encode any amount of information in an arbitrarily short signal. (Well, there's a limit to that, too, for which see Georg Cantor.) In practice, you can't use all of that space, because your instruments might distinguish quite well between 0.001 and 0.002, but they can't reliably tell the difference between 0.001 and 0.0005. On top of that, there is noise, which is also a big topic, but you can think of it as a random fluctuation in the signal. If the ambient noise varies between 0.0 and 0.0005 in the same example, you can't even reliably tell the difference between 0.001 and 0.002.

What the parent is getting at is that laws of physics, being derived from observations of nature with limited precision, might occasionally be overturned by better observations. Fundamental mathematical principles, on the other hand, are much more reliable. There might be a difference between rest mass and inertial mass that we could exploit for thrustless propulsion. It's extremely unlikely, but it can't be ruled out. But there is zero possibility that 2 + 2 will ever equal anything other than four. Shannon's limit and, for that matter, the Nyquist sampling theorem are a little more complex than a simple integer sum, but the actual math for both would fit on an index card with plenty of room to spare to blather on about "infinite" analog signals. We use digital signals most of the time these days because it makes the hardware easier to design, but neither digital nor analog can be used to make an end run around the Second Law.

What the researchers in TFA claim to have figured out is another way to use part of the signal outside of the frequency domain to stuff data into. It's a really ingenious approach that might be quite useful if it pans out in actual practice, but it's not magic, and it's not infinite.

Spectrum not overcrowded, mismanged

[colsandurz45]

I did my MS thesis on wideband spectrum sensing (just about everything under 2.2 GHz). Turns out the spectrum isn't actually overcrowded, it's underutilized, especially over 500 MHz. Look at some papers by the Shared Spectrum Company www.sharedspectrum.com/. This is common misperception and it's the result of FCC policies (that they're working on changing). The underlying problem is that institutions that have spectrum allocated for them now actually need it, just not most of time.

Re:Shannon-Hartley still in effect.

[YoopDaDum]

CDMA is different than this. CDMA is just a multiplexing mechanism, splitting a single channel capacity between users. Here the scheme adds new independent channels.

And CDMA has been removed from DOCSIS 3.0. It had been added in DOCSIS 2.0, then people eventually realized it was a dumb idea over cable, and then removed it. The company that had pushed it went bankrupt, but not before its share peaked and some people made a lot of money selling at the right time...
What you mention (channel bonding) is also called carrier aggregation in HSPA and LTE (LTE advanced, not the current one). It's just adding the capacity of different physical channels and treating them as one logical pipe. Very similar to Ethernet bonding, although it's more complex when you get to the details. But it has nothing to do with CDMA.

CDMA is the most hyped multiplexing technology. It's been hyped to death, so much some people think it's some form of magic. But it's not, and it's our past now. CDMA key point was that it was the first mechanism that enabled deploying cellular over a single frequency, which maximized at the time cellular capacity. This was very useful in cellular system, but it's a non issue in cable (there's no cell, duh). So CDMA over cable is a marketing/hype driven monstrosity that should never have happened (CDMA may by useful for a contention channel though). And even in cellular there are better schemes which have become practical now. All 4G system are based on OFDMA for example, with just the contention channel using some form of code multiplexing to be more robust to collisions.
Even HSPA, which is still CDMA based, went back to something closer to TDM in spirit than CDMA: there are still codes, but they're usually allocated to a single user over a short duration, and multiplexing is mostly TDM. Instead of having multiple user at the same time using different codes, which is the essence of CDMA. The HSPA way to send with more density over a shorter period of time instead of spreading the signal is more power efficient.