Wireless Breakthrough: One Frequency, Multiple Signals - Page 1
Forum section: electronics / general

author: SgtRock
author_rank: Unknown
timestamp: March 03, 2012, 01:23:30 am
content: Greetings EEVBees:--Please see below for a link to and article in The Register about using "orbital angular momentum" to get multiple signals on one frequency.http://www.theregister.co.uk/2012/03/02/radio_breakthrough/--I am not sure, this can be called a broadcasting discovery, since it seems to be pretty much line of sight. But I invite those who are knowledgeable in this area, to comment, as to the potential usefulness of this system."Marconi invented the radio but he had to wait years and years till anything decent was on."Johnny Carson 1925 - 1992Best RegardsClear Ether

author: sonicj
author_rank: Frequent Contributor
timestamp: March 03, 2012, 02:04:34 am
content: so they built a Circular Polarized Antenna?-sj

author: vk6zgo
author_rank: Super Contributor
timestamp: March 03, 2012, 02:29:34 am
content: Quote from: SgtRock on March 03, 2012, 01:23:30 amSarge,the link went "walkies"

author: buffalo960
author_rank: Contributor
timestamp: March 03, 2012, 02:33:22 am
content: I think this is probably the article he was talking about.http://www.bbc.co.uk/news/science-environment-17221490

author: vk6zgo
author_rank: Super Contributor
timestamp: March 03, 2012, 04:20:13 am
content: Quote from: sonicj on March 03, 2012, 02:04:34 amExactly!Hey,I wonder if Anthony Sutera & Chamtec are associated with this,too!

author: SgtRock
author_rank: Unknown
timestamp: March 03, 2012, 04:57:44 am
content: Greetings EEVBees:--My apologies for omitting the link. Link now included in the original post, as well as here below.http://www.theregister.co.uk/2012/03/02/radio_breakthrough/The link posted by Buffalo, indeed appears to be concerned with the same experiment."If it weren't for Philo T. Farnsworth, inventor of television, we'd still be eating frozen radio dinners."Johnny Carson 1925 - 1992Best RegardsClear Ether

author: vk6zgo
author_rank: Super Contributor
timestamp: March 03, 2012, 07:59:23 am
content: I'm a bit bemused!The initial comments in the report gave me the impression that it was something to do with atom spins--like in NMR & MRI machines, but further reading,plus the picture of the butchered dish,seems to indicate that all they are referring to is,as sonicj says:Circular PolarisationThis is a known & well used technique in Radio Communication,but it is normally utilised to compensate for polarisation changes over the path between the local Tx & the remote Rx.If linear polarisation is used,& there is substantial polarisation change in the path,the received signal can be reduced progressively,until if the polarisation change reaches 90degrees,the signal at the Rx input will be -20dB wrt that when the original polarisation is maintained.Circular polarisation does not suffer this effect,providing both antennas are Circularly polarised in the same sense.A Righthand Circular Polarisation (RHCP) Tx transmitting to a LHCP Rx will normally cause the same 20dB drop in signal at the Rx terminals,compared to the case where both are RHCP or LHCP.This presents the possibility of sending one signal on a RHCP-RHCP system,& the other on a LHCP-LHCP system,where they will theoretically have 20dB of isolation,but I don't think it has been done commercially.The report only showed one antenna,so it doesn't seem that is what they are doing.It would seem these Physicists are so narrowly specified to what they do,that they have no knowledge of around 70 years of Radio Communications Theory & Practice.

author: ejeffrey
author_rank: Super Contributor
timestamp: March 03, 2012, 09:47:02 am
content: This is not circular polarization. It is the use of Laguerre-Gauss modes which are spatial modes that have integer units of angular momentum (L = +/-1, +/-2, ...). It doesn't have anything to do with polarization, and could be combined with both polarizations. This is a phase shift that varies across the wave front, with a net value of 2*pi*L if you make a complete loop around the beam axis. At the center of the beam axis there is a phase singularity, and thus the intensity is zero. They are called orbital angular momentum states in contrast to polarization which is considered spin angular momentum (since it is related to the spin-1 nature of the photon).Anyway, I haven't looked at the paper, so I won't comment on how significant this work is. OAM states have been demonstrated in optical communication for some time now. I would have expected it to be quite difficult to use OAM on radio communication in any useful fashion. You normally need to collect most or all of the beam in order to resolve the spatial mode. This is no problem in optics where a laser can easily propagate in a beam-like fashion for miles, but is more troublesome with RF due to the much stronger diffraction.

author: vk6zgo
author_rank: Super Contributor
timestamp: March 03, 2012, 11:16:29 am
content: OK,that is interesting!Way beyond "little moi",though!I wonder how they achieve such a phase shift,& if any such relationships can be maintained at the frequencies used for Radio Communications.The report does not show any way of obtaining this phase shift in any way which could have separate intelligence impressed upon it .The picture is of a dish antenna which has been quite roughly cut & bent,in a manner which suggests Circular Polarisation!It looks quite dodgy!Propagation from a conventional antenna pretty much follows the Inverse Square Law,so that,even allowing for the dish directivity,at a reasonably long distance the original wavefront is spread over a very large area.With a conventional system it doesn't matter,as if we intercept any part of the wavefront,the information is the same as any other,except for amplitude,(& any propagation anomalies),but in the proposed system,as you remarked,you need to collect most of the beam.This implies a very large antenna array, each element of which is only sensitive to the part of the wave front it is supposed to see---reflections & side lobes need not apply!

author: SoftwareSamurai
author_rank: Regular Contributor
timestamp: March 03, 2012, 03:10:53 pm
content: Although interesting, this appears to be a point-to-point transmission architecture. And if that's the case, I'm not sure how this is a "breakthrough".Are they proposing that they can transmit two or more signals on the same frequencyon the same antenna? No, I don't think so. So the best that can be achieved here is two or more antennas, spacially separated and aimed at the same receiving point. All that's necessary at the receiving point is to align a separate parabolic dish towards each transmitting antenna and toss the shrimp on the barbie. Done and done.Am I missing something?

author: Simon
author_rank: Unknown
timestamp: March 03, 2012, 10:44:54 pm
content: well I'm not expert on RF and this sort of thing but looks like total bullshit to me, I guess there will be a spray on version coming out soon too......

author: vk6zgo
author_rank: Super Contributor
timestamp: March 04, 2012, 05:08:33 am
content: Quote from: Simon on March 03, 2012, 10:44:54 pmI dunno,ejeffrey's explanation makes sense as to the basic concept.What I can't see,is quite how the people in the report achieved the kind of phase relationship needed,simply by butchering a dish,or how they modulated the carrier so each channel's information only appeared on that part of the signal which had the correct phase relationship.The description said something about timing the different information,but surely this would limit the data rate to that of a normal system using TDM.Quadrature modulation would seem to do the same job,with a better data rate.

author: gregariz
author_rank: Frequent Contributor
timestamp: March 04, 2012, 06:57:13 am
content: This looks similar to an idea (twisted light) that came up a few years ago in a few talks at the conferences. I remember seeing an array developed by one of the defence companies however it looked to me to be only really practical at the higher GHz/THz zones (because of the need for an array to form the twist)IIRC the idea was that it was propagating in normal mode ie Linear or circular polarity however it added the additional trick of traveling in somewhat of a corkscrew fashion (rather than direct point to point).... but I havent seen much discussion of this since then so my memory may be a bit flawed here. I dont think for low frequency comms in the sweet zone (ie to 3GHz) that this is going to change the world.This looks a bit like it here;http://www.newscientist.com/article/dn16591-twisted-radio-beams-could-untangle-the-airwaves.html

author: Simon
author_rank: Unknown
timestamp: March 04, 2012, 07:26:58 am
content: as far as I know you can have clockwise and anticlockwise polarisation but that involve spinning diopoles, the basic concepts might be sound but i suspect they have been "twisted" (pun intended) to suit the fancy looking dish. This looks along the lines of the usual scam: "people will not believe this one simple trick and we are going to show it to you....."

author: IanB
author_rank: Super Contributor
timestamp: March 04, 2012, 07:30:40 am
content: Quote from: SoftwareSamurai on March 03, 2012, 03:10:53 pmYes, that is basically what was demonstrated in the experiments. Quoting from the BBC article:In a highly publicised event in 2011, the team used a normal antenna and their modified antenna to send waves of 2.4 GHz - a band used by wi-fi - to send two audio signals within the bandwidth normally required by one. They repeated the experiment later with two television signals.

author: vk6zgo
author_rank: Super Contributor
timestamp: March 04, 2012, 10:35:52 am
content: I found a link to a more complete article had been posted on QRZ.com:http://iopscience.iop.org/1367-2630/14/3/033001/articleYou will note in section (2) of this article,that one "channel" was transmitted with a linearly polarised yagi,& the other with the butchered dish.The diagram at the bottom doesn't show the yagi,though!

author: Simon
author_rank: Unknown
timestamp: March 04, 2012, 10:40:51 am
content: so the evidence of the test working was a projection on a building wall that says "signal received" yes very sensationalistic, not very scientific, what about the other signal ?

author: vk6zgo
author_rank: Super Contributor
timestamp: March 04, 2012, 11:26:09 am
content: Over 442m,you would probably get 20dB of rejection with one vertically & one horizontally polarised antenna,with a lot less drama!Oh well, I guess we will have to see how it turns out.Wecouldtry the same trick & see if it works,I suppose!It can't be that hard to ruin a dish antenna!

author: SoftwareSamurai
author_rank: Regular Contributor
timestamp: March 05, 2012, 03:31:28 pm
content: Quote from: IanB on March 04, 2012, 07:30:40 amFromthis article, they stated that they usedtwoantennae: One was a Yagi-Uda, and the other was their butchered parabolic dish."The ? = 0 source was radiated with linear polarization by a commercial 16.5 dBi gain Yagi–Uda antenna . To generate the ? = 1 vortex beam, we mechanically modified a 26 dBi commercial off-axis parabolic antenna, with diameter D = 80 cm, to attain an off-axis spiral parabolic-shaped phase mask reflector."Perhaps I'm missing the part of the experiment where they attached the two identical-frequency FM transmitters to their butchered parabolic dish?

author: SgtRock
author_rank: Unknown
timestamp: March 06, 2012, 06:44:37 pm
content: Greetings EEVBees:--Science Daily has an article, "Pasta-Shaped Radio Waves Beamed Across Venice", about this experiment which includes some additional explanation, see below link.--http://www.sciencedaily.com/releases/2012/03/120302083011.htm--The parts about the dish and the pasta I understand, but where is the tasty red gravy."Before I came here I was confused about this subject. Having listened to your lecture I am still confused. But on a higher level."Enrico Fermi 1901 1954Best RegardsClear Ether