Back in March, a group of Italian and Swedish radio researchers demonstrated that a characteristic of radio waves called orbital angular momentum (OAM) can be used as a multiplexing technique, vastly increasing the theoretical capacity of wireless transmissions. Now, according to Nature Photonics, the same technique has been …
I have to ask
Are they putting a spin on how they are reporting this? It almost sounds like they want us to believe that they will be able to scale bandwidth indefinitely. Perhaps some of the brighter lights here can explain why this isn't just multiplexing.
Re: I have to ask
What they've demonstrated isn't multiplexing as we used to it as that's about sharing multiple data lines or signals over a single carrier. What they've done with OAM is to use the same carrier multiple times concurrently.
They've demontrated that they can fill a single carrier (radio or optical) with 8 times the capacity we know today by putting 8 signals over it at the same time using the 'spins' without changing the carrier frequency and without those signals interfering with each other as they would normally.
As for infinite - RTFA. "...OAM is hailed as offering “infinite” bandwidth, it’s limited by how finely “spin” can be manipulated and detected."
Intuitively (= I don't know any of the details of the science involved) I would picture it as a series of slightly out-of-phase helices wrapped together. So you would need separation in the order of the wavelength used, which is quite a large number of waveforms, then x 2 because of the inverse set. So not indefinite scaling, but certainly a chunky increase.
I was about to say that exact same thing :D
Equally, it fails in multi-path
Once it gets reflected about a bit, the information on the OAM gets lost very quickly and is no longer recoverable.
Thus this technique is really only usable for point-to-point tight-beam transmission.
Which really means it's a bit 'meh' for radio-frequency below the tens-to-hundreds of GHz (when you have to do point-to-point tight-beam anyway as nothing else has reasonable power requirements for decent range), and equally not that useful in optical until you're in free space as the photons get scattered by the atmosphere.
I do think it would work pretty well in single-mode fibre though, which is interesting - if they're at a petabit/s scale in fibre already with OAM, OAM might add another order of magnitude.
Re: Equally, it fails in multi-path
Hence the in-space applications.
Re: Equally, it fails in multi-path
Bugger, that should read "..if they're at a petabit/s scale in fibre already without OAM..."
The fail is clearly me..
Infinite bandwidth... pfffft.
Sure, I could give you infinite bandwith if I had a zero noise channel and an infinitely powerful DSP at either end.
As a bear of little brain, this appears to be another mode of orthogonality.
Shannon still applies.
How noisy is and OAM modulated channel? If it is related to the relative polarisation of the carrier, the channel is going to be good and noisy particularly in a multipath environment.
Not naysaying, quite the contrary: WTG you folks, nice one! But tone down the rhetoric for pity's sake.
Re: Infinite bandwidth... pfffft.
First time I have seen the name Shannon mentioned anywhere near this research! Yay!
Physical expertise required!
I didn't bother to read up on the science, but the thing you discuss is nothing else than using polarization as an extra degree of freedom for data transmission. Unfortunately, El Reg / the author has failed to grasp the rather simple physics behind this story and therefore offers a lot of very unphysical comments.
"Orbital angular momentum in waves is difficult to explain, because frankly, any non-mathematical explanation is at best an imperfect metaphor."
No it's not difficult to explain: If you add a beam with polarization in the x-axis and another with polarization in the y-axis (the z-axis being the propagation direction), you get a 3-dimensional wave in space and time. Upon modulation of both signals, the resulting wave may look quite complex (e.g., producing a helical wavefront), but the signal can always be decomposed into the x and y polarized components -- quite simple really. Everything else, including the moniker "orbital angular momentum" is spin, not of the physical type, but rather of the advertisement type.
"El Reg notes that while OAM is hailed as offering “infinite” bandwidth, it’s limited by how finely “spin” can be manipulated and detected. It is, however, a very young technology."
Photons have no spin, neither does any other electromagnetic wave. The angular momentum in the polarization shaped beam adds information density relative to a linearly (or circularly) polarized beam. Indeed, the information content is easily established because any helical polarization can be created as the sum of two linearly polarized beams. The information content is therefore exactly twice as large as that of the linearly polarized case. Apart from that, the information content is a question of bandwidth -- and this in principle infinite if you assume that you have emitters / receivers with infinite bandwidth. Nothing new here.
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