
 'Twisted' wireless OAM beams carry 2.5 terabits per second - ukdm
http://www.extremetech.com/extreme/131640-infinite-capacity-wireless-vortex-beams-carry-2-5-terabits-per-second
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tanvach
For my research, we looked at optical vortex modulation schemes for a while.
With polarisation (spin angular momentum), there are only two orthogonal
states. With orbital angular momentum you can have infinite integer "charge"
states that are orthogonal (yes you can have negative charge as well). So in
principal by encoding each stream with one of these states, a bunch can be
transmitted at the same time in the same beam.

I might venture a guess that this was done over a short distance in a closed
room. Problem arrises when these beams counter strong turbulence or
scattering, then the vortices are either destroyed or coupled together. So the
channel leakage and signal to noise becomes a problem.

Still neat though, bundling 8 channels is impressive.

~~~
femto
In a previous discussion here, it was figured out that "twisting" is
equivalent to MIMO. Is that your understanding? If so, it will be subject the
same capacity limitations, meaning that capacity is approximately linear with
degrees of freedom, then tapers off to a limit, the limit being related to the
volume occupied by the receiver and transmitter.

Presumably the number of "integer charge" states is in some way related to the
volume of space occupied?

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recursive
> According to Thide, OAM should allow us to twist together an "infinite
> number" of conventional transmission protocols without using any more
> spectrum. In theory,

I don't know much about the science, but anyone who claims their new
breakthrough has an infinite capacity immediately sets off my skepticism.

~~~
DiabloD3
Take a 4TB harddrive back in time to 1979, and tell them it is infinite. They
will not argue with your assessment, even if its not completely correct.

~~~
wolf550e
you are incredibly wrong. "More than I think you'll need" and "infinite" are
not the same. Capped "unlimited" data plans are a good example. Company
expense accounts are another example.

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jws
This test showed 95 bps/hz of spectrum. For cpmparison LTE is 16 bps/hz. They
allude to problems with turbulence, that may make long range terrestrial use
problematic.

But I remember 1 bps/hz being a limit, we've come a long way.

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kalleboo
If this works as well as the hype, it'll still be highly directional, right?
So it'll be great for things like backhaul networks, satellite linkups etc,
but it won't solve the problem of mobile wireless access (WiFi/LTE/etc). Or am
I missing something?

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Stwerp
No, you're right. From everything I've read on this, it is essentially limited
to line of sight as you need the "middle" reference for the other "orbits", or
whatever the heck they're called. (Assuming this is the same technique as the
experiment the Italians displayed in Venice a few months ago.)

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aortega
bps/hz measurements in TFA are bullshit without noise figures. Per Shannon-
Hartley theorem, spectral density depends on the Bandwidth of the channel
(assume infinite for free air) and noise. So what is the noise? of course
DVB-T (Digital TV) has low bps/hz, the antenna is tens of KM away and there is
a lot of noise! the experiment used two antennas located 1 meter away, almost
zero noise. EDIT: You might as well cite the spectral density of the voyager
II spacecraft onboard antenna. Hint: It's very very low.

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DanielBMarkham
Stuff like this is why current SETI efforts seem at best provincial.

We aren't even beginning to understand how intelligent extra-solar beings
would communicate with each other. Current efforts are like a colony of ants
all by themselves on an island somewhere in the middle of the Pacific building
a giant super-pheromone nose on top of a 3-inch anthill.

~~~
mikeash
SETI is not aimed at listening in on communications between aliens, which
would be impractical for the reason you mention and more. SETI's goal is to
listen for deliberate "anybody out there?" transmissions, which one can expect
to be much simpler, since they would be deliberately designed to be universal
and easily understood.

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ImprovedSilence
I really want to see a technical paper on this. the Nature article is pay
walled... also, the image on the bottom right kinda looks like it's spinning
when looked at out of the corner of your eye (ie, when reading the article)

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Stwerp
In a discussion on Slashdot, I was linked to an IEEE article that claims that
all OAM type transmissions are subsets of MIMO. I haven't read this in detail
yet, but so far it seems interesting:

[http://lup.lub.lu.se/luur/download?func=downloadFile&rec...](http://lup.lub.lu.se/luur/download?func=downloadFile&recordOId=2062936&fileOId=2339120)

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nemo1618
It seems like turbulence is an issue when the beam is sent through the air,
but could it be transmitted through a cable over longer distances? I certainly
wouldn't mind upgrading my internet speed from 25 Mb/s to 2500000 Mb/s.

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mikecane
Some day phone makers will subsidize carriers. All towers will have been
built, bandwidth will be extreme (note I did not say "infinite" although I
wouldn't object if that happened!), prices will collapse. Buy a phone, get
included service free.

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adventureful
So there's a nice opportunity: software for OAM.

Build something useful there and write your ticket in an acquisition.

~~~
ovi256
Given that OAM is a modulation technique, which is a physical-layer tech,
nothing above firmware will even know about it. At most software will be able
to read/set a flag enabling use, similar to FX/DX on Ethernet.

~~~
JoeAltmaier
I think its more than that. When you exceed by an order of magnitude the
bitrate, different APIs will be needed to take advantage. Sure you can just
send on a socket, but it will max out at the rate you can call that API
(including kernel switch, copying data, waiting for a completion interrupt
etc).

Some kind of virtual hardware or mapped memory will be desirable to achieve
great leaps in bandwidth usage.

