
Li-Fi Gets Ready to Compete with Wi-Fi - moreati
http://spectrum.ieee.org/telecom/internet/lifi-gets-ready-to-compete-with-wifi#.VG6wkfFznwI.hackernews
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wyc
I know that even cheap LEDs have rise and fall times of easily under 5 ns with
the right circuitry[0]. This means that a transmitter of 100 MHz can be easily
built with COTS parts, so the speeds they mention make sense to me.

However, when I built a cheap optical communications system with LEDs and
$0.10 diodes, the tough part was always getting the photodiode capacitance low
enough to receive high frequencies while still sensitive enough to detect the
signal accurately[1]. This is why lasers are very popular for communications:
a concentrated beam can deliver a very high signal-to-noise ratio. With LEDs,
it's tougher because the beam is dispersed and you don't want to blind people
with high power output, so the signal is a lot harder to receive. It looks
like they went with nice avalanche diodes instead of cheap ones, and I'm sure
that have a well-designed integrated circuit helps a lot too. I'm very excited
to see how this plays out.

[0]:
[http://arxiv.org/ftp/arxiv/papers/1011/1011.1954.pdf](http://arxiv.org/ftp/arxiv/papers/1011/1011.1954.pdf)

[1]:
[http://www.thorlabs.us/tutorials.cfm?tabID=31760](http://www.thorlabs.us/tutorials.cfm?tabID=31760)

~~~
Luc
> This is why lasers are very popular for communications: a concentrated beam
> can deliver a very high signal-to-noise ratio. With LEDs, it's tougher
> because the beam is dispersed and you don't want to blind people with high
> power output

I suppose I am missing something obvious... but laser beams are shaped into a
beam by way of a lens at the end of a tube, right? The laser diodes I've seen
emit light in all directions. You could just as well stick an LED at the
bottom of that tube.

~~~
dfox
Classic lasers directly emit coherent beam without the need for additional
optics. Output aperture of most lasers might seem to emit light into all
directions, but most of the output power is concentrated into the direction of
the resonant cavity. Due to manufacturing tolerances low power laser diodes
tend to produce somewhat irregular output beam (both diverging and uneven)
which is then compensated for using some external optics that also serve to
set required width of the final output beam.

It is impractical to convert light from some arbitrary "point-like" source
(ie. LED) into beam that has low divergence by lenses or similar optics.

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brianpgordon
So the obvious question is... what happens when line-of-sight is lost between
the endpoints? With near-visual wavelengths, someone walking past could
interrupt the connection. I could see this being useful for _very_ limited
applications in settings where radio is completely unusable, but it's a
stretch.

> In 25 years, every lightbulb in your house will have the processing power of
> your cellphone today

And in 50 years, every hammer and wrench in your toolbox will be able to beat
Ken Jennings on Jeopardy!

~~~
TheCoreh
I have another question, could be seriously stupid: Do you need to have a
strong, glowing light on the top of your computer for this to work? Could it
use the monitor backlight? Imagine if the Apple logo in a MacBook is actually
a Li-Fi "antenna".

~~~
Sanddancer
The article says they are using 5 milliwatt LEDs for this, so it doesn't seem
like you need that much brightness at all.

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discardorama
Whatever happened to UWB? Ten years ago, there was so much talk about UWB, and
how it would provide so much bandwidth, etc. And now it's nowhere to be found.
One _huge_ advantage of radio is that it can work through walls; "Li-Fi"
won't.

~~~
kristoffer
UWB is used in the 803.15 WPAN (wireless personal area network) standards.

~~~
bornabox
802.15 WPAN - It's kinda stalled for some time. And UWB really did get good
press with some very promising features / performance back in the day.

If I remember they ended up using it in some system for firefighters that can
'see' through walls, and it might've been used for such a system in the army
as well.

But overall UWB didn't deliver on it's promise.

~~~
amreact
UWB is making steady progress in active tracking systems and passive tracking
systems. I work for a company that does active tracking:
[http://pluslocation.com/](http://pluslocation.com/).

PLUS is an offshoot of Time Domain,
[http://www.timedomain.com/](http://www.timedomain.com/). Time Domain does
radar; I don't think they do any active tracking (could be wrong though). IIRC
Time Domain is the company that did the see-through-walls system.

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efqdalton
It's interesting how things go back and forth in science...

A Brazilian priest studied about using light to transmit data and achieved
that around 1 century ago, even before Marconi

Unfortunately his government didn't supported him, because he "was a mad
priest" that wanted to send messages to other worlds, he said:

"Give me a vibratory movement as large as the distance that separates us from
these other worlds that roll our heads, or below our feet, and I will make the
sound of my voice reach there."

[http://www.landelldemoura.com.br/lm04.htm](http://www.landelldemoura.com.br/lm04.htm)

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cpach
Look here if you want to build your own optical communication system:
[http://ronja.twibright.com/](http://ronja.twibright.com/)

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jodrellblank
Time for the Timex Datalink Watch to make a comeback?

[http://en.wikipedia.org/wiki/Timex_Datalink#Optical_sensor](http://en.wikipedia.org/wiki/Timex_Datalink#Optical_sensor)

 _Upon closer inspection however a small lens at the position of 12 o ' clock
on the watch face indicated the mode of the wireless data transmission through
visible light.[4][16] Data was transmitted from the CRT of the computer
through a series of pulsating horizontal bars,[17][18] that were then focused
by the tiny lens and inputted into the watch EEPROM memory through an
optoelectronic transducer operating in the visible light spectrum and
employing optical scanning technology.[19][20] The CRT synchronization was
possible only for systems operating on Windows 95 and Windows 98. The watch
was compatible with Schedule+ and for the Datalink 70 model the time needed to
download seventy phone numbers was about twenty seconds.[14][17]_

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amjaeger
When my brother and I were younger we would sneak up to the door of each
other's room and flip the light switch. It was mildly annoying. The kids of
the future are going to go ballistic when a younger sibling flicks off their
internet.

~~~
_RPM
I find that this frustrates me now. When I have to restart my router for some
arbitrary reason, I get impatient.

~~~
esaym
Here's my router:
[http://shorewall.net/XenMyWay.html](http://shorewall.net/XenMyWay.html) I
once went 450 days without restarting it :)

~~~
amscanne
And less than half that time was spent configuring it! ;)

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ArchD
The article mentions industrial applications being the first to use this. What
are some example industrial applications where you need a high-bandwidth, low-
latency, link that has to be wireless, that WiFi is not good enough?

~~~
jwilliams
A lot of industrial processes generate immense interference - electroplating
and arc welding spring to mind. I guess those would be a logical starting
point.

~~~
mistercow
True, but arc welding will also create enormous interference in the visible
spectrum.

~~~
electromagnetic
That's easily shielded against by use of an aperture, where as RF interference
is not nearly as easily shielded against.

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NatW
Contrary to the article's title, LiFi doesn't _compete_ with the majority of
WiFi use cases due to its limited propagation characteristics.

The article confirms as much: "“We don’t want to replace Wi-Fi,” he says.
“That’s not our goal.” But Deicke says Li-Fi could complement existing
communications technologies including WiFi"

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fakeyfake
Going to repost this talk they held at UoE about it:
[https://www.youtube.com/watch?v=WRG9iXZbuAc](https://www.youtube.com/watch?v=WRG9iXZbuAc)
(2014-04-23)

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walterbell
Would this be less susceptible to local surveillance than wifi, e.g. within a
few hundred feet? How would it deal with interference from overlapping beams?

~~~
wyc
Line-of-sight communications are definitely less susceptible to surveillance
than WiFi that has omnidirectional broadcast. Even if the signal bleeds due to
refraction, at such high frequencies, this loss of power is extremely
detrimental to making sense of the waveforms.

Multipath interference can be managed with existing communications techniques,
as light isn't the only EM wave that bounces off of things.

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WalterBright
I'd like Li-Fi even for desktops. It's one less wire in that rat's nest behind
the computer.

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Igglyboo
Won't this technology be very distracting with the lights constantly
flickering/pulsating?

~~~
teraflop
At the signaling rates they're talking about, the flicker frequency will be
orders of magnitude too fast to be perceptible.

~~~
Dylan16807
Think about the packet rate. Little flickers whenever it has something to
send.

~~~
teraflop
Hmm, true, but that could be remedied by transmitting the signal with
something like Manchester encoding, so that the average intensity is constant
whether or not data is being sent.

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bch
see also: Reasonable Optical Near Joint Access (RONJA) --
[http://en.wikipedia.org/wiki/RONJA](http://en.wikipedia.org/wiki/RONJA)

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gima
Let the headaches begin.

