LED light bulb 'li-fi' closer, say Chinese scientists

Wi-fi connectivity from a light bulb - or "li-fi" - has come a step closer, according to Chinese scientists.

A microchipped bulb can produce data speeds of up to 150 megabits per second (Mbps), Chi Nan, IT professor at Shanghai's Fudan University told Xinhua News.

http://www.bbc.co.uk/news/technology-24579776

The idea isn't new- infrared communication (between buildings for example) has been possible, though not common afaik, for decades. Tendency to go pear-shaped when it's foggy or raining hard.

eta- Also not very secure even with a tightly focussed beam... in light rain or mist the link may still work, but enough of the light is simply scattered for anyone to pick up.

And as the article says- break the beam and you break the link, so if used indoors you'd have to be very careful where a) you put the sender and receiver and b) you walked

Could be useful, but a bit of a niche idea I'd think.

Edited by billford (Sat 19-Oct-13 10:04:33)

isnt this how remote controls work, they just dont use visible light.

It is, I hadn't thought of that.

Though I suspect the data rate is closer to dial-up than it is to fibre, and they also demonstrate the line-of-sight problem.

They also demonstrate another possible limitation- if there's too much direct light on the receiver (eg sunlight through a window) it saturates and the link doesn't work. Another thing to be borne in mind when positioning it.

Morning Bill

Why should it be "closer to dial-up", when similar devices are used to drive fibre-optic connections etc?

FTTx?

Re the possible WiFi use, I wonder to what degree other infra-red devices experience interference and noise from such random sources as open/solid fuel fires?

In reply to a post by eckiedoo:

Why should it be "closer to dial-up", when similar devices are used to drive fibre-optic connections etc?

A slight exaggeration on my part

But there are major differences- for a fibre the diode output is tightly coupled to the fibre, there's very little loss between sender and receiver (in particular the inverse square law is irrelevant), hopefully no ambient light so not much power required.

With a remote the light is sprayed out over quite a wide angle, inverse square law takes its toll, the level at the receiver has to be high enough not to be swamped by ambient light so a fair amount of power is needed. It's harder to switch high powers than low ones.

There's also multi-path propagation (echoes) to think about- I can often use my remotes pointing directly away from the receiver by bouncing the light off a wall or two. So there can be several metres path length differences, say 10nsec difference in arrival times which could make anything above about 20Mbps distinctly iffy.

The same applies with fibre- if you want the maximum possible speed and to hell with the expense you use single-mode low-dispersion fibre with a highly coherent laser, not multi-mode with a diode. The multi-mode effectively means multi-path, and higher dispersion means the light velocity varies with frequency, thus different "colours" sent at the same instant are separated by the time they get to the receiver.

Oops, forgot about:

In reply to a post by eckiedoo:

I wonder to what degree other infra-red devices experience interference and noise from such random sources as open/solid fuel fires?

I'm not entirely sure about this, but afaik the diodes in remotes, whilst not lasers, only emit over a fairly narrow band and the the cover over the receiver only passes a (narrow-ish) range of wavelengths. Fires etc are broadband emitters (not far off black bodies), so there's not much power in the frequency band that the remote uses.

In theory that applies to the sun as well, but in practice it starts off with a hell of a lot more power than your average fire, solid fuel or electric!

Many years ago I arrived home with my fine new Mercedes-Benz to find my infra-red transmitter failed to open the garage door. Of course, I immediately swapped with the one from my wife's Renault ... which didn't work either, but mine worked in the Renault. Turned out the Merc had the new infra-red suppressing windscreen glass to keep it cool on hot days. We had to change to radio remotes.

The wonders of competing technologies

Back to the HE and TE modes in waveguides!