[Ronja] 100Mbit over LEDs

[Gregory Maxwell]

How much thought has been given to the possibility of achieving
100mbit using LEDs?

In many countries laser emitters are subject to much more regulation
than other visible light sources.  For example, in the United States
any laser with more than 5mw output is at risk of complications from
regulations, even if the aperture is so large that only a tiny amount
of light could enter the eye. Non-laser sources are not subject to the
same scrutiny. Also, for a given output power non-laser sources are
much cheaper (red-orange Luxeon 3W outputs >50mw of light and costs
$10 .. I'm not aware of any laser diode with price performance near
that)

Many years ago I measured the bandwidths of some leds that I had and
found that they had a decent response to about 20MHz. I do not know
what the ronja hardware delivers, but I would assume that it is around
that, since about that much is required for 10mbit performance.   This
is not anywhere near enough bandwidth to do 100mbit using the binary
NRZ encoding that ronja uses.  It is also not enough to do 100mbit
using the three level encoding used on 100TX (which needs about
35MHz).

It should, however, be possible to encode 100mbit into that bandpass
by using more levels of amplitude modulation.   Using multi-level
amplitude modulation will require better signal to noise ratios and
good driver and photo-cell linearity.  I would not expect SNR to be a
problem for ronja, but the use of multi-level encoding would add a lot
of complexity to the electronics.

Has this line of thinking been explored already?