[Laser] fundmentals

[James Whitfield]

Glenn,

Thank you for the response.

I disagree that the "one and only" difference is that radio "wavelengths are
somewhat longer."  Unless someone has amateur tools that I have not heard
about yet, we cannot use phase or frequency modulation techniques on the
photon stream that are commonly used for communications with radio waves.
If you want to have a fair comparison, you need to restrict the radio
transmit systems to AM or CW which is an extreme form of AM.  You also need
to limit your receiver systems to simple diode detectors - no product
detectors.  Then limit the Q of RF filters, if you have any.  RF amplifiers
are not allowed, but you can have a really big, and very directional
antenna.  You can have several stages of audio amplification, but they have
to be capacitively coupled.

I will have to take a little time to digest the probability math numbers.
There are some differences in the way that I think of probabilites and what
you must be trying to suggest.  I was taught that the worst probability for
an event is 0.5.   If the probability of you being correct is less than 50
percent, then you simply use the inverse which by definition will then have
probability greater than 50 percent.  That does not seem to relate to what
you said.  I accept that FEC works quite well.  ( It seems a little more
complicated than how a middle value selector works in a fly-by-wire flight
control system, but I have worked with those for two decades. )

Maybe what I am suggesting is that we try to connect an A to D converter of
equivalent to sound card quality, or better, directly to a photodiode
receiving an ON-OFF-Keyed data stream.  Any amplification of the signal from
the photodiode would need to be DC coupled and have no more noise than is
introduced by a K3PGP preamp and sound card.

That could then be compared to sound card processing of MCW, tone PSK,
multi-tone FSK, or any other system that looks promising.  The comparison to
a multi-tone system would need to match data rate:  if you have four tones
in an FSK system the OOK system would need to send two bits in the same time
as one tone interval.  A sixty four data tone system would send six bits.
Something like MT63 sends 64 simultaneous BPSK signals, so 64 bits are sent
in the same time slot.

The idea is to get reasonable numbers for comparing different coding
schemes.  Is there a clear benefit to BPSK at 250 Hz on an optical link
compared to 2300 Hz?  Is QPSK at 125 Hz better?  Is OOK demonstrably better
or worse for optical paths than any single tone system?  Multi-tone?

I  also think that I was wrong to suggest that sampling of a square wave
could be done with only two states..  I don't understand the number of bits
needed to sample a signal, and the practical sampling rate seems to be
about three times the highest frequency component.

James
N5GUI