[Laser] Troposcatter and Information Theory-2

[TWOSIG at aol.com]

I am still trying to absorb the ideas on posted on this topic (and recovering 
from Field Day).

I see the benefit of slow transmission rate at 136 KHz, it is obviously a 
bandwidth limited channel.  Similarly a phone modem cannot transmit fast data.

What is not clear to me, is the reason why very long closely spaced tones 
have an advantage sending information through air on a laser beam, which is not a 
bandwidth limited channel.  It seems to me sending the same message with 
shorter tones, but repeating the message, so long as the total throughput is 
constant, should provide the same probability that the message is correctly 
received. 

I think that part of the reason I am not understanding is that things are 
described in radio communications terms or techniques that have been used for 
weak signal radio reception and Very Low Frequency work.  I know that there are 
similarities, but there are also hugh differences when we are talking about 
light beams.

Here is an example of a difference.  The laser diodes that I have been 
working with are notoriously non-linear.  I use a 555 audio frequency square wave 
generator to modulate it, and with CW, I start and stop the generator.  If I did 
that on any RF frequency, I would get terrible key clicks.  Are they on my 
laser signal?  Probably.  Do they interfere with my communication or with anyone 
else's communication?  No.  So I probably do not need to find a way to 
suppress the key clicks.

Another example.  In theory, I could key a crystal oscillator with the 
control line of my Ramsey laser transmitter.  It would be a simple on-off keying of 
the oscillator at about 18 kHz, pulse width modulated.  I could then tune a 
receiver to the crystal frequency and using AM mode, listen to the sounds picked 
up by the microphone.  For that matter, at close range, I could use an 
un-tuned wire for an antenna, a chunk of galena, a cat-whisker, and high impedance 
headphone.  Would it work?  Yes.  Should you use it on HF?  No.

My point is that the differences between laser work and VLF, MF, HF, VHF, 
UHF, SHF, weak signal, and all the rest, are not being communicated, and maybe 
are not being considered.  If a technique works for VLF weak signal, another 
technique might suit laser weak signal better.


James
N5GUI