[Laser] 5mw laser tranceiver kit

[Tim Toast]

Hi Tom

Yes, i believe there might be a couple ways to partially cancel 
out atmospheric scintillation by using a full duplex link. I 
think it would amount to a fast ALC at the transmitter using 
feedback from the distant receiver "target". Or a fast AGC at 
the receiver which uses a pilot subcarrier tone from the 
transmitter to control it. A kind of real-time active feedback 
of channel conditions from one station to the other.  

1.
The distant receiver supplies the "feedback" by sending out a 
pilot tone, say 1 khz, with constant amplitude. While traveling 
through the atmospheric path, the tone is amplitude modulated 
with the attenuation that is occuring at that instant. The 
transmitting station uses that data to "pre-adapt" its own 
modulation amplitude according to the current conditions of the 
channel. If the channel is fading, the transmitted audio is 
increased. If the channel is peaking, the audio is reduced. 
The result is an outgoing audio stream that tries to pre-
compensate for channel conditions in real time. Since the 
transmitter and receiver both share aproximately the same 
atmospheric path, signals coming from either station would 
contain nearly the same path-loss data. 

This ties up one of the duplex paths of course. On the reverse 
communication, the roles are reversed and the other station 
transmits the tone signal. So whichever station is listening, 
sends out an unmodulated tone for the other station to use as 
feedback. 

To restore the full duplex, the feedback pilot tone could be 
moved out of the baseband, to a higher frequency where it would 
not interfere. Like a 20 khz tone. This amounts to an unmodulated 
subcarrier. At the transmitter where the tone is used, it is 
detected and the low frequency envelope is used to drive the 
transmitter ALC. So, each receiver has two channels, one for the 
tone and the other for regular audio. Idealy if the the channel 
fades 10 dB, the transmitter increases amplitude by the same 
amount, 10 dB. 

2.
Another way to do this would be for the transmitting station to 
send the reference tone signal piggy-backed on its own audio as 
a subcarrier. The receiver would use this pilot tone to drive its 
AGC, so that it's receiver gain is changed to compensate for the 
transmitted tone scintillations (and thus the received baseband 
audio signal since it shares the exact same path literally). So 
the receiver station modulates its receiver gain in accordance 
with the pilot tone variations. In this case the transmitter sends 
plain audio that is uncompensated and the pilot subcarrier. It is 
at the receiver where the scintillations are cancelled. This is 
really the tradishional receiver AGC although it is not derived 
from the base band audio but the pilot carrier or subcarrier 
signal instead. This second method may be a bit better than the 
first since it uses a reference tone that travels in the same 
path through the entire atmospheric path, whereas the first 
method is offset a little from the same path and so is a bit 
different. A combination of both a receiver subcarrier AGC and 
feedback ALC at the transmitter could increase the combined 
effects. In this case both stations are supplying a reference 
signal the other uses. 

Other ways to do this are diversity reception and actually using 
the whole signal instead of only part of it. Diversity reception 
amounts to using more than one small piece of the light beam. An 
extreme case of diversity reception is: the receiver lens captures 
the entire light beam from the transmitter, where it will be 
averaged on its photodetector (if it is big enough). This is the 
type of thing Yves talks about when he removes scintillation by 
looking at the beam spot reflected by some object. The entire beam 
spot is focused on the detector. And so most of the scintillation 
averages out and goes away. 

Maybe this type of thing is practical to do, i dont know for sure 
since i havent tried it. Maybe it has already been tried before by 
those who know more about it than me. When the observitories use 
the artificial star method, and the complex adaptive optics, they 
are compensating for the direction the light is coming from in 2
dimensions x-y. A third channel would be the amplitude but i think 
the detector itself is integrating the amplitude directly (with 
film or a ccd) and they are not worried with amplitude changes in 
the signal for the most part.


> 
> I wonder if scintillation can be corrected, akin to a
> terrestrial 
> observatory using a laser to produce an artificial star
> whose motion is 
> removed from the objective image?  Could, for
> instance, twin paths - or 
> full-duplex colinear paths - be useful to remove the
> effect?
> 
> Tom
> http://www.ustream.tv/channel/bowcam
> http://www.ustream.tv/channel/cape-coral-marine-radio
> VHF
> http://67.207.143.181/vlf9.m3u Lightning, spherics
>