[Laser] NLOS Optical Comms

[TWOSIG at aol.com]

I got to thinking a little more about NLOS communication, particularly the 
suggestion of pointing a laser straight up.  There are in common use by amateur 
astronomers green laser pointers that provide a clearly visible line in the 
night sky.  It would seem a simple task to apply any of the modulation schemes 
previously discussed here and elsewhere to such a laser and to provide a sytem 
of transmission that would be visible (and supposedly de-modulate-able) at a 
considerable distance.

I am thinking that it is probably not very efficient to use the scatter of a 
beam that is only a few milli-radians wide and pointed straight up.  That 
would seem to depend on energy scattered at more than 90 degrees from the 
direction of beam radiation.  I have done some limited tinkering with my red laser 
pointers which lead me to believe that scattering is stronger a small angles to 
the beam travel, both back scatter and forward scatter.  For now, I will skip 
consideration of back scatter.

For communications from point A to point B, it would seem that transmission 
beam that could be used vertically, would provide a stronger signal if it were 
tilted in the direction of B.  Since we are considering NLOS, then there is 
something that prevents B from seeing A, but not the scattered energy from the 
beam.

Suppose, however, that point A needs to broadcast in a 360 degree circle 
instead of to point B in a known direction.  The first suggestion that comes to my 
mind is to spin the beam around the verical axis, which, if the beam is 
pointed up would have no real effect.  If the beam were pointed a few degrees off 
the spin axis, the covered area would be a cone.  The further from "up" the 
beam is pointed, the flatter the cone of coverage.  If the beam is at 90 degrees 
from the spin axis, then the coverage would degenerate from a cone to a plane. 
 Practical situations, such as transmitter location, would dictate the 
necessary angle for an upward cone to clear any obstructions on the "horizon".

A rotating beam does cause some problems with the modulation of information 
in the system.  The amount of scattered energy detectable at any one location 
would be a small fraction of the full rotation.  Hence the beam is by its 
nature pulse modulated at that location.  Depending on the information to be 
transmitted, varying the speed of rotation ( the detector would see it as pulse 
frequency modulation ) could be used. Communication quality voice data could be 
sent if the beam rotated at 30,000 RPS ( 1.8 million revolutions per minute ) 
modulated from 25,000 to 35,000 RPS.  Not a very practical idea.  However, if 
not all 360 degrees needs to be covered with one beam, it might be more 
practical.

I have a laser level that has line generator.  This will provide an output 
beam that is more than 50 degrees wide.  I do not think that my Ramsey receiver 
and laser level line generator are going to make a daytime cross town NLOS 
voice communication system.  But I do think there is benefit in thinking about 
what can be done.

James
N5GUI