[Laser] Troposcatter and Information Theory-2

[Andrew T. Flowers, K0SM]

Bob,

I've also had fun playing with Argo around here.  It's exactly that that 
put me on this project.  I am aware of the QRSS CW stuff that being used 
for Lowers here and 136Khz in the rest of the world.  CW and DFCW are 
still relatively inefficient compared pretty inefficent in terms of 
throughput compared to what you can get with a a multi-tone modulation 
scheme.  As an example, imagine that your communications path allows you 
to just barely communicate using the 3-second "dit" length using QRSS 
CW.  In this idea case, you could use frequency bins as small as 0.33Hz 
(I don't want to get into the keying sidebands due to OOK) and produce 
copy--either the bin has a signal in it or it doesn't, producing the 
"1's and 0's" that make up morse code.  Each character needs several 
bits before it is understood as a character--on average about 12 bits. 
 DFCW is a little faster since it converts all dashes (3 bits) to one 
bit.  It is more efficent to use the adjecent frequency bins and have 
one "bit" per character, thus improving throughput tremendously (or 
conversely, you could decrease the S/N and get the same throughput).  

The folks who brave LF have known this for a long time, though 
generating such a signal at RF with the accuracy and stability at RF is 
not a trivial task.  In those cases all you often have is a CW 
transmitter, so QRSSCW makes lots of sense.  For those with more 
elaborate setups, the coding scheme often has some sort of frequency 
synchonization to account for tunning differences.  While necessary, it 
does sacrifice S/N.  Neat stuff!

Andy

riese-k3djc at juno.com wrote:

>Andy
>
>
>I am not sure if you are aware of some of the existing QRSS programs or
>not
>these will go down to the milHz range and at 120 second dot length are
>able
>to go 30 Db under the noise to see the signal. The people that developed
>the 
>programs may be able to help with your questions.. These programs are
>used to 
>work on LF at 135 Khz as well as the band 160 to 190 Khz where part 15
>operators
>can run 1 watt to a 50 ft antenna, with these limitations beacons have
>been copied
>over a 1000 miles,, thats a really low ERP.
>
>suggest checking the Long Wave Club of America at    
>http://www.lwca.org/
>you will find links to the software 
>
>Using ARGO I have been able to copy the sub carrier of the ramsie Tx from
>a double wall bounce
>HAR ,,,,the Tx was in a different room and the dot was placed on a wall
>reflected several times
>before entering the shack and the K3GPG detector which directly fed my LF
>converter, the soundcard
>was fed by the IF rcv
>
>HAve Fun
>
>Bob  K3DJC
>