[Lowfer] Spatial and time diversity for QRSS

[JD]

Hi Craig. White noise differs from other random noise mainly in its spectral 
composition.  Summing two random sources of equal level doubles the noise 
power (3 dB), whereas summing two coherent sources in phase doubles the 
signal voltage (6 dB).  Thus, if you can achieve phase coherence of two 
copies of the signal, and the noise accompanying each is random, then you 
get a net 3 dB S/N advantage with each doubling of sources.  If the noise is 
not truly random in the two copies (ie, powerline buzz) or the signal cannot 
be lined up in precise phase match, the advantage will be less.

I'm not sure about your time diversity ideas where the same signal is 
overlaid on itself with time delay. That will null certain specific 
frequencies each time, according to the delay period, but it will also 
introduce smear in the signal rise and fall times, and has the need (as you 
noted) to keep the signal carrier in phase with itself or it too will suffer 
cancellation part of the time along with the noise.

>>> The third time diversity option I'm testing is to fold in 2 or more 
>>> copies of the same QRSS signal from adjacent iterations of the full call 
>>> being sent.

Doing this with adjacent iterations of the call at the display stage can be 
quite effective.  As you noted, NDBfinder was a good example, but it really 
wasn't designed for QRSS.  I'm not sure if anyone has been working on an 
improved version.

I wish I were enough of a software guru to take a long-term set of FFT data 
from a spectrum program and try overlaying the bins periodically to display 
a summed result.  It'd take some user input to line up the instances, 
though, unless the repetition rate were known very precisely; and of course, 
if the transmitted signal or the receiver drifts in frequency during the 
capture it wouldn't work at all because adjacent iterations wouldn't be 
depositing energy in the same set of bins.

John