[600MRG] WSQ - A Weak Signal QSO mode for LF/MF [-25dB snr]

[Ralph Wallio, W0RPK]

http://www.qsl.net/zl1bpu/SOFT/WSQ.htm

Amateur Radio enthusiasts who are interested in the LF and MF bands will 
be familiar with WSPR. Used as a propagation probe or beacon, WSPR 
allows very weak signals to be detected, frequently as weak as -27dB SNR 
or lower. However, once you have established that a signal path exists, 
how then do you hold a QSO?

But what QSO mode matches WSPR for sensitivity?

WSPR is not a QSO mode, and is very slow, so you cast around for 
something else - to find that no 'chat' modes offer performance anywhere 
near the -27dB sensitivity of WSPR. Not Morse, PSK31 or even CMSK will 
reach this sensitivity. Even DominoEX4 will only manage about -18dB SNR. 
The best offering is JASON, with a sensitivity of about -25dB, but it is

V... E... R... Y...     S... L... O... W...

How frustrating!

So, Con ZL2AFP and Murray ZL1BPU decided to do something about this 
problem - to take on the challenge to design a QSO mode capable of 
sensitivity performance near that of WSPR, and much faster than JASON: 
fast enough to allow real QSOs. To do this required a completely new 
approach. Using a new IFK design with more tones, plus text compression 
techniques, we have achieved not quite a comfortable 'chat' speed, but 
at least enough typing speed to allow contact and to exchange 
information within a few minutes. Novice CW operators have been doing 
much the same for decades (although not at -25dB SNR!).