FYI, message from Joe/K1JT  (permission to forward it to the group).

73, Stefan/DK7FC

-------- Original-Nachricht --------

Betreff:	Re: Fwd: LF: Experimental software for WSPR-8 and -32
Datum:	Sat, 22 Sep 2012 14:11:10 -0400
Von:	Joe Taylor
An:	Stefan Schäfer

[...]

I had some free time recently to think further about a more sensitive 
mode for MF and LF use.  Here is a very brief and preliminary planning 
statement, including simulation results for the decoding thresholds that 
should be reached.

############################## JT8 ###################################

JT8 is a mode designed for amateur QSOs and beacon-like transmissions at 
MF and LF.  The mode uses the same 72-bit user messages as JT65, 
augmented by a 12-bit cyclic redundancy check (CRC).  Error-control 
coding uses a convolutional code with constraint length K=16, rate 
r=1/2, and a zero tail, leading to an encoded message length of 
(72+12+15)*2 = 198 bits.  Modulation is 8-FSK, so a transmission 
requires 198/3 = 66 information-carrying channel symbols.  Two 8x8 
Costas arrays are added to each transmission for purposes of time and 
frequency synchronization.  A full transmission thus contains 66 + 16 = 
82 symbols.  Tone spacing df of the 8-FSK modulation is equal to the 
keying rate; symbol duration tsym = 1/df, and the total occupied 
bandwidth is 8*df.  The actual transmission length TxT is slightly less 
than the T/R sequence time, to allow for possible message decoding 
before the next transmission starts.

Parameters of the five JT8 sub-modes are summarized in the following 
table, along with S/N thresholds measured by simulation on an AWGN 
(additive white Gaussian noise) channel.

Mode   T/R   TxT    df    tsym   BW    S/N*
       (m)   (s)   (Hz)    (s)  (Hz)   (dB)
-----------------------------------------------
JT8-1    1    52  1.577   0.63  12.6  -26.9
JT8-2    2   112  0.732   1.37   5.9  -30.2
JT8-5    5   292  0.281   3.56   2.2  -34.4
JT8-10  10   592  0.139   7.22   1.1  -37.5
JT8-30  30  1792  0.046  21.85   0.4  -42.3
-----------------------------------------------
* Noise power measured in a 2500 Hz bandwidth.

########################################################################

As you can see, with 2-minute transmissions JT8-2 has 1-2 dB better 
sensitivity than WSPR.  This is because 8-FSK is more efficient than 
4-FSK and because in JT8 less Tx energy is "wasted" on the 
synchronization task.  JT8-30 does about 12 dB better than JT8-2.

As time is available in coming weeks, I hope to start some coding.

Your comments would of course be welcome!

	-- 73, Joe, K1JT