Re: LF: JT-8 !

[Roger Lapthorn <rogerlapthorn@gmail.com>]

Hi Stefan,

Thank you for forwarding the message from Joe K1JT. 

This is most encouraging: JT8-30 at ~12dB better than WSPR2 would be so useful on LF/MF.  I hope Joe can fit this exciting development around his busy work schedule. His superb weak signal software is such an asset to the amateur community.

73s
Roger G3XBM

On 22 September 2012 20:07, Stefan Schäfer <Stefan.Schaefer@iup.uni-heidelberg.de> wrote:

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

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