----- Original Message -----
Sent: Saturday, September 22, 2012 7:07
PM
Subject: LF: JT-8 !
FYI, message from Joe/K1JT (permission to forward it to
the group).
73, Stefan/DK7FC
-------- Original-Nachricht
--------
[...]
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
