Hi all,Lots of work has been going on (off list) on optimizations of the JT65 decoder for cases where Doppler spreading is significant. A summary graph of my most recent simulation results is attached.
I've come to believe that using any JT65 submode (including C or even a "D" submode) is not a good choice for Doppler spreads (full limb-to-limb widths) greater than about 10-20 Hz.
In the best case of very low Doppler spreads, JT65 provides 50% decoding probability down to between -24 and -25 dB. This is about 2 dB better than JT4 under low-spread conditions.
(In the graph I have not included JT4 results for Averaging or Deep Search. But you can guess pretty well where those curves should go.)
However, as shown in the attached graph JT4F with the Fano decoder is about 1 dB better than JT65C with the FT decoder at Doppler spread 32 Hz, and by even more at larger spreads.
To me this suggests that for EME purposes a dividing line between JT65C and JT4F should occur somewhere around 2-3 GHz. Use JT65C for 1296 MHz EME, for sure; and use JT4F at 5.7 GHz and above. At 2.3 and 3.4 GHz, take your pick -- but maybe we should agree to recommend one or the other, to reduce confusion?
For skeds you can always try to pick a time of low Doppler spread. Then, even up to 10 GHz you might do well to use JT65C.
Does this make sense? Have I missed anything important?
-- Joe, K1JT
JT65_Simulations.pdf
Description: Adobe PDF document
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