Re: FT8-Call HF Path Tests

Ev Tupis
 

Hi Tony,

As for a slow data rates allowing more computing time, I doubt it.  The computing time is a function of how long the previous TX interval is allowed to continue.  Computer-processed/augmented, delayed decoding would make any data mode more robust.

It's that sort of conjecture that leads me to ask what makes the mode "Magical".

Ev, W2EV




On Monday, September 24, 2018, 9:25:25 PM EDT, Tony <DXDX@...> wrote:


Ev:

I think the use of a robust modulation type that concentrates it's energy into an efficient narrow-band signal plays a big part. Being able to detect the signal at low s/n ratios gets you so far so I suspect the heavy lifting must come from the code.

I've read that the slow data rate allows more computing time to be spent on correcting errors which helps to decode further into the noise.

There's probably a good reason why they didn't chose PSK modulation. Maybe it's sensitivity to phase distortion would restrict it's a ability to perform as well as MFSK.

Tony -K2MO  



On 9/23/2018 8:02 AM, Ev Tupis via Groups.Io wrote:
Very interesting!  As I reflect on this, I wonder if these results are because of the modulation, the protocol or the demodulation-strategy (or a little of each).

"Would PSK31 (or any other) modulation" deliver similar result if the payload were structured the same and be post-processed prior to display?

"Where does the magic happen?"

Ev, W2EV


On Saturday, September 22, 2018, 9:19:18 PM EDT, Tony <DXDX@...> wrote:


All:

I spent some time testing FT-8 Call to see how well it does in the
noise. I found that it's certainly capable of error-free decodes down to
-22db s/n (3KHz b/w) on AWGN channels. See FT8AWGN screenshot.

It did require better s/n ratios when subjected to multi-path
simulations (common with any mode when s/n ratios are near the decode
threshold) but still managed -18db s/n, error-free decodes. See FT8MP
screenshot.

I also did some weak signal tests on the 40 meter band between New York
and New Zealand. Multi-path is inevitable on such a long path, but the
mode produced fairly consistent decodes down to -23db s/n. See: ZLNY
screenshots 1,2 and 3.

The New Zealand station (ZL2AUB) was running 20 watts to a dipole at 30
feet while I was using 10 watts with a rotatable dipole at 75 feet. I
kept reducing power to get the reports down into the -20db range.

I may be wrong, but I don't believe there's another chat-mode protocol
that can duplicate this type of weak signal performance on HF. Not at
that bandwidth and at that speed. Or is there?

Tony -K2MO








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