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When designing a waveform for packet radio usage, it'd be helpful to know a few things, mainly

  • distribution of coherency bandwidths
  • distribution of coherency times

Whilst such statistics are reasonably possible to generate for higher frequencies based on own measurements, for terrestrial HF propagation, due to the large distances involved and the low useable bandwidths, a sufficient measurement campaign would be infeasible.

HF propagation models are very light on details or vary wildly when it comes to the relevant aspects that would allow for simulation of the same (that being delay spread and doppler spread). Mostly, they're early cold war studies that care only about amplitude attenuation.

  1. Are there more modern channel models for HF propagation?
  2. Alternatively, statistics over the coherency properties?
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  • $\begingroup$ Good question. Only thing I found were approx. cold war period studies on phase and delay modulation due to something like multi-path refraction/reflection from multiple fading/moving layers. One had a simulation model. $\endgroup$ – hotpaw2 Jan 16 at 17:36
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    $\begingroup$ "measurement campaign would be infeasible" why? It might be possible to gather some interesting statistics using something like a crowd-sourced SDR reverse beacon protocol designed for that purpose. So a new question might be how to design an experimental (and legal) modulation scheme that would tease out HF doppler spread and delay variation statistics over large distances. $\endgroup$ – hotpaw2 Jan 16 at 20:19
  • $\begingroup$ I like that idea! The problem is that sufficient statistics actually require a lot of coordinated work to get useful statistics instead of just producing much statistics of "Good" station positions (which coincidentially is where people put their antennas :) ) and very few of the ones where you actually usually lose your APRS connectivity $\endgroup$ – Marcus Müller Jan 16 at 20:56
  • $\begingroup$ But: the measurements would be easy enough to design; the question is how we can legally send measurement beacons $\endgroup$ – Marcus Müller Jan 16 at 20:57
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    $\begingroup$ Different question, close to the answer: ham.stackexchange.com/a/15371/218 $\endgroup$ – Phil Frost - W8II Jan 18 at 0:51
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There are plenty of modern studies of HF propagation. For instance, in the last 10 years, I have seen new models for both sporatic E and selective fading.

For selective fading, the new models look at how faraday rotation causes circular polarization. The modem used by freedv is specifically designed to resist this, so they must have a model for selective fading. I believe that the group designing freedv have published several scholarly papers from their research.

You may also want to research the various HF beacon networks such as wspr, many of which are designed specifically to research propagation. There may also be isosonde data available, but I couldn't find any with a quick search.

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