[aprssig] New VARA-HF Softmodem A Spectacular Improvement For HF APRS

[Jess Haas]

One of the cool things about software modems is that it is possible for
igates to receive multiple modes.  I had a 70cm digipeater/igate going for
a bit that could receive 9600, 1200 or 300 baud. The idea was that if you
couldn't get in but needed to send something you could drop down to a more
robust mode.

On HF this concept seems to have even more potential.

-Jess
K5LLI


On Thu, Jan 12, 2023, 5:15 PM Scott Howard <showard at nd.edu> wrote:

> On Wed, Jan 11, 2023 at 3:40 PM Stephen H Smith via aprssig
> <aprssig at lists.tapr.org> wrote:
> > YES!   Here is the preliminary write-up on the planned test in February.
> >
> >     <http://wa8lmf.net/APRS_VARA_MFSK/>
>
> Thanks, these are interesting results and I'm looking forward to
> hearing how they go!
>
> After VARA HF was released, fldigi added "OFDM" modes [1] which I
> believe are based on the same underlying idea as VARA. If it is
> possible, it could be interesting to see how the fldigi OFDM modes
> (e.g., OFDM500F) compare to VARA HF as well since fldigi is common and
> open-source. John Phelps (KL4YFD, the author of the OFDM modes in
> fldigi) has a nice write up describing these modes here [3, 4].
>
> A note: OFDM modes won't work for connected mode AX.25 because they
> are designed to be robust in extreme fading links and therefore won't
> be able to acknowledge packets within the timeout window. However,
> APRS's unconnected mode (AX.25 UI frames) don't need a response, so
> that caveat shouldn't affect APRS.
>
> With SDR becoming so easily available (heck, people even figured out
> how to do it on a tiny microcontroller [5]!), and DSP/FEC pretty much
> ubiquitous, you can tailor whatever mode you want for specific link
> properties (how fast do you want to go? how low SNR? how much fading
> do you want to be resilient to? how long do you want your time on air
> per packet? what power efficiency do you need?). The hard part is
> picking one, or at least picking the parameters that matter. I think
> that's partly why APRS was so successful: the bell 202 modem was so
> common and the de facto standard in other areas, so the available
> hardware probably made it a relatively easier choice. Everything could
> be designed around the bell 202 modem. The current abundance of choice
> is probably a good and new challenge for the community & APRS
> Foundation that didn't exist when APRS was created.
>
> -Scott KD9PDP
>
> [1] http://www.w1hkj.com/FldigiHelp/OFDM_page.html
> [2] https://rosmodem.files.wordpress.com/2017/09/levels.jpg
> [3] https://groups.io/g/NBEMS-HI/message/1273
> [4] https://sourceforge.net/p/fldigi/mailman/message/37278628/
> [5] https://dl2man.de/
>
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