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

[Scott Howard]

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/