Any graceful degradation scheme should take into account that the main
problem we face is frequency-selective attenuation due to fading. The
mechanism, I am told, is multiple depths of reflection from the ionosphere
leading to multiple phases combined at the antenna. There is a lot of
literature saying that one can actually reduce the fading with circular
receive antenna polarization. I've not gotten to test it.
If you plan to make this work with conventional antennas, you can
experiment with carrier spacing on the theory that some number of carriers
would not be in a fade at one time.
Thanks
Bruce
On Sun, Sep 6, 2015 at 9:01 AM, Tomas Härdin <tjop...@acc.umu.se> wrote:
> Hi
>
> (I tried sending this a few days ago but forgot to confirm my
> subscription to this list - oops! So resending)
>
> My name is Tomas and I've been following the development of codec2 on
> and off for a few years now, and since getting my ham license earlier
> this year I find myself thinking about it more. A few days ago I had
> what I thought was a clever idea for fixing the "digital cliff" problem
> Mike mentioned in a talk that's up on YouTube (I forget which). Today I
> see on the roadmap post[1] that this is currently being worked on using
> two GMSK streams, but I thought "hey, maybe someone will find it
> interesting". So here goes:
>
> The idea assumes that codec2 can make use of bitrate peeling. That is,
> that we can split the stream up into two or more streams where the first
> one provides a rough but usable quality, and subsequent streams improve
> upon this. H.264's SVC would be an example from the video world
>
> So the idea is take these streams and modulate them onto a hierarchical
> QAM system. The simplest would be to take the current FDMDV modem and
> instead of using 14x QPSK (aka 4-QAM) carriers you use 7x 16-QAM
> carriers. You then code the more important bitstream into the most
> significant bits in each 16-QAM symbol, and the improvement stream into
> the lower bits (assuming the two streams have identical bitrate). Since
> you now have half the number of carriers you can put twice the amount of
> power into each carrier
>
> That's about it. Lots of variants are of course possible, but this
> should get the point across. I may experiment with the idea once I get
> some suitable SSB equipment, but for now I'm interested in feedback even
> if it's just shooting it down :)
>
> /Tomas, SA2TMS
>
> [1] http://www.rowetel.com/blog/?p=3931
>
>
>
>
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