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Re: FEC on over North America



kevin schuchmann wrote:

> Hi Phil,
>   good blocks below a squint of 45 is relatively easy, how well was it doing 
> earlier when the squint was 45 or higher??

I didn't stick around for the high squint angles yesterday, but some 
other stations that have been experimenting with FEC report good results 
  up to a squint of 93 degrees. Above that, frames start to be lost but 
many are still received up to 103.

The FEC doesn't even start breathing hard until about 90 degrees; by 
that, I mean up to 90 degrees the inner Viterbi decoder does nearly all 
of the work, with few if any errors left for the outer Reed-Solomon code 
to fix. Above 90, the Reed-Solomon decoder starts to kick in regularly.

These "concatenated" codes have very steep thresholds. There is a pretty 
well-defined SNR above which they just work perfectly, and below which 
not at all. This is a property shared by all strong FEC schemes, as 
required by Shannon's information theory.

Because of the interleaving in the AO-40 FEC, it's the average SNR of a 
block that mostly counts, not the minimum. That's what makes it so 
powerful against deep spin fading.

Digital satellite broadcasting systems like DirecTV use very similar FEC 
codes to the one I designed for AO-40, though without interleaving since 
those signals don't suffer from spin fading. If you have such a system 
you may have noticed how a very small decrease in signal strength near 
the threshold takes it from "perfect picture" to "garbled blocks" to no 
picture at all.

--Phil

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