On 25/01/2018 19:23, Don Goldston wrote:
Sirs,
I have not delved into your software, and being old and retired I may
not do so. From the superficial descriptions of FT8 I have found,
your group seems to know very well what they are doing.
https://users.ece.cmu.edu/~koopman/roses/dsn04/koopman04_crc_poly_embedded.pdf
<https://users.ece.cmu.edu/%7Ekoopman/roses/dsn04/koopman04_crc_poly_embedded.pdf>
Here is a link to a paper on CRCs that you should consider for future
design work. The paper gives the best CRC of a given length for a
data block of a given length. You can potentially have equal or
better corruption detection with a smaller CRC, Examine table 3
carefully. I believe you could use an 8 bit CRC (0x97) for data
blocks of up to length 119 bits and achieve a hamming distance of 4,
which is equal to or better than the performance achievable with the
optimal 12 bit CRC at data block lengths above 53 bits in length.
It may be too late to impact FT8, but the principles outlined in the
paper can save you a few bits in future development.
As a general caution, be aware that previous published "standard" and
"good" CRCs, may not be the best even at long block lengths.
If you were already aware of this paper, please forgive me for wasting
your time.
Is there a more detailed description of the various modes available to
members of this list? I am a new ham, but did waveform design and
optimal detection for many years until I retired last May. I may be
able to help in some manner.
73,
Don Goldston, AE0AG
Hi Don,
we are indeed aware of that paper and web site, in fact it is a current
topic of discussion amongst the development team for a few reasons. We
are also aware that a 12-bit CRC is probably more that is necessary, in
fact our CRC polynomial is not the one we intended due to a small
development misunderstanding and is only effectively an 11-bit CRC and
not optimal to boot. Simulations do show it is effective and in theory
substituting a proper optimal 12-bit CRC should improve the already
excellent decoding error rate by a dB or two. It may get changed or we
may re-purpose the "wasted" bit. Clearly a change in the CRC length or
polynomial would obsolete the current protocol so there has to be a
really good reason to make such a disruptive change given the number of
active stations.
73
Bill
G4WJS.
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