Hi Don,
Thanks for your message and your interest in FT8, etc.
We're well aware of the tables of "good CRC polynomials" published by
Koopman, though admittedly we have not always used them to best advantage.
Steve Franke, K9AN, did an extensive series of tests with different CRC
generators when used with the 75-bit information block in FT8. The
CRC12 used in FT8 is slightly sub-optimal, but the resulting increase in
undetected error rate is only a factor of 1.6. Since the error rate is
already very low, the consequences are not serious.
-- Joe, K1JT
On 1/25/2018 2:23 PM, 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
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
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