Hi Take-san,
> Additional gain comes from:
>
> Adopt QRA64 or QRA128 utilizing 3kHz wide fox TX bandwidth : +4dB gain
I don’t think that there is 4 dB to gain here.
Nico showed that the 50% decoding threshold of QRA64 on the AWGN channel is
about -26.5 dB under ideal circumstances. With sync losses, the number is about
-26 dB. The duration of a QRA64 message is 48.4s.
The 50% decoding threshold of standard FT8 on the AWGN channel (including sync
losses) is about -20.5 dB and the duration of an FT8 signal is 12.6 seconds.
Note that the QRA64 signal contains a 72-bit message payload, whereas FT8
conveys a 75-bit payload. Let’s ignore the fact that FT8 conveys more bits for
the moment.
If we shorten the QRA64 signal by a factor of 48.4/12.6=3.84 so that it has the
same 12.6s duration as an FT8 signal, it’s decoding threshold would be
increased by 10*log10(3.84)=5.8 dB, resulting in a decoding threshold of -26.0
+ 5.8 = -20.2 dB, about 0.3 dB worse than FT8. The bandwidth of the sped-up
QRA64 signal would be 3.84*111 Hz = 426 Hz, or a factor of 8.5 times the
bandwidth of FT8.
The nearly equal sensitivity of FT8 and QRA64 (scaled to the same duration as
FT8) would be maintained if both signals were sped up by a factor of 5, say. In
that case, the decoding threshold of both signals would increase by 7 dB. The
bandwidth of the sped-up FT8 signal would be 250 Hz, and the bandwidth of the
QRA64 counterpart would be 5*426=2.1 kHz.
> 65% packed message length base on Bill’s suggestion : +4dB gain
Of course, I agree that some gain would be available from careful source
encoding, rather than just sending 5 standard messages serially.
73 Steve, k9an
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