Hi Steve,
I am glad I have received a memo from Urbana-Champaign.
I am sorry I confused you by the misuse of the terminology about FTXX and
QRAXX. I meant QRAXX as “Q-ary Repeat-Accumulate Codes for Weak Signal
Communications” in Nico’s literature but I do not have any intent to modify
wsjt-X “QRA64” mode to this discussion. If your are comfortable by the change
to FT64 and FT128, then I will use them from now on.
By the increase of symbol length to 64mS (FT64) and 74.7mS (FT128) from 32mS
(from 160mS FT8 symbol length speed up by factor 5), the gain is 10LOG(64/32) =
+3dB and 10LOG(74.7/32) = +4dB. The necessary occupied bandwidth of FT64 is
1,000Hz and FT128 is 1,714Hz and both are within 3kHz TX limitation.
As we have already included 7dB loss from the speed up by factor 5 in number
“-14 “, the following figure becomes total +1dB gain even at Nslots=5.
“the figure becomes -14 + 7 + 4 + 4 = +1 “
I hope the above explanation is satisfactory for you.
Regards,
take
de JA5AEA
Sent from Mail<https://go.microsoft.com/fwlink/?LinkId=550986> for Windows 10
From: Steven Franke via wsjt-devel <wsjt-devel@lists.sourceforge.net>
Sent: Wednesday, July 4, 2018 9:48:02 AM
To: WSJT software development
Cc: Steven Franke
Subject: Re: [wsjt-devel] Observation on Expedition Mode
Hi Take-san,
Additional gain comes from:
1. 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.
2. 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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