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Clarify some User Guide text about AP decoding.

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git-svn-id: svn+ssh://svn.code.sf.net/p/wsjt/wsjt/branches/wsjtx@8179 ab8295b8-cf94-4d9e-aec4-7959e3be5d79
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Joe Taylor 2017-10-23 13:41:11 +00:00
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doc/user_guide/en/decoder_notes.adoc
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=== AP Decoding
With the QRA64 decoder Nico Palermo, IV3NWV, introduced a technique
for decoding with the aid of information that naturally accumulates
during a minimal QSO. This _a priori_ (AP) information can be
used to increase the sensitivity of the decoder.
Our decoders for QRA64 and FT8 include optional procedures to use
information that naturally accumulates during a minimal QSO. This _a
priori_ (AP) information can increase the sensitivity of the decoder.
When an operator decides to answer a CQ, he already knows his own
callsign and that of his potential QSO partner. He therefore knows
what to expect for at least 56 of the 72 message bits in a
standard-format response to his call. The _WSJT-X_ decoders for QRA64
and FT8 can use these and similar AP bits to decode messages
containing them with higher sensitivity than otherwise possible.
We have implemented AP decoding in slightly different ways in QRA64
and FT8. To provide some explicit examples for users, we provide here
a brief description of the FT8 behavior.
For example, when an operator decides to answer a CQ, he already knows
his own callsign and that of his potential QSO partner. He therefore
knows what to expect for at least 57 of the 72 message bits in a
standard-format response to his call. The task of the decoder can
therefore be reduced to determining the remaining 15 bits of the
message and ensuring that the resulting decode is reliable. We have
implemented AP decoding in slightly different ways in QRA64 and FT8.
To provide some explicit examples for users, we provide here a brief
description of the FT8 behavior.
AP decoding attempts effectively set the AP bits to the hypothesized
values, as if they had been received correctly. The decoder then