Hi Take,
On 7/5/2018 9:55 PM, Tsutsumi Takehiko JA5AEA wrote:
I am waiting your response to my previous message to recalculate
additional gain for myself.
To make sure my intention, I described the inquiry as follow.
One FT8 frame has 7x7x3 =147 bits synch words and I understand current
DX Pedition mode locates them in each FDM slot. Thus, we can shrink them
from 147 x 5 (= 735) to 147 bit in TDM frame at N=5 slots.
/– Synchronization: 7×7 Costas arrays at start, middle, and end/
I am waiting your response soon.
Evidently you do not understand what is meant by a "7x7 Costas Array".
This name refers to a sequence of 7 channel symbols at 7 different tone
frequencies and possessing ideal auto-correlation properties in two
dimensions. The numbers 7x7=49 and 7x7x3=147 are wholly irrelevant.
The Costas arrays have nothing to do with "bits"; these symbols do not
carry any message information. Their only purpose in the FT8 protocol
is to allow the receiving software to determine the frequency offset and
starting point of a received signal waveform.
As described in the WSJT-X User Guide, an FT8 transmission consists of
79 symbols: 58 3-bit symbols that convey message information, and 21
symbols in three 7x7 Costas Arrays. If time and frequency
synchronization were provided in some other way, so that synchronizing
symbols were unnecessary, the information-carrying symbols could be made
longer in the ratio 79/58. With no energy devoted to synchronization,
sensitivity would be improved by only 10log(79/58) = 1.3 dB.
You advocate a five-fold reduction in the fraction of signal energy
devoted to synchronization. The consequence of such a change would be
that sync losses would dominate all other causes of decoding failure.
Finally: you seem to suggest that we have not taken care to optimize the
FT8 synchronization scheme, that we do not understand the dependence of
threshold decoding sensitivity on various protocol design parameters,
and that we have not done controlled, reliable simulation experiments to
confirm our theoretical understanding. None of these things is true.
Many further details, including results of sensitivity-measuring
simulations, can be found in these papers:
http://physics.princeton.edu/pulsar/K1JT/JT65.pdf
http://physics.princeton.edu/pulsar/k1jt/FrankeTaylor_QEX_2016.pdf
http://physics.princeton.edu/pulsar/k1jt/MSK144_Protocol_QEX.pdf
http://physics.princeton.edu/pulsar/k1jt/Work_the_World_part2.pdf
I think that by now we have exhausted the potential benefits of this dialog.
With best wishes,
-- 73, Joe, K1JT
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