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LF: [TECH] WOLF, FDK, AFK, etc.

To: [email protected]
Subject: LF: [TECH] WOLF, FDK, AFK, etc.
From: "Stewart Nelson" <[email protected]>
Date: Mon, 16 Apr 2001 16:41:41 +0200
Organization: SC Group
References: <[email protected]> <[email protected]> <[email protected]>
Reply-to: [email protected]
Sender: <[email protected]>
Hi Steve and all,

Sorry that some statements on my WOLF page might be misleading.

Speaking about "multitone FSK", I meant COFDM (coded orthogonal frequency
division multiplexing), and similar systems where the simultaneous tones
are independently controlled.  In comparison with conventional FSK, m-ary FSK,
MSK, OQPSK, and other constant-envelope forms of FM, I believe that the
penalty I described is accurate.

You are correct, of course, in observing that PSK also has a penalty, if it
is filtered to keep the bandwidth requirement reasonable.  I never stated
that there was none, but that might have been implied by the initial absence
of information about it.  I did not omit this information to make what I was
"selling" look good, it was because early versions of WOLF did not offer any
filtering!  When the envelope shaping was added, information about signal
loss was duly included.  However, because it is described on a different
section of the page, it might still be taken as misleading.  The "multitone"
section has now been updated for additional clarity.

You are also correct that in the special case of dual tones symmetrically
located about the carrier frequency, in the absence of filtering, the penalty
can be avoided.  However, I don't think that this is particularly interesting,
because if the number of frequency differences is small (2 or 4), the signal
is very similar to WOLF (BPSK with some redundancy added to aid tracking).
If it's large, e.g. a pair for each letter of the alphabet, I don't see a
good way to track the signal when it is very weak.  Your web page doesn't
have any information on how such tracking might be accomplished.  So, IMO,
although a strong FDK signal can be received without the need for good
frequency accuracy, in the weak signal case it has needless complexity
with no real benefit.

That brings us around to AFK (known in the literature as m-ary FSK), which
is IMO the technically best system for weak signal LF.  I will admit that
BPSK was chosen for WOLF, for the simple reason that many LowFERs already had
the ability to transmit it, and it would be relatively easy for others to
gain the capability.  Like almost everything else in engineering, it was
an economic tradeoff.

But I really believe that if it is possible to complete a transatlantic LF
QSO in one hour, with a signal that is about 1 Hz wide, AFK is the way to go.
The main problem is that it can presently only be generated with an HF SSB
transceiver feeding an LF transverter, plus taking special measures to ensure
excellent stability.  There are very few stations with this capability.
Some have suggested that dynamically reprogramming a DDS would work, but
it would be very difficult to maintain the phase coherency required for
good performance.

So IMO the real challenge for AFK is to develop a very simple exciter
which can generate this format, as well as others.  I have some ideas
on the subject, and would be glad to start a discussion, if any are
interested.

73,

Stewart KK7KA

There is just one thing I don't understand on the site:-

start quote :-

"Multitone FSK, etc.
I believe that sending multiple tones at once is not suitable for
weak-signal LF work. Virtually all PA's in use are limited by PEP (rather
than thermally). Therefore, sending two tones requires that each be 6 dB
below the level permitted for a single tone, to avoid clipping the envelope.
Compared with sending the tones sequentially, there is a still a 3 dB loss,
even after allowing for the longer durations. "

end quote :-

The thing I don't understand is how did you manage to create a BPSK signal
(which I understand WOLF is) without creating two symmetrical sidebands ???
I thought if you generated a BPSK signal at some rate it would create at
least one pair of sidebands (two tones).    All my simulations do this.
How did you manage to eliminate the two tones ?

The second thing I don't understand is that you say there is a 6dB penalty.
This is true for linear systems.    However, one of the recommended methods
for WOLF (apparently widely used} is to use a XOR gate phase switcher.
Using this scheme the penalty is 3.92dB, not 6dB.    Taking into account the
longer durations (which you correctly identify gives 3dB gain) the penalty
is only 0.92dB compared to a single tone.



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