I agree completely with John here, and I can add that ANY psk mode is
unsuitable for EME because of
the phase instability of the path. This is also why coherent CW does not work
for EME.
Rein PA0R
Hi Mike,
Psk125r (and the other r psk modes for that matter) are simply a standard
bpsk mode with the following changes:
1. FEC with a rate of 1/2 which means that we send two bits for every bit of
data. This redundancy is what provides most of it's extra robustness but of
course at the cost of the effective data speed. So psk125r is about (more on
this later) the same typing rate as psk63.
2. Convolutional encoder to spread the bits around so that noise has less
effect
3. Soft-decoder which takes into account the phase and amplitude of the signal
received to decide if it is a strong 1 or 0 or a weak one. Since we
send two bits per data bit, the decoder on the receiver end can than decide
which one is of better quality and has more chances to be the real thing
rather than noise.
4. MFSK varicode for the simple reason that is has for some patterns of
characters about 13% speed gain on the standard psk varicode.
So in conclusion you may ask why bother with double the bandwidth (and
therefore a 3dB power handicap)?
The coding gain from all the above is theoretically 5dB and the tests
performed in the lab show that this is pretty right and therefore +5 -3 =
+2dB of advantage in white noise conditions.
But also (and probably more importantly) the spreading of the bits in time
allows for the impact of noise bursts to be reduced since it is less probable
to have two noise bursts at exactly the same time for the first bit and it's
redundant counterpart which is sent later on.
This set of modes was created mainly for ARQ applications like Pskmail and
Flarq as we wanted to close a gap between the MFSK/IFSK modes which are robust
but slow (while remaining below 500Hz bandwidth) and the psk125,250 and 500
modes which are fast but can be more easily disturbed by QRM and need good s/n
ratios. Remembering that for ARQ applications, one bad bit is most likely one
bad frame that needs to be retransmitted.
Regarding QPSK modes: they have also FEC but the spreading of bit is more
limited and the 90 degree instead of 180 degree spreading of the phase changes
make it only marginally better than bpsk in my experience. In some cases they
can be worse in fact.
Now for your third question: I doubt that this would be a good mode for EME
due to it's level of sensitivity.
I am no expert in EME but the little I know it that you need to have a mode
that has a very low minimum s/n as the path losses are very high. The JT65A
mode has a minimum s/n of -23dB versus something around -13 or -14dB for
psk125r.
Even the slowest psk mode that I know of, PSK10AM (in Patrick's Multipsk) has
a -19.5 minimum s/n, so still a few dBs below JT65A.
If these king of modes had any chance in EME then it would need to be slowed
down even further, plus I don't know what the EME path is like regarding phase
distortion which is a major negative for these modes since they rely and a
phase change to encode a 1 or 0.
Hope this helps.
Best 73s,
John (VK2ETA)
--- In digitalradio@yahoogroups.com, Mike Lebo wrote:
How does pak-125r work? Does it use the same varicode? Does it have error
correcting code like QPSK-125? How many phases does it use? Would it work
well for EME?
n6ief
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