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# Re: Saudisat status

```Wellllllll,   I guess you could put it that way.
On a good day, with strong signals, the G3RUH is
indeed GMSK compatible since GMSK is a filtered
FSK.   This tells less than half the story.

MSK, or FFSK, is the version of 2 level FSK (two tones
encoding zero or one), where the shift is exactly equal
to one half the baud rate.  So, 9600 bps MSK would have
a shift of 4800 Hz.    GMSK  means that before the pulse
is allowed to modulate the FM transmitter, it has been
filtered by a filter whose shape is  gaussian like and greatly
reduces the energy that is present in the sidebands for
this signal type.

In both cases,  MSK and GMSK, the following signal condition is
true.   From the center of one baud, to the center of the next
baud, in a properly equalized signal,  the phase shift is
exactly +90 degrees or -90 degrees.   This fact can be used
in two ways.

(a)  It is  pretty easy to coherently demodulate this signal.
That means follow the center frequency and the phase from
baud to baud.      This is a BIG improvement over the way
G3RUH, K9NG, and most other demodulators do the job
which is simply to measure the deviation from center frequency
at the proper baud time.

(b)   Let me use an example to demonstrate the other gain.
Suppose I transmit  000 in succession and 001 in succession.
From the beginning of the tribit,  I will have incremented phase
-90, -90, -90 for a total of -270.      In the latter I will have
incremented phase -90, -90, +90 for a total of -90.    Thus for
one bit difference, the place where we wind up on the circle
we are wrapping around in the farthest possible point.  With
knowledge of the filter applied by the transmitter, receiver,
and the channel,  we can make use of this kind of information
to recover the most likely path around the circle as we receive
the data.    This is a dynamic program and is commonly called
a viterbi decoder or equalizer.

Both of these things taken together will allow a MUCH less capable
station (hardware wise:  antenna, preamps, etc.) to receive the
data from a GMSK transmission from a satellite at the same bit
error rate as the G3RUH demod.

Stephen Hodgart has been brilliant again, and given us a reasonably
easy to code version the coherent, maximum likelihood path demodulator.
I am not certain this will run on the DSP56002 experimenter boards but
I believe it will.   I coded two G3RUH style modems to run in parallel
with processing power to spare for the DSP2232 (one for each port).
And added a fractionally spaced equalizer ON THE RECEIVE SIDE and should
work for  GMSK until we can get a coherent demodulators (such as Hodgart's)
done.

BEWARE!!!    The coherent demodulator and the Hodgart demodulator
require an  SSB receiver with sufficient bandwidth!!   An FM receiver
will never work.  The only way to use an FM receiver is  the noncoherent
demodulator (G3RUH, K9NG, or the one Pavel did for the Finnish DSP56000
board, or the EVM, or lastly the N4HY DSP2232 demods).  Of  these
the one Pavel did for the Finnish DSP 56000 board or the I did for the
DSP2232 demods are theoretically the best since they have an equalizer in
them but that needs testing to prove.  For the well equipped satellite station,
the G3RUH will work fine as will the others.

Bob

At 01:11 PM 09/27/2000 Wednesday, Jim White wrote:
>GMSK is G3RUH compatible.
>Jim
>wd0e@amsat.org
>
>
>At 11:34 AM 09/27/2000 -0500, you wrote:
>
>> > 9k6 FM GMSK.  The data will look much like the other 9k6 digital
>>
>>
>>OK what is " GMSK " ???
>>
>>John
>
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