[amsat-bb] Re: Can D-Star work on VO-52?

[Edward Cole]

GMSK: <from wikipedia>


Gaussian minimum shift keying or GMSK is a kind 
of continuous phase 
<http://en.wikipedia.org/wiki//wiki/Frequency-shift_keying>frequency-shift 
keying. The baseband 
<http://en.wikipedia.org/wiki//wiki/Modulation>modulation 
is generated by starting with a 
<http://en.wikipedia.org/wiki//wiki/Bitstream>bitstream 
0/1 and a bit-clock giving a 
<http://en.wikipedia.org/wiki//w/index.php?title=Timeslice&action=edit>timeslice 
for each 
<http://en.wikipedia.org/wiki//wiki/Bit>bit. This 
is the type of modulation used in 
<http://en.wikipedia.org/wiki//wiki/Global_System_for_Mobile_Communications>Global 
System for Mobile Communications (GSM) - although 
that system includes 
<http://en.wikipedia.org/wiki//wiki/Differential_encoding>differential 
encoding of the bitstream, which makes the 
overall modulation behave like a kind of 
continuous phase 
<http://en.wikipedia.org/wiki//wiki/Binary_Phase_Shift_Keying>Binary 
Phase Shift Keying with constant 
<http://en.wikipedia.org/wiki//wiki/Envelope_detector>envelope.

The baseband 
<http://en.wikipedia.org/wiki//wiki/Signal_%28information_theory%29>signal 
is generated by first transforming the zero/one 
encoded bits into -1/+1 encoded bits. This -1/+1 
signal is then 
<http://en.wikipedia.org/wiki//wiki/Electronic_filter>filtered 
in such a way that the "boxcar" shaped +1/-1 
<http://en.wikipedia.org/wiki//wiki/Pulse>pulses 
are transformed into 
<http://en.wikipedia.org/wiki//wiki/Normal_distribution>Gaussian-shaped 
signals. The baseband signal is then modulated 
using 
<http://en.wikipedia.org/wiki//wiki/Frequency_modulation>frequency 
modulation, producing a complete GMSK signal. If 
the Gaussian shapes do not overlap, then the 
modulation form is called 1-GMSK. If the slots 
overlap 50% (½), the modulation is called 2-GMSK, and so on.

The more the bits overlap, the more significant 
<http://en.wikipedia.org/wiki//wiki/Intersymbol_interference>intersymbol 
interference (ISI) from adjacent bits will be, 
and for 4-GMSK and up, the ISI seen at any 
particular point in time is stronger than the 
signal from the bit currently being decoded. By 
looking at greater parts of the signal using 
advanced decoder techniques (including 
<http://en.wikipedia.org/wiki//wiki/Viterbi_algorithm>Viterbi 
algorithm decoders), high density codings can be 
decoded efficiently. Currently the highest density coding being used is 5-GMSK.

GMSK has high 
<http://en.wikipedia.org/wiki//wiki/Spectral_efficiency>spectral 
efficiency, but it needs higher 
<http://en.wikipedia.org/wiki//wiki/Electrical_power>power 
level than for instance 
<http://en.wikipedia.org/wiki//wiki/Quadrature_phase-shift_keying>Quadrature 
phase-shift keying 
(<http://en.wikipedia.org/wiki//wiki/QPSK>QPSK) 
to reliably 
<http://en.wikipedia.org/wiki//wiki/Communicate>communicate 
the same amount of <http://en.wikipedia.org/wiki//wiki/Data>data.

<Don't ask me what it means>


73,
Ed - KL7UW
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