It is equivalent to QPSK with a good implementation.

On Sun, Mar 21, 2021 at 4:10 PM Chris Thompson <chrisethompson@gmail.com> wrote:
Very interesting Bob and I learnt something.   I saw for example that the Chinese CAS spacecraft are using GMSK and I had not understood the advantage other than bandwidth and power efficiency.   It suggests that a dedicated GMSK decoder will perform better in a side by side comparison and would be a nice project to try. 

Thanks.

73
Chris
G0KLA / AC2CZ

On Sun, Mar 21, 2021 at 1:03 PM Robert McGwier <rwmcgwier@gmail.com> wrote:
Chris:

This is incomplete.  It can be detected and demodulated by an FSK receiver.  This is a form of incoherent processing.  But GMSK is also offset Quadrature Shift Keying with a weird shape function that changes the rate of the phasor goes around the circle between the points on the constellation.  This slowing of the rate of change of phase is how the bandwidth is controlled.  But this also means that treated this way, a more complex demodulator doing coherent demodulation which keeps tracking of the phase angle can perform lots better. Further since the phase shifts are constrained to plus or -90 degrees at the transition from one baud to the next.  This means you can flow the constellation points through a Viterbi decoder and some error correction happens.   This allows GMSK demodulation done this is several dB better than noncoherent FSK detection.

73s
Bob
N4HY


On Sat, Mar 20, 2021 at 5:36 PM Chris Thompson <chrisethompson@gmail.com> wrote:
Tim,

My understanding is that GMSK is just Frequency Shift Keying.  The acronym stands for Gaussian Minimum Shift Keying. The minimum shift part means that the data and carrier are synchronized so that the data changes happen when the carrier is zero. This removes phase discontinuities. The data is run through a Gaussian filter before it is transmitted.  The main objective is to reduce the sideband power. This makes it a very power efficient mode and it is part of the GSM Mobile Phone standard. It is a good choice for something battery powered, like a phone or a small spacecraft. With that said it is harder to decode than straight FSK with a Root Raised Cosine Filter.  So it might be better with strong signals than weak ones. That is just my conjecture. I'm sure there is bit error data somewhere that can answer that question. 

73

Chris 

G0KLA / AC2CZ



On Sat, Mar 20, 2021, 18:14 Timothy Tapio <timothytapio1@gmail.com> wrote:
I know nothing about GMSK but am interested in learning how to decode it...can someone point me to a good URL for information? I did a search on duckduckgo but wasn't finding anything that was a simple enough explanation for me to understand....

73 de Tim  K4SHF


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--
Bob McGwier
Founder, Federated Wireless, Inc
Founder and Technical Advisor, HawkEye 360, Inc
ARDC, member Board of Directors 
Adjunct Professor Virginia Tech
Former Chief Scientist:  The Ted and Karyn Hume Center for National Security and Technology
Senior Member IEEE, Facebook: N4HYBob, ARS: N4HY


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--
Chris E. Thompson
chrisethompson@gmail.com
g0kla@arrl.net


--
Bob McGwier
Founder, Federated Wireless, Inc
Founder and Technical Advisor, HawkEye 360, Inc
ARDC, member Board of Directors 
Adjunct Professor Virginia Tech
Former Chief Scientist:  The Ted and Karyn Hume Center for National Security and Technology
Senior Member IEEE, Facebook: N4HYBob, ARS: N4HY