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
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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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
Sent via AMSAT-BB(a)amsat.org. AMSAT-NA makes this open forum available to all interested persons worldwide without requiring membership. Opinions expressed are solely those of the author, and do not reflect the official views of AMSAT-NA. Not an AMSAT-NA member? Join now to support the amateur satellite program!
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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
Sent via AMSAT-BB(a)amsat.org. AMSAT-NA makes this open forum available to all interested persons worldwide without requiring membership. Opinions expressed are solely those of the author, and do not reflect the official views of AMSAT-NA. Not an AMSAT-NA member? Join now to support the amateur satellite program!
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Sent via AMSAT-BB(a)amsat.org. AMSAT-NA makes this open forum available to all interested persons worldwide without requiring membership. Opinions expressed are solely those of the author, and do not reflect the official views of AMSAT-NA. Not an AMSAT-NA member? Join now to support the amateur satellite program!
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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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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
Sent via AMSAT-BB(a)amsat.org. AMSAT-NA makes this open forum available to all interested persons worldwide without requiring membership. Opinions expressed are solely those of the author, and do not reflect the official views of AMSAT-NA. Not an AMSAT-NA member? Join now to support the amateur satellite program!
View archives of this mailing list at https://mailman.amsat.org/hyperkitty/list/amsat-bb@amsat.org To unsubscribe send an email to amsat-bb-leave(a)amsat.org Manage all of your AMSAT-NA mailing list preferences at https://mailman.amsat.org
Sent via AMSAT-BB(a)amsat.org. AMSAT-NA makes this open forum available to all interested persons worldwide without requiring membership. Opinions expressed are solely those of the author, and do not reflect the official views of AMSAT-NA. Not an AMSAT-NA member? Join now to support the amateur satellite program!
View archives of this mailing list at https://mailman.amsat.org/hyperkitty/list/amsat-bb@amsat.org To unsubscribe send an email to amsat-bb-leave(a)amsat.org Manage all of your AMSAT-NA mailing list preferences at https://mailman.amsat.org
-- 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
Sent via AMSAT-BB(a)amsat.org. AMSAT-NA makes this open forum available to all interested persons worldwide without requiring membership. Opinions expressed are solely those of the author, and do not reflect the official views of AMSAT-NA. Not an AMSAT-NA member? Join now to support the amateur satellite program!
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-- Chris E. Thompson chrisethompson@gmail.com g0kla@arrl.net
On 3/22/21 3:38 PM, Robert McGwier wrote:
It is equivalent to QPSK with a good implementation.
Hello Bob,
I disagree with your statement of equivalence, but only slightly, for a couple of reasons: QPSK encodes 2 bits per symbol, so you should have stated BPSK (1 bit per symbol).
Additionally, GMSK's performance (probability of error) is close, but not equal, to BPSK for a given Eb/N0. The difference is up to several dB of required signal power for the same bit error rate. These numbers assume no channel coding to correct for errors on either system.
There's always a trade-off between bandwidth and power. GMSK trades off power for better-constrained bandwidth, while BPSK does the opposite. Not having to have a well-linear amplification system helps out with system efficiency, but noise performance offsets this gain.
An interesting paper by a pair of JPL employees explores this very subject:
"GMSK Modulation for Deep Space Applications" https://trs.jpl.nasa.gov/bitstream/handle/2014/44994/12-0053_A1b.pdf?sequenc...
--- Zach N0ZGO
Thanks for your input Zach. Unfortunately, it is mostly irrelevant to the question. You are the first person in this conversation to mention BPSK.
I clearly stated that GMSK is a form of staggered QPSK with an unusual shaping filter. Accompanied with Viterbi decoding picking out the most likely path through the phase transitions of +/- 90 degrees at every baud time it has essentially the same performance as QPSK. I've taught digital communications at Virginia Tech at the undergraduate and graduate level and had graduate students doing digital communications research for over a decade. But thank you for your lecture.
Cheers, Bob
On Mon, Mar 22, 2021 at 4:11 PM Zach Metzinger zmetzing@pobox.com wrote:
On 3/22/21 3:38 PM, Robert McGwier wrote:
It is equivalent to QPSK with a good implementation.
Hello Bob,
I disagree with your statement of equivalence, but only slightly, for a couple of reasons: QPSK encodes 2 bits per symbol, so you should have stated BPSK (1 bit per symbol).
Additionally, GMSK's performance (probability of error) is close, but not equal, to BPSK for a given Eb/N0. The difference is up to several dB of required signal power for the same bit error rate. These numbers assume no channel coding to correct for errors on either system.
There's always a trade-off between bandwidth and power. GMSK trades off power for better-constrained bandwidth, while BPSK does the opposite. Not having to have a well-linear amplification system helps out with system efficiency, but noise performance offsets this gain.
An interesting paper by a pair of JPL employees explores this very subject:
"GMSK Modulation for Deep Space Applications"
https://trs.jpl.nasa.gov/bitstream/handle/2014/44994/12-0053_A1b.pdf?sequenc...
--- Zach N0ZGO
Sent via AMSAT-BB(a)amsat.org. AMSAT-NA makes this open forum available to all interested persons worldwide without requiring membership. Opinions expressed are solely those of the author, and do not reflect the official views of AMSAT-NA. Not an AMSAT-NA member? Join now to support the amateur satellite program!
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On 3/27/21 2:18 AM, Robert McGwier wrote:
Thanks for your input Zach. Unfortunately, it is mostly irrelevant to the question. You are the first person in this conversation to mention BPSK.
I clearly stated that GMSK is a form of staggered QPSK with an unusual shaping filter. Accompanied with Viterbi decoding picking out the most likely path through the phase transitions of +/- 90 degrees at every baud time it has essentially the same performance as QPSK. I've taught digital communications at Virginia Tech at the undergraduate and graduate level and had graduate students doing digital communications research for over a decade. But thank you for your lecture.
Thanks for clearing that up, Bob.
Your learned, considerate response is exactly what I've come to expect from academia.
--- Zach N0ZGO
participants (4)
-
Chris Thompson -
Robert McGwier -
Timothy Tapio -
Zach Metzinger