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.


On Mon, Mar 22, 2021 at 4:11 PM Zach Metzinger <> 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"

--- Zach


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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
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