Why not use matched filter in GMSK demodulator

Dear all,
I am reading the GMSK demodulator code (gmsk.py) and I have a question.
In
the modulator, Gaussian filter is used before the FM modulator. In the
demodulator, no matched filter is used. The signal go through FM
demodulator, then clock recovery, then make the decision. Why not use a
matched filter after the demodulator, as in standard diagrams of the
receivers? Thank you very much!

On Wed, Jul 29, 2009 at 1:43 PM, Shizheng Li [email protected]
wrote:

Dear all,
I am reading the GMSK demodulator code (gmsk.py) and I have a question. In
the modulator, Gaussian filter is used before the FM modulator. In the
demodulator, no matched filter is used. The signal go through FM
demodulator, then clock recovery, then make the decision. Why not use a
matched filter after the demodulator, as in standard diagrams of the
receivers? Thank you very much!

I think that one reason may be is that the output from the USRP to the
host
is not normalized. With a matched filter, you’re going to convolve the
input signal with your coefficients, and if your input signal is not
normalized then a higher input signal to the USRP will generate stronger
output from the matched filter (whether it’s a match or not). Therefore
you’re going to have trouble thresholding to determine the peak in
synchronization.

  • George

On Wed, Jul 29, 2009 at 2:46 PM, George N. [email protected] wrote:

receivers? Thank you very much!

I think that one reason may be is that the output from the USRP to the host
is not normalized. With a matched filter, you’re going to convolve the
input signal with your coefficients, and if your input signal is not
normalized then a higher input signal to the USRP will generate stronger
output from the matched filter (whether it’s a match or not). Therefore
you’re going to have trouble thresholding to determine the peak in
synchronization.

actually, I’m thinking an acquisition matched filter, so you might want
to
ignore that :slight_smile:

  • George

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