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Re: [Discuss-gnuradio] gsm gmsk demodulation

From: Ben Wojtowicz
Subject: Re: [Discuss-gnuradio] gsm gmsk demodulation
Date: Fri, 6 Jun 2008 08:41:04 -0500

I agree with Bob, most gsm demodulators I have seen use a viterbi equalizer (sometimes called MLSE equalization).


On 6/6/08, Bob McGwier <address@hidden> wrote:
This is not my professional experience.  The sounding data is used to find the channel and then the data symbols are soft detected through a "viterbi equalizer" in every implementation I am aware of that is any good at all with the exception of one I wrote several years ago which estimates the data given the channel and then restimates the channel and then the data and then the channel and then the data, etc.  MMSE and not MLE is the goal and this was a suboptimal implementation of the EM algorithm.  It was suboptimal since it did not estimate the data bauds using ALL observations but only those between sounding data.  Further,  assumptions that the conditional distributions of the data given the observations could be described in 1st and 2nd product moments (not Gaussian but having similar properties).  This has been published by many.  The computational complexity is on a par with the viterbi equalizer and it outperforms it.

Most of the cell phones I know use the Viterbi equalizer.


Steven Clark wrote:
On Thu, Jun 5, 2008 at 7:45 AM, isaacgerg <address@hidden> wrote:
 Concerning GSM GMSK demodulation, due to the ISI, I initially thought many
folks were using the Viterbi algorithm on the waveform to demodulate it
properly.  After doing some lit review, I am finding that this is not the
case and that when most folks talk about Viterbi concerning GSM GMSK
demodulation, they are referring to undoing the convolution encoding and not
referring to demodultion in the face of ISI.  Can anyone please confirm

I see many demodulators simply just ignoring the ISI and just demodulating
as if it wasnt present.  It seems they just rely on the convolutional
encoding for error correction and therefore dont need to worry about the
ISI......Is this true?

What you have said is true in my experience as well. More often than
not, Viterbi decoding is just used as part of the forward error
correction scheme.

Please look for my next email to the mailing list for an alternate
GMSK demodulator that reduces ISI...



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