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.
Bob
Steven Clark wrote:
On Thu, Jun 5, 2008 at 7:45 AM, isaacgerg <[EMAIL PROTECTED]> wrote:
Hi,
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
this?
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...
-Steven
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