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GMSK demodulator

Started by Pavel Schukin September 5, 2007
Hello, everybody.
Who knows how to demodulate baseband GSM signal?
I know that it can be realized by
1)typical complex FM discriminator,timing recovery and slicer or by
2)Viterbi algorithm
I have filtered baseband GSM signal with 1 sample per symbol. Should i
interpolate or not to get 2x or 4x sample rate? I must provide error free
bitstream recovery with 10dB SNR at the input.
Thank you. Pavel Schukin.


On Sep 5, 2:28 pm, "Pavel Schukin" <pav8...@yahoo.com> wrote:
> Hello, everybody. > Who knows how to demodulate baseband GSM signal? > I know that it can be realized by > 1)typical complex FM discriminator,timing recovery and slicer or by > 2)Viterbi algorithm > I have filtered baseband GSM signal with 1 sample per symbol. Should i > interpolate or not to get 2x or 4x sample rate? I must provide error free > bitstream recovery with 10dB SNR at the input. > Thank you. Pavel Schukin.
Option 1 likely won't perform well due to the ISI induced in the modulator -- the eye opening is small. Option 2 is nontrivial, but will work well *if* you can obtain a trellis representation of the modulation, FEC, and channel, plus phaselock. That is what the midamble is for. John

Pavel Schukin wrote:

> Hello, everybody. > Who knows how to demodulate baseband GSM signal?
I do. However it is going to be for money.
> I know that it can be realized by > 1)typical complex FM discriminator,timing recovery and slicer or by
Suboptimal solution. However it works quite well up to BT ~ 1/2. For the smaller BT, you will need a DFE after the discriminator.
> 2)Viterbi algorithm
This is better.
> I have filtered baseband GSM signal with 1 sample per symbol. Should i > interpolate or not to get 2x or 4x sample rate?
It depends. How accurate is the symbol sync?
> I must provide error free > bitstream recovery with 10dB SNR at the input.
It depends. What is BT, what is the encoding. Vladimir Vassilevsky DSP and Mixed Signal Design Consultant http://www.abvolt.com
Vladimir Vassilevsky wrote:
> > > Pavel Schukin wrote: > >> Hello, everybody. >> Who knows how to demodulate baseband GSM signal? > > I do. However it is going to be for money. > >> I know that it can be realized by >> 1)typical complex FM discriminator,timing recovery and slicer or by > > Suboptimal solution. However it works quite well up to BT ~ 1/2. For the > smaller BT, you will need a DFE after the discriminator.
For GSM, BT is ~0.25. As you say, the FM demod needs to be followed something to deal with the ISI. This can be a DFE or a Viterbi decoder. In this situation the Viterbi decoder is not the same as the one used in Option 2. John