GMSK demodulator

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