gmsk modulate/demodulate

[whit]

communication-gmsk

Hi

I heard one of my colleague said that gmsk modulate/demodulate are just
a A/D and D/A convertor, It that right, I do appreciate someone can explain
it in more detail, or can pls refer me some book on this subject, having
circuit example is more prefer.

Thanks

 

[amir_rastegar2003]

gmsk phase trellis

hi

see digital communications. by " Proakis" 4th edition.

regards

 

[cabone]

gmsk filter

what your friend said, about GMSK mod. and demod. is not totally wrong and in the same time it's not true because in any digital communication system we have the data in binary form which can't be transmitted on air so we have to convert the data to analouge form, that'a what your friend said but in fact the difference betwen different types of digital modulation technique is how to represent that digital data into analouge, so we find in BPSK that we represent the binary '1' with a sinewave with pahse 180 and binary '0' with sinewave with phase 0, in GMSK we represent binary '1' with a sinewave with frequency f1(for example) and binary '0' with frequency f2
to further understand on how we choose f1 and f2 you can read any text book about digital communication
i helped you

 

[whit]

understanding gmsk

Hi

I saw a circuit block about a baseband interface ic and
it shows that for transmit path baseband bit stream first pass
the gmsk mod then to a DAC+filter and come out to be I/Q;
for receive path, I/Q go into sigma-delta mod then pass the FIR
filter and finally go into baseband interface.

Something confusing me is :
1. why does need a DAC in transmit path, is the signal after a gmsk mod
still a digital signal?
2. in receive path, why no gmsk demod block insite?

The ic i refer is PCF50732.

Can you help me out for these puzzles?

 

[bcl]

gmsk mod

The DAC on the modulator side brings the digital information into analog domain for transmission, and the ADC on the demodulator side brings the analog signal into digital domain for decoding. The modulation and demodulation process is a whole lot more complicated then just these two small but important steps.

 

[lincolndsp]

gmsk & isi

type GMSK modulation in the EDA requests forum and lots of
needed articles on the topic will be

 

[whit]

gmsk true table

Hi
Thanks all for your kind help.
For my understanding, in this transmit
path, it is using a look up table to
stand the gmsk modulated signal, so
it uses a dac to convert it to analog
signal, am i right?

 

[bcl]

look up table gmsk

You are mostly correct. There are different ways to generate a GMSK waveform and an LUT is certainly one of them. The main idea is to filter the bits with a gaussian filter, and spread the phase transition of that filtered bit over the 1/(B*T) symbols (Gaussian filter parameter). Since the phase transition of every bit is spread in the same fashion, cintrolled "ISI" is intrduced intentionally. The larger the 1/(B*T) number, the more complex decoding phase trellis get in the demodulator.