What's the use of Gaussian LPF and Accumulator in GMSK modulator?

[peichinlim]

Hi,

In order to design GMSK modulator, the NRZ signal needs to pass through Gaussian low pass filter and accumulator before forming I and Q channel. What's the use of Gaussian LPF and Accumulator?

Thanks

 

[PaulHolland]

gmsk spectrum

GMSK is a Gaussian Minimum Shift Keying. You need to shape your bitstream before transmission to get a waveform that is GMSK. otherwise you would get FSK. The spectrum of an FSK transmitter can be converter into an GMSK spectrum but its much easyer to do this by shaping your bit stream before.

regards,

Paul.

 

[mission]

gmsk modulation,gaussian filter

Gaussian pulse shaping filter is used to convert the abrupt transitions in the waveform into smooth transition. When pulse shaping of this nature is used the resulting spectrum main lobe confines all the information and this helps in reducing the frequency band occupied by the modulated signal. In essence gaussian filter is used to confine 95% of the energy in the main lobe of the spectrum.
This enables efficient usage of the spectrum that means more number of channels can be accomodated in a given bandwidth as against the FSK spectrum which is not very well confined.