Hi all
I'm working on GMSK modem simulation in MATLAB. I'm almost done with
the modulation part albeit i'm not sure if the model I used is
correct
adding to my trouble is the ambiguity with the Power Spectral Density
of the GMSK modulated signal (also the MSK modulated signal). Here
I'm
writing the algorithm I followed to do the simulation:
bitrate=4000bps BT=0.3
1.calculated the impulse response of the GaussianLPF with the formula
g(t)= convolution(gaussian pulse , rectangular pulse of width T) as
specified in the GSM 5.04 version 8.1.2 Release 1999.
In fact I directly used the impulse response specified in the
following link
2. then I sampled the incoming binary data stream exactly in the
middle of the bit period, then over sampled the stream with an over
sampling rate of 41
(here in a bit period first 20 samples will be 0 the 21st sample will
be +/-1 depending on the data thenext 20 will be 0 again)
3. I convolved these samples with the Impulse response I calculated
earlier.
4.then integrated the resulting signal to get 'y'
5.then calculated the I and Q channel streams as I=cos(y),Q=sin(y)
then modulated it with a I/Q modulator with Fc(carrier frequency=5000)
then I got the modulated signal as shown in the link below for the
data:
Does a GMSK modulated signal look like that???
It shows constant amplitude and frequency deviations
the Ichannel vs Q channel (baseband) plot is as shown in the
following
link
It is a circle with radius 1 as is expected of a GMSK signal
I calculated its PSD of the above signal using FFT and scaled its
frequency axis to 0 to Fs/2 (here Fs=16400Hz)
It is in the zoomed in figure(for viewability) shown in following link
http://aycu20.webshots.com/image/46459/2003353100129112203_rs.jpg
My doubt here is:
In many text books and technical papers the PSD of GMSK signal shows
a peak at the carrier frequency but in my signal's PSD shows a dip
there I'm not getting where I went wrong.
PLEASE GUIDE ME WHERE I WENT WRONG AND PLEASE TELL ME THE CORRECT WAY
OUT OF THIS PROBLEM