FIR filter , Clock recovery and Quadrature demodulation


#1

Hi,
The following are some code segments of gsm-scan.py
basically
used for gsm data reception. I have a few questions about the difference
between input_rate and sps,

clock_rate = 64e6
#set the default input rate, we will check with the USRP if it is being
used
input_rate = clock_rate / options.decim
gsm_symb_rate = 1625000.0 / 6.0
sps = input_rate/gsm_symb_rate

configure the processing blocks

configure channel filter

filter_cutoff = 145e3 #135,417Hz is GSM bandwidth
filter_t_width = 10e3

#Only DSP adjust for offset on datafile, adjust tuner for USRP
#TODO: see if we can change this offset at runtime based on freq
detection
if options.inputfile:
offset = self.offset
else:
offset = 0.0

filter_taps = gr.firdes.low_pass(1.0, input_rate, filter_cutoff,
filter_t_width, gr.firdes.WIN_HAMMING)
self.filter = gr.freq_xlating_fir_filter_ccf(1, filter_taps, offset,
input_rate)

           # configure demodulator

adjust the phase gain for sampling rate

self.demod = gr.quadrature_demod_cf(sps);

#configure clock recovery
gain_mu = 0.01
gain_omega = .25 * gain_mu * gain_mu # critically damped
self.clocker = gr.clock_recovery_mm_ff( sps,
gain_omega,
0.5, #mu
gain_mu,
0.3) #omega_relative_limit,

self.burst = gsm.burst_ff()
self.connect(self.filter, self.demod, self.clocker, self.burst)

. Can anyone please give me an idea why " gr.freq_xlating_fir_filter_ccf
"
is used here and ‘input_rate’ is used as an argument in stead of ‘sps’.
Whereas, in quadrature demod and Clock_recovery_mm ‘sps’ is used for
sampling rate and why clock recovery is used after demodulation block ?
Are I and Q samples are flowing out of clocker block or are they the
encoded
bits ?

Thanks,

M.Awais


#2

On Sun, 2009-04-26 at 22:30 +0500, M. Awais Arshad wrote:

being used

self.filter = gr.freq_xlating_fir_filter_ccf(1, filter_taps, offset,
gain_omega,
gr.freq_xlating_fir_filter_ccf " is used here and ‘input_rate’ is used
as an argument in stead of ‘sps’.
Well, because a freq_translating_fir_filter needs the input rate.
It knows nothing about symbols so sps (samples per symbol) makes no
sense here.

Whereas, in quadrature demod and Clock_recovery_mm ‘sps’ is used for
sampling rate
sps is not used for samplingrate.
Did you even bother looking up what the parameters mean.
In quadrature_demod the parameter means gain (not sampling rate)
See:
http://gnuradio.org/doc/doxygen/classgr__quadrature__demod__cf.html

In clock_recovery_mm_ff the first parameter means omega (not sampling
rate)
http://gnuradio.org/doc/doxygen/classgr__clock__recovery__mm__ff.html

and why clock recovery is used after demodulation block ?
Because you want to recover the symbol clock.
As long as the samples are not demodulated there exists no symbols.

Are I and Q samples are flowing out of clocker block or are they the
encoded bits ?

Mueller and Moeller (M&M) based clock recovery block with float input,
float output.

This implements the Mueller and Moeller (M&M) discrete-time
error-tracking synchronizer.

See “Digital Communication Receivers: Synchronization, Channel
Estimation and Signal Processing” by Heinrich Meyr, Marc Moeneclaey, &
Stefan Fechtel. ISBN 0-471-50275-8.