Hi guys,
I'm reading the code of Airprobe and I found this function.
double gsm_receiver_cf::compute_freq_offset(const gr_complex * input,
unsigned first_sample, unsigned last_sample)
{
double phase_sum = 0;
unsigned ii;
for (ii = first_sample; ii < last_sample; ii++) {
double phase_diff = compute_phase_diff(input[ii], input[ii-1]) - (M_PI /
2) / d_OSR;
phase_sum += phase_diff;
}
double phase_offset = phase_sum / (last_sample - first_sample);
double freq_offset = phase_offset * 1625000.0 / (12.0 * M_PI);
return freq_offset;
}
It calculates the frequency offset with "freq_offset = phase_offset *
1625000.0 / (12.0 * M_PI);" I guess the (1625000.0/12*pi) equals
(1625000/6)/(2pi) and 1625/6 kbit/s is the modulation rate of GSM.
So, what is the principle of this calculation? How to calculate the
freq_offset with phase_offset. I tried some derivation but failed. Can
anyone help me to find out the derivation ?
Best wishes,
Zhenhua
2014-03-04 9:28 GMT+08:00 zhenhua han <hzhua...@gmail.com>:
> Hi Perper,
>
> I took a look at your code, and I found this
>
> //set_frequency(d_freq_offset);
>
> It seems you have turned off the the frequency offset correction as you
> said.
> Do you know the reason of the introduced instability by this part?
>
> Best,
> Zhenhua
>
> 2014-03-04 1:42 GMT+08:00 Perper <per...@o2.pl>:
>
> W dniu 28.02.2014 15:31, zhenhua han pisze:
>> > Hi,
>> >
>> > As I have known, GSM uses GMSK modulation which BT = 0.3 and it
>> > uses Viterbi algorithm for demodulation. And I took a look at the code
>> > of GMSK demod code in GNU Radio, it use quadrature_demod but not
>> > Viterbi as demodulation method. So which one is better in doing
>> > demodulating GMSK?
>> >
>> > Moreover, GSM uses Viterbi algorithm to decode the convolution
>> > encoding. Would it be possible to use quadrature_demod to demodulate
>> > GSM signal instead of Viterbi? What about the convolution decoding
>> > part (maybe by some other method)?
>> >
>> > Cheers,
>> > Zhenhua
>> >
>> >
>> > _______________________________________________
>> > Discuss-gnuradio mailing list
>> > Discuss-gnuradio@gnu.org
>> > https://lists.gnu.org/mailman/listinfo/discuss-gnuradio
>> Hello all,
>>
>> Regarding usage of viterbi algorithm for maximum likelihood sequence
>> estimation in GSM - you may look at and try gsm-receiver that I once
>> submitted to airprobe. I've prepared version where I thrown out all of
>> decoding stuff and left only synchronization, channel estimation and
>> detection of bits with viterbi algorithm. It's available for download in
>> here:
>> https://github.com/Jakotako/gr-gsm
>>
>> It compiles with newest gnu-radio and it has gnuradio companion xml file
>> as well. It can be easily connected to different sources in gnuradio
>> (file, rtl-sdr, usrp) and it has a message sink where burst bits and
>> gsmtap headers are sent. I've corrected some issues with synchronization
>> and turned off frequency offset correction that introduced instability
>> of the receiver. After compiling and installing you can start with
>> examples.
>>
>> --
>> Best Regards,
>> Piotr Krysik
>>
>> _______________________________________________
>> Discuss-gnuradio mailing list
>> Discuss-gnuradio@gnu.org
>> https://lists.gnu.org/mailman/listinfo/discuss-gnuradio
>>
>
>
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