OK, I've got it. Thank you for your help. Cheers, Zhenhua
2014-03-04 10:28 GMT+08:00 Tom Tsou <t...@tsou.cc>: > On Mon, Mar 3, 2014 at 8:32 PM, zhenhua han <hzhua...@gmail.com> wrote: > > 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) > > > ~ > > > > 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 ? > > The offset is derived assuming the signal is a constant tone. The code > is averaging the phase shift between subsequent samples in a burst. > Frequency is the first derivative of phase with respect to time, so > the phase shift multiplied by the sample rate will give you the > frequency of the tone. > > -TT >
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