131 lines
3.6 KiB
C
131 lines
3.6 KiB
C
/* AFC (Automatic Frequency Correction) Implementation */
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/* (C) 2010 by Harald Welte <laforge@gnumonks.org>
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*
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* All Rights Reserved
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*
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* This program is free software; you can redistribute it and/or modify
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* it under the terms of the GNU General Public License as published by
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* the Free Software Foundation; either version 2 of the License, or
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* (at your option) any later version.
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*
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* This program is distributed in the hope that it will be useful,
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* but WITHOUT ANY WARRANTY; without even the implied warranty of
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* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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* GNU General Public License for more details.
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*
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* You should have received a copy of the GNU General Public License along
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* with this program; if not, write to the Free Software Foundation, Inc.,
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* 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
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*
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*/
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#include <stdint.h>
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#include <stdio.h>
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#include <debug.h>
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#include <osmocom/gsm/gsm_utils.h>
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#include <layer1/afc.h>
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#include <layer1/avg.h>
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#include <calypso/dsp.h>
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#define AFC_INITIAL_DAC_VALUE -700
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/* Over how many TDMA frames do we want to average? (this may change in dedicated mode) */
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#define AFC_PERIOD 40
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/* How many of our measurements have to be valid? */
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#define AFC_MIN_MUN_VALID 8
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/* The actual AFC code */
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struct afc_state {
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struct running_avg ravg; /* running average */
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int16_t dac_value; /* current DAC output value */
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uint16_t arfcn;
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};
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static void afc_ravg_output(struct running_avg *ravg, int32_t avg);
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static struct afc_state afc_state = {
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.ravg = {
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.outfn = &afc_ravg_output,
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.period = AFC_PERIOD,
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.min_valid = AFC_MIN_MUN_VALID,
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},
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.dac_value = AFC_INITIAL_DAC_VALUE,
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};
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/* The AFC DAC in the ABB has to be configured as follows:
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* DAC = 1MHz / 947MHz * FreqErr(Hz) / AFCslop(ppm/LSB)
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* where:
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* 947 MHz is the center of EGSM
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* AFCslope is coded F1.15, thus a normalization factor of 2^15 applies
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*/
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#define AFC_NORM_FACTOR_GSM ((1<<15) / 947)
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#define AFC_NORM_FACTOR_DCS ((1<<15) / 1894)
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/* we assume 8.769ppb per LSB, equals 0.008769 * 32768 == 287 */
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//#define AFC_SLOPE 320
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#define AFC_SLOPE 287
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/* The DSP can measure the frequency error in the following ranges:
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* FB_MODE0: +/- 20 kHz
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* FB_MODE1: +/- 4 kHz
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* Sync Burst: +/- 1 kHz
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* Normal Burst: +/- 400 Hz
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*/
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/* Update the AFC with a frequency error, bypassing averaging */
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void afc_correct(int16_t freq_error, uint16_t arfcn)
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{
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int32_t afc_norm_factor;
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int16_t delta;
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switch (gsm_arfcn2band(arfcn)) {
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case GSM_BAND_900:
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case GSM_BAND_850:
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afc_norm_factor = AFC_NORM_FACTOR_GSM;
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break;
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default:
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afc_norm_factor = AFC_NORM_FACTOR_DCS;
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}
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delta = (int16_t) ((afc_norm_factor * (int32_t)freq_error) / AFC_SLOPE);
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printd("afc_correct(error=%dHz, arfcn=%u): delta=%d, afc_dac(old=%d,new=%d)\n",
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freq_error, arfcn, delta, afc_state.dac_value, afc_state.dac_value+delta);
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afc_state.dac_value += delta;
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/* The AFC DAC has only 13 bits */
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if (afc_state.dac_value > 4095)
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afc_state.dac_value = 4095;
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else if (afc_state.dac_value < -4096)
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afc_state.dac_value = -4096;
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}
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void afc_reset(void)
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{
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afc_state.dac_value = AFC_INITIAL_DAC_VALUE;
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}
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void afc_input(int32_t freq_error, uint16_t arfcn, int valid)
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{
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afc_state.arfcn = arfcn;
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runavg_input(&afc_state.ravg, freq_error, valid);
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runavg_check_output(&afc_state.ravg);
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}
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/* callback function for runavg */
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static void afc_ravg_output(struct running_avg *ravg, int32_t avg)
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{
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afc_correct(avg, afc_state.arfcn);
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}
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/* Update DSP with new AFC DAC value to be used for next TDMA frame */
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void afc_load_dsp(void)
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{
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dsp_api.db_w->d_afc = afc_state.dac_value;
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dsp_api.db_w->d_ctrl_abb |= (1 << B_AFC);
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}
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