First shot at frequency synchronization
Supports only the BPSK part so far (ramp-up, preamble).
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@ -17,7 +17,7 @@
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#define RRC_DELAY 7 // delay in symbols
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#define RRC_BETA 0.2f
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#define TRANSMISSION_RAMP_UP_LEN 32 // symbols
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#define TRANSMISSION_RAMP_UP_LEN 128 // symbols
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#define TRANSMISSION_RAMP_DOWN_LEN 32 // symbols
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#endif // CONFIG_H
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@ -1,6 +1,7 @@
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#include <stdio.h>
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#include <string.h>
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#include <assert.h>
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#include <math.h>
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#include <liquid/liquid.h>
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#include "utils.h"
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@ -19,9 +20,9 @@ int main(void)
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channel_cccf channel = channel_cccf_create();
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float snr = 20.0f;
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float snr = 50.0f;
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channel_cccf_add_awgn(channel, -snr, snr);
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channel_cccf_add_carrier_offset(channel, 0.01f, 1.00f);
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channel_cccf_add_carrier_offset(channel, 0.10f, 1.00f);
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packet_mod_ctx_t pmod;
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@ -77,12 +78,93 @@ int main(void)
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channel_cccf_execute_block(channel, whole_burst, burst_len, msg_received);
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dump_array_cf(msg_received, burst_len, 1.0f, "/tmp/rx.cpx");
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// create NCO for carrier frequency compensation
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nco_crcf carrier_nco = nco_crcf_create(LIQUID_NCO);
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nco_crcf_set_frequency(carrier_nco, 0.00f);
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nco_crcf_set_phase(carrier_nco, 0.0f);
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// create symbol synchronizer
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symsync_crcf symsync = symsync_crcf_create_rnyquist(LIQUID_FIRFILT_RRC, RRC_SPS, RRC_DELAY, RRC_BETA, 32);
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float complex symsync_out[burst_len];
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unsigned int symsync_out_len;
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symsync_crcf_execute(symsync, msg_received, burst_len, symsync_out, &symsync_out_len);
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unsigned int symsync_out_len = 0;
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#define FREQ_EST_L 16
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float phase_history[FREQ_EST_L];
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memset(phase_history, 0, sizeof(phase_history));
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for(unsigned int i = 0; i < burst_len; i++) {
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// Mix the input signal with the carrier NCO, which oscillates at the
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// frequency estimated so far.
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float complex mixed_sample;
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nco_crcf_step(carrier_nco);
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nco_crcf_mix_down(carrier_nco, msg_received[i], &mixed_sample);
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// run the timing synchronizer (works even with shifted frequency
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unsigned int out_len;
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symsync_crcf_execute(symsync, &mixed_sample, 1, symsync_out + symsync_out_len, &out_len);
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if(out_len != 0) {
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// for all the output samples produced, run the frequency
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// estimator. This is an implementation that works with unknown
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// BPSK symbols and therefore can be used during ramp-up and
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// preamble.
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if(out_len < FREQ_EST_L) {
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memmove(phase_history,
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phase_history + out_len,
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(FREQ_EST_L-out_len) * sizeof(phase_history[0]));
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}
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for(unsigned int j = 0; j < out_len; j++) {
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float complex *psymbol = symsync_out + symsync_out_len + j;
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// square the symbol to remove BPSK ambiguity
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float phase = cargf((*psymbol) * (*psymbol));
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phase_history[FREQ_EST_L - out_len + j] = phase;
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}
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// update the frequency estimate
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float unwrapped_phase_history[FREQ_EST_L];
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memcpy(unwrapped_phase_history, phase_history, sizeof(unwrapped_phase_history));
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liquid_unwrap_phase(unwrapped_phase_history, FREQ_EST_L);
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// calculate slope of LMS-fitted line
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float mean_index = (FREQ_EST_L-1) / 2.0f;
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float mean_phase = 0.0f;
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for(unsigned int j = 0; j < FREQ_EST_L; j++) {
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mean_phase += unwrapped_phase_history[j];
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}
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mean_phase /= FREQ_EST_L;
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float numerator = 0.0f;
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float denominator = 0.0f;
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for(unsigned int j = 0; j < FREQ_EST_L; j++) {
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float delta_index = j - mean_index;
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numerator += delta_index * (unwrapped_phase_history[j] - mean_phase);
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denominator += delta_index*delta_index;
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}
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float lms_phase_change = numerator / denominator;
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nco_crcf_adjust_frequency(carrier_nco, lms_phase_change / RRC_SPS / 32);
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printf("Phase change: %.6f - carrier frequency: %.6f\n", lms_phase_change, nco_crcf_get_frequency(carrier_nco));
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if(i/RRC_SPS == 2*FREQ_EST_L) {
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float complex tmp[FREQ_EST_L];
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for(unsigned int j = 0; j < FREQ_EST_L; j++) {
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tmp[j] = unwrapped_phase_history[j];
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}
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dump_array_cf(tmp, FREQ_EST_L, 1.0f, "/tmp/freq_est.cpx");
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printf("MARK\n");
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}
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}
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symsync_out_len += out_len;
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}
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dump_array_cf(symsync_out, symsync_out_len, 1.0f, "/tmp/rx.cpx");
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#if 0
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@ -124,6 +206,10 @@ int main(void)
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printf("%u bit errors detected.\n", bit_errors);
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#endif
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nco_crcf_destroy(carrier_nco);
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symsync_crcf_destroy(symsync);
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fec_destroy(q);
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modem_destroy(demod);
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