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First shot at frequency synchronization

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Supports only the BPSK part so far (ramp-up, preamble).
This commit is contained in:
Thomas Kolb 2022-01-30 20:05:20 +01:00
parent 09465591ed
commit c1dd039d6b
2 changed files with 91 additions and 5 deletions
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2
impl/src/config.h
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@ -17,7 +17,7 @@
#define RRC_DELAY 7 // delay in symbols
#define RRC_BETA 0.2f
#define TRANSMISSION_RAMP_UP_LEN 32 // symbols
#define TRANSMISSION_RAMP_UP_LEN 128 // symbols
#define TRANSMISSION_RAMP_DOWN_LEN 32 // symbols
#endif // CONFIG_H

94
impl/src/main.c
View file

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