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https://github.com/radio95-rnt/fm95.git
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66 lines
2.0 KiB
C
66 lines
2.0 KiB
C
#include "qpsk.h"
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#include <stdlib.h>
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#include <string.h>
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#include <math.h>
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#define OQPSK_RRC_DELAY 6 // filter delay in symbols (liquid's "m" param)
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static void oqpsk_next_dibit(OQPSKMod *m, float *i, float *q) {
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uint8_t b0 = m->last_bits[0], b1 = m->last_bits[1], bit;
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if (bit_ring_read1(m->bitring, &bit)) b0 = bit;
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if (bit_ring_read1(m->bitring, &bit)) b1 = bit;
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m->last_bits[0] = b0;
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m->last_bits[1] = b1;
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*i = b0 ? -1.0f : 1.0f;
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*q = b1 ? -1.0f : 1.0f;
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}
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void oqpsk_init(OQPSKMod *m, float if_freq, float sample_rate, float symbol_rate, float alpha, bit_ring_t *bitring) {
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memset(m, 0, sizeof(*m));
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init_oscillator(&m->carrier, if_freq, sample_rate);
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m->sps = (int)roundf(sample_rate / symbol_rate);
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if (m->sps < 2) m->sps = 2;
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if (m->sps % 2 != 0) m->sps += 1; // must be even for the half-symbol Q delay
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m->fir_i = firfilt_rrrf_create_rnyquist(LIQUID_FIRFILT_RRC, m->sps, OQPSK_RRC_DELAY, alpha, 0.0f);
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m->fir_q = firfilt_rrrf_create_rnyquist(LIQUID_FIRFILT_RRC, m->sps, OQPSK_RRC_DELAY, alpha, 0.0f);
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m->q_delay_len = m->sps / 2;
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m->q_delay = calloc(m->q_delay_len, sizeof(float));
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m->q_delay_pos = 0;
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m->symbol_clock = 0;
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m->sym_i = m->sym_q = 1.0f;
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m->bitring = bitring;
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}
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float oqpsk_process(OQPSKMod *m) {
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float impulse_i = 0.0f, impulse_q = 0.0f;
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if (m->symbol_clock == 0) {
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oqpsk_next_dibit(m, &m->sym_i, &m->sym_q);
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impulse_i = m->sym_i * (float)m->sps;
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impulse_q = m->sym_q * (float)m->sps;
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}
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m->symbol_clock = (m->symbol_clock + 1) % m->sps;
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float filt_i, filt_q_raw;
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firfilt_rrrf_push(m->fir_i, impulse_i);
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firfilt_rrrf_execute(m->fir_i, &filt_i);
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firfilt_rrrf_push(m->fir_q, impulse_q);
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firfilt_rrrf_execute(m->fir_q, &filt_q_raw);
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float filt_q = m->q_delay[m->q_delay_pos];
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m->q_delay[m->q_delay_pos] = filt_q_raw;
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m->q_delay_pos = (m->q_delay_pos + 1) % m->q_delay_len;
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float c = get_oscillator_cos_sample(&m->carrier);
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float s = get_oscillator_sin_sample(&m->carrier);
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return filt_i * c - filt_q * s;
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}
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void oqpsk_destroy(OQPSKMod *m) {
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firfilt_rrrf_destroy(m->fir_i);
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firfilt_rrrf_destroy(m->fir_q);
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free(m->q_delay);
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} |