kopia lustrzana https://github.com/kgoba/ft8_lib
370 wiersze
12 KiB
C
370 wiersze
12 KiB
C
#include "decode.h"
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#include "constants.h"
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#include "crc.h"
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#include "ldpc.h"
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#include "unpack.h"
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#include <stdbool.h>
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#include <math.h>
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/// Compute log likelihood log(p(1) / p(0)) of 174 message bits for later use in soft-decision LDPC decoding
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/// @param[in] power Waterfall data collected during message slot
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/// @param[in] cand Candidate to extract the message from
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/// @param[in] code_map Symbol encoding map
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/// @param[out] log174 Output of decoded log likelihoods for each of the 174 message bits
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static void extract_likelihood(const waterfall_t *power, const candidate_t *cand, float *log174);
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static float max2(float a, float b);
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static float max4(float a, float b, float c, float d);
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static void heapify_down(candidate_t heap[], int heap_size);
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static void heapify_up(candidate_t heap[], int heap_size);
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static void decode_symbol(const uint8_t *power, int bit_idx, float *log174);
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static void decode_multi_symbols(const uint8_t *power, int num_bins, int n_syms, int bit_idx, float *log174);
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static int get_index(const waterfall_t *power, int block, int time_sub, int freq_sub, int bin)
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{
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return ((((block * power->time_osr) + time_sub) * power->freq_osr + freq_sub) * power->num_bins) + bin;
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}
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int find_sync(const waterfall_t *power, int num_candidates, candidate_t heap[], int min_score)
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{
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int heap_size = 0;
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int sym_stride = power->time_osr * power->freq_osr * power->num_bins;
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// Here we allow time offsets that exceed signal boundaries, as long as we still have all data bits.
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// I.e. we can afford to skip the first 7 or the last 7 Costas symbols, as long as we track how many
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// sync symbols we included in the score, so the score is averaged.
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for (int time_sub = 0; time_sub < power->time_osr; ++time_sub)
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{
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for (int freq_sub = 0; freq_sub < power->freq_osr; ++freq_sub)
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{
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for (int time_offset = -12; time_offset < 24; ++time_offset)
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{
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for (int freq_offset = 0; freq_offset + 7 < power->num_bins; ++freq_offset)
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{
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int score = 0;
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int num_average = 0;
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// Compute average score over sync symbols (m+k = 0-7, 36-43, 72-79)
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for (int m = 0; m <= 72; m += 36)
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{
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for (int k = 0; k < 7; ++k)
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{
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int block = time_offset + m + k;
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// Check for time boundaries
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if (block < 0)
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continue;
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if (block >= power->num_blocks)
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break;
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int offset = get_index(power, block, time_sub, freq_sub, freq_offset);
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const uint8_t *p8 = power->mag + offset;
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// Weighted difference between the expected and all other symbols
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// Does not work as well as the alternative score below
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// score += 8 * p8[kFT8_Costas_pattern[k]] -
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// p8[0] - p8[1] - p8[2] - p8[3] -
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// p8[4] - p8[5] - p8[6] - p8[7];
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// ++num_average;
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// Check only the neighbors of the expected symbol frequency- and time-wise
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int sm = kFT8_Costas_pattern[k]; // Index of the expected bin
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if (sm > 0)
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{
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// look at one frequency bin lower
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score += p8[sm] - p8[sm - 1];
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++num_average;
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}
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if (sm < 7)
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{
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// look at one frequency bin higher
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score += p8[sm] - p8[sm + 1];
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++num_average;
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}
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if ((k > 0) && (block > 0))
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{
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// look one symbol back in time
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score += p8[sm] - p8[sm - sym_stride];
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++num_average;
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}
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if ((k < 6) && ((block + 1) < power->num_blocks))
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{
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// look one symbol forward in time
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score += p8[sm] - p8[sm + sym_stride];
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++num_average;
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}
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}
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}
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if (num_average > 0)
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score /= num_average;
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if (score < min_score)
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continue;
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// If the heap is full AND the current candidate is better than
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// the worst in the heap, we remove the worst and make space
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if (heap_size == num_candidates && score > heap[0].score)
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{
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heap[0] = heap[heap_size - 1];
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--heap_size;
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heapify_down(heap, heap_size);
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}
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// If there's free space in the heap, we add the current candidate
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if (heap_size < num_candidates)
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{
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heap[heap_size].score = score;
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heap[heap_size].time_offset = time_offset;
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heap[heap_size].freq_offset = freq_offset;
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heap[heap_size].time_sub = time_sub;
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heap[heap_size].freq_sub = freq_sub;
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++heap_size;
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heapify_up(heap, heap_size);
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}
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}
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}
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}
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}
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// Sort the candidates by sync strength - here we benefit from the heap structure
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int len_unsorted = heap_size;
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while (len_unsorted > 1)
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{
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candidate_t tmp = heap[len_unsorted - 1];
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heap[len_unsorted - 1] = heap[0];
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heap[0] = tmp;
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len_unsorted--;
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heapify_down(heap, len_unsorted);
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}
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return heap_size;
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}
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void extract_likelihood(const waterfall_t *power, const candidate_t *cand, float *log174)
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{
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int sym_stride = power->time_osr * power->freq_osr * power->num_bins;
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int offset = get_index(power, cand->time_offset, cand->time_sub, cand->freq_sub, cand->freq_offset);
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// Go over FSK tones and skip Costas sync symbols
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const int n_syms = 1;
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const int n_bits = 3 * n_syms;
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const int n_tones = (1 << n_bits);
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for (int k = 0; k < FT8_ND; k += n_syms)
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{
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// Add either 7 or 14 extra symbols to account for sync
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int sym_idx = (k < FT8_ND / 2) ? (k + 7) : (k + 14);
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int bit_idx = 3 * k;
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int block = cand->time_offset + sym_idx;
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// Check for time boundaries
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if ((block < 0) || (block >= power->num_blocks))
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{
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log174[bit_idx] = 0;
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}
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else
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{
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// Pointer to 8 bins of the current symbol
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const uint8_t *ps = power->mag + offset + (sym_idx * sym_stride);
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decode_symbol(ps, bit_idx, log174);
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}
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}
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// Compute the variance of log174
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float sum = 0;
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float sum2 = 0;
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for (int i = 0; i < FT8_LDPC_N; ++i)
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{
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sum += log174[i];
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sum2 += log174[i] * log174[i];
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}
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float inv_n = 1.0f / FT8_LDPC_N;
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float variance = (sum2 - (sum * sum * inv_n)) * inv_n;
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// Normalize log174 distribution and scale it with experimentally found coefficient
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float norm_factor = sqrtf(24.0f / variance);
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for (int i = 0; i < FT8_LDPC_N; ++i)
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{
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log174[i] *= norm_factor;
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}
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}
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bool decode(const waterfall_t *power, const candidate_t *cand, message_t *message, int max_iterations, decode_status_t *status)
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{
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float log174[FT8_LDPC_N]; // message bits encoded as likelihood
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extract_likelihood(power, cand, log174);
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uint8_t plain174[FT8_LDPC_N]; // message bits (0/1)
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bp_decode(log174, max_iterations, plain174, &status->ldpc_errors);
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// ldpc_decode(log174, max_iterations, plain174, &status->ldpc_errors);
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if (status->ldpc_errors > 0)
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{
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return false;
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}
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// Extract payload + CRC (first FT8_LDPC_K bits) packed into a byte array
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uint8_t a91[FT8_LDPC_K_BYTES];
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pack_bits(plain174, FT8_LDPC_K, a91);
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// Extract CRC and check it
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status->crc_extracted = extract_crc(a91);
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// [1]: 'The CRC is calculated on the source-encoded message, zero-extended from 77 to 82 bits.'
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a91[9] &= 0xF8;
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a91[10] &= 0x00;
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status->crc_calculated = ft8_crc(a91, 96 - 14);
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if (status->crc_extracted != status->crc_calculated)
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{
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return false;
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}
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status->unpack_status = unpack77(a91, message->text);
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if (status->unpack_status < 0)
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{
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return false;
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}
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// Reuse binary message CRC as hash value for the message
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message->hash = status->crc_extracted;
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return true;
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}
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static float max2(float a, float b)
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{
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return (a >= b) ? a : b;
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}
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static float max4(float a, float b, float c, float d)
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{
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return max2(max2(a, b), max2(c, d));
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}
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static void heapify_down(candidate_t heap[], int heap_size)
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{
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// heapify from the root down
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int current = 0;
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while (true)
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{
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int largest = current;
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int left = 2 * current + 1;
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int right = left + 1;
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if (left < heap_size && heap[left].score < heap[largest].score)
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{
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largest = left;
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}
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if (right < heap_size && heap[right].score < heap[largest].score)
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{
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largest = right;
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}
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if (largest == current)
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{
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break;
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}
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candidate_t tmp = heap[largest];
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heap[largest] = heap[current];
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heap[current] = tmp;
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current = largest;
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}
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}
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static void heapify_up(candidate_t heap[], int heap_size)
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{
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// heapify from the last node up
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int current = heap_size - 1;
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while (current > 0)
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{
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int parent = (current - 1) / 2;
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if (heap[current].score >= heap[parent].score)
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{
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break;
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}
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candidate_t tmp = heap[parent];
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heap[parent] = heap[current];
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heap[current] = tmp;
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current = parent;
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}
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}
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// Compute unnormalized log likelihood log(p(1) / p(0)) of 3 message bits (1 FSK symbol)
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static void decode_symbol(const uint8_t *power, int bit_idx, float *log174)
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{
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// Cleaned up code for the simple case of n_syms==1
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float s2[8];
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for (int j = 0; j < 8; ++j)
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{
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s2[j] = (float)power[kFT8_Gray_map[j]];
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}
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log174[bit_idx + 0] = max4(s2[4], s2[5], s2[6], s2[7]) - max4(s2[0], s2[1], s2[2], s2[3]);
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log174[bit_idx + 1] = max4(s2[2], s2[3], s2[6], s2[7]) - max4(s2[0], s2[1], s2[4], s2[5]);
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log174[bit_idx + 2] = max4(s2[1], s2[3], s2[5], s2[7]) - max4(s2[0], s2[2], s2[4], s2[6]);
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}
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// Compute unnormalized log likelihood log(p(1) / p(0)) of bits corresponding to several FSK symbols at once
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static void decode_multi_symbols(const uint8_t *power, int num_bins, int n_syms, int bit_idx, float *log174)
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{
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const int n_bits = 3 * n_syms;
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const int n_tones = (1 << n_bits);
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float s2[n_tones];
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for (int j = 0; j < n_tones; ++j)
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{
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int j1 = j & 0x07;
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if (n_syms == 1)
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{
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s2[j] = (float)power[kFT8_Gray_map[j1]];
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continue;
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}
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int j2 = (j >> 3) & 0x07;
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if (n_syms == 2)
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{
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s2[j] = (float)power[kFT8_Gray_map[j2]];
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s2[j] += (float)power[kFT8_Gray_map[j1] + 4 * num_bins];
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continue;
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}
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int j3 = (j >> 6) & 0x07;
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s2[j] = (float)power[kFT8_Gray_map[j3]];
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s2[j] += (float)power[kFT8_Gray_map[j2] + 4 * num_bins];
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s2[j] += (float)power[kFT8_Gray_map[j1] + 8 * num_bins];
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}
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// Extract bit significance (and convert them to float)
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// 8 FSK tones = 3 bits
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for (int i = 0; i < n_bits; ++i)
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{
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if (bit_idx + i >= FT8_LDPC_N)
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{
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// Respect array size
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break;
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}
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uint16_t mask = (n_tones >> (i + 1));
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float max_zero = -1000, max_one = -1000;
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for (int n = 0; n < n_tones; ++n)
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{
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if (n & mask)
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{
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max_one = max2(max_one, s2[n]);
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}
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else
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{
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max_zero = max2(max_zero, s2[n]);
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}
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}
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log174[bit_idx + i] = max_one - max_zero;
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}
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}
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