kopia lustrzana https://github.com/F5OEO/ft8_lib
297 wiersze
8.1 KiB
C++
297 wiersze
8.1 KiB
C++
//
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// LDPC decoder for FT8.
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//
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// given a 174-bit codeword as an array of log-likelihood of zero,
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// return a 174-bit corrected codeword, or zero-length array.
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// last 87 bits are the (systematic) plain-text.
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// this is an implementation of the sum-product algorithm
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// from Sarah Johnson's Iterative Error Correction book.
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// codeword[i] = log ( P(x=0) / P(x=1) )
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//
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#include <stdio.h>
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#include <math.h>
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#include <stdlib.h>
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#include "constants.h"
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namespace ft8 {
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static int ldpc_check(uint8_t codeword[]);
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static float fast_tanh(float x);
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static float fast_atanh(float x);
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// Packs a string of bits each represented as a zero/non-zero byte in plain[],
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// as a string of packed bits starting from the MSB of the first byte of packed[]
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void pack_bits(const uint8_t plain[], int num_bits, uint8_t packed[]) {
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int num_bytes = (num_bits + 7) / 8;
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for (int i = 0; i < num_bytes; ++i) {
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packed[i] = 0;
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}
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uint8_t mask = 0x80;
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int byte_idx = 0;
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for (int i = 0; i < num_bits; ++i) {
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if (plain[i]) {
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packed[byte_idx] |= mask;
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}
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mask >>= 1;
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if (!mask) {
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mask = 0x80;
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++byte_idx;
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}
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}
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}
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// codeword is 174 log-likelihoods.
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// plain is a return value, 174 ints, to be 0 or 1.
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// max_iters is how hard to try.
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// ok == 87 means success.
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void ldpc_decode(float codeword[], int max_iters, uint8_t plain[], int *ok) {
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float m[ft8::M][ft8::N]; // ~60 kB
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float e[ft8::M][ft8::N]; // ~60 kB
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int min_errors = ft8::M;
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for (int j = 0; j < ft8::M; j++) {
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for (int i = 0; i < ft8::N; i++) {
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m[j][i] = codeword[i];
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e[j][i] = 0.0f;
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}
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}
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for (int iter = 0; iter < max_iters; iter++) {
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for (int j = 0; j < ft8::M; j++) {
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for (int ii1 = 0; ii1 < kNrw[j]; ii1++) {
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int i1 = kNm[j][ii1] - 1;
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float a = 1.0f;
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for (int ii2 = 0; ii2 < kNrw[j]; ii2++) {
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int i2 = kNm[j][ii2] - 1;
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if (i2 != i1) {
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a *= fast_tanh(-m[j][i2] / 2.0f);
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}
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}
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e[j][i1] = logf((1 - a) / (1 + a));
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}
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}
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for (int i = 0; i < ft8::N; i++) {
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float l = codeword[i];
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for (int j = 0; j < 3; j++)
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l += e[kMn[i][j] - 1][i];
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plain[i] = (l > 0) ? 1 : 0;
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}
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int errors = ldpc_check(plain);
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if (errors < min_errors) {
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// Update the current best result
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min_errors = errors;
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if (errors == 0) {
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break; // Found a perfect answer
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}
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}
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for (int i = 0; i < ft8::N; i++) {
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for (int ji1 = 0; ji1 < 3; ji1++) {
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int j1 = kMn[i][ji1] - 1;
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float l = codeword[i];
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for (int ji2 = 0; ji2 < 3; ji2++) {
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if (ji1 != ji2) {
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int j2 = kMn[i][ji2] - 1;
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l += e[j2][i];
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}
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}
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m[j1][i] = l;
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}
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}
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}
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*ok = min_errors;
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}
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//
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// does a 174-bit codeword pass the FT8's LDPC parity checks?
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// returns the number of parity errors.
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// 0 means total success.
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//
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static int ldpc_check(uint8_t codeword[]) {
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int errors = 0;
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for (int j = 0; j < ft8::M; ++j) {
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uint8_t x = 0;
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for (int i = 0; i < kNrw[j]; ++i) {
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x ^= codeword[kNm[j][i] - 1];
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}
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if (x != 0) {
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++errors;
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}
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}
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return errors;
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}
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void bp_decode(float codeword[], int max_iters, uint8_t plain[], int *ok) {
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float tov[ft8::N][3];
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float toc[ft8::M][7];
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int min_errors = ft8::M;
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int nclast = 0;
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int ncnt = 0;
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// initialize messages to checks
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for (int i = 0; i < ft8::M; ++i) {
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for (int j = 0; j < kNrw[i]; ++j) {
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toc[i][j] = codeword[kNm[i][j] - 1];
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}
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}
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for (int i = 0; i < ft8::N; ++i) {
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for (int j = 0; j < 3; ++j) {
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tov[i][j] = 0;
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}
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}
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for (int iter = 0; iter < max_iters; ++iter) {
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float zn[ft8::N];
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// Update bit log likelihood ratios (tov=0 in iter 0)
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for (int i = 0; i < ft8::N; ++i) {
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zn[i] = codeword[i] + tov[i][0] + tov[i][1] + tov[i][2];
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plain[i] = (zn[i] > 0) ? 1 : 0;
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}
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// Check to see if we have a codeword (check before we do any iter)
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int errors = ldpc_check(plain);
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if (errors < min_errors) {
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// we have a better guess - update the result
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min_errors = errors;
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if (errors == 0) {
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break; // Found a perfect answer
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}
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}
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// Send messages from bits to check nodes
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for (int i = 0; i < ft8::M; ++i) {
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for (int j = 0; j < kNrw[i]; ++j) {
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int ibj = kNm[i][j] - 1;
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toc[i][j] = zn[ibj];
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for (int kk = 0; kk < 3; ++kk) {
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// subtract off what the bit had received from the check
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if (kMn[ibj][kk] - 1 == i) {
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toc[i][j] -= tov[ibj][kk];
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}
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}
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}
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}
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// send messages from check nodes to variable nodes
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for (int i = 0; i < ft8::M; ++i) {
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for (int j = 0; j < kNrw[i]; ++j) {
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toc[i][j] = fast_tanh(-toc[i][j] / 2);
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}
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}
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for (int i = 0; i < ft8::N; ++i) {
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for (int j = 0; j < 3; ++j) {
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int ichk = kMn[i][j] - 1; // kMn(:,j) are the checks that include bit j
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float Tmn = 1.0f;
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for (int k = 0; k < kNrw[ichk]; ++k) {
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if (kNm[ichk][k] - 1 != i) {
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Tmn *= toc[ichk][k];
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}
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}
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tov[i][j] = 2 * fast_atanh(-Tmn);
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}
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}
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}
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*ok = min_errors;
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}
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// https://varietyofsound.wordpress.com/2011/02/14/efficient-tanh-computation-using-lamberts-continued-fraction/
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// http://functions.wolfram.com/ElementaryFunctions/ArcTanh/10/0001/
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// https://mathr.co.uk/blog/2017-09-06_approximating_hyperbolic_tangent.html
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// thank you Douglas Bagnall
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// https://math.stackexchange.com/a/446411
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static float fast_tanh(float x) {
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if (x < -4.97f) {
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return -1.0f;
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}
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if (x > 4.97f) {
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return 1.0f;
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}
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float x2 = x * x;
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//float a = x * (135135.0f + x2 * (17325.0f + x2 * (378.0f + x2)));
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//float b = 135135.0f + x2 * (62370.0f + x2 * (3150.0f + x2 * 28.0f));
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//float a = x * (10395.0f + x2 * (1260.0f + x2 * 21.0f));
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//float b = 10395.0f + x2 * (4725.0f + x2 * (210.0f + x2));
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float a = x * (945.0f + x2 * (105.0f + x2));
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float b = 945.0f + x2 * (420.0f + x2 * 15.0f);
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return a / b;
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}
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static float fast_atanh(float x) {
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float x2 = x * x;
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//float a = x * (-15015.0f + x2 * (19250.0f + x2 * (-5943.0f + x2 * 256.0f)));
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//float b = (-15015.0f + x2 * (24255.0f + x2 * (-11025.0f + x2 * 1225.0f)));
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//float a = x * (-1155.0f + x2 * (1190.0f + x2 * -231.0f));
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//float b = (-1155.0f + x2 * (1575.0f + x2 * (-525.0f + x2 * 25.0f)));
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float a = x * (945.0f + x2 * (-735.0f + x2 * 64.0f));
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float b = (945.0f + x2 * (-1050.0f + x2 * 225.0f));
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return a / b;
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}
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static float pltanh(float x) {
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float isign = +1;
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if (x < 0) {
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isign = -1;
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x = -x;
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}
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if (x < 0.8f) {
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return isign * 0.83 * x;
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}
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if (x < 1.6f) {
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return isign * (0.322f * x + 0.4064f);
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}
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if (x < 3.0f) {
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return isign * (0.0524f * x + 0.8378f);
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}
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if (x < 7.0f) {
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return isign * (0.0012f * x + 0.9914f);
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}
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return isign*0.9998f;
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}
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static float platanh(float x) {
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float isign = +1;
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if (x < 0) {
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isign = -1;
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x = -x;
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}
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if (x < 0.664f) {
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return isign * x / 0.83f;
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}
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if (x < 0.9217f) {
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return isign * (x - 0.4064f) / 0.322f;
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}
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if (x < 0.9951f) {
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return isign * (x - 0.8378f) / 0.0524f;
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}
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if (x < 0.9998f) {
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return isign * (x - 0.9914f) / 0.0012f;
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}
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return isign * 7.0f;
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}
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} // namespace
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