158 lines
3.9 KiB
C++
158 lines
3.9 KiB
C++
/* -*- c++ -*- */
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/*
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* gr-satnogs: SatNOGS GNU Radio Out-Of-Tree Module
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*
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* Copyright (C) 2016, 2017, 2018
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* Libre Space Foundation <http://librespacefoundation.org/>
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*
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* This program is free software: you can redistribute it and/or modify
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* it under the terms of the GNU General Public License as published by
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* the Free Software Foundation, either version 3 of the License, or
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* (at your option) any later version.
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*
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* This program is distributed in the hope that it will be useful,
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* but WITHOUT ANY WARRANTY; without even the implied warranty of
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* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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* GNU General Public License for more details.
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*
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* You should have received a copy of the GNU General Public License
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* along with this program. If not, see <http://www.gnu.org/licenses/>.
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*/
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#ifdef HAVE_CONFIG_H
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#include "config.h"
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#endif
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#include <satnogs/golay24.h>
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#include <satnogs/utils.h>
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namespace gr {
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namespace satnogs {
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/*
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* Matrix P was retrieved by:
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* Lin & Costello, pg 128, Ch4, "Error Control Coding", 2nd ed, Pearson.
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*
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* Matrix mentioned by Morelos-Zaragoza, Robert H. "The art of error correcting coding."
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* John Wiley & Sons, 2006 was not suitable.
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*/
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const std::vector<uint32_t> golay24::G_P = {
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0x8ED, 0x1DB, 0x3B5, 0x769, 0xED1, 0xDA3, 0xB47, 0x68F, 0xD1D, 0xA3B, 0x477,
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0xFFE
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};
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const std::vector<uint32_t> golay24::G_I = {
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0x800, 0x400, 0x200, 0x100, 0x080, 0x040, 0x020, 0x010, 0x008, 0x004, 0x002,
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0x001
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};
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golay24::golay24()
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{
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}
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golay24::~golay24()
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{
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}
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static inline uint32_t
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weight(uint32_t x)
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{
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return bit_count(x & 0xFFF);
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}
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static inline uint32_t
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syndrome(uint16_t x, uint16_t y)
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{
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uint32_t s = 0;
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for (size_t i = 0; i < 12; i++) {
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s = (s << 1) | (weight(y & golay24::G_P[i]) % 2);
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}
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s ^= x;
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return s;
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}
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/**
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* Encodes a 12-bit message
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* @param in the input 12-bit message. The message should be placed at the
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* 12 LS bits
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* @param lsb_parity if set to true, the parity part is placed on the 12 LSB,
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* whereas if set to false it is placed on the 12 MSB of the 24 bit coded word
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* @return the coded 24-bit message. The message is placed at the 24 LS bits
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*/
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uint32_t
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golay24::encode12(uint16_t in, bool lsb_parity)
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{
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uint32_t c[2] =
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{ 0x0, 0x0 };
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c[0] = in & 0xFFF;
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for (size_t i = 0; i < 12; i++) {
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uint32_t tmp = 0;
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for (size_t j = 0; j < 12; j++) {
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tmp ^= (((c[0] & G_P[i]) >> j) & 0x01);
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}
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c[1] = (c[1] << 1) ^ tmp;
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}
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if (lsb_parity) {
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return ((c[0] & 0xFFF) << 12) | (c[1] & 0xFFF);
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}
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return ((c[1] & 0xFFF) << 12) | (c[0] & 0xFFF);
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}
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/**
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* Decodes a single Golay (24, 12, 8) codeword
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*
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* @param out the 24-bit decoded message. The placement of the parity is
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* implementation specific.
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*
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* @param in the coded 24 bit code word. The message should be placed at the
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* 24 LS bits
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* @return true if the decoding was successful, false in case the error correction
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* could not be performed
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*/
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bool
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golay24::decode24(uint32_t *out, const uint32_t in)
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{
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uint32_t e[2] = {0x0, 0x0};
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uint32_t r[2] = {0x0, 0x0};
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uint32_t c[2] = {0x0, 0x0};
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uint32_t s = 0x0;
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r[0] = (in >> 12) & 0xFFF;
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r[1] = in & 0xFFF;
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s = syndrome(r[0], r[1]);
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if (weight(s) <= 3) {
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*out = ((r[0] ^ s) << 12) | (r[1]);
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return true;
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}
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for (size_t i = 0; i < 12; i++) {
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const uint16_t tmp = s ^ G_P[i];
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if (weight(tmp) <= 2) {
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*out = ((r[0] ^ tmp) << 12) | (r[1] ^ G_I[i]);
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return true;
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}
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}
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/* Compute the sP vector */
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uint32_t sP = 0;
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for (size_t i = 0; i < 12; i++) {
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sP = (sP << 1) | (weight(s & G_P[i]) % 2);
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}
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if (weight(sP) == 2 || weight(sP) == 3) {
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*out = (r[0] << 12) | (r[1] ^ sP);
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return true;
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}
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for (size_t i = 0; i < 12; i++) {
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const uint16_t tmp = sP ^ G_P[i];
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if (weight(tmp) == 2) {
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*out = ((r[0] ^ G_I[i]) << 12) | (r[1] ^ tmp);
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return true;
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}
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}
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return false;
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}
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} /* namespace satnogs */
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} /* namespace gr */
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