243 lines
7.2 KiB
C++
243 lines
7.2 KiB
C++
/***************************************************************************
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* Copyright (C) 2021 - 2022 by Federico Amedeo Izzo IU2NUO, *
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* Niccolò Izzo IU2KIN *
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* Frederik Saraci IU2NRO *
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* Silvano Seva IU2KWO *
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* *
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* Adapted from original code written by Phil Karn KA9Q *
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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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#ifndef M17_VITERBI_H
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#define M17_VITERBI_H
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#ifndef __cplusplus
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#error This header is C++ only!
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#endif
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#include <cstdint>
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#include <cstddef>
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#include <array>
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#include <bitset>
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#include "M17Utils.hpp"
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namespace M17
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{
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/**
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* Hard decision Viterbi decoder tailored on M17 protocol specifications,
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* that is for decoding of data encoded with a convolutional encoder with a
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* coder rate R = 1/2, a constraint length K = 5 and polynomials G1 = 0x19 and
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* G2 = 0x17.
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*/
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class M17Viterbi
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{
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public:
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/**
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* Constructor.
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*/
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M17Viterbi() : prevMetrics(&prevMetricsData), currMetrics(&currMetricsData)
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{ }
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/**
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* Destructor.
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*/
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~M17Viterbi() { }
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/**
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* Decode unpunctured convolutionally encoded data.
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*
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* @param in: input data.
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* @param out: destination array where decoded data are written.
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* @return number of bit errors corrected.
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*/
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template < size_t IN, size_t OUT >
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uint16_t decode(const std::array< uint8_t, IN >& in,
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std::array< uint8_t, OUT >& out)
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{
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static_assert(IN*4 < 244, "Input size exceeds max history");
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currMetricsData.fill(0x00);
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prevMetricsData.fill(0x00);
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size_t pos = 0;
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for (size_t i = 0; i < IN*8; i += 2)
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{
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uint8_t s0 = getBit(in, i) ? 2 : 0;
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uint8_t s1 = getBit(in, i + 1) ? 2 : 0;
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decodeBit(s0, s1, pos);
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pos++;
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}
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return chainback(out, pos) / ((K - 1) >> 1);
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}
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/**
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* Decode punctured convolutionally encoded data.
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*
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* @param in: input data.
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* @param out: destination array where decoded data are written.
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* @return number of bit errors corrected.
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*/
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template < size_t IN, size_t OUT, size_t P >
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uint16_t decodePunctured(const std::array< uint8_t, IN >& in,
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std::array< uint8_t, OUT >& out,
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const std::array< uint8_t, P >& punctureMatrix)
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{
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static_assert(IN*4 < 244, "Input size exceeds max history");
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currMetricsData.fill(0x00);
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prevMetricsData.fill(0x00);
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size_t histPos = 0;
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size_t punctIndex = 0;
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size_t bitPos = 0;
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uint16_t punctBitCnt = 0;
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while(bitPos < IN*8)
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{
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uint8_t sym[2] = {1, 1};
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for(uint8_t i = 0; i < 2; i++)
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{
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if(punctureMatrix[punctIndex++])
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{
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sym[i] = getBit(in, bitPos) ? 2 : 0;
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bitPos++;
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}
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else
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{
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punctBitCnt++;
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}
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if(punctIndex >= P) punctIndex = 0;
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}
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decodeBit(sym[0], sym[1], histPos);
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histPos++;
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}
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return (chainback(out, histPos) - punctBitCnt) / ((K - 1) >> 1);
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}
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private:
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/**
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* Decode one bit and update trellis.
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*
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* @param s0: cost of the first symbol.
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* @param s1: cost of the second symbol.
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* @param pos: bit position in history.
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*/
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void decodeBit(uint8_t s0, uint8_t s1, size_t pos)
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{
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static constexpr uint8_t COST_TABLE_0[] = {0, 0, 0, 0, 2, 2, 2, 2};
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static constexpr uint8_t COST_TABLE_1[] = {0, 2, 2, 0, 0, 2, 2, 0};
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for(uint8_t i = 0; i < NumStates/2; i++)
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{
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uint16_t metric = std::abs(COST_TABLE_0[i] - s0)
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+ std::abs(COST_TABLE_1[i] - s1);
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uint16_t m0 = (*prevMetrics)[i] + metric;
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uint16_t m1 = (*prevMetrics)[i + NumStates/2] + (4 - metric);
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uint16_t m2 = (*prevMetrics)[i] + (4 - metric);
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uint16_t m3 = (*prevMetrics)[i + NumStates/2] + metric;
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uint8_t i0 = 2 * i;
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uint8_t i1 = i0 + 1;
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if(m0 >= m1)
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{
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history[pos].set(i0, true);
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(*currMetrics)[i0] = m1;
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}
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else
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{
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history[pos].set(i0, false);
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(*currMetrics)[i0] = m0;
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}
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if(m2 >= m3)
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{
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history[pos].set(i1, true);
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(*currMetrics)[i1] = m3;
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}
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else
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{
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history[pos].set(i1, false);
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(*currMetrics)[i1] = m2;
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}
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}
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std::swap(currMetrics, prevMetrics);
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}
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/**
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* History chainback to obtain final byte array.
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*
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* @param out: destination byte array for decoded data.
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* @param pos: starting position for the chainback.
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* @return minimum Viterbi cost at the end of the decode sequence.
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*/
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template < size_t OUT >
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uint16_t chainback(std::array< uint8_t, OUT >& out, size_t pos)
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{
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uint8_t state = 0;
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size_t bitPos = OUT*8;
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while(bitPos > 0)
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{
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bitPos--;
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pos--;
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bool bit = history[pos].test(state >> 4);
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state >>= 1;
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if(bit) state |= 0x80;
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setBit(out, bitPos, bit);
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}
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uint16_t cost = (*prevMetrics)[0];
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for(size_t i = 0; i < NumStates; i++)
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{
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uint16_t m = (*prevMetrics)[i];
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if(m < cost) cost = m;
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}
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return cost;
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}
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static constexpr size_t K = 5;
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static constexpr size_t NumStates = (1 << (K - 1));
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std::array< uint16_t, NumStates > *prevMetrics;
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std::array< uint16_t, NumStates > *currMetrics;
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std::array< uint16_t, NumStates > prevMetricsData;
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std::array< uint16_t, NumStates > currMetricsData;
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std::array< std::bitset< NumStates >, 244 > history;
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};
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} // namespace M17
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#endif // M17_VITERBI_H
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