diff --git a/debian/copyright b/debian/copyright index 50b5d4c4..6b6fd367 100644 --- a/debian/copyright +++ b/debian/copyright @@ -52,6 +52,10 @@ License: LGPL-2.1 License (LGPL) version 2.1 can be found in the file '/usr/share/common-licenses/LGPL-2.1'. +Files: lib/viterbi/* +Copyright: Min Xu +License: Apache-2.0 + Files: python/ccsds/* Copyright: 2019 Athanasios Theocharis License: GPL-3+ diff --git a/lib/CMakeLists.txt b/lib/CMakeLists.txt index a11b5e64..be2b6254 100644 --- a/lib/CMakeLists.txt +++ b/lib/CMakeLists.txt @@ -76,6 +76,7 @@ list(APPEND satellites_sources radecoder/ra_decoder_gen.c radecoder/ra_lfsr.c radecoder/ra_encoder.c + viterbi/viterbi.cc ) set(satellites_sources "${satellites_sources}" PARENT_SCOPE) diff --git a/lib/viterbi/LICENSE.md b/lib/viterbi/LICENSE.md new file mode 100644 index 00000000..8dada3ed --- /dev/null +++ b/lib/viterbi/LICENSE.md @@ -0,0 +1,201 @@ + Apache License + Version 2.0, January 2004 + http://www.apache.org/licenses/ + + TERMS AND CONDITIONS FOR USE, REPRODUCTION, AND DISTRIBUTION + + 1. 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"1" : "0"; + } + } +} + +int ViterbiCodec::BranchMetric(const std::string& bits, + int source_state, + int target_state) const +{ + assert(bits.size() == num_parity_bits()); + assert((target_state & ((1 << (constraint_ - 2)) - 1)) == source_state >> 1); + const std::string output = Output(source_state, target_state >> (constraint_ - 2)); + + return HammingDistance(bits, output); +} + +std::pair ViterbiCodec::PathMetric(const std::string& bits, + const std::vector& prev_path_metrics, + int state) const +{ + int s = (state & ((1 << (constraint_ - 2)) - 1)) << 1; + int source_state1 = s | 0; + int source_state2 = s | 1; + + int pm1 = prev_path_metrics[source_state1]; + if (pm1 < std::numeric_limits::max()) { + pm1 += BranchMetric(bits, source_state1, state); + } + int pm2 = prev_path_metrics[source_state2]; + if (pm2 < std::numeric_limits::max()) { + pm2 += BranchMetric(bits, source_state2, state); + } + + if (pm1 <= pm2) { + return std::make_pair(pm1, source_state1); + } else { + return std::make_pair(pm2, source_state2); + } +} + +void ViterbiCodec::UpdatePathMetrics(const std::string& bits, + std::vector* path_metrics, + Trellis* trellis) const +{ + std::vector new_path_metrics(path_metrics->size()); + std::vector new_trellis_column(1 << (constraint_ - 1)); + for (int i = 0; i < path_metrics->size(); i++) { + std::pair p = PathMetric(bits, *path_metrics, i); + new_path_metrics[i] = p.first; + new_trellis_column[i] = p.second; + } + + *path_metrics = new_path_metrics; + trellis->push_back(new_trellis_column); +} + +std::string ViterbiCodec::Decode(const std::string& bits) const +{ + // Compute path metrics and generate trellis. + Trellis trellis; + std::vector path_metrics(1 << (constraint_ - 1), + std::numeric_limits::max()); + path_metrics.front() = 0; + for (int i = 0; i < bits.size(); i += num_parity_bits()) { + std::string current_bits(bits, i, num_parity_bits()); + // If some bits are missing, fill with trailing zeros. + // This is not ideal but it is the best we can do. + if (current_bits.size() < num_parity_bits()) { + current_bits.append( + std::string(num_parity_bits() - current_bits.size(), '0')); + } + UpdatePathMetrics(current_bits, &path_metrics, &trellis); + } + + // Traceback. + std::string decoded; + int state = + std::min_element(path_metrics.begin(), path_metrics.end()) - path_metrics.begin(); + for (int i = trellis.size() - 1; i >= 0; i--) { + decoded += state >> (constraint_ - 2) ? "1" : "0"; + state = trellis[i][state]; + } + std::reverse(decoded.begin(), decoded.end()); + + // Remove (constraint_ - 1) flushing bits. + return decoded.substr(0, decoded.size() - constraint_ + 1); +} diff --git a/lib/viterbi/viterbi.h b/lib/viterbi/viterbi.h new file mode 100644 index 00000000..78159543 --- /dev/null +++ b/lib/viterbi/viterbi.h @@ -0,0 +1,94 @@ +// Viterbi Codec. +// +// Author: Min Xu +// Date: 01/30/2015 + +#ifndef VITERBI_H_ +#define VITERBI_H_ + +#include +#include +#include +#include + +// This class implements both a Viterbi Decoder and a Convolutional Encoder. +class ViterbiCodec +{ +public: + // Note about Polynomial Descriptor of a Convolutional Encoder / Decoder. + // A generator polymonial is built as follows: Build a binary number + // representation by placing a 1 in each spot where a connection line from + // the shift register feeds into the adder, and a zero elsewhere. There are 2 + // ways to arrange the bits: + // 1. msb-current + // The MSB of the polynomial corresponds to the current input, while the + // LSB corresponds to the oldest input that still remains in the shift + // register. + // This representation is used by MATLAB. See + // http://radio.feld.cvut.cz/matlab/toolbox/comm/tutor124.html + // 2. lsb-current + // The LSB of the polynomial corresponds to the current input, while the + // MSB corresponds to the oldest input that still remains in the shift + // register. + // This representation is used by the Spiral Viterbi Decoder Software + // Generator. See http://www.spiral.net/software/viterbi.html + // We use 2. + ViterbiCodec(int constraint, const std::vector& polynomials); + + std::string Encode(const std::string& bits) const; + + std::string Decode(const std::string& bits) const; + + int constraint() const { return constraint_; } + + const std::vector& polynomials() const { return polynomials_; } + +private: + // Suppose + // + // Trellis trellis; + // + // Then trellis[i][s] is the state in the (i - 1)th iteration which leads to + // the current state s in the ith iteration. + // It is used for traceback. + typedef std::vector> Trellis; + + int num_parity_bits() const; + + void InitializeOutputs(); + + int NextState(int current_state, int input) const; + + std::string Output(int current_state, int input) const; + + int BranchMetric(const std::string& bits, int source_state, int target_state) const; + + // Given num_parity_bits() received bits, compute and returns path + // metric and its corresponding previous state. + std::pair PathMetric(const std::string& bits, + const std::vector& prev_path_metrics, + int state) const; + + // Given num_parity_bits() received bits, update path metrics of all states + // in the current iteration, and append new traceback vector to trellis. + void UpdatePathMetrics(const std::string& bits, + std::vector* path_metrics, + Trellis* trellis) const; + + const int constraint_; + const std::vector polynomials_; + + // The output table. + // The index is current input bit combined with previous inputs in the shift + // register. The value is the output parity bits in string format for + // convenience, e.g. "10". For example, suppose the shift register contains + // 0b10 (= 2), and the current input is 0b1 (= 1), then the index is 0b110 (= + // 6). + std::vector outputs_; +}; + +std::ostream& operator<<(std::ostream& os, const ViterbiCodec& codec); + +int ReverseBits(int num_bits, int input); + +#endif // VITERBI_H_