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204 lines
4.7 KiB
C++
204 lines
4.7 KiB
C++
// Copyright 2021 Rob Riggs <rob@mobilinkd.com>
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// All rights reserved.
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#pragma once
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#include "IirFilter.h"
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#include <algorithm>
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#include <array>
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#include <cstdint>
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#include <cmath>
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#include <cstddef>
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#include <type_traits>
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#include <tuple>
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#include <limits>
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#include <iostream>
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#include "export.h"
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namespace modemm17 {
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struct MODEMM17_API Correlator
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{
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static const size_t SYMBOLS = 8;
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static const size_t SAMPLES_PER_SYMBOL = 10;
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using buffer_t = std::array<float, SYMBOLS * SAMPLES_PER_SYMBOL>;
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using sync_t = std::array<int8_t, SYMBOLS>;
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using sample_filter_t = BaseIirFilter<3>;
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buffer_t buffer_;
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float limit_ = 0.;
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size_t symbol_pos_ = 0;
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size_t buffer_pos_ = 0;
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size_t prev_buffer_pos_ = 0;
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int code = -1;
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// IIR with Nyquist of 1/240.
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static const std::array<float,3> b;
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static const std::array<float,3> a;
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sample_filter_t sample_filter{b, a};
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std::array<int, SYMBOLS> tmp;
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void sample(float value)
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{
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limit_ = sample_filter(std::abs(value));
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buffer_[buffer_pos_] = value;
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prev_buffer_pos_ = buffer_pos_;
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if (++buffer_pos_ == buffer_.size()) {
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buffer_pos_ = 0;
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}
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}
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float correlate(sync_t sync)
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{
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float result = 0.;
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size_t pos = prev_buffer_pos_ + SAMPLES_PER_SYMBOL;
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for (size_t i = 0; i != sync.size(); ++i)
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{
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if (pos >= buffer_.size()) {
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pos -= buffer_.size(); // wrapped
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}
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result += sync[i] * buffer_[pos];
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pos += SAMPLES_PER_SYMBOL;
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}
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return result;
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}
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float limit() const {return limit_;}
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size_t index() const {return prev_buffer_pos_ % SAMPLES_PER_SYMBOL;}
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/**
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* Get the average outer symbol levels at a given index. This makes trhee
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* assumptions.
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*
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* 1. The max symbol value is above 0 and the min symbol value is below 0.
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* 2. The samples at the given index only contain outer symbols.
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* 3. The index is a peak correlation index.
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*
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* The first should hold true except for extreme frequency errors. The
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* second holds true for the sync words used for M17. The third will
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* hold true if passed the timing index from a triggered sync word.
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*/
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std::tuple<float, float> outer_symbol_levels(size_t sample_index)
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{
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float min_sum = 0;
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float max_sum = 0;
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size_t min_count = 0;
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size_t max_count = 0;
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size_t index = 0;
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for (size_t i = sample_index; i < buffer_.size(); i += SAMPLES_PER_SYMBOL)
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{
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tmp[index++] = buffer_[i] * 1000.;
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max_sum += buffer_[i] * ((buffer_[i] > 0.));
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min_sum += buffer_[i] * ((buffer_[i] < 0.));
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max_count += (buffer_[i] > 0.);
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min_count += (buffer_[i] < 0.);
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}
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return std::make_tuple(min_sum / min_count, max_sum / max_count);
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}
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template <typename F>
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void apply(F func, uint8_t index)
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{
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for (size_t i = index; i < buffer_.size(); i += SAMPLES_PER_SYMBOL)
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{
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func(buffer_[i]);
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}
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}
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};
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struct SyncWord
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{
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using buffer_t = std::array<int8_t, Correlator::SYMBOLS>;
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using sample_buffer_t = std::array<float, Correlator::SAMPLES_PER_SYMBOL>;
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buffer_t sync_word_;
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sample_buffer_t samples_;
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size_t pos_ = 0;
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size_t timing_index_ = 0;
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bool triggered_ = false;
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int8_t updated_ = 0;
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float magnitude_1_ = 1.0f;
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float magnitude_2_ = -1.0f;
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SyncWord(
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buffer_t&& sync_word,
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float magnitude_1,
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float magnitude_2 = std::numeric_limits<float>::lowest()
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) :
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sync_word_(std::move(sync_word)),
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magnitude_1_(magnitude_1),
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magnitude_2_(magnitude_2)
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{}
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float triggered(Correlator& correlator)
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{
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float limit_1 = correlator.limit() * magnitude_1_;
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float limit_2 = correlator.limit() * magnitude_2_;
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auto value = correlator.correlate(sync_word_);
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return (value > limit_1 || value < limit_2) ? value : 0.0;
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}
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size_t operator()(Correlator& correlator)
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{
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auto value = triggered(correlator);
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float peak_value = 0.0f;
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if (value != 0)
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{
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if (!triggered_)
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{
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samples_.fill(0);
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triggered_ = true;
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}
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samples_[correlator.index()] = value;
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}
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else
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{
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if (triggered_)
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{
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// Calculate the timing index on the falling edge.
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triggered_ = false;
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timing_index_ = 0;
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peak_value = value;
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uint8_t index = 0;
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for (auto f : samples_)
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{
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if (abs(f) > abs(peak_value))
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{
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peak_value = f;
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timing_index_ = index;
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}
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index += 1;
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}
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updated_ = peak_value > 0 ? 1 : -1;
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}
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}
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return timing_index_;
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}
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int8_t updated()
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{
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auto result = updated_;
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updated_ = 0;
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return result;
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}
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};
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} // modemm17
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