signal_processor.cpp

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// SPDX-License-Identifier: GPL-2.0-only
//
// Copyright (c) 2006-2008 Marc Brooker and Michael Inggs
// Copyright (c) 2008-present FERS Contributors (see AUTHORS.md).
//
// See the GNU GPLv2 LICENSE file in the FERS project root for more information.
 
/**
 * @file signal_processor.cpp
 * @brief Implementation for receiver-side signal processing and rendering.
 *
 * This file contains the implementation of functions that perform digital signal
 * processing on received radar signals, as declared in signal_processor.h.
 */
 
#include "signal_processor.h"
 
#include <algorithm>
#include <cmath>
#include <future>
#include <queue>
#include <tuple>
#include <vector>
 
#include "core/parameters.h"
#include "core/thread_pool.h"
#include "noise/noise_generators.h"
#include "serial/response.h"
 
namespace
{
    /**
     * @brief Simulate an ADC quantization process on a window of complex samples.
     * @param data A span of ComplexType objects representing the window to quantize.
     * @param bits The number of bits used for quantization.
     * @param fullscale The full-scale value used for quantization.
     */
    void adcSimulate(std::span<ComplexType> data, const unsigned bits, const RealType fullscale) noexcept
    {
        const RealType levels = std::pow(2, bits - 1);
 
        for (auto& sample : data)
        {
            auto [i, q] = std::tuple{std::clamp(std::floor(levels * sample.real() / fullscale) / levels, -1.0, 1.0),
                                     std::clamp(std::floor(levels * sample.imag() / fullscale) / levels, -1.0, 1.0)};
            sample = ComplexType(i, q);
        }
    }
 
    /**
     * @brief Process a single response and add its rendered data to a local window buffer.
     * @param resp A pointer to a Response object to process.
     * @param localWindow A reference to a vector of ComplexType representing the local window.
     * @param rate The sample rate in Hz.
     * @param start The start time of the window in seconds.
     * @param fracDelay The fractional delay of the window in seconds.
     * @param localWindowSize The size of the local window in samples.
     */
    void processResponse(const serial::Response* resp, std::vector<ComplexType>& localWindow, const RealType rate,
                         const RealType start, const RealType fracDelay, const std::size_t localWindowSize)
    {
        unsigned psize;
        RealType prate;
        const auto array = resp->renderBinary(prate, psize, fracDelay);
        int start_sample = static_cast<int>(std::round(rate * (resp->startTime() - start)));
        const auto roffset = start_sample < 0 ? static_cast<std::size_t>(-start_sample) : std::size_t{0};
        const auto start_offset = static_cast<std::size_t>(std::max(start_sample, 0));
 
        for (std::size_t i = roffset; i < psize && i + start_offset < localWindowSize; ++i)
        {
            localWindow[i + start_offset] += array[i];
        }
    }
}
 
namespace processing
{
    void renderWindow(std::vector<ComplexType>& window, const RealType length, const RealType start,
                      const RealType fracDelay, const std::span<const std::unique_ptr<serial::Response>> responses)
    {
        const RealType end = start + length;
        std::queue<serial::Response*> work_list;
 
        for (const auto& response : responses)
        {
            if (response->startTime() <= end && response->endTime() >= start)
            {
                work_list.push(response.get());
            }
        }
 
        const RealType rate = params::rate() * params::oversampleRatio();
        const auto local_window_size = static_cast<std::size_t>(std::ceil(length * rate));
 
        std::vector local_window(local_window_size, ComplexType{});
 
        while (!work_list.empty())
        {
            const auto* resp = work_list.front();
            work_list.pop();
            processResponse(resp, local_window, rate, start, fracDelay, local_window_size);
        }
 
        for (std::size_t i = 0; i < local_window_size; ++i)
        {
            window[i] += local_window[i];
        }
    }
 
    void applyThermalNoise(std::span<ComplexType> window, const RealType noiseTemperature, std::mt19937& rngEngine)
    {
        if (noiseTemperature == 0)
        {
            return;
        }
 
        const RealType b = params::rate() / (2.0 * params::oversampleRatio());
        const RealType total_power = params::boltzmannK() * noiseTemperature * b;
 
        // Split total power equally between the I and Q channels
        const RealType per_channel_power = total_power / 2.0;
        const RealType stddev = std::sqrt(per_channel_power);
 
        noise::WgnGenerator generator(rngEngine, stddev);
        for (auto& sample : window)
        {
            sample += ComplexType(generator.getSample(), generator.getSample());
        }
    }
 
    RealType quantizeAndScaleWindow(std::span<ComplexType> window)
    {
        RealType max_value = 0;
        for (const auto& sample : window)
        {
            const RealType real_abs = std::fabs(sample.real());
            const RealType imag_abs = std::fabs(sample.imag());
 
            max_value = std::max({max_value, real_abs, imag_abs});
        }
 
        if (const auto adc_bits = params::adcBits(); adc_bits > 0)
        {
            adcSimulate(window, adc_bits, max_value);
        }
        else if (max_value != 0)
        {
            for (auto& sample : window)
            {
                sample /= max_value;
            }
        }
        return max_value;
    }
}