mayhem-firmware/firmware/baseband/proc_adsbrx.cpp

/*
 * Copyright (C) 2014 Jared Boone, ShareBrained Technology, Inc.
 * Copyright (C) 2017 Furrtek
 *
 * This file is part of PortaPack.
 *
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation; either version 2, or (at your option)
 * any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program; see the file COPYING.  If not, write to
 * the Free Software Foundation, Inc., 51 Franklin Street,
 * Boston, MA 02110-1301, USA.
 */

#include "proc_adsbrx.hpp"
#include "portapack_shared_memory.hpp"
#include "sine_table_int8.hpp"
#include "event_m4.hpp"

#include <cstdint>
#include <cstddef>

using namespace adsb;

void ADSBRXProcessor::execute(const buffer_c8_t& buffer) {
    int8_t re, im;
    uint32_t mag;
    uint32_t c;
    uint8_t bit, byte{};

    // This is called at 2M/2048 = 977Hz
    // One pulse = 500ns = 2 samples
    // One bit = 2 pulses = 1us = 4 samples

    if (!configured) return;

    for (size_t i = 0; i < buffer.count; i++) {
        // Compute sample's magnitude
        re = (int32_t)buffer.p[i].real();
        im = (int32_t)buffer.p[i].imag();
        mag = ((uint32_t)(re * re) + (uint32_t)(im * im));

        if (decoding) {
            // Decode

            // 1 bit lasts 2 samples
            if (sample_count & 1) {
                if (bit_count >= msgLen) {
                    const ADSBFrameMessage message(frame, amp);
                    shared_memory.application_queue.push(message);
                    decoding = false;
                    bit = (prev_mag > mag) ? 1 : 0;
                } else {
                    bit = (prev_mag > mag) ? 1 : 0;
                }

                byte = bit | (byte << 1);
                bit_count++;

                // Perform checks at the end of the first byte
                if (!(bit_count & 7)) {
                    // Store the byte
                    frame.push_byte(byte);

                    // Check at the end of the first byte of the message
                    uint8_t df = (byte >> 3);
                    if ((bit_count == 8) && !(df & 0x10)) {
                        msgLen = 56;  // DFs 16 or greater are long 112. DFs 15 or less are short 56.
                    }

                    // Abondon all frames that arent DF17 or DF18 extended squitters
                    if ((bit_count == 8) && !((df == 17) || (df == 18))) {
                        decoding = false;
                        bit = (prev_mag > mag) ? 1 : 0;
                        frame.clear();
                    }
                }  // last bit of a byte
            }      // Second sample of each bit
            sample_count++;
        }

        // Continue looking for preamble even if in a packet
        // switch if new preamble is higher magnitude

        // Shift the preamble
        for (c = 0; c < (ADSB_PREAMBLE_LENGTH); c++) {
            shifter[c] = shifter[c + 1];
        }
        shifter[ADSB_PREAMBLE_LENGTH] = mag;

        // First check of relations between the first 10 samples
        // representing a valid preamble. We don't even investigate further
        // if this simple test is not passed
        if (shifter[0] < shifter[1] &&
            shifter[1] > shifter[2] &&
            shifter[2] < shifter[3] &&
            shifter[3] > shifter[4] &&
            shifter[4] < shifter[1] &&
            shifter[5] < shifter[1] &&
            shifter[6] < shifter[1] &&
            shifter[7] < shifter[1] &&
            shifter[8] > shifter[9] &&
            shifter[9] < shifter[10] &&
            shifter[10] > shifter[11]) {
            // The samples between the two spikes must be < than the average
            // of the high spikes level. We don't test bits too near to
            // the high levels as signals can be out of phase so part of the
            // energy can be in the near samples
            int32_t thisAmp = (shifter[1] + shifter[3] + shifter[8] + shifter[10]);
            uint32_t high = thisAmp / 9;
            if (
                shifter[5] < high &&
                shifter[6] < high &&
                // Similarly samples in the range 11-13 must be low, as it is the
                // space between the preamble and real data. Again we don't test
                // bits too near to high levels, see above
                shifter[12] < high &&
                shifter[13] < high &&
                shifter[14] < high) {
                if ((decoding == false) ||                    // New preamble
                    ((decoding == true) && (thisAmp > amp)))  // Higher power than existing packet
                {
                    decoding = true;
                    msgLen = 112;
                    amp = thisAmp;
                    sample_count = 0;
                    bit_count = 0;
                    frame.clear();
                }
            }  // 4 & 5 low and 11-14 low
        }      // Check for preamble pattern

        // Store mag for next time
        prev_mag = mag;
    }
}

void ADSBRXProcessor::on_message(const Message* const message) {
    if (message->id == Message::ID::ADSBConfigure) {
        bit_count = 0;
        sample_count = 0;
        decoding = false;
        configured = true;
    }
}

#ifndef _WIN32
int main() {
    EventDispatcher event_dispatcher{std::make_unique<ADSBRXProcessor>()};
    event_dispatcher.run();
    return 0;
}
#endif