portapack-mayhem/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