gr-air-modes/lib/air_modes_slicer.cc

/*
# Copyright 2010 Nick Foster
# Copyright 2013 Nicholas Corgan
# 
# This file is part of gr-air-modes
# 
# gr-air-modes 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 3, or (at your option)
# any later version.
# 
# gr-air-modes 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 gr-air-modes; see the file COPYING.  If not, write to
# the Free Software Foundation, Inc., 51 Franklin Street,
# Boston, MA 02110-1301, USA.
# 
*/

#ifdef HAVE_CONFIG_H
#include "config.h"
#endif

#include <ciso646>
#include <air_modes_slicer.h>
#include <gr_io_signature.h>
#include <air_modes_types.h>
#include <sstream>
#include <iomanip>
#include <modes_crc.h>
#include <iostream>
#include <gr_tags.h>

extern "C"
{
#include <stdio.h>
#include <string.h>
}

air_modes_slicer_sptr air_make_modes_slicer(int channel_rate, gr_msg_queue_sptr queue)
{
	return air_modes_slicer_sptr (new air_modes_slicer(channel_rate, queue));
}

air_modes_slicer::air_modes_slicer(int channel_rate, gr_msg_queue_sptr queue) :
    gr_sync_block ("modes_slicer",
                   gr_make_io_signature (1, 1, sizeof(float)), //stream 0 is received data, stream 1 is binary preamble detector output
                   gr_make_io_signature (0, 0, 0) )
{
	set_rate(channel_rate);
	d_queue = queue;
}

void air_modes_slicer::set_rate(int channel_rate)
{
	d_chip_rate = 2000000; //2Mchips per second
	d_samples_per_chip = 2;//FIXME this is constant now channel_rate / d_chip_rate;
	d_samples_per_symbol = d_samples_per_chip * 2;
	d_check_width = 120 * d_samples_per_symbol; //how far you will have to look ahead
	set_output_multiple(d_check_width*2); //how do you specify buffer size for sinks?
}

//this slicer is courtesy of Lincoln Labs. supposedly it is more resistant to mode A/C FRUIT.
//see http://adsb.tc.faa.gov/WG3_Meetings/Meeting8/Squitter-Lon.pdf
static slice_result_t slicer(const float bit0, const float bit1, const float ref) {
	slice_result_t result;

	//3dB limits for bit slicing and confidence measurement
	float highlimit=ref*1.414;
	float lowlimit=ref*0.707;
	
	bool firstchip_inref  = ((bit0 > lowlimit) && (bit0 < highlimit));
	bool secondchip_inref = ((bit1 > lowlimit) && (bit1 < highlimit));

	if(firstchip_inref && !secondchip_inref) {
		result.decision = 1;
		result.confidence = 1;
	}
	else if(secondchip_inref && !firstchip_inref) {
		result.decision = 0;
		result.confidence = 1;
	} 
	else if(firstchip_inref && secondchip_inref) {
		result.decision = bit0 > bit1;
		result.confidence = 0;
	}
	else {//if(!firstchip_inref && !secondchip_inref) {
		result.decision = bit0 > bit1;
		if(result.decision) {
			if(bit1 < lowlimit * 0.5) result.confidence = 1;
			else result.confidence = 0;
		} else {
			if(bit0 < lowlimit * 0.5) result.confidence = 1;
			else result.confidence = 0;
		}
	}
	return result;
}

int air_modes_slicer::work(int noutput_items,
                          gr_vector_const_void_star &input_items,
		                  gr_vector_void_star &output_items)
{
	const float *in = (const float *) input_items[0];
	int size = noutput_items - d_check_width; //since it's a sync block, i assume that it runs with ninput_items = noutput_items

	if(0) std::cout << "Slicer called with " << size << " samples" << std::endl;
	
	std::vector<gr_tag_t> tags;
	uint64_t abs_sample_cnt = nitems_read(0);
	get_tags_in_range(tags, 0, abs_sample_cnt, abs_sample_cnt + size, pmt::pmt_string_to_symbol("preamble_found"));
	std::vector<gr_tag_t>::iterator tag_iter;

	for(tag_iter = tags.begin(); tag_iter != tags.end(); tag_iter++) {
		uint64_t i = tag_iter->offset - abs_sample_cnt;
		modes_packet rx_packet;

		memset(&rx_packet.data, 0x00, 14 * sizeof(unsigned char));
		memset(&rx_packet.lowconfbits, 0x00, 24 * sizeof(unsigned char));
		rx_packet.numlowconf = 0;

		//let's use the preamble to get a reference level for the packet
		//fixme: a better thing to do is create a bi-level avg 1 and avg 0
		//through simple statistics, then take the median for your slice level
		//this won't improve decoding but will improve confidence
		rx_packet.reference_level = (in[i]
								   + in[i+2]
								   + in[i+7]
								   + in[i+9]) / 4.0;

		i += 16; //move on up to the first bit of the packet data
		//now let's slice the header so we can determine if it's a short pkt or a long pkt
		unsigned char pkt_hdr = 0;
		for(int j=0; j < 5; j++) {
			slice_result_t slice_result = slicer(in[i+j*2], in[i+j*2+1], rx_packet.reference_level);
			if(slice_result.decision) pkt_hdr += 1 << (4-j);
		}
		if(pkt_hdr == 16 or pkt_hdr == 17 or pkt_hdr == 20 or pkt_hdr == 21) rx_packet.type = Long_Packet;
		else rx_packet.type = Short_Packet;
		int packet_length = (rx_packet.type == framer_packet_type(Short_Packet)) ? 56 : 112;

		//it's slice time!
		//TODO: don't repeat your work here, you already have the first 5 bits
		for(int j = 0; j < packet_length; j++) {
			slice_result_t slice_result = slicer(in[i+j*2], in[i+j*2+1], rx_packet.reference_level);

			//put the data into the packet
			if(slice_result.decision) {
				rx_packet.data[j/8] += 1 << (7-(j%8));
			}
			//put the confidence decision into the packet
			if(slice_result.confidence) {
				//rx_packet.confidence[j/8] += 1 << (7-(j%8));
			} else {
				if(rx_packet.numlowconf < 24) rx_packet.lowconfbits[rx_packet.numlowconf++] = j;
			}
		}
			
		rx_packet.timestamp = pmt_to_double(tag_iter->value);
			
		//traverse the whole packet and if you find all 0's, just toss it. don't know why these packets turn up, but they pass ECC.
		bool zeroes = 1;
		for(int m = 0; m < 14; m++) {
			if(rx_packet.data[m]) { zeroes = 0; break; }
		}
		if(zeroes) {continue;} //toss it

		rx_packet.message_type = (rx_packet.data[0] >> 3) & 0x1F; //get the message type to make decisions on ECC methods

		if(rx_packet.type == Short_Packet && rx_packet.message_type != 11 && rx_packet.numlowconf > 0) {continue;}
		if(rx_packet.message_type == 11 && rx_packet.numlowconf >= 10) {continue;}
			
		rx_packet.crc = modes_check_crc(rx_packet.data, packet_length);

		//crc for packets that aren't type 11 or type 17 is encoded with the transponder ID, which we don't know
		//therefore we toss 'em if there's syndrome
		//crc for the other short packets is usually nonzero, so they can't really be trusted that far
		if(rx_packet.crc && (rx_packet.message_type == 11 || rx_packet.message_type == 17)) {continue;}

		d_payload.str("");
		for(int m = 0; m < packet_length/8; m++) {
			d_payload << std::hex << std::setw(2) << std::setfill('0') << unsigned(rx_packet.data[m]);
		}

		d_payload << " " << std::setw(6) << rx_packet.crc << " " << std::dec << rx_packet.reference_level
		          << " " << std::setprecision(10) << std::setw(10) << rx_packet.timestamp;
			gr_message_sptr msg = gr_make_message_from_string(std::string(d_payload.str()));
		d_queue->handle(msg);
	}
	if(0) std::cout << "Slicer consumed " << size << ", returned " << size << std::endl;
	return size;
}