219 lines
8.0 KiB
C++
219 lines
8.0 KiB
C++
/*
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# Copyright 2010 Nick Foster
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#
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# This file is part of gr-air-modes
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#
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# gr-air-modes is free software; you can redistribute it and/or modify
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# it under the terms of the GNU General Public License as published by
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# the Free Software Foundation; either version 3, or (at your option)
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# any later version.
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#
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# gr-air-modes is distributed in the hope that it will be useful,
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# but WITHOUT ANY WARRANTY; without even the implied warranty of
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# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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# GNU General Public License for more details.
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#
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# You should have received a copy of the GNU General Public License
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# along with gr-air-modes; see the file COPYING. If not, write to
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# the Free Software Foundation, Inc., 51 Franklin Street,
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# Boston, MA 02110-1301, USA.
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#
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*/
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#ifdef HAVE_CONFIG_H
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#include "config.h"
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#endif
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#include <air_modes_slicer.h>
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#include <gr_io_signature.h>
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#include <air_modes_types.h>
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#include <sstream>
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#include <iomanip>
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#include <modes_crc.h>
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#include <iostream>
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#include <gr_tags.h>
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extern "C"
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{
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#include <stdio.h>
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#include <string.h>
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}
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air_modes_slicer_sptr air_make_modes_slicer(int channel_rate, gr_msg_queue_sptr queue)
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{
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return air_modes_slicer_sptr (new air_modes_slicer(channel_rate, queue));
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}
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air_modes_slicer::air_modes_slicer(int channel_rate, gr_msg_queue_sptr queue) :
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gr_sync_block ("modes_slicer",
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gr_make_io_signature (1, 1, sizeof(float)), //stream 0 is received data, stream 1 is binary preamble detector output
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gr_make_io_signature (0, 0, 0) )
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{
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//initialize private data here
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d_chip_rate = 2000000; //2Mchips per second
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d_samples_per_chip = 2;//FIXME this is constant now channel_rate / d_chip_rate;
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d_samples_per_symbol = d_samples_per_chip * 2;
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d_check_width = 120 * d_samples_per_symbol; //how far you will have to look ahead
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d_queue = queue;
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set_output_multiple(1+d_check_width); //how do you specify buffer size for sinks?
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}
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//this slicer is courtesy of Lincoln Labs. supposedly it is more resistant to mode A/C FRUIT.
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//see http://adsb.tc.faa.gov/WG3_Meetings/Meeting8/Squitter-Lon.pdf
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static slice_result_t slicer(const float bit0, const float bit1, const float ref) {
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slice_result_t result;
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//3dB limits for bit slicing and confidence measurement
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float highlimit=ref*2;
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float lowlimit=ref*0.5;
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bool firstchip_inref = ((bit0 > lowlimit) && (bit0 < highlimit));
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bool secondchip_inref = ((bit1 > lowlimit) && (bit1 < highlimit));
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if(firstchip_inref && !secondchip_inref) {
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result.decision = 1;
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result.confidence = 1;
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}
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else if(secondchip_inref && !firstchip_inref) {
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result.decision = 0;
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result.confidence = 1;
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}
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else if(firstchip_inref && secondchip_inref) {
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result.decision = bit0 > bit1;
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result.confidence = 0;
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}
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else {//if(!firstchip_inref && !secondchip_inref) {
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result.decision = bit0 > bit1;
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if(result.decision) {
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if(bit1 < lowlimit * 0.5) result.confidence = 1;
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else result.confidence = 0;
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} else {
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if(bit0 < lowlimit * 0.5) result.confidence = 1;
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else result.confidence = 0;
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}
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}
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return result;
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}
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int air_modes_slicer::work(int noutput_items,
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gr_vector_const_void_star &input_items,
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gr_vector_void_star &output_items)
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{
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const float *in = (const float *) input_items[0];
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int size = noutput_items - d_check_width; //since it's a sync block, i assume that it runs with ninput_items = noutput_items
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std::vector<gr_tag_t> tags;
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uint64_t abs_sample_cnt = nitems_read(0);
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get_tags_in_range(tags, 0, abs_sample_cnt, abs_sample_cnt + size, pmt::pmt_string_to_symbol("preamble_found"));
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std::vector<gr_tag_t>::iterator tag_iter;
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for(tag_iter = tags.begin(); tag_iter != tags.end(); tag_iter++) {
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uint64_t i = tag_iter->offset - abs_sample_cnt;
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modes_packet rx_packet;
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memset(&rx_packet.data, 0x00, 14 * sizeof(unsigned char));
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memset(&rx_packet.lowconfbits, 0x00, 24 * sizeof(unsigned char));
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rx_packet.numlowconf = 0;
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//let's use the preamble to get a reference level for the packet
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//fixme: a better thing to do is create a bi-level avg 1 and avg 0
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//through simple statistics, then take the median for your slice level
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//this won't improve decoding but will improve confidence
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rx_packet.reference_level = (in[i]
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+ in[i+2]
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+ in[i+7]
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+ in[i+9]) / 4.0;
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i += 16; //move on up to the first bit of the packet data
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//now let's slice the header so we can determine if it's a short pkt or a long pkt
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unsigned char pkt_hdr = 0;
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for(int j=0; j < 5; j++) {
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slice_result_t slice_result = slicer(in[i+j*2], in[i+j*2+1], rx_packet.reference_level);
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if(slice_result.decision) pkt_hdr += 1 << (4-j);
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}
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if(pkt_hdr == 17) rx_packet.type = Long_Packet;
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else rx_packet.type = Short_Packet;
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int packet_length = (rx_packet.type == framer_packet_type(Short_Packet)) ? 56 : 112;
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//it's slice time!
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//TODO: don't repeat your work here, you already have the first 5 bits
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for(int j = 0; j < packet_length; j++) {
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slice_result_t slice_result = slicer(in[i+j*2], in[i+j*2+1], rx_packet.reference_level);
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//put the data into the packet
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if(slice_result.decision) {
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rx_packet.data[j/8] += 1 << (7-(j%8));
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}
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//put the confidence decision into the packet
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if(slice_result.confidence) {
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//rx_packet.confidence[j/8] += 1 << (7-(j%8));
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} else {
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if(rx_packet.numlowconf < 24) rx_packet.lowconfbits[rx_packet.numlowconf++] = j;
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}
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}
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/******************** BEGIN TIMESTAMP BS ******************/
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rx_packet.timestamp = pmt_to_double(tag_iter->value);
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/******************* END TIMESTAMP BS *********************/
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//increment for the next round
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//here you might want to 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.
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bool zeroes = 1;
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for(int m = 0; m < 14; m++) {
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if(rx_packet.data[m]) zeroes = 0;
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}
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if(zeroes) {continue;} //toss it
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rx_packet.message_type = (rx_packet.data[0] >> 3) & 0x1F; //get the message type for the parser to conveniently use, and to make decisions on ECC methods
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if(rx_packet.type == Short_Packet && rx_packet.message_type != 11 && rx_packet.numlowconf > 0) {continue;}
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if(rx_packet.message_type == 11 && rx_packet.numlowconf >= 10) {continue;}
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rx_packet.crc = modes_check_crc(rx_packet.data, packet_length);
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//crc for packets that aren't type 11 or type 17 is encoded with the transponder ID, which we don't know
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//therefore we toss 'em if there's syndrome
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//crc for the other short packets is usually nonzero, so they can't really be trusted that far
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if(rx_packet.crc && (rx_packet.message_type == 11 || rx_packet.message_type == 17)) {continue;}
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//we no longer attempt to brute force error correct via syndrome. it really only gets you 1% additional returns,
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//at the expense of a lot of CPU time and complexity
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//we'll replicate some data by sending the message type as the first field, followed by the first 8+24=32 bits of the packet, followed by
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//56 long packet data bits if applicable (zero-padded if not), followed by crc
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d_payload.str("");
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d_payload << std::dec << std::setw(2) << std::setfill('0') << rx_packet.message_type << std::hex << " ";
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for(int m = 0; m < 4; m++) {
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d_payload << std::setw(2) << std::setfill('0') << unsigned(rx_packet.data[m]);
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}
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d_payload << " ";
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if(packet_length == 112) {
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for(int m = 4; m < 11; m++) {
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d_payload << std::setw(2) << std::setfill('0') << unsigned(rx_packet.data[m]);
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}
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d_payload << " ";
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for(int m = 11; m < 14; m++) {
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d_payload << std::setw(2) << std::setfill('0') << unsigned(rx_packet.data[m]);
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}
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} else {
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for(int m = 4; m < 11; m++) {
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d_payload << std::setw(2) << std::setfill('0') << unsigned(0);
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}
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d_payload << " ";
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for(int m = 4; m < 7; m++) {
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d_payload << std::setw(2) << std::setfill('0') << unsigned(rx_packet.data[m]);
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}
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}
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d_payload << " " << std::setw(6) << rx_packet.crc << " " << std::dec << rx_packet.reference_level
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<< " " << std::setprecision(10) << std::setw(10) << rx_packet.timestamp;
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gr_message_sptr msg = gr_make_message_from_string(std::string(d_payload.str()));
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d_queue->handle(msg);
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}
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return size;
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}
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