gr-air-modes/lib/preamble_impl.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 "preamble_impl.h"
#include <gnuradio/io_signature.h>
#include <string.h>
#include <iostream>
#include <gnuradio/tags.h>

namespace gr {

air_modes::preamble::sptr air_modes::preamble::make(int channel_rate, float threshold_db) {
    return gnuradio::get_initial_sptr(new air_modes::preamble_impl(channel_rate, threshold_db));
}

air_modes::preamble_impl::preamble_impl(int channel_rate, float threshold_db) :
        gr::block ("preamble",
           gr::io_signature::make2 (2, 2, sizeof(float), sizeof(float)), //stream 0 is received data, stream 1 is moving average for reference
           gr::io_signature::make (1, 1, sizeof(float))) //the output soft symbols
{
    d_chip_rate = 2000000; //2Mchips per second
    set_rate(channel_rate);
    set_threshold(threshold_db);

    std::stringstream str;
    str << name() << unique_id();
    d_me = pmt::string_to_symbol(str.str());
    d_key = pmt::string_to_symbol("preamble_found");
}

void air_modes::preamble_impl::set_rate(int channel_rate) {
    d_samples_per_chip = channel_rate / d_chip_rate;
    d_samples_per_symbol = d_samples_per_chip * 2;
    d_check_width = 120 * d_samples_per_symbol;
    d_secs_per_sample = 1.0/channel_rate;
    set_output_multiple(1+d_check_width*2);
    set_history(d_samples_per_symbol);
}

void air_modes::preamble_impl::set_threshold(float threshold_db) {
    d_threshold_db = threshold_db;
    d_threshold = powf(10., threshold_db/20.); //the level that the sample must be above the moving average in order to qualify as a pulse
}

float air_modes::preamble_impl::get_threshold(void) {
    return d_threshold_db;
}

int air_modes::preamble_impl::get_rate(void) {
    return d_samples_per_chip * d_chip_rate;
}

static void integrate_and_dump(float *out, const float *in, int chips, int samps_per_chip) {
    for(int i=0; i<chips; i++) {
        float acc = 0;
        for(int j=0; j<samps_per_chip; j++) {
            acc += in[i*samps_per_chip+j];
        }
        out[i] = acc;
    }
}

//the preamble pattern in bits
//fixme goes in .h
static const bool preamble_bits[] = {1, 0, 1, 0, 0, 0, 0, 1, 0, 1};
static double correlate_preamble(const float *in, int samples_per_chip) {
    double corr = 0.0;
    for(int i=0; i<10; i++) {
        for(int j=0; j<samples_per_chip;j++)
            if(preamble_bits[i]) corr += in[i*samples_per_chip+j];
    }
    return corr;
}

//todo: make it return a pair of some kind, otherwise you can lose precision
static double tag_to_timestamp(gr::tag_t tstamp, uint64_t abs_sample_cnt, double secs_per_sample) {
    uint64_t ts_sample, last_whole_stamp;
    double last_frac_stamp;

    if(tstamp.key == NULL || pmt::symbol_to_string(tstamp.key) != "rx_time") return 0;

    last_whole_stamp = pmt::to_uint64(pmt::tuple_ref(tstamp.value, 0));
    last_frac_stamp = pmt::to_double(pmt::tuple_ref(tstamp.value, 1));
    ts_sample = tstamp.offset;

    double tstime = double(abs_sample_cnt * secs_per_sample) + last_whole_stamp + last_frac_stamp;
    if(0) std::cout << "HEY WE GOT A STAMP AT " << tstime << " TICKS AT SAMPLE " << ts_sample << " ABS SAMPLE CNT IS " << abs_sample_cnt << std::endl;
    return tstime;
}

int air_modes::preamble_impl::general_work(int noutput_items,
                          gr_vector_int &ninput_items,
                          gr_vector_const_void_star &input_items,
                          gr_vector_void_star &output_items)
{
    const float *in = (const float *) input_items[0];
    const float *inavg = (const float *) input_items[1];

    int mininputs = std::min(ninput_items[0], ninput_items[1]); //they should be matched but let's be safe
    //round number of input samples down to nearest d_samples_per_chip
    //we also subtract off d_samples_per_chip to allow the bit center finder some leeway
    const int ninputs = std::max(mininputs - (mininputs % d_samples_per_chip) - d_samples_per_chip, 0);
    if (ninputs <= 0) { consume_each(0); return 0; }

    float *out = (float *) output_items[0];

    if(0) std::cout << "Preamble called with " << ninputs << " samples" << std::endl;

    //fixme move into .h
    const int pulse_offsets[4] = {    0,
                                  int(2 * d_samples_per_chip),
                                  int(7 * d_samples_per_chip),
                                  int(9 * d_samples_per_chip)
                                 };

    uint64_t abs_sample_cnt = nitems_read(0);
    std::vector<gr::tag_t> tstamp_tags;
    get_tags_in_range(tstamp_tags, 0, abs_sample_cnt, abs_sample_cnt + ninputs, pmt::string_to_symbol("rx_time"));
    //tags.back() is the most recent timestamp, then.
    if(tstamp_tags.size() > 0) {
        d_timestamp = tstamp_tags.back();
    }

    for(int i=0; i < ninputs; i++) {
        float pulse_threshold = inavg[i] * d_threshold;
        if(in[i] > pulse_threshold) { //hey we got a candidate
            if(in[i+1] > in[i]) continue; //wait for the peak
            //check to see the rest of the pulses are there
            if( in[i+pulse_offsets[1]] < pulse_threshold ) continue;
            if( in[i+pulse_offsets[2]] < pulse_threshold ) continue;
            if( in[i+pulse_offsets[3]] < pulse_threshold ) continue;

            //get a more accurate bit center by finding the correlation peak across all four preamble bits
            bool late, early;
            int how_late = 0;
            do {
                double now_corr = correlate_preamble(in+i, d_samples_per_chip);
                double late_corr = correlate_preamble(in+i+1, d_samples_per_chip);
                double early_corr = correlate_preamble(in+i-1, d_samples_per_chip);
                late = (late_corr > now_corr);
                //early = (early_corr > now_corr);
                if(late) { i++; how_late++; }
                //if(early && i>0) { std::cout << "EARLY " << i << std::endl; i--; }
            } while(late and how_late < d_samples_per_chip);// xor early);

            if(0) std::cout << "We were " << how_late << " samples late" << std::endl;

            //now check to see that the non-peak symbols in the preamble
            //are below the peaks by threshold dB
            float avgpeak = ( in[i+pulse_offsets[0]]
                            + in[i+pulse_offsets[1]]
                            + in[i+pulse_offsets[2]]
                            + in[i+pulse_offsets[3]]) / 4.0;

            float space_threshold = inavg[i] + (avgpeak - inavg[i])/d_threshold;
            bool valid_preamble = true; //f'in c++
            for( int j=1.5*d_samples_per_symbol; j<=3*d_samples_per_symbol; j++)
                if(in[i+j] > space_threshold) valid_preamble = false;
            for( int j=5*d_samples_per_symbol; j<=7.5*d_samples_per_symbol; j++)
                if(in[i+j] > space_threshold) valid_preamble = false;
            if(!valid_preamble) continue;

            //be sure we've got enough room in the input buffer to copy out a whole packet
            if(ninputs-i < 240*d_samples_per_chip) {
                consume_each(std::max(i-1,0));
                if(0) std::cout << "Preamble consumed " << std::max(i-1,0) << ", returned 0 (no room)" << std::endl;
                return 0;
            }

            //all right i'm prepared to call this a preamble
            for(int j=0; j<240; j++) {
                out[j] = in[i+j*d_samples_per_chip] - inavg[i];
            }

            //get the timestamp of the preamble
            double tstamp = tag_to_timestamp(d_timestamp, abs_sample_cnt + i, d_secs_per_sample);

            //now tag the preamble
            add_item_tag(0, //stream ID
                     nitems_written(0), //sample
                     d_key,      //frame_info
                     pmt::from_double(tstamp),
                     d_me        //block src id
                    );

            //std::cout << "PREAMBLE" << std::endl;

            //produce only one output per work call -- TODO this should probably change
            if(0) std::cout << "Preamble consumed " << i+240*d_samples_per_chip << "with i=" << i << ", returned 240" << std::endl;

            consume_each(i+240*d_samples_per_chip);
            return 240;
        }
    }

    //didn't get anything this time
    if(0) std::cout << "Preamble consumed " << ninputs << ", returned 0" << std::endl;
    consume_each(ninputs);
    return 0;
}

} //namespace gr