path: root/src/lib/gsm_receiver_cf.cc

/* -*- c++ -*- */
/*
 * @file
 * @author Piotr Krysik <pkrysik@stud.elka.pw.edu.pl>
 * @section LICENSE
 *
 * 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 3, 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.
 */

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

#include <gr_io_signature.h>
#include <gr_math.h>
#include <math.h>
#include <Assert.h>
#include <list>
#include <boost/circular_buffer.hpp>
#include <algorithm>
#include <gsm_receiver_cf.h>
#include <viterbi_detector.h>
#include <sch.h>

#define SYNC_SEARCH_RANGE 30
#define TRAIN_SEARCH_RANGE 40

//TODO: this shouldn't be here - remove it when gsm receiver's interface will be ready
void gsm_receiver_cf::process_normal_burst(burst_counter burst_nr, const unsigned char * burst_binary)
{
  if (burst_nr.get_timeslot_nr() == 0) {
//     printf("burst = [ ");
//     for (int i = 0; i < BURST_SIZE ; i++) {
//       printf(" %d", burst_binary[i]);
//     }
//     printf("];\n");
//     std::cout  << " t2: " << burst_nr.get_t2() << "\n";
    GS_process(&d_gs_ctx, TIMESLOT0, 6, &burst_binary[3], burst_nr.get_frame_nr());
  }
}
//TODO: this shouldn't be here also - the same reason
void gsm_receiver_cf::configure_receiver()
{
  d_channel_conf.set_multiframe_type(TSC0, multiframe_51);
  
  d_channel_conf.set_burst_types(TSC0, TEST_CCH_FRAMES, sizeof(TEST_CCH_FRAMES) / sizeof(unsigned), normal_burst);
  d_channel_conf.set_burst_types(TSC0, FCCH_FRAMES, sizeof(FCCH_FRAMES) / sizeof(unsigned), fcch_burst);
  
//   d_channel_conf.set_multiframe_type(TIMESLOT6, multiframe_26);
//   d_channel_conf.set_burst_types(TIMESLOT6, TRAFFIC_CHANNEL_F, sizeof(TRAFFIC_CHANNEL_F) / sizeof(unsigned), normal_burst);
}


typedef std::list<float> list_float;
typedef std::vector<float> vector_float;

typedef boost::circular_buffer<float> circular_buffer_float;

gsm_receiver_cf_sptr
gsm_make_receiver_cf(gr_feval_dd *tuner, int osr)
{
  return gsm_receiver_cf_sptr(new gsm_receiver_cf(tuner, osr));
}

static const int MIN_IN = 1; // mininum number of input streams
static const int MAX_IN = 1; // maximum number of input streams
static const int MIN_OUT = 0; // minimum number of output streams
static const int MAX_OUT = 1; // maximum number of output streams

/*
 * The private constructor
 */
gsm_receiver_cf::gsm_receiver_cf(gr_feval_dd *tuner, int osr)
    : gr_block("gsm_receiver",
               gr_make_io_signature(MIN_IN, MAX_IN, sizeof(gr_complex)),
               gr_make_io_signature(MIN_OUT, MAX_OUT, 142 * sizeof(float))),
    d_OSR(osr),
    d_chan_imp_length(CHAN_IMP_RESP_LENGTH),
    d_tuner(tuner),
    d_counter(0),
    d_fcch_start_pos(0),
    d_freq_offset(0),
    d_state(first_fcch_search),
    d_burst_nr(osr)
{
  int i;
  gmsk_mapper(SYNC_BITS, N_SYNC_BITS, d_sch_training_seq, gr_complex(0.0, -1.0));

  for (i = 0; i < TRAIN_SEQ_NUM; i++) {
    gmsk_mapper(train_seq[i], N_TRAIN_BITS, d_norm_training_seq[i], gr_complex(1.0, 0.0));
  }
  
  /* Initialize GSM Stack */
  GS_new(&d_gs_ctx); //TODO: remove it! it'a not right place for a decoder
}

/*
 * Virtual destructor.
 */
gsm_receiver_cf::~gsm_receiver_cf()
{
}

void gsm_receiver_cf::forecast(int noutput_items, gr_vector_int &nitems_items_required)
{
  nitems_items_required[0] = noutput_items * floor((TS_BITS + 2 * GUARD_PERIOD) * d_OSR);
}

int
gsm_receiver_cf::general_work(int noutput_items,
                              gr_vector_int &nitems_items,
                              gr_vector_const_void_star &input_items,
                              gr_vector_void_star &output_items)
{
  const gr_complex *input = (const gr_complex *) input_items[0];
  //float *out = (float *) output_items[0];
  int produced_out = 0;  //how many output elements were produced - this isn't used yet
  //probably the gsm receiver will be changed into sink so this variable won't be necessary

  switch (d_state) {
      //bootstrapping
    case first_fcch_search:
      if (find_fcch_burst(input, nitems_items[0])) { //find frequency correction burst in the input buffer
        set_frequency(d_freq_offset);                //if fcch search is successful set frequency offset
        //produced_out = 0;
        d_state = next_fcch_search;
      } else {
        //produced_out = 0;
        d_state = first_fcch_search;
      }
      break;

    case next_fcch_search: {                         //this state is used because it takes a bunch of buffered samples
        //before previous set_frequqency cause change
        float prev_freq_offset = d_freq_offset;
        if (find_fcch_burst(input, nitems_items[0])) {
          if (abs(prev_freq_offset - d_freq_offset) > FCCH_MAX_FREQ_OFFSET) {
            set_frequency(d_freq_offset);              //call set_frequncy only frequency offset change is greater than some value
          }
          //produced_out = 0;
          d_state = sch_search;
        } else {
          //produced_out = 0;
          d_state = next_fcch_search;
        }
        break;
      }
    case sch_search: {
        vector_complex channel_imp_resp(CHAN_IMP_RESP_LENGTH*d_OSR);
        int t1, t2, t3;
        int burst_start = 0;
        unsigned char output_binary[BURST_SIZE];

        if (reach_sch_burst(nitems_items[0])) {                              //wait for a SCH burst
          burst_start = get_sch_chan_imp_resp(input, &channel_imp_resp[0]); //get channel impulse response from it
          detect_burst(input, &channel_imp_resp[0], burst_start, output_binary); //detect bits using MLSE detection
          if (decode_sch(&output_binary[3], &t1, &t2, &t3, &d_ncc, &d_bcc) == 0) { //decode SCH burst
            DCOUT("sch burst_start: " << burst_start);
            DCOUT("bcc: " << d_bcc << " ncc: " << d_ncc << " t1: " << t1 << " t2: " << t2 << " t3: " << t3);
            d_burst_nr.set(t1, t2, t3, 0);                                  //set counter of bursts value

            //configure the receiver - tell him where to find which burst type
            d_channel_conf.set_multiframe_type(TIMESLOT0, multiframe_51);  //in the timeslot nr.0 bursts changes according to t3 counter
            configure_receiver();//TODO: this shouldn't be here - remove it when gsm receiver's interface will be ready
            d_channel_conf.set_burst_types(TIMESLOT0, FCCH_FRAMES, sizeof(FCCH_FRAMES) / sizeof(unsigned), fcch_burst);  //tell where to find fcch bursts
            d_channel_conf.set_burst_types(TIMESLOT0, SCH_FRAMES, sizeof(SCH_FRAMES) / sizeof(unsigned), sch_burst);     //sch bursts
            d_channel_conf.set_burst_types(TIMESLOT0, BCCH_FRAMES, sizeof(BCCH_FRAMES) / sizeof(unsigned), normal_burst);//!and maybe normal bursts of the BCCH logical channel
            d_burst_nr++;

            consume_each(burst_start + BURST_SIZE * d_OSR);   //consume samples up to next guard period
            d_state = synchronized;
          } else {
            d_state = next_fcch_search;                       //if there is error in the sch burst go back to fcch search phase
          }
        } else {
          d_state = sch_search;
        }
        break;
      }
      //in this state receiver is synchronized and it processes bursts according to burst type for given burst number
    case synchronized: {
        vector_complex channel_imp_resp(CHAN_IMP_RESP_LENGTH*d_OSR);
        int burst_start;
        int offset = 0;
        int to_consume = 0;
        unsigned char output_binary[BURST_SIZE];

        burst_type b_type = d_channel_conf.get_burst_type(d_burst_nr); //get burst type for given burst number

        switch (b_type) {
          case fcch_burst: {                                                                    //if it's FCCH  burst
              const unsigned first_sample = ceil((GUARD_PERIOD + 2 * TAIL_BITS) * d_OSR) + 1;
              const unsigned last_sample = first_sample + USEFUL_BITS * d_OSR;
              double freq_offset = compute_freq_offset(input, first_sample, last_sample);       //extract frequency offset from it
              if (abs(freq_offset) > FCCH_MAX_FREQ_OFFSET) {
                d_freq_offset -= freq_offset;                                                   //and adjust frequency if it have changed beyond
                set_frequency(d_freq_offset);                                                   //some limit
                DCOUT("Adjusting frequency, new frequency offset: " << d_freq_offset << "\n");
              }
            }
            break;
          case sch_burst: {                                                                    //if it's SCH burst
              int t1, t2, t3, d_ncc, d_bcc;
              burst_start = get_sch_chan_imp_resp(input, &channel_imp_resp[0]);                //get channel impulse response
              detect_burst(input, &channel_imp_resp[0], burst_start, output_binary);           //MLSE detection of bits
              if (decode_sch(&output_binary[3], &t1, &t2, &t3, &d_ncc, &d_bcc) == 0) {         //and decode SCH data
                // d_burst_nr.set(t1, t2, t3, 0);                                              //but only to check if burst_start value is correct
                DCOUT("bcc: " << d_bcc << " ncc: " << d_ncc << " t1: " << t1 << " t2: " << t2 << " t3: " << t3);
                offset =  burst_start - floor((GUARD_PERIOD) * d_OSR);                         //compute offset from burst_start - burst should start after a guard period
                DCOUT(offset);
                to_consume += offset;                                                          //adjust with offset number of samples to be consumed
              }
            }
            break;

          case normal_burst:                                                                  //if it's normal burst
            burst_start = get_norm_chan_imp_resp(input, &channel_imp_resp[0], TRAIN_SEARCH_RANGE, d_bcc); //get channel impulse response for given training sequence number - d_bcc
            detect_burst(input, &channel_imp_resp[0], burst_start, output_binary);            //MLSE detection of bits
            process_normal_burst(d_burst_nr, output_binary); //TODO: this shouldn't be here - remove it when gsm receiver's interface will be ready
            break;

          case rach_burst:
            //implementation of this channel isn't possible in current gsm_receiver
            //it would take some realtime processing, counter of samples from USRP to
            //stay synchronized with this device and possibility to switch frequency from  uplink
            //to C0 (where sch is) back and forth

            break;
          case dummy:                                                         //if it's dummy
            burst_start = get_norm_chan_imp_resp(input, &channel_imp_resp[0], TRAIN_SEARCH_RANGE, TS_DUMMY); //read dummy
            detect_burst(input, &channel_imp_resp[0], burst_start, output_binary);   // but as far as I know it's pointless
            break;
          case empty:   //if it's empty burst
            break;      //do nothing
        }

        d_burst_nr++;   //go to next burst

        to_consume += TS_BITS * d_OSR + d_burst_nr.get_offset();  //consume samples of the burst up to next guard period
        //and add offset which is introduced by
        //0.25 fractional part of a guard period
        //burst_number computes this offset
        //but choice of this class to do this was random
        consume_each(to_consume);
      }
      break;
  }

  return produced_out;
}

bool gsm_receiver_cf::find_fcch_burst(const gr_complex *input, const int nitems)
{
  circular_buffer_float phase_diff_buffer(FCCH_HITS_NEEDED * d_OSR); //circular buffer used to scan throug signal to find
  //best match for FCCH burst
  float phase_diff = 0;
  gr_complex conjprod;
  int start_pos = -1;
  int hit_count = 0;
  int miss_count = 0;
  float min_phase_diff;
  float max_phase_diff;
  double best_sum = 0;
  float lowest_max_min_diff = 99999;

  int to_consume = 0;
  int sample_number = 0;
  bool end = false;
  bool result = false;
  circular_buffer_float::iterator buffer_iter;

  /**@name Possible states of FCCH search algorithm*/
  //@{
  enum states {
    init,               ///< initialize variables
    search,             ///< search for positive samples
    found_something,    ///< search for FCCH and the best position of it
    fcch_found,         ///< when FCCH was found
    search_fail         ///< when there is no FCCH in the input vector
  } fcch_search_state;
  //@}

  fcch_search_state = init;

  while (!end) {
    switch (fcch_search_state) {

      case init: //initialize variables
        hit_count = 0;
        miss_count = 0;
        start_pos = -1;
        lowest_max_min_diff = 99999;
        phase_diff_buffer.clear();
        fcch_search_state = search;

        break;

      case search: // search for positive samples
        sample_number++;

        if (sample_number > nitems - FCCH_HITS_NEEDED * d_OSR) { //if it isn't possible to find FCCH because
          //there's too few samples left to look into,
          to_consume = sample_number;                            //don't do anything with those samples which are left
          //and consume only those which were checked
          fcch_search_state = search_fail;
        } else {
          phase_diff = compute_phase_diff(input[sample_number], input[sample_number-1]);

          if (phase_diff > 0) {                                 //if a positive phase difference was found
            to_consume = sample_number;
            fcch_search_state = found_something;                //switch to state in which searches for FCCH
          } else {
            fcch_search_state = search;
          }
        }

        break;

      case found_something: {// search for FCCH and the best position of it
          if (phase_diff > 0) {
            hit_count++;       //positive phase differencies increases hits_count
          } else {
            miss_count++;      //negative increases miss_count
          }

          if ((miss_count >= FCCH_MAX_MISSES * d_OSR) && (hit_count <= FCCH_HITS_NEEDED * d_OSR)) {
            //if miss_count exceeds limit before hit_count
            fcch_search_state = init;       //go to init
            continue;
          } else if (((miss_count >= FCCH_MAX_MISSES * d_OSR) && (hit_count > FCCH_HITS_NEEDED * d_OSR)) || (hit_count > 2 * FCCH_HITS_NEEDED * d_OSR)) {
            //if hit_count and miss_count exceeds limit then FCCH was found
            fcch_search_state = fcch_found;
            continue;
          } else if ((miss_count < FCCH_MAX_MISSES * d_OSR) && (hit_count > FCCH_HITS_NEEDED * d_OSR)) {
            //find difference between minimal and maximal element in the buffer
            //for FCCH this value should be low
            //this part is searching for a region where this value is lowest
            min_phase_diff = * (min_element(phase_diff_buffer.begin(), phase_diff_buffer.end()));
            max_phase_diff = * (max_element(phase_diff_buffer.begin(), phase_diff_buffer.end()));

            if (lowest_max_min_diff > max_phase_diff - min_phase_diff) {
              lowest_max_min_diff = max_phase_diff - min_phase_diff;
              start_pos = sample_number - FCCH_HITS_NEEDED * d_OSR - FCCH_MAX_MISSES * d_OSR; //store start pos
              best_sum = 0;

              for (buffer_iter = phase_diff_buffer.begin();
                   buffer_iter != (phase_diff_buffer.end());
                   buffer_iter++) {
                best_sum += *buffer_iter - (M_PI / 2) / d_OSR;   //store best value of phase offset sum
              }
            }
          }

          sample_number++;

          if (sample_number >= nitems) {    //if there's no single sample left to check
            fcch_search_state = search_fail;//FCCH search failed
            continue;
          }

          phase_diff = compute_phase_diff(input[sample_number], input[sample_number-1]);
          phase_diff_buffer.push_back(phase_diff);
          fcch_search_state = found_something;
        }
        break;

      case fcch_found: {
          DCOUT("fcch found on position: " << d_counter + start_pos);
          to_consume = start_pos + FCCH_HITS_NEEDED * d_OSR + 1; //consume one FCCH burst

          d_fcch_start_pos = d_counter + start_pos;

          //compute frequency offset
          double phase_offset = best_sum / FCCH_HITS_NEEDED;
          double freq_offset = phase_offset * 1625000.0 / (12.0 * M_PI);
          d_freq_offset -= freq_offset;
          DCOUT("freq_offset: " << d_freq_offset);

          end = true;
          result = true;
          break;
        }

      case search_fail:
        end = true;
        result = false;
        break;
    }
  }

  d_counter += to_consume;
  consume_each(to_consume);

  return result;
}

double gsm_receiver_cf::compute_freq_offset(const gr_complex * input, unsigned first_sample, unsigned last_sample)
{
  double phase_sum = 0;
  unsigned ii;

  for (ii = first_sample; ii < last_sample; ii++) {
    double phase_diff = compute_phase_diff(input[ii], input[ii-1]) - (M_PI / 2) / d_OSR;
    phase_sum += phase_diff;
  }

  double phase_offset = phase_sum / (last_sample - first_sample);
  double freq_offset = phase_offset * 1625000.0 / (12.0 * M_PI);
  return freq_offset;
}

void gsm_receiver_cf::set_frequency(double freq_offset)
{
  d_tuner->calleval(freq_offset);
}

inline float gsm_receiver_cf::compute_phase_diff(gr_complex val1, gr_complex val2)
{
  gr_complex conjprod = val1 * conj(val2);
  return gr_fast_atan2f(imag(conjprod), real(conjprod));
}

bool gsm_receiver_cf::reach_sch_burst(const int nitems)
{
  //it just consumes samples to get near to a SCH burst
  int to_consume = 0;
  bool result = false;
  unsigned sample_nr_near_sch_start = d_fcch_start_pos + (FRAME_BITS - SAFETY_MARGIN) * d_OSR;

  //consume samples until d_counter will be equal to sample_nr_near_sch_start
  if (d_counter < sample_nr_near_sch_start) {
    if (d_counter + nitems >= sample_nr_near_sch_start) {
      to_consume = sample_nr_near_sch_start - d_counter;
    } else {
      to_consume = nitems;
    }
    result = false;
  } else {
    to_consume = 0;
    result = true;
  }

  d_counter += to_consume;
  consume_each(to_consume);
  return result;
}

int gsm_receiver_cf::get_sch_chan_imp_resp(const gr_complex *input, gr_complex * chan_imp_resp)
{
  vector_complex correlation_buffer;
  vector_float power_buffer;
  vector_float window_energy_buffer;

  int strongest_window_nr;
  int burst_start = 0;
  int chan_imp_resp_center = 0;
  float max_correlation = 0;
  float energy = 0;

  for (int ii = SYNC_POS * d_OSR; ii < (SYNC_POS + SYNC_SEARCH_RANGE) *d_OSR; ii++) {
    gr_complex correlation = correlate_sequence(&d_sch_training_seq[5], N_SYNC_BITS - 10, &input[ii]);
    correlation_buffer.push_back(correlation);
    power_buffer.push_back(pow(abs(correlation), 2));
  }

  //compute window energies
  vector_float::iterator iter = power_buffer.begin();
  bool loop_end = false;
  while (iter != power_buffer.end()) {
    vector_float::iterator iter_ii = iter;
    energy = 0;

    for (int ii = 0; ii < (d_chan_imp_length) *d_OSR; ii++, iter_ii++) {
      if (iter_ii == power_buffer.end()) {
        loop_end = true;
        break;
      }
      energy += (*iter_ii);
    }
    if (loop_end) {
      break;
    }
    iter++;
    window_energy_buffer.push_back(energy);
  }

  strongest_window_nr = max_element(window_energy_buffer.begin(), window_energy_buffer.end()) - window_energy_buffer.begin();
//   d_channel_imp_resp.clear();

  max_correlation = 0;
  for (int ii = 0; ii < (d_chan_imp_length) *d_OSR; ii++) {
    gr_complex correlation = correlation_buffer[strongest_window_nr + ii];
    if (abs(correlation) > max_correlation) {
      chan_imp_resp_center = ii;
      max_correlation = abs(correlation);
    }
//     d_channel_imp_resp.push_back(correlation);
    chan_imp_resp[ii] = correlation;
  }

  burst_start = strongest_window_nr + chan_imp_resp_center - 48 * d_OSR - 2 * d_OSR + 2 + SYNC_POS * d_OSR;
  return burst_start;
}

void gsm_receiver_cf::detect_burst(const gr_complex * input, gr_complex * chan_imp_resp, int burst_start, unsigned char * output_binary)
{
  float output[BURST_SIZE];
  gr_complex rhh_temp[CHAN_IMP_RESP_LENGTH*d_OSR];
  gr_complex rhh[CHAN_IMP_RESP_LENGTH];
  gr_complex filtered_burst[BURST_SIZE];
  int start_state = 3;
  unsigned int stop_states[2] = {4, 12};

  autocorrelation(chan_imp_resp, rhh_temp, d_chan_imp_length*d_OSR);
  for (int ii = 0; ii < (d_chan_imp_length); ii++) {
    rhh[ii] = conj(rhh_temp[ii*d_OSR]);
  }

  mafi(&input[burst_start], BURST_SIZE, chan_imp_resp, d_chan_imp_length*d_OSR, filtered_burst);

  viterbi_detector(filtered_burst, BURST_SIZE, rhh, start_state, stop_states, 2, output);

  for (int i = 0; i < BURST_SIZE ; i++) {
    output_binary[i] = (output[i] > 0);
  }
}

//TODO consider placing this funtion in a separate class for signal processing
void gsm_receiver_cf::gmsk_mapper(const unsigned char * input, int nitems, gr_complex * gmsk_output, gr_complex start_point)
{
  gr_complex j = gr_complex(0.0, 1.0);

  int current_symbol;
  int encoded_symbol;
  int previous_symbol = 2 * input[0] - 1;
  gmsk_output[0] = start_point;

  for (int i = 1; i < nitems; i++) {
    //change bits representation to NRZ
    current_symbol = 2 * input[i] - 1;
    //differentially encode
    encoded_symbol = current_symbol * previous_symbol;
    //and do gmsk mapping
    gmsk_output[i] = j * gr_complex(encoded_symbol, 0.0) * gmsk_output[i-1];
    previous_symbol = current_symbol;
  }
}

//TODO consider use of some generalized function for correlation and placing it in a separate class  for signal processing
gr_complex gsm_receiver_cf::correlate_sequence(const gr_complex * sequence, int length, const gr_complex * input)
{
  gr_complex result(0.0, 0.0);
  int sample_number = 0;

  for (int ii = 0; ii < length; ii++) {
    sample_number = (ii * d_OSR) ;
    result += sequence[ii] * conj(input[sample_number]);
  }

  result = result / gr_complex(length, 0);
  return result;
}

//computes autocorrelation for positive arguments
//TODO consider placing this funtion in a separate class for signal processing
inline void gsm_receiver_cf::autocorrelation(const gr_complex * input, gr_complex * out, int nitems)
{
  int i, k;
  for (k = nitems - 1; k >= 0; k--) {
    out[k] = gr_complex(0, 0);
    for (i = k; i < nitems; i++) {
      out[k] += input[i] * conj(input[i-k]);
    }
  }
}

//TODO consider use of some generalized function for filtering and placing it in a separate class  for signal processing
inline void gsm_receiver_cf::mafi(const gr_complex * input, int nitems, gr_complex * filter, int filter_length, gr_complex * output)
{
  int ii = 0, n, a;

  for (n = 0; n < nitems; n++) {
    a = n * d_OSR;
    output[n] = 0;
    ii = 0;

    while (ii < filter_length) {
      if ((a + ii) >= nitems*d_OSR)
        break;
      output[n] += input[a+ii] * filter[ii];
      ii++;
    }
  }
}

int gsm_receiver_cf::get_norm_chan_imp_resp(const gr_complex *input, gr_complex * chan_imp_resp, unsigned search_range, int bcc)
{
  vector_complex correlation_buffer;
  vector_float power_buffer;
  vector_float window_energy_buffer;

  int strongest_window_nr;
  int burst_start = 0;
  int chan_imp_resp_center = 0;
  float max_correlation = 0;
  float energy = 0;

  int search_center = (int)((TRAIN_POS + GUARD_PERIOD) * d_OSR);
  int search_start_pos = search_center + 1;
  int search_stop_pos = search_center + d_chan_imp_length * d_OSR + 2 * d_OSR;

  for (int ii = search_start_pos; ii < search_stop_pos; ii++) {
    gr_complex correlation = correlate_sequence(&d_norm_training_seq[bcc][TRAIN_BEGINNING], N_TRAIN_BITS - 10, &input[ii]);

    correlation_buffer.push_back(correlation);
    power_buffer.push_back(pow(abs(correlation), 2));
  }

  //compute window energies
  vector_float::iterator iter = power_buffer.begin();
  bool loop_end = false;
  while (iter != power_buffer.end()) {
    vector_float::iterator iter_ii = iter;
    energy = 0;

    for (int ii = 0; ii < (d_chan_imp_length)*d_OSR; ii++, iter_ii++) {
      if (iter_ii == power_buffer.end()) {
        loop_end = true;
        break;
      }
      energy += (*iter_ii);
    }
    if (loop_end) {
      break;
    }
    iter++;

    window_energy_buffer.push_back(energy);
  }

  strongest_window_nr = max_element(window_energy_buffer.begin(), window_energy_buffer.end()) - window_energy_buffer.begin();
//   d_channel_imp_resp.clear();

  max_correlation = 0;
  for (int ii = 0; ii < (d_chan_imp_length)*d_OSR; ii++) {
    gr_complex correlation = correlation_buffer[strongest_window_nr + ii];
    if (abs(correlation) > max_correlation) {
      chan_imp_resp_center = ii;
      max_correlation = abs(correlation);
    }
//     d_channel_imp_resp.push_back(correlation);
    chan_imp_resp[ii] = correlation;
  }
  // We want to use the first sample of the impulseresponse, and the
  // corresponding samples of the received signal.
  // the variable sync_w should contain the beginning of the used part of
  // training sequence, which is 3+57+1+6=67 bits into the burst. That is
  // we have that sync_t16 equals first sample in bit number 67.

  burst_start = search_start_pos + chan_imp_resp_center + strongest_window_nr - TRAIN_POS * d_OSR;

  // GMSK modulator introduces ISI - each bit is expanded for 3*Tb
  // and it's maximum value is in the last bit period, so burst starts
  // 2*Tb earlier
  burst_start -= 2 * d_OSR;
  burst_start += 2;
  //std::cout << " burst_start: " << burst_start << " center: " << ((float)(search_start_pos + strongest_window_nr + chan_imp_resp_center)) / d_OSR << " stronegest window nr: " <<  strongest_window_nr << "\n";

  return burst_start;

}