path: root/gsm-receiver/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 <boost/circular_buffer.hpp>
#include <algorithm>
#include <numeric>
#include <gsm_receiver_cf.h>
#include <viterbi_detector.h>
#include <string.h>
#include <sch.h>


#include "RxBurst.h"
#include "GSMCommon.h"

#define SYNC_SEARCH_RANGE 30
// #define TRAIN_SEARCH_RANGE 40
//FIXME: decide to use this define or not

//TODO: this shouldn't be here - remove it when gsm receiver's interface will be ready
void decrypt(const unsigned char * burst_binary, byte * KC, unsigned char * decrypted_data, unsigned FN)
{
  byte AtoB[2*DATA_BITS];

  /* KC is all zero: no decryption */

  if(KC[0] == 0 && KC[1] == 0 && KC[2] == 0 && KC[3] == 0 &
     KC[4] == 0 && KC[5] == 0 && KC[6] == 0 && KC[7] == 0) {
    for (int i = 0; i < 148; i++) {
      decrypted_data[i] = burst_binary[i];
    }
    return;
  }

  keysetup(KC, FN);
  runA51(AtoB);

  /* guard bits */
  for (int i = 0; i < 3; i++) {
    decrypted_data[i] = burst_binary[i];
  }

  for (int i = 0; i < 57; i++) {
    decrypted_data[i+3] = AtoB[i] ^ burst_binary[i+3];
  }

  /* stealing bits and midamble */
  for (int i = 60; i < 88; i++) {
    decrypted_data[i] = burst_binary[i];
  }

  for (int i = 0; i < 57; i++) {
    decrypted_data[i+88] = AtoB[i+57] ^ burst_binary[i+88];
  }

  /* guard bits */
  for (int i = 145; i < 148; i++) {
    decrypted_data[i] = burst_binary[i];
  }
}

//TODO: this shouldn't be here */
void dump_bits(const unsigned char * burst_binary, unsigned char * decrypted_data, burst_counter burst_nr, bool first_burst)
{
  int i;

  /* Cipher bits */
  printf("C%d %d %d: ", first_burst, burst_nr.get_frame_nr(), burst_nr.get_frame_nr_mod());
  for (int i = 0; i < 57; i++)
    printf("%d", burst_binary[i+3]);
  for (int i = 0; i < 57; i++)
    printf("%d", burst_binary[i+88]);
  printf("\n");

  /* Plain bits */
  printf("P%d %d %d: ", first_burst, burst_nr.get_frame_nr(), burst_nr.get_frame_nr_mod());
  for (int i = 0; i < 57; i++)
    printf("%d", decrypted_data[i+3]);
  for (int i = 0; i < 57; i++)
    printf("%d", decrypted_data[i+88]);
  printf("\n");

  /* Keystream bits */
  printf("S%d %d %d: ", first_burst, burst_nr.get_frame_nr(), burst_nr.get_frame_nr_mod());
  for (int i = 0; i < 57; i++)
    printf("%d", burst_binary[i+3] ^ decrypted_data[i+3]);
  for (int i = 0; i < 57; i++)
    printf("%d", burst_binary[i+88] ^ decrypted_data[i+88]);
  printf("\n");
}

void gsm_receiver_cf::read_key(std::string key)
{
  printf("Key: '%s'\n", key.c_str());

  int i;
  int b;
  for (i = 0;i < 8;i++) {
    b = d_hex_to_int[(char)key[(i)*2]]*16 + d_hex_to_int[(char)key[i*2+1]];
    d_KC[i] = (byte)b;
  }  
}

void gsm_receiver_cf::read_configuration(std::string configuration)
{
  printf("Configuration: '%s'\n", configuration.c_str());

  if ((char)configuration[0] == 0) {
    printf("  No configuration set.\n");
    return;
  }

  /* get timeslot */
  int ts = atoi(configuration.c_str());
  if (ts < 0 || ts > 7) {
    printf("  Invalid TS: %d\n", ts);
    return;
  }

  printf("  Configuration TS: %d\n", ts);

  d_tch_mode = TM_NONE;
  if((char)configuration[1] == 'C')
    d_gs_ctx.ts_ctx[ts].type = TST_FCCH_SCH_BCCH_CCCH_SDCCH4;
  else if((char)configuration[1] == 'B')
    d_gs_ctx.ts_ctx[ts].type = TST_FCCH_SCH_BCCH_CCCH;
  else if((char)configuration[1] == 'S')
    d_gs_ctx.ts_ctx[ts].type = TST_SDCCH8;
  else if((char)configuration[1] == 'T') {
    d_gs_ctx.ts_ctx[ts].type = TST_TCHF;
    if((char)configuration[2] == 'E')
      d_tch_mode = TM_SPEECH_EFR;
    else
      d_tch_mode = TM_SPEECH_FR;
  } else {
    printf("  Invalid configuration: %c\n", (char)configuration[1]);
    return;
  }

  /* any other timeslot than 0: turn TS0 off */
  if(ts != 0) {
    d_gs_ctx.ts_ctx[0].type = TST_OFF;
    d_trace_sch = false;
    printf("  TS0 is turned off\n");
  }
}

void gsm_receiver_cf::process_normal_burst(burst_counter burst_nr, const unsigned char * burst_binary, bool first_burst)
{
  unsigned char decrypted_data[148];
  float decrypted_data_float[148];
  unsigned char * voice_frame;
  int ts = burst_nr.get_timeslot_nr();

  /* no processing if turned off*/
  if (d_gs_ctx.ts_ctx[ts].type == TST_OFF)
    return;

  /* handle traffic timeslots */
#if 0
  /* always try to decrypt and decode traffic in TS 1...7 */
  /* TODO: this will fail if there is unencrypted traffic in more than one TS */
  if (burst_nr.get_timeslot_nr() >= 1 && burst_nr.get_timeslot_nr() <= 7) {
#else
  if (d_gs_ctx.ts_ctx[ts].type == TST_TCHF) {
#endif
    decrypt(burst_binary, d_KC, decrypted_data, burst_nr.get_frame_nr_mod());

    if (burst_nr.get_t2() == 12 || burst_nr.get_t2() == 25) { /* SACCH of Full Rate TCH */
      if (ts % 2 == 0) /* SACH position and start depends on the timeslot */
        first_burst = (burst_nr.get_frame_nr() % 104) == (12 + 26 * (ts / 2));
      else
        first_burst = (burst_nr.get_frame_nr() % 104) == (25 + 26 * ((ts - 1) / 2));
#if 0 /* dump cipher, plain and keystream bits */
      dump_bits(burst_binary, decrypted_data, burst_nr, first_burst);
#endif
      GS_process(&d_gs_ctx, TIMESLOT0 + ts, NORMAL, &decrypted_data[3], burst_nr.get_frame_nr(), first_burst);
    } else {
      int i;
      for (i = 0; i< 148; i++)
	decrypted_data_float[i] = decrypted_data[i];

      GSM::Time time(burst_nr.get_frame_nr(), ts);
      GSM::RxBurst rxbrst(decrypted_data_float, time);
      if (ts - TIMESLOT1 >= 0 && ts - TIMESLOT1 < N_TCH_DECODER) {
	if ( d_tch_decoder[ts - TIMESLOT1]->processBurst( rxbrst ) == true)
	  fwrite(d_tch_decoder[ts - TIMESLOT1]->get_voice_frame(), 1 , d_tch_decoder[ts - TIMESLOT1]->get_voice_frame_length(), d_speech_file);
	else if(rxbrst.Hl() || rxbrst.Hu()) {
	  /* Stolen bits are set, might be FACCH */
	  GS_process(&d_gs_ctx, TIMESLOT0 + ts, NORMAL, &decrypted_data[3], burst_nr.get_frame_nr(), first_burst);
        }
      }
    }
  }

  /* handle SDCCH/8 timeslots */
  if (d_gs_ctx.ts_ctx[ts].type == TST_SDCCH8) {
    decrypt(burst_binary, d_KC, decrypted_data, burst_nr.get_frame_nr_mod());
    #if 1 /* dump cipher, plain and keystream bits */
    dump_bits(burst_binary, decrypted_data, burst_nr, first_burst);
    #endif
    GS_process(&d_gs_ctx, TIMESLOT0 + ts, NORMAL, &decrypted_data[3], burst_nr.get_frame_nr(), first_burst);
  }

  /* TS0 is special (TODO) */
  if (ts == 0) {
    memcpy(decrypted_data, burst_binary, sizeof(decrypted_data));
    if (d_gs_ctx.ts_ctx[ts].type == TST_FCCH_SCH_BCCH_CCCH_SDCCH4) {
      if (SDCCH_SACCH_4_MAP[burst_nr.get_frame_nr() % 51] != 0) { /* SDCCH/4 or SACCH/4 */
        decrypt(burst_binary, d_KC, decrypted_data, burst_nr.get_frame_nr_mod());
        #if 1 /* dump cipher, plain and keystream bits */
        dump_bits(burst_binary, decrypted_data, burst_nr, first_burst);
        #endif
      }
    }
    GS_process(&d_gs_ctx, TIMESLOT0 + ts, NORMAL, &decrypted_data[3], burst_nr.get_frame_nr(), first_burst);
  }

}
//TODO: this shouldn't be here also - the same reason
void gsm_receiver_cf::configure_receiver()
{
  int ts;
  printf("configure_receiver\n");

  /* configure TS0, TS0 is special (TODO)  */

  d_channel_conf.set_multiframe_type(TIMESLOT0, multiframe_51);
  d_channel_conf.set_burst_types(TIMESLOT0, TEST_CCH_FRAMES, TEST_CCH_FIRST, sizeof(TEST_CCH_FRAMES) / sizeof(unsigned), normal_burst);
  /* FCCH bursts */
  d_channel_conf.set_burst_types(TIMESLOT0, FCCH_FRAMES, sizeof(FCCH_FRAMES) / sizeof(unsigned), fcch_burst);
  /* SCH bursts */
  d_channel_conf.set_burst_types(TIMESLOT0, SCH_FRAMES, sizeof(SCH_FRAMES) / sizeof(unsigned), sch_burst);

  /* configure TS1...TS7 */

  for (ts = TIMESLOT1; ts < TIMESLOT7; ts++) {
    if (d_gs_ctx.ts_ctx[ts].type == TST_TCHF) {
      d_channel_conf.set_multiframe_type(ts, multiframe_26);
      d_channel_conf.set_burst_types(ts, TRAFFIC_CHANNEL_F, sizeof(TRAFFIC_CHANNEL_F) / sizeof(unsigned), dummy_or_normal);
      /* SACH position depends on the timeslot */
      if (ts % 2 == 0) {
	d_channel_conf.set_single_burst_type(ts, 12, normal_burst); /* SACCH for even timeslots */
	d_channel_conf.set_single_burst_type(ts, 25, empty); /* IDLE for even timeslots */
      } else {
	d_channel_conf.set_single_burst_type(ts, 12, empty); /* IDLE for odd timeslots */
	d_channel_conf.set_single_burst_type(ts, 25, normal_burst); /* SACCH for odd timeslots */
      }
    }
    else if (d_gs_ctx.ts_ctx[ts].type == TST_SDCCH8) {
      d_channel_conf.set_multiframe_type(ts, multiframe_51);
      d_channel_conf.set_burst_types(ts, SDCCH_SACCH_8_FRAMES, SDCCH_SACCH_8_FIRST, sizeof(SDCCH_SACCH_8_FRAMES) / sizeof(unsigned), dummy_or_normal);  
    }
  }
}


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, gr_feval_dd *synchronizer, int osr, std::string key, std::string configuration)
{
  return gsm_receiver_cf_sptr(new gsm_receiver_cf(tuner, synchronizer, osr, key, configuration));
}

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, gr_feval_dd *synchronizer, int osr, std::string key, std::string configuration)
    : 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),
    d_failed_sch(0),
    d_trace_sch(true)
{
  const unsigned char amr_nb_magic[6] = { 0x23, 0x21, 0x41, 0x4d, 0x52, 0x0a };
  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++) {
    gr_complex startpoint;
    if (i == 6 || i == 7) {                           //this is nasty hack
      startpoint = gr_complex(-1.0, 0.0);   //if I don't change it here all bits of normal bursts for BTSes with bcc=6 will have reversed values
    } else {
      startpoint = gr_complex(1.0, 0.0);    //I've checked this hack for bcc==0,1,2,3,4,6
    }                                       //I don't know what about bcc==5 and 7 yet
    //TODO:find source of this situation - this is purely mathematical problem I guess

    gmsk_mapper(train_seq[i], N_TRAIN_BITS, d_norm_training_seq[i], startpoint);
  }

  for (i = 0; i < N_TCH_DECODER; i++)
    d_tch_decoder[i] = new GSM::TCHFACCHL1Decoder(GSM::gFACCH_TCHFMapping);

  d_hex_to_int['0'] = 0; //!!
  d_hex_to_int['4'] = 4; //!!
  d_hex_to_int['8'] = 8; //!!
  d_hex_to_int['c'] = 0xc; //!!
  d_hex_to_int['1'] = 1; //!!
  d_hex_to_int['5'] = 5; //!!
  d_hex_to_int['9'] = 9; //!!
  d_hex_to_int['d'] = 0xd; //!!
  d_hex_to_int['2'] = 2; //!!
  d_hex_to_int['6'] = 6; //!!
  d_hex_to_int['a'] = 0xa; //!!
  d_hex_to_int['e'] = 0xe; //!!
  d_hex_to_int['3'] = 3; //!!
  d_hex_to_int['7'] = 7; //!!
  d_hex_to_int['b'] = 0xb; //!!
  d_hex_to_int['f'] = 0xf; //!!
  read_key(key); //!!

  /* Initialize GSM Stack, clear d_gs_ctx */
  GS_new(&d_gs_ctx); //TODO: remove it! it's not a right place for a decoder

  /* configuration is stored in d_gs_ctx */
  read_configuration(configuration);

  /* open speech file and configure d_tch_decoders */
  switch (d_tch_mode) {

  case TM_SPEECH_FR:
    d_speech_file = fopen( "speech.au.gsm", "wb" );
    for (i = 0; i < N_TCH_DECODER; i++)
        d_tch_decoder[i]->setMode(GSM::MODE_SPEECH_FR);
    break;

  case TM_SPEECH_EFR:
    d_speech_file = fopen( "speech.amr", "wb" );
    fwrite(amr_nb_magic, 1, 6, d_speech_file); /* Write header */
    for (i = 0; i < N_TCH_DECODER; i++)
        d_tch_decoder[i]->setMode(GSM::MODE_SPEECH_EFR);
    break;

  default:
    d_speech_file = NULL;

  }

  configure_receiver();
}

/*
 * 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 some time (a bunch of buffered samples)
        float prev_freq_offset = d_freq_offset;        //before previous set_frequqency cause change
        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
            if(d_trace_sch)
            {
              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

            #if 0 /* Dieter: now done in constructor */
            //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
            // Dieter: don't call it, otherwise overwrites configuration of configure_receiver()
            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
            #endif

            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
        bool first_burst = d_channel_conf.get_first_burst(d_burst_nr); // first burst of four ?

        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 - TAIL_BITS * d_OSR;
              double freq_offset = compute_freq_offset(input, first_sample, last_sample);       //extract frequency offset from it

              d_freq_offset_vals.push_front(freq_offset);

              if (d_freq_offset_vals.size() >= 10) {
                double sum = std::accumulate(d_freq_offset_vals.begin(), d_freq_offset_vals.end(), 0);
                double mean_offset = sum / d_freq_offset_vals.size();                           //compute mean
                d_freq_offset_vals.clear();
                if (abs(mean_offset) > FCCH_MAX_FREQ_OFFSET) {
                  d_freq_offset -= mean_offset;                                                 //and adjust frequency if it have changed beyond
                  set_frequency(d_freq_offset);                                                 //some limit
                  DCOUT("mean_offset: " << mean_offset);
                  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
                d_failed_sch = 0;
                offset =  burst_start - floor((GUARD_PERIOD) * d_OSR);                         //compute offset from burst_start - burst should start after a guard period
                if(d_trace_sch)
                {
                  DCOUT("bcc: " << d_bcc << " ncc: " << d_ncc << " t1: " << t1 << " t2: " << t2 << " t3: " << t3);
                  DCOUT(offset);
                }
                to_consume += offset;                                                          //adjust with offset number of samples to be consumed
              } else {
                d_failed_sch++;
                if (d_failed_sch >= MAX_SCH_ERRORS) {
//                   d_state = next_fcch_search;        //TODO: this isn't good, the receiver is going wild when it goes back to next_fcch_search from here
//                   d_freq_offset_vals.clear();
                  DCOUT("many sch decoding errors");
                }
              }
            }
            break;

          case normal_burst:                                                                  //if it's normal burst
            burst_start = get_norm_chan_imp_resp(input, &channel_imp_resp[0], 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, first_burst); //TODO: this shouldn't be here - remove it when gsm receiver's interface will be ready
            break;

          case dummy_or_normal: {
              burst_start = get_norm_chan_imp_resp(input, &channel_imp_resp[0], TS_DUMMY);
              detect_burst(input, &channel_imp_resp[0], burst_start, output_binary);

              std::vector<unsigned char> v(20);
              std::vector<unsigned char>::iterator it;
              it = std::set_difference(output_binary + TRAIN_POS, output_binary + TRAIN_POS + 16, &train_seq[TS_DUMMY][5], &train_seq[TS_DUMMY][21], v.begin());
              int different_bits = (it - v.begin());

              if (different_bits > 2) {
                burst_start = get_norm_chan_imp_resp(input, &channel_imp_resp[0], d_bcc);
                detect_burst(input, &channel_imp_resp[0], burst_start, output_binary);
                if (!output_binary[0] && !output_binary[1] && !output_binary[2]) {
                  process_normal_burst(d_burst_nr, output_binary, first_burst); //TODO: this shouldn't be here - remove it when gsm receiver's interface will be ready
                }
              }
            }
          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], 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++;
    }
  }
}

//TODO: get_norm_chan_imp_resp is similar to get_sch_chan_imp_resp - consider joining this two functions
//TODO: this is place where most errors are introduced and can be corrected by improvements to this fuction
//especially computations of strongest_window_nr
int gsm_receiver_cf::get_norm_chan_imp_resp(const gr_complex *input, gr_complex * chan_imp_resp, 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_start_pos = search_center -  d_chan_imp_length * d_OSR;
  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 - 2)*d_OSR; ii++, iter_ii++) {
//    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);
  }
  //!why doesn't this work
  strongest_window_nr = max_element(window_energy_buffer.begin(), window_energy_buffer.end()) - window_energy_buffer.begin();
  strongest_window_nr = 3; //! so I have to override it here

  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;
}