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airprobe/gsm-receiver/src/lib/gsm_receiver_cf.cc

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/* -*- 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);
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;
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());
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 , 33, d_gsm_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);
}
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)
{
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_gsm_file = fopen( "speech.au.gsm", "wb" ); //!!
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);
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;
}
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