/* -*- c++ -*- */
/*
* @file
* @author Piotr Krysik
* @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.
*/
/*
* viterbi_detector:
* This part does the detection of received sequnece.
* Employed algorithm is viterbi Maximum Likehood Sequence Estimation.
* At this moment it gives hard decisions on the output, but
* it was designed with soft decisions in mind.
*
* SYNTAX: void viterbi_detector(
* const gr_complex * input,
* unsigned int samples_num,
* gr_complex * rhh,
* unsigned int start_state,
* const unsigned int * stop_states,
* unsigned int stops_num,
* float * output)
*
* INPUT: input: Complex received signal afted matched filtering.
* samples_num: Number of samples in the input table.
* rhh: The autocorrelation of the estimated channel
* impulse response.
* start_state: Number of the start point. In GSM each burst
* starts with sequence of three bits (0,0,0) which
* indicates start point of the algorithm.
* stop_states: Table with numbers of possible stop states.
* stops_num: Number of possible stop states
*
*
* OUTPUT: output: Differentially decoded hard output of the algorithm:
* -1 for logical "0" and 1 for logical "1"
*
* SUB_FUNC: none
*
* TEST(S): Tested with real world normal burst.
*/
#include
#include
#include
#define PATHS_NUM (1 << (CHAN_IMP_RESP_LENGTH-1))
void viterbi_detector(const gr_complex * input, unsigned int samples_num, gr_complex * rhh, unsigned int start_state, const unsigned int * stop_states, unsigned int stops_num, float * output)
{
float increment[8];
float path_metrics1[16];
float path_metrics2[16];
float paths_difference;
float * new_path_metrics;
float * old_path_metrics;
float * tmp;
float trans_table[BURST_SIZE][16];
float pm_candidate1, pm_candidate2;
bool real_imag;
float input_symbol_real, input_symbol_imag;
unsigned int i, sample_nr;
/*
* Setup first path metrics, so only state pointed by start_state is possible.
* Start_state metric is equal to zero, the rest is written with some very low value,
* which makes them practically impossible to occur.
*/
for(i=0; i max_stop_state_metric){
max_stop_state_metric = stop_state_metric;
best_stop_state = stop_states[i];
}
}
/*
* This table was generated with hope that it gives a litle speedup during
* traceback stage.
* Received bit is related to the number of state in the trellis.
* I've numbered states so their parity (number of ones) is related
* to a received bit.
*/
static const unsigned int parity_table[PATHS_NUM] = { 0, 1, 1, 0, 0, 1, 1, 0, 0, 1, 1, 0, 0, 1, 1, 0, };
/*
* Table of previous states in the trellis diagram.
* For GMSK modulation every state has two previous states.
* Example:
* previous_state_nr1 = prev_table[current_state_nr][0]
* previous_state_nr2 = prev_table[current_state_nr][1]
*/
static const unsigned int prev_table[PATHS_NUM][2] = { {0,8}, {0,8}, {1,9}, {1,9}, {2,10}, {2,10}, {3,11}, {3,11}, {4,12}, {4,12}, {5,13}, {5,13}, {6,14}, {6,14}, {7,15}, {7,15}, };
/*
* Traceback and differential decoding of received sequence.
* Decisions stored in trans_table are used to restore best path in the trellis.
*/
sample_nr=samples_num;
unsigned int state_nr=best_stop_state;
unsigned int decision;
bool out_bit=0;
while(sample_nr>0){
sample_nr--;
decision = (trans_table[sample_nr][state_nr]>0);
if(decision != out_bit)
output[sample_nr]=-trans_table[sample_nr][state_nr];
else
output[sample_nr]=trans_table[sample_nr][state_nr];
out_bit = out_bit ^ real_imag ^ parity_table[state_nr];
state_nr = prev_table[state_nr][decision];
real_imag = !real_imag;
}
}