gnss-sdr/src/algorithms/telemetry_decoder/libs/convolutional.h

/* File convolutional.h
   
   Description: General functions used to implement convolutional encoding.   

   Copyright (C) 2006-2008, Matthew C. Valenti

   Last updated on May 22, 2008

   The functions in this file are part of the Iterative Solutions 
   Coded Modulation Library. The Iterative Solutions Coded Modulation 
   Library is free software; you can redistribute it and/or modify it 
   under the terms of the GNU Lesser General Public License as published 
   by the Free Software Foundation; either version 2.1 of the License, 
   or (at your option) any later version.

   This library 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
   Lesser General Public License for more details.
  
   You should have received a copy of the GNU Lesser General Public
   License along with this library; if not, write to the Free Software
   Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA  02110-1301  USA

*/

/* define constants used throughout the library */
#define MAXLOG 1e7  /* Define infinity */

/* function itob()

  Description: Converts an integer symbol into a vector of bits

	Output parameters:
		binvec_p: The binary vector
		
    Input parameters:
	    symbol:  The integer-valued symbol
		length:  The length of the binary vector
	
  This function is used by conv_encode()  */

void itob(
						int	binvec_p[],
						int symbol,
						int length )
{
	int counter;

	/* Go through each bit in the vector */
	for (counter=0;counter<length;counter++) {
		binvec_p[length-counter-1] = (symbol&1);
		symbol = symbol>>1;
	}

	return;
}

/* function parity_counter()

  Description: Determines if a symbol has odd (1) or even (0) parity

	Output parameters:
		(returned int): The symbol's parity = 1 for odd and 0 for even
		
    Input parameters:
	    symbol:  The integer-valued symbol
		length:  The highest bit position in the symbol
	
  This function is used by nsc_enc_bit(), rsc_enc_bit(), and rsc_tail()  */

int parity_counter( int symbol, int length )
{
	int counter;
	int temp_parity = 0;

	for (counter=0;counter<length;counter++) {
		temp_parity = temp_parity^(symbol&1);
		symbol = symbol>>1;
	}

	return( temp_parity );
}


/* Function nsc_enc_bit() 

  Description: Convolutionally encodes a single bit using a rate 1/n encoder.
  Takes in one input bit at a time, and produces a n-bit output.
  
	Input parameters:
		input		The input data bit (i.e. a 0 or 1).
		state_in	The starting state of the encoder (an int from 0 to 2^m-1).
		g[]			An n-element vector containing the code generators in binary form.
		KK			The constraint length of the convolutional code.

	Output parameters:
		output_p[]		An n-element vector containing the encoded bits.
		state_out_p[]	An integer containing the final state of the encoder	
						(i.e. the state after encoding this bit)
	
  This function is used by rsc_encode(), nsc_transit(), rsc_transit(), and nsc_transit() */

static int nsc_enc_bit(
				   int	state_out_p[],
				   int	input,
				   int	state_in, 
				   int  g[],
				   int  KK,
				   int  nn )
{
	/* declare variables */
	int state, i;
	int out = 0;	

	/* create a word made up of state and new input */
	state = (input<<(KK-1))^state_in;
	
	/* AND the word with the generators */
	for (i=0;i<nn;i++)
    {		
		/* update output symbol */
		out = (out<<1) + parity_counter( state&g[i], KK ); 		
    }
	
	/* shift the state to make the new state */
	state_out_p[0] = state>>1; 	
	return(out);
}

/* like nsc_enc_bit() but for a RSC code */
static int rsc_enc_bit(
				   int	state_out_p[],
				   int	input,
				   int	state_in, 
				   int  g[],
				   int  KK,
				   int  nn )
{
	/* declare variables */
	int state, i, out, a_k;	

	/* systematic output */
	out = input;

	/* determine feedback bit */
	a_k = input^parity_counter( g[0]&state_in, KK );

	/* create a word made up of state and feedback bit */
	state = (a_k<<(KK-1))^state_in;

	/* AND the word with the generators */
	for (i=1;i<nn;i++)
    {		
		/* update output symbol */
		out = (out<<1) + parity_counter( state&g[i], KK ); 		
    }
	
	/* shift the state to make the new state */
	state_out_p[0] = state>>1; 	
	return(out);
}

/* function that creates the transit and output vectors */
static void nsc_transit(
						int		output_p[],
						int		trans_p[],
						int		input,
						int     g[],
						int     KK,
						int     nn )
{
	int nextstate[1];
	int state, states;
	states = (1<<(KK-1));  /* The number of states: 2^mm */

	/* Determine the output and next state for each possible starting state */
	for(state=0;state<states;state++) {
		output_p[state]  = nsc_enc_bit( nextstate, input, state, g, KK, nn ); 
		trans_p[state]  = nextstate[0];
	}
	return;
}

/* Function rsc_transit()

  Description: Calculates the "transition matrix" for the trellis.
  This information tells the decoder what the next state and output bits
  will be given the current state and input bit.

	Input parameters:
		input		Either 0 or 1 --- the input data bit.
		g[]			A two element vector containing the code generators.
		KK			The constraint length of the convolutional code.

	Output parameters:
		output_p[]	A vector of length max_states = 2^(KK-1) containing
		            the output symbols.
		trans_p[]   A vector of length max_states that tells the decoder
					what the next state will be given the input and current state.
	
  This function is used by turbo_decode()   */

static void rsc_transit(
						int	output_p[],
						int trans_p[],
						int	input,
						int g[],
						int KK,
						int nn )
{
	int nextstate[1];
	int state, states; 

	states = 1 << (KK-1); /* The number of states: 2^mm */

	/* Determine the output and next state for each possible starting state */
	for(state=0;state<states;state++) {
		output_p[state] = rsc_enc_bit( nextstate, input, state, g, KK, nn ); 
		trans_p[state]  = nextstate[0];
	}	
	return;
}

/* determine the tail for a RSC code */
static void rsc_tail(
						int	tail_p[],
						int g[],
						int max_states,
						int mm )
{
	int state;

	/* Determine the tail for each state */
	for(state=0;state<max_states;state++) {		
		/* determine feedback word */
		tail_p[state] = parity_counter( g[0]&state, mm );
	}
	return;
}

/* perform convolutional encoding */
static void conv_encode(
	     int		output_p[],
	     int		input[],
		 int		out0[], 
		 int		state0[], 
		 int		out1[], 
		 int		state1[],
         int		tail[],	 
         int        KK,
         int        LL,
		 int        nn )
{
  int i, j, outsym;
  int *bin_vec;
  int state = 0;

  /* Negative value in "tail" is a flag that this is 
  a tail-biting NSC code.  Determine initial state */

  if ( tail[0] < 0 ) {
	  for (i=LL-KK+1;i<LL;i++) {  
		  if (input[i]) {			  
			  /* Determine next state */
			  state = state1[state];
		  } else {	  
			  /* Determine next state */
			  state = state0[state];
		  }
	  }
  }

  bin_vec = (int*)calloc( nn, sizeof(int) );

  /* encode data bits one bit at a time */
  for (i=0;i<LL;i++) {  
	  if (input[i]) {
		  /* Input is a one */
		  outsym = out1[state];  /* The output symbol */
		  
		  /* Determine next state */
		  state = state1[state];
	  } else {
		  /* Input is a zero */
		  outsym = out0[state];  /* The output symbol */
		  
		  /* Determine next state */
		  state = state0[state];
	  }

	  /* Convert symbol to a binary vector	*/
	  itob( bin_vec, outsym, nn );
		  
	  /* Assign to output */
	  for (j=0;j<nn;j++)
		  output_p[nn*i+j] = bin_vec[j];
  }

  /* encode tail if needed */
  if (tail[0] >= 0) {
	  for (i=LL;i<LL+KK-1;i++) {
		  if (tail[state]) {
			  /* Input is a one */
			  outsym = out1[state];  /* The output symbol */
			  
			  /* Determine next state */
			  state = state1[state];
		  } else {
			  /* Input is a zero */
			  outsym = out0[state];  /* The output symbol */
			  
			  /* Determine next state */
			  state = state0[state];
		  }
		  
		  /* Convert symbol to a binary vector	*/
		  itob( bin_vec, outsym, nn );
		  
		  /* Assign to output */
		  for (j=0;j<nn;j++)
			  output_p[nn*i+j] = bin_vec[j];
	  }
  }

  free(bin_vec);

  return;
}


/* function Gamma()
  
	Description: Computes the branch metric used for decoding.

	Output parameters:
		(returned float) 	The metric between the hypothetical symbol and the recevieved vector
	
	Input parameters:
		rec_array			The received vector, of length nn
		symbol				The hypothetical symbol
		nn					The length of the received vector
	
  This function is used by siso()  */


static float Gamma(float  rec_array[],
				   int    symbol,
				   int    nn )
{
	float rm = 0;
	int i;
	int mask;
	
	mask = 1;
	for (i=0;i<nn;i++) {
		if (symbol&mask)
			rm += rec_array[nn-i-1];
		mask = mask<<1;
	} 
	
	return(rm);
} 


/* Function Viterbi()

  Description: Uses the Viterbi algorithm to perform hard-decision decoding of a convolutional code.

	Input parameters:
		out0[]		The output bits for each state if input is a 0 (generated by rsc_transit).
		state0[]	The next state if input is a 0 (generated by rsc_transit).
		out1[]		The output bits for each state if input is a 1 (generated by rsc_transit).
		state1[]	The next state if input is a 1 (generated by rsc_transit).
		r[]			The received signal in LLR-form. For BPSK, must be in form r = 2*a*y/(sigma^2).
		KK			The constraint length of the convolutional code.
		LL			The number of data bits.

	Output parameters:
		output_u_int[]		Hard decisions on the data bits
	
*/

static void Viterbi(
				int    output_u_int[],
				int    out0[], 
				int    state0[], 
				int    out1[], 
				int    state1[],
				double  input_c[],
				int    KK,
				int    nn,
				int    LL
				)
{
	int i, t, state, mm, states;
	int number_symbols;
	float metric;
	float *prev_section, *next_section;
	int *prev_bit;
	int *prev_state;
	float *metric_c;	/* Set of all possible branch metrics */
	float *rec_array;   /* Received values for one trellis section */
	float max_val;
	
	/* some derived constants */
	mm = KK-1;
	states = 1 << mm;			/* 2^mm */
	number_symbols = 1 << nn;	    /* 2^nn */
	
	/* dynamically allocate memory */ 
	prev_section = (float*)calloc( states, sizeof(float) );
	next_section = (float*)calloc( states, sizeof(float) );
	prev_bit = (int*)calloc( states*(LL+mm), sizeof(int) );
	prev_state = (int*)calloc( states*(LL+mm), sizeof(int) );
	rec_array = (float*)calloc( nn, sizeof(float) );
	metric_c = (float*)calloc( number_symbols, sizeof(float) );
	
	/* initialize trellis */
	for (state=0;state<states;state++) {
		prev_section[state] = -MAXLOG; 
		next_section[state] = -MAXLOG;
	}
	prev_section[0] = 0; /* start in all-zeros state */
	
	/* go through trellis */
	for (t=0;t<LL+mm;t++) {
		for (i=0;i<nn;i++)
			rec_array[i] = (float)input_c[nn*t+i];
		
		/* precompute all possible branch metrics */
		for (i=0;i<number_symbols;i++)
			metric_c[i] = Gamma( rec_array, i, nn ); 
		
		/* step through all states */
		for (state=0;state<states;state++) {
			
			/* hypothesis: info bit is a zero */
			metric = prev_section[state] + metric_c[ out0[ state ] ];
			
			/* store new metric if more than metric in storage */
			if ( metric > next_section[state0[state]] ) {
				next_section[state0[state]] = metric;
				prev_state[t*states+state0[state]] = state;
				prev_bit[t*states+state0[state]] = 0;
			}
			
			/* hypothesis: info bit is a one */
			metric = prev_section[state] + metric_c[ out1[ state ] ];
			
			/* store new metric if more than metric in storage */
			if ( metric > next_section[state1[state]] ) {
				next_section[state1[state]] = metric;
				prev_state[t*states+state1[state]] = state;
				prev_bit[t*states+state1[state]] = 1;
			}
		}

		/* normalize */
		max_val = 0;
		for (state=0;state<states;state++) {
			if (next_section[state]>max_val){
				max_val = next_section[state];
			}
		}
		for (state=0;state<states;state++) {
			prev_section[state] = next_section[state] - max_val;
			next_section[state] = -MAXLOG;
		}
	}
	
	/* trace-back operation */
	state = 0;

	/* tail, no need to output */
	for (t=LL+mm-1; t>=LL; t--) {
		state = prev_state[t*states+state];
	}

	for (t=LL-1; t>=0; t--) {		
		output_u_int[t] = prev_bit[t*states+state];
		state = prev_state[t*states+state];
	}
	
	/* free the dynamically allocated memory */
	free(prev_section);
	free(next_section);
	free(prev_bit);
	free(prev_state);
	free(rec_array);
	free(metric_c); 
	
}

/* Function ViterbiTb()

  Description: Uses the Viterbi algorithm to perform hard-decision decoding of a tail-biting convolutional code.

	Input parameters:
		out0[]		The output bits for each state if input is a 0 (generated by rsc_transit).
		state0[]	The next state if input is a 0 (generated by rsc_transit).
		out1[]		The output bits for each state if input is a 1 (generated by rsc_transit).
		state1[]	The next state if input is a 1 (generated by rsc_transit).
		r[]			The received signal in LLR-form. For BPSK, must be in form r = 2*a*y/(sigma^2).
		KK			The constraint length of the convolutional code.
		LL			The number of data bits.
		depth		head and tail decoding length [Ref. W. Sung, Electronics Letters, vol. 36, no. 7]

	Output parameters:
		output_u_int[]		Hard decisions on the data bits
  
*/


static void ViterbiTb(
				int    output_u_int[],
				int    out0[], 
				int    state0[], 
				int    out1[], 
				int    state1[],
				double  input_c[],
				int    KK,
				int    nn,
				int    LL,
				int	   depth
				)
{
	int i, t, state, mm, states, max_state;
	int number_symbols, starting_bit;
	float metric;
	float *prev_section, *next_section;
	int *prev_bit;
	int *prev_state;
	float *metric_c;	/* Set of all possible branch metrics */
	float *rec_array;   /* Received values for one trellis section */
	float max_val;
	
	/* some derived constants */
	mm = KK-1;
	states = 1 << mm;			/* 2^mm */
	number_symbols = 1 << nn;	    /* 2^nn */
	
	/* dynamically allocate memory */ 
	prev_section = (float*)calloc( states, sizeof(float) );
	next_section = (float*)calloc( states, sizeof(float) );
	prev_bit = (int*)calloc( states*(LL+depth), sizeof(int) );
	prev_state = (int*)calloc( states*(LL+depth), sizeof(int) );
	rec_array = (float*)calloc( nn, sizeof(float) );
	metric_c = (float*)calloc( number_symbols, sizeof(float) );
	
	/* initialize trellis */
	for (state=0;state<states;state++) {
		prev_section[state] = 0; /* equally likely starting state */
		next_section[state] = -MAXLOG;
	}
	
	/* go through trellis */
	for (t=-depth;t<LL+depth;t++) {
		/* determine the corresponding data bits */
		starting_bit = nn*(t%LL);
		if (starting_bit < 0 )
			starting_bit = nn*LL + starting_bit;
		
		/* printf( "start at %d\n", starting_bit ); */
		for (i=0;i<nn;i++) {
			rec_array[i] = (float)input_c[starting_bit+i];
			/* printf( "%1f\n", rec_array[i] ); */
		}

		/* precompute all possible branch metrics */
		for (i=0;i<number_symbols;i++)
			metric_c[i] = Gamma( rec_array, i, nn ); 
		
		/* step through all states */
		for (state=0;state<states;state++) {
			
			/* hypothesis: info bit is a zero */
			metric = prev_section[state] + metric_c[ out0[ state ] ];
			
			/* store new metric if more than metric in storage */
			if ( metric > next_section[state0[state]] ) {
				next_section[state0[state]] = metric;
				if (t>=0) {				
					prev_state[t*states+state0[state]] = state;
					prev_bit[t*states+state0[state]] = 0;
				}
			}
			
			/* hypothesis: info bit is a one */
			metric = prev_section[state] + metric_c[ out1[ state ] ];
			
			/* store new metric if more than metric in storage */
			if ( metric > next_section[state1[state]] ) {
				next_section[state1[state]] = metric;
				if (t>=0) {				
					prev_state[t*states+state1[state]] = state;				
					prev_bit[t*states+state1[state]] = 1;
				}
			}
		}
		
		/* normalize */
		max_val = 0;
		for (state=0;state<states;state++) {
			if (next_section[state]>max_val){
				max_val = next_section[state];
				max_state = state;
			}
		}
		for (state=0;state<states;state++) {
			prev_section[state] = next_section[state] - max_val;
			next_section[state] = -MAXLOG;
		}
	}
	
	/* trace-back operation */
	state = max_state;

	/* tail, no need to output */
	for (t=LL+depth-1; t>=LL; t--) {
		state = prev_state[t*states+state];
	}

	for (t=LL-1; t>=0; t--) {		
		output_u_int[t] = prev_bit[t*states+state];
		state = prev_state[t*states+state];
	}
	
	/* free the dynamically allocated memory */
	free(prev_section);
	free(next_section);
	free(prev_bit);
	free(prev_state);
	free(rec_array);
	free(metric_c); 
	
}