convolutional.h

各种卷积码、turbo码的迭代解码平台

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

   Copyright (C) 2006, Matthew C. Valenti

   Last updated on Jan. 11, 2006

   Functions  itob, parity_counter, nsc_enc_bit, rsc_enc_bit, nsc_transit, 
   rsc_transit, rsc_tail, and conv_encode 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

*/

/* 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()

  Copyright 2003, Matthew C. Valenti.
	
  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() 

  (c) Copyright 2003, Matthew C. Valenti.

  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);
}

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()

  Copyright 2001, Matthew C. Valenti.
	
  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;
}

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;
}

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, inbit, outsym;
  int *bin_vec;
  int state = 0;

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

  /* encode one bit at a time */
  for (i=0;i<LL+KK-1;i++) {
	  if (i<LL)
		  inbit = input[i]; /* data bit */
	  else
		  inbit = tail[state]; /* tail bit */
	  
	  if (inbit) {
		  /* 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;
}