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/* File: Capacity.c
Description: Determines capacity of M-ary modulation
The calling syntax is:
[output] = capacity( input, data )
Where:
output = Instantaneous capacity of this frame
input = M by N matrix of symbol likelihoods
data = 1 by N*log2(M) vector of data bits
Note: if M = 1, then the input must be bitwise LLRs.
Copyright (C) 2005-2006, Matthew C. Valenti
Last updated on Jan. 11, 2006
Function Capacity is 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
*/
#include <math.h>
#include <mex.h>
#include <Matrix.h>
#include <stdlib.h>
#include "./include/maxstar.h"
/* Input Arguments
prhs[0] is input
prhs[1] is data */
/* Output Arguments
plhs[0] is output */
/* main function that interfaces with MATLAB */
void mexFunction(
int nlhs,
mxArray *plhs[],
int nrhs,
const mxArray *prhs[] )
{
int M;
int number_symbols, number_bits, bits_per_symbol;
double *input, *output, *data;
int i,j,index,temp_int;
double cap_sum, temp_cap;
/* make sure there are enough inputs */
if (nrhs<2)
mexErrMsgTxt("Usage: [output] = capacity( input, data )");
/* read in input received symbols */
number_symbols = mxGetN(prhs[0]);
M = mxGetM(prhs[0]);
input = mxGetPr(prhs[0]);
/* read in data bits */
number_bits = mxGetN(prhs[1]);
if ( mxGetM(prhs[1])!=1)
mexErrMsgTxt("data must be a row vector");
data = mxGetPr(prhs[1]);
/* determine the number of bits per symbol */
if (M == 1)
bits_per_symbol = 1; /* input is bitwise LLRs */
else {
bits_per_symbol = 0;
temp_int = M;
while (temp_int>1) {
temp_int = temp_int/2;
bits_per_symbol++;
}
}
/* printf( "number of symbols = %d\n", number_symbols);
printf( "number of bits = %d\n", number_bits );
printf( "M = %d\n", M);
printf( "Bits per symbol = %d\n", bits_per_symbol ); */
/* make sure that number of bits is consistent */
if ( number_bits%bits_per_symbol )
mexErrMsgTxt( "Number of bits does not divide log_2(M)" );
if ( (number_symbols*bits_per_symbol) != number_bits )
mexErrMsgTxt( "Number of bits inconsistent with number of symbols" );
/* output is a real scalar */
plhs[0] = mxCreateDoubleMatrix(1, 1, mxREAL);
output = mxGetPr( plhs[0] );
/* determine capacity */
cap_sum = 0;
if ( M == 1 ) {
/* the input is bitwise LLRs */
for (i=0;i<number_symbols;i++) {
if ( data[i] > 0 )
cap_sum += max_star4( 0, -input[i] );
else
cap_sum += max_star4( 0, input[i] );
}
/* printf( "cap_sum = %f\n", cap_sum ); */
output[0] = 1 - cap_sum/(number_symbols*log(2));
} else {
for (i=0;i<number_symbols;i++) { /* create each modulated symbol */
/* determine the index */
index = 0;
for (j=0;j<bits_per_symbol;j++) { /* go through each associated bit */
index = index << 1; /* shift to the left (multiply by 2) */
index += data[i*bits_per_symbol+j];
}
temp_cap = -1000000;
for (j=0;j<M;j++) {
/* printf( "metric[%d] = %f\n", M, input[i*M+j]-input[i*M+index] ); */
temp_cap = max_star4( temp_cap, input[i*M+j] - input[i*M+index] );
}
/* printf( "temp_cap = %f\n\n", temp_cap ); */
cap_sum += temp_cap;
}
output[0] = 1 - cap_sum/(number_symbols*log(M));
}
/* printf( "cap_sum = %f\n", cap_sum ); */
/* printf( "instantaneous capacity = %f\n", output[0] ); */
}
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