Hello everyone,
I have a couple of questions on GNU Radio implementation of the Butterfly
macros. I'm somehow familiar with doing butterfly on paper (the trellis)
but could someone explain to me how this macro works? I also don't
understand how the list of butterflies for a specific code (for example the
CCSDS CC(K= 7, R = 1/2) butterflies generated by the program below the
butterfly macro) are generated (in particular, how to derive the partab
lookup).
Regards,
#define BUTTERFLY(i, sym) \
{ \
int m0, m1; \
\
/* ACS for 0 branch */ \
m0 = state[i].metric + mets[sym]; /* 2*i */ \
m1 = state[i + 32].metric + mets[3 ^ sym]; /* 2*i + 64 */ \
if (m0 > m1) { \
next[2 * i].metric = m0; \
next[2 * i].path = state[i].path << 1; \
} else { \
next[2 * i].metric = m1; \
next[2 * i].path = (state[i + 32].path << 1) | 1; \
} \
/* ACS for 1 branch */ \
m0 = state[i].metric + mets[3 ^ sym]; /* 2*i + 1 */ \
m1 = state[i + 32].metric + mets[sym]; /* 2*i + 65 */ \
if (m0 > m1) { \
next[2 * i + 1].metric = m0; \
next[2 * i + 1].path = state[i].path << 1; \
} else { \
next[2 * i + 1].metric = m1; \
next[2 * i + 1].path = (state[i + 32].path << 1) | 1; \
} \
}
*/* Generate the inline BUTTERFLY macro calls for viterbi.c */*
*/* The two generator polynomials for the NASA Standard K=7 code */ *
*#include <stdio.h>*
*#define POLYA 0x4f*
*#define POLYB 0x6d *
*unsigned char Partab[] = {*
* 0, 1, 1, 0, 1, 0, 0, 1,*
* 1, 0, 0, 1, 0, 1, 1, 0,*
* 1, 0, 0, 1, 0, 1, 1, 0,*
* 0, 1, 1, 0, 1, 0, 0, 1,*
* 1, 0, 0, 1, 0, 1, 1, 0,*
* 0, 1, 1, 0, 1, 0, 0, 1,*
* 0, 1, 1, 0, 1, 0, 0, 1,*
* 1, 0, 0, 1, 0, 1, 1, 0,*
* 1, 0, 0, 1, 0, 1, 1, 0,*
* 0, 1, 1, 0, 1, 0, 0, 1,*
* 0, 1, 1, 0, 1, 0, 0, 1,*
* 1, 0, 0, 1, 0, 1, 1, 0,*
* 0, 1, 1, 0, 1, 0, 0, 1,*
* 1, 0, 0, 1, 0, 1, 1, 0,*
* 1, 0, 0, 1, 0, 1, 1, 0,*
* 0, 1, 1, 0, 1, 0, 0, 1,*
* 1, 0, 0, 1, 0, 1, 1, 0,*
* 0, 1, 1, 0, 1, 0, 0, 1,*
* 0, 1, 1, 0, 1, 0, 0, 1,*
* 1, 0, 0, 1, 0, 1, 1, 0,*
* 0, 1, 1, 0, 1, 0, 0, 1,*
* 1, 0, 0, 1, 0, 1, 1, 0,*
* 1, 0, 0, 1, 0, 1, 1, 0,*
* 0, 1, 1, 0, 1, 0, 0, 1,*
* 0, 1, 1, 0, 1, 0, 0, 1,*
* 1, 0, 0, 1, 0, 1, 1, 0,*
* 1, 0, 0, 1, 0, 1, 1, 0,*
* 0, 1, 1, 0, 1, 0, 0, 1,*
* 1, 0, 0, 1, 0, 1, 1, 0,*
* 0, 1, 1, 0, 1, 0, 0, 1,*
* 0, 1, 1, 0, 1, 0, 0, 1,*
* 1, 0, 0, 1, 0, 1, 1, 0,*
*};*
*int main(int argc, char *argv[]) *
*{ *
* int e,i;*
* int nNonInv = 1;*
* for(i=0;i<32;i++){ *
* if(nNonInv) {*
* e = Partab[2*i & POLYA] << 1; *
* e |= Partab[2*i & POLYB]; *
* }*
* else {*
* // CCSDS Poly V27POLYB, -V27POLYA*
* e = Partab[2*i & POLYA] << 1; *
* e |= 1^Partab[2*i & POLYB]; *
* }*
* printf("BUTTERFLY(%d,%d); ",i,e);*
* if (i % 4 ==3) {*
* printf("\n");*
* } *
* } *
* return 0;*
*} *
