Floating point --> Fixed point --> Bit string conversion for constellation code lookup tables

lut-helper.c

#include <stdlib.h>
#include <stdio.h>
#include <math.h>

const double qpsk[] = {-1.0, 1.0};
const double qam16[] = {-3.0, -1.0, 3.0, 1.0};
const double qam64[] = {-7.0, -5.0, -1.0, -3.0, 7.0, 5.0, 1.0, 3.0};

/* Generic function for converting short integers to bit string
 * representation.
 *
 * Example:
 *
 * short s = 19; char output[17];
 * short_to_bitstring(s, output);
 *
 * Output would now contain: "0000000000010011"
*/
void short_to_bitstring(const short s, char *output) {
  short mask, i, il;
  for (i=0,il=sizeof(s)*8;i<il;++i) {
    mask = 1 << i;
    output[il-1-i] = ((s & mask) == 0) ? '0' : '1';
  }
  output[il] = '\0';
}

void print_normalized_lut(const double *arr, const short size,
                          const double ncoef) {
  short i, fp;
  char binary[17];
  double f;

  for (i=0;i<size;++i) {
    f = arr[i] * ncoef;
    // Convert normalized constellation code to s,1,14 fixed point format
    fp = (signed short)(f * (1<<14) + (f >=0 ? 0.5 : -0.5));
    short_to_bitstring(fp, binary);
    printf("%s\n", binary);
  }
}

int main() {
  printf("BPSK\n");
  print_normalized_lut(qpsk, 2, 1.0);
  printf("QPSK\n");
  print_normalized_lut(qpsk, 2, 1.0 / sqrt(2.0));
  printf("16QAM\n");
  print_normalized_lut(qam16, 4, 1.0 / sqrt(10));
  printf("64QAM\n");
  print_normalized_lut(qam64, 8, 1.0 / sqrt(42));

  return 0;
}