QAM add noise

QAM/qam.c

@@ -41,6 +41,23 @@ void init_constellation (qam_system* qam) {
     }
 }
 
+// Calcul du bruit gaussien pour un sigma donné
+// Formule de Box-Muller
+double gaussian_noise (double sigma) {
+    double u1 = (rand() + 1) / ((double)RAND_MAX + 2);
+    double u2 = (rand() + 1) / ((double)RAND_MAX + 2);
+    return sigma * sqrt(-2 * log(u1)) * cos(2 * M_PI * u2);
+}
+
+// Ajout du bruit
+void add_noise (double complex* s, int len, double sigma) {
+    for (int i = 0; i < len; i++) {
+        double nr = gaussian_noise(sigma);
+        double ni = gaussian_noise(sigma);
+        s[i] += nr + I * ni;
+    }
+}
+
 // Changer le tableau de bits en boolen ou alors la represenation binaire et shifter pour extraire les bits (pas bien si M plus grand)
 void bits_to_symbols (qam_system* qam, uint8_t* bits, int nb_bits, double complex* symbols) {
     int nb_symbols = nb_bits / qam->k;
@@ -129,7 +146,7 @@ int main (int argc, char *argv[]) {
     qam.M = 256;
     qam.k = (int)log2((double)(qam.M));
     qam.Fs = 44100;
-    qam.Ts = 0.00003;
+    qam.Ts = 0.003;
     qam.N = (int)qam.Fs * qam.Ts;
     qam.Fc = 2000;
     init_constellation(&qam);
@@ -155,6 +172,13 @@ int main (int argc, char *argv[]) {
     int total_samples = qam.N * nb_symbols;
     double complex* s = (double complex*)malloc(sizeof(double complex) * total_samples);
     modulate(&qam, symbols, nb_symbols, s);
+    // Ajout du bruit 
+    double signal_power = (2.0/3.0)*(qam.M-1); // puissance moyenne avant échelle
+    double snr_dB = -27; // Signal to noise ratio
+    double snr_lin = pow(10.0, snr_dB / 10.0);
+    double sigma = sqrt(signal_power / snr_lin);
+    printf("Ajout du bruit \n       puissance du signal : %f\n       SNR db : %f\n       sigma : %f\n", signal_power, snr_dB, sigma);
+    add_noise(s, total_samples, sigma);
 
     printf("Demodulation\n");