Merge pull request #41 from marojevic/master

Optimization of soft-demodulation algorithm

lte/phy/lib/modem/src/soft_algs.c

@@ -54,36 +54,30 @@ void llr_approx(const _Complex float *in, float *out, int N, int M, int B,
     _Complex float *symbols, uint32_t (*S)[6][32], float sigma2) {
   int i, s, b;
   float num, den;
-  float new_num, new_den;
-  float idiff0, qdiff0, idiff1, qdiff1;
   int change_sign = -1;
+  float x, y, d[64];
 
   for (s=0; s<N; s++) {  	/* recevied symbols */
+	  /* Compute the distances squared d[i] between the received symbol and all constellation points */
+	  for (i=0; i<M; i++) {
+		x = __real__ in[s] - __real__ symbols[i];
+		y = __imag__ in[s] - __imag__ symbols[i];
+		d[i] = x*x + y*y;
+	  }
+
 	  for (b=0; b<B; b++) {/* bits per symbol*/
 	  	/* initiate num[b] and den[b] */
-    idiff0 = __real__ in[s] - __real__ symbols[S[0][b][0]];
-    qdiff0 = __imag__ in[s] - __imag__ symbols[S[0][b][0]];
-          num = idiff0*idiff0 + qdiff0*qdiff0;
-
-          idiff1 = __real__ in[s] - __real__ symbols[S[1][b][0]];
-          qdiff1 = __imag__ in[s] - __imag__ symbols[S[1][b][0]];
-          den = idiff1*idiff1 + qdiff1*qdiff1;
-
-          /* half the constellation symbols have '1'|'0' at any bit pos. */
-          for (i=1; i<M/2; i++) {
-            idiff0 = __real__ in[s] - __real__ symbols[S[0][b][i]];
-            qdiff0 = __imag__ in[s] - __imag__ symbols[S[0][b][i]];
-            new_num = idiff0*idiff0 + qdiff0*qdiff0;
+		num = d[S[0][b][0]];
+		den = d[S[1][b][0]];
 
-            idiff1 = __real__ in[s] - __real__ symbols[S[1][b][i]];
-            qdiff1 = __imag__ in[s] - __imag__ symbols[S[1][b][i]];
-            new_den = idiff1*idiff1 + qdiff1*qdiff1;
-
-            if (new_num < num) {
-              num = new_num;
+		/* Minimum distance squared search between recevied symbol and a constellation point with a
+		 '1' and a '0' for each bit position */
+	        for (i=1; i<M/2; i++) { /* half the constellation points have '1'|'0' at any given bit position */
+	        	if (d[S[0][b][i]] < num) {
+	        		num = d[S[0][b][i]];
 	        	}
-            if (new_den < den) {
-              den = new_den;
+	        	if (d[S[1][b][i]] < den) {
+	        		den = d[S[1][b][i]];
 	        	}
 	        }
 	        /* Theoretical LLR and approximate LLR values are positive if
@@ -93,6 +87,7 @@ void llr_approx(const _Complex float *in, float *out, int N, int M, int B,
 	        out[s*B+b] = change_sign*(den-num)/sigma2;
 	  }
   }
+
 }
 
 /**
@@ -115,23 +110,26 @@ void llr_exact(const _Complex float *in, float *out, int N, int M, int B,
     _Complex float *symbols, uint32_t (*S)[6][32], float sigma2) {
   int i, s, b;
   float num, den;
-  float idiff0, qdiff0, idiff1, qdiff1;
   int change_sign = -1;
+  float x, y, d[64];
 
   for (s=0; s<N; s++) {  	/* recevied symbols */
+    	/* Compute exp{·} of the distances squared d[i] between the received symbol and all constellation points */
+    	for (i=0; i<M; i++) {
+    		x = __real__ in[s] - __real__ symbols[i];
+    		y = __imag__ in[s] - __imag__ symbols[i];
+    		d[i] = exp(-1*(x*x + y*y)/sigma2);
+    	}
+
+	/* Sum up the corresponding d[i]'s for each bit position */
     	for (b=0; b<B; b++) {/* bits per symbol*/
     		/* initiate num[b] and den[b] */
     		num = 0;
     		den = 0;
-          /* half the constellation symbols have '1'|'0' at any bit pos. */
-          for (i=0; i<M/2; i++) {
-            idiff0 = __real__ in[s] - __real__ symbols[S[0][b][i]];
-            qdiff0 = __imag__ in[s] - __imag__ symbols[S[0][b][i]];
-            num += exp(-1*(idiff0*idiff0 + qdiff0*qdiff0)/sigma2);
     	    	
-            idiff1 = __real__ in[s] - __real__ symbols[S[1][b][i]];
-            qdiff1 = __imag__ in[s] - __imag__ symbols[S[1][b][i]];
-            den += exp(-1*(idiff1*idiff1 + qdiff1*qdiff1)/sigma2);
+    	        for (i=0; i<M/2; i++) { /* half the constellation points have '1'|'0' at any given bit position */
+    	        	num += d[S[0][b][i]];
+    	        	den += d[S[1][b][i]];
     	        }
     	        /* Theoretical LLR and approximate LLR values are positive if
     	         * symbol(s) with '0' is/are closer and negative if symbol(s)