Reduce rounding error in rshift of sum_det

 - Use a uint64_t
 - move shift after the sum
 - add bias before shifting to reduce rounding error

openair1/PHY/NR_TRANSPORT/nr_ulsch_demodulation.c

@@ -493,10 +493,12 @@ static simde__m128i nr_ulsch_comp_muli_sum(simde__m128i input_x,
   //print_ints("rx_re:",(int32_t*)&xy_re_128[0]);
   //print_ints("rx_Img:",(int32_t*)&xy_im_128[0]);
   //divide by matrix det and convert back to Q15 before packing
-  uint32_t sum_det = 0;
+  uint64_t sum_det = 0;
   for (int k = 0; k < 4; k++) {
-    sum_det += (((uint32_t *)&det)[k]) >> 2;
+    sum_det += (((uint32_t *)&det)[k]);
   }
+  // Add bias to reduce rounding error
+  sum_det = (sum_det + 2) >> 2;
 
   int b = log2_approx(sum_det) - 8;
   if (b > 0) {
@@ -944,10 +946,12 @@ static uint8_t nr_ulsch_mmse_2layers(NR_DL_FRAME_PARMS *frame_parms,
 
     // Magnitude computation
     if (mod_order > 2) {
-      uint32_t sum_det = 0;
+      uint64_t sum_det = 0;
       for (int k = 0; k < 4; k++) {
-        sum_det += (((uint32_t *)&determ_fin_128[0])[k]) >> 2;
+        sum_det += (((uint32_t *)&determ_fin_128[0])[k]);
       }
+      // Add bias to reduce rounding error
+      sum_det = (sum_det + 2) >> 2;
 
       int b = log2_approx(sum_det) - 8;
       if (b > 0) {