Check for DCI false positive

by comparing the received bits with re-encoded payload bits

openair1/PHY/NR_UE_TRANSPORT/dci_nr.c

@@ -874,6 +874,33 @@ void nr_pdcch_unscrambling(int16_t *z,
 
 
 #ifdef NR_PDCCH_DCI_RUN
+/* This function compares the received DCI bits with
+ * re-encoded DCI bits and returns the number of mismatched bits
+ */
+uint16_t nr_dci_false_detection(uint64_t *dci,
+                            int16_t *soft_in,
+                            t_nrPolar_params *polar_param,
+                            int encoded_length,
+                            int rnti) {
+
+  uint32_t encoder_output[NR_MAX_DCI_SIZE_DWORD];
+  polar_encoder_fast(dci, (void*)encoder_output, rnti, 1, polar_param);
+  uint8_t *enout_p = (int16_t*)encoder_output;
+  uint16_t x = 0;
+
+  for (int i=0; i<encoded_length/8; i++) {
+    x += ( enout_p[i] & 1 ) ^ ( ( soft_in[i*8] >> 15 ) & 1);
+    x += ( ( enout_p[i] >> 1 ) & 1 ) ^ ( ( soft_in[i*8+1] >> 15 ) & 1 );
+    x += ( ( enout_p[i] >> 2 ) & 1 ) ^ ( ( soft_in[i*8+2] >> 15 ) & 1 );
+    x += ( ( enout_p[i] >> 3 ) & 1 ) ^ ( ( soft_in[i*8+3] >> 15 ) & 1 );
+    x += ( ( enout_p[i] >> 4 ) & 1 ) ^ ( ( soft_in[i*8+4] >> 15 ) & 1 );
+    x += ( ( enout_p[i] >> 5 ) & 1 ) ^ ( ( soft_in[i*8+5] >> 15 ) & 1 );
+    x += ( ( enout_p[i] >> 6 ) & 1 ) ^ ( ( soft_in[i*8+6] >> 15 ) & 1 );
+    x += ( ( enout_p[i] >> 7 ) & 1 ) ^ ( ( soft_in[i*8+7] >> 15 ) & 1 );
+  }
+  return x;
+}
+
 uint8_t nr_dci_decoding_procedure(PHY_VARS_NR_UE *ue,
                                   UE_nr_rxtx_proc_t *proc,
                                   fapi_nr_dci_indication_t *dci_ind) {
@@ -920,6 +947,12 @@ uint8_t nr_dci_decoding_procedure(PHY_VARS_NR_UE *ue,
         if (crc == n_rnti) {
           LOG_D(PHY, "(%i.%i) Received dci indication (rnti %x,dci format %d,n_CCE %d,payloadSize %d,payload %llx)\n",
                 proc->frame_rx, proc->nr_slot_rx,n_rnti,rel15->dci_format_options[k],CCEind,dci_length,*(unsigned long long*)dci_estimation);
+          uint16_t mb = nr_dci_false_detection(dci_estimation,tmp_e,currentPtrDCI,L*108,n_rnti);
+          if (mb > 10) {
+            LOG_W(PHY,"DCI false positive. Dropping DCI index %d. Mismatched bits: %d/%d\n",j,mb,L*108);
+            continue;
+          }
+          else {
             dci_ind->SFN = proc->frame_rx;
             dci_ind->slot = proc->nr_slot_rx;
             dci_ind->dci_list[dci_ind->number_of_dcis].rnti        = n_rnti;
@@ -929,6 +962,7 @@ uint8_t nr_dci_decoding_procedure(PHY_VARS_NR_UE *ue,
             memcpy((void*)dci_ind->dci_list[dci_ind->number_of_dcis].payloadBits,(void*)dci_estimation,8);
             dci_ind->number_of_dcis++;
             break;    // If DCI is found, no need to check for remaining DCI lengths
+          }
         } else {
           LOG_D(PHY,"(%i.%i) Decoded crc %x does not match rnti %x for DCI format %d\n", proc->frame_rx, proc->nr_slot_rx, crc, n_rnti, rel15->dci_format_options[k]);
         }