Selecting TA based on first significant tap instead of based on absolute max

openair1/PHY/INIT/nr_init.c

@@ -138,6 +138,9 @@ void phy_init_nr_gNB(PHY_VARS_gNB *gNB)
   AssertFatal(ret_loader == 0, "error loading LDPC library\n");
 
   gNB->max_nb_pdsch = MAX_MOBILES_PER_GNB;
+  fp->delay_table = calloc(2 * MAX_DELAY_COMP + 1, sizeof(*fp->delay_table));
+  fp->delay_table128 = calloc(2 * MAX_DELAY_COMP + 1, sizeof(*fp->delay_table128));
+  AssertFatal(fp->delay_table && fp->delay_table128, "Failed to allocate delay tables\n");
   init_delay_table(fp->ofdm_symbol_size, MAX_DELAY_COMP, NR_MAX_OFDM_SYMBOL_SIZE, fp->delay_table);
   init_delay_table(128, MAX_DELAY_COMP, 128, fp->delay_table128);
 
@@ -283,6 +286,9 @@ void phy_free_nr_gNB(PHY_VARS_gNB *gNB)
 
   free_nrLDPC_coding_interface(&gNB->nrLDPC_coding_interface);
 
+  NR_DL_FRAME_PARMS *fp = &gNB->frame_parms;
+  free_and_zero(fp->delay_table);
+  free_and_zero(fp->delay_table128);
 }
 
 void nr_phy_config_request_sim(PHY_VARS_gNB *gNB,

openair1/PHY/INIT/nr_init_ue.c

@@ -336,6 +336,11 @@ void term_nr_ue_signal(PHY_VARS_NR_UE *ue, int nb_connected_gNB)
     free_and_zero(ue->prs_vars[idx]);
   }
 
+  free(ue->frame_parms.delay_table);
+  ue->frame_parms.delay_table = NULL;
+  free(ue->frame_parms.delay_table128);
+  ue->frame_parms.delay_table128 = NULL;
+
   sl_ue_free(ue);
 }
 
@@ -489,6 +494,9 @@ void clean_UE_harq(PHY_VARS_NR_UE *UE)
 
 void phy_init_nr_top(PHY_VARS_NR_UE *ue) {
   NR_DL_FRAME_PARMS *frame_parms = &ue->frame_parms;
+  frame_parms->delay_table = calloc(2 * MAX_DELAY_COMP + 1, sizeof(*frame_parms->delay_table));
+  frame_parms->delay_table128 = calloc(2 * MAX_DELAY_COMP + 1, sizeof(*frame_parms->delay_table128));
+  AssertFatal(frame_parms->delay_table && frame_parms->delay_table128, "Failed to allocate delay tables\n");
   init_delay_table(frame_parms->ofdm_symbol_size, MAX_DELAY_COMP, NR_MAX_OFDM_SYMBOL_SIZE, frame_parms->delay_table);
   crcTableInit();
   init_byte2m128i();

openair1/PHY/defs_nr_common.h

@@ -80,7 +80,7 @@
 #define MAX_NUM_NR_SRS_AP 4
 
 
-#define MAX_DELAY_COMP 20
+#define MAX_DELAY_COMP 160
 
 #define PBCH_MAX_RE_PER_SYMBOL (20 * 12)
 
@@ -239,10 +239,10 @@ struct NR_DL_FRAME_PARMS {
   /// sequence used to compensate the phase rotation due to timeshifted OFDM symbols
   /// First dimenstion is for different CP lengths
   c16_t timeshift_symbol_rotation[4096*2] __attribute__ ((aligned (16)));
-  /// Table used to apply the delay compensation in DL/UL
-  c16_t delay_table[2 * MAX_DELAY_COMP + 1][NR_MAX_OFDM_SYMBOL_SIZE];
-  /// Table used to apply the delay compensation in PUCCH2
-  c16_t delay_table128[2 * MAX_DELAY_COMP + 1][128];
+  /// Table used to apply the delay compensation in DL/UL (dynamically allocated)
+  c16_t (*delay_table)[NR_MAX_OFDM_SYMBOL_SIZE];
+  /// Table used to apply the delay compensation in PUCCH2 (dynamically allocated)
+  c16_t (*delay_table128)[128];
   /// Power used by SSB in order to estimate signal strength and path loss
   int ss_PBCH_BlockPower;
 

openair1/PHY/nr_phy_common/src/nr_phy_common.c

@@ -384,7 +384,30 @@ void nr_est_delay(int ofdm_symbol_size, const c16_t *ls_est, c16_t *ch_estimates
   // estimated delay, and causing the delay compensation to worsen the result instead of improving it. After analyzing several
   // peaks, and doing many tests, a PEAK_DETECT_THRESHOLD = 15 is an adequate value, to apply delay compensation only when there is
   // clearly a peak
-  delay->est_delay = mean_val > 0 && max_val / mean_val > PEAK_DETECT_THRESHOLD ? max_pos - sync_pos : 0;
+  if (mean_val == 0 || max_val / mean_val <= PEAK_DETECT_THRESHOLD) {
+    delay->est_delay = 0;
+    return;
+  }
+
+  // In multipath channels, the strongest tap may be a late reflection rather than the
+  // first arrival. Aligning to a late tap shifts the FFT window so early paths fall
+  // before the CP, worsening ISI. Instead, find the first significant tap (-6 dB
+  // relative to the peak in power) which keeps the most multipath energy inside the CP.
+  // TODO: could scan from max_pos - scan_range to max_pos instead of -scan_range to
+  // +scan_range since the first path is always <= peak delay
+  const int threshold = max_val >> 2;
+  const int scan_range = MAX_DELAY_COMP < ofdm_symbol_size / 2 ? MAX_DELAY_COMP : ofdm_symbol_size / 2 - 1;
+  int first_sig_delay = max_pos;
+  for (int d = -scan_range; d <= scan_range; d++) {
+    int idx = d < 0 ? d + ofdm_symbol_size : d;
+    int power = c16amp2(ch_estimates_time[idx]) >> 1;
+    if (power > threshold) {
+      first_sig_delay = d;
+      break;
+    }
+  }
+
+  delay->est_delay = first_sig_delay - sync_pos;
 }
 
 unsigned int nr_get_tx_amp(int power_dBm, int power_max_dBm, int total_nb_rb, int nb_rb)