Introduce SRS oversampling for better accuracy in estimating nanoseconds

openair1/PHY/NR_ESTIMATION/nr_measurements_gNB.c

@@ -37,12 +37,12 @@
 #define I0_SKIP_DC 1
 
 int nr_est_timing_advance_srs(const NR_DL_FRAME_PARMS *frame_parms,
-                              const c16_t srs_estimated_channel_time[][frame_parms->ofdm_symbol_size])
+                              const c16_t srs_estimated_channel_time[][NR_SRS_IDFT_OVERSAMP_FACTOR * frame_parms->ofdm_symbol_size])
 {
   int timing_advance = 0;
   int max_val = 0;
 
-  for (int i = 0; i < frame_parms->ofdm_symbol_size; i++) {
+  for (int i = 0; i < NR_SRS_IDFT_OVERSAMP_FACTOR * frame_parms->ofdm_symbol_size; i++) {
     int temp = 0;
     for (int aa = 0; aa < frame_parms->nb_antennas_rx; aa++) {
       int Re = ((c16_t*)srs_estimated_channel_time[aa])[i].r;
@@ -55,9 +55,10 @@ int nr_est_timing_advance_srs(const NR_DL_FRAME_PARMS *frame_parms,
     }
   }
 
-  if (timing_advance > frame_parms->ofdm_symbol_size/2) {
-    timing_advance = timing_advance - frame_parms->ofdm_symbol_size;
+  if (timing_advance > (NR_SRS_IDFT_OVERSAMP_FACTOR * frame_parms->ofdm_symbol_size) / 2) {
+    timing_advance = timing_advance - (NR_SRS_IDFT_OVERSAMP_FACTOR * frame_parms->ofdm_symbol_size);
   }
+  timing_advance = timing_advance / NR_SRS_IDFT_OVERSAMP_FACTOR;
 
   LOG_I(NR_PHY, "Estimated SRS ToA %d\n", timing_advance);
 

openair1/PHY/NR_ESTIMATION/nr_ul_channel_estimation.c

@@ -780,8 +780,8 @@ int nr_srs_channel_estimation(int ant,
                               c16_t srs_received_signal[ofdm_symbol_size * N_symb_SRS],
                               c16_t srs_received_noise[ofdm_symbol_size * N_symb_SRS],
                               c16_t srs_estimated_channel_freq[ofdm_symbol_size * N_symb_SRS],
-                              c16_t srs_estimated_channel_time[ofdm_symbol_size],
-                              c16_t srs_estimated_channel_time_shifted[ofdm_symbol_size],
+                              c16_t srs_estimated_channel_time[NR_SRS_IDFT_OVERSAMP_FACTOR * ofdm_symbol_size],
+                              c16_t srs_estimated_channel_time_shifted[NR_SRS_IDFT_OVERSAMP_FACTOR * ofdm_symbol_size],
                               uint32_t *signal_power,
                               uint32_t *noise_power,
                               int16_t *noise_power_per_rb)
@@ -959,15 +959,27 @@ int nr_srs_channel_estimation(int ant,
 #endif
 
       // Convert to time domain
-      freq2time(ofdm_symbol_size, (int16_t *)srs_estimated_channel_freq, (int16_t *)srs_estimated_channel_time);
+      int16_t ofdm_symbol_size_half = ofdm_symbol_size >> 1;
+      int16_t ofdm_os_size = NR_SRS_IDFT_OVERSAMP_FACTOR * ofdm_symbol_size;
+      int16_t ofdm_os_size_half = ofdm_os_size >> 1;
+      int16_t start_offset = ofdm_os_size - ofdm_symbol_size_half;
 
-      memcpy(srs_estimated_channel_time_shifted,
-             &srs_estimated_channel_time[ofdm_symbol_size >> 1],
-             (ofdm_symbol_size >> 1) * sizeof(c16_t));
+      c16_t chF_interpol[ofdm_os_size] __attribute__((aligned(32)));
+      memset(chF_interpol, 0, sizeof(chF_interpol));
 
-      memcpy(&srs_estimated_channel_time_shifted[ofdm_symbol_size >> 1],
-             srs_estimated_channel_time,
-             (ofdm_symbol_size >> 1) * sizeof(c16_t));
+      // Place SRS channel estimates in FFT shifted format for oversampling
+      memcpy(&chF_interpol[0], &srs_estimated_channel_freq[0], ofdm_symbol_size_half * sizeof(c16_t));
+      memcpy(&chF_interpol[start_offset],
+             &srs_estimated_channel_freq[ofdm_symbol_size_half],
+             ofdm_symbol_size_half * sizeof(c16_t));
+
+      // Convert to time domain oversampled
+      freq2time(ofdm_os_size, (int16_t *)chF_interpol, (int16_t *)srs_estimated_channel_time);
+
+      // Do FFT shift
+      memcpy(srs_estimated_channel_time_shifted, &srs_estimated_channel_time[ofdm_os_size_half], ofdm_os_size_half * sizeof(c16_t));
+
+      memcpy(&srs_estimated_channel_time_shifted[ofdm_os_size_half], srs_estimated_channel_time, ofdm_os_size_half * sizeof(c16_t));
 
       // Compute wideband SNR
       int tot_subcarriers = m_SRS_b * NR_NB_SC_PER_RB;

openair1/PHY/NR_ESTIMATION/nr_ul_estimation.h

@@ -59,8 +59,9 @@ void dump_nr_I0_stats(FILE *fd,PHY_VARS_gNB *gNB);
 
 void gNB_I0_measurements(PHY_VARS_gNB *gNB, int slot, int first_symb, int num_symb, uint32_t rb_mask_ul[14][9]);
 
-int nr_est_timing_advance_srs(const NR_DL_FRAME_PARMS *frame_parms,
-                              const c16_t srs_estimated_channel_time[][frame_parms->ofdm_symbol_size]);
+int nr_est_timing_advance_srs(
+    const NR_DL_FRAME_PARMS *frame_parms,
+    const c16_t srs_estimated_channel_time[][NR_SRS_IDFT_OVERSAMP_FACTOR * frame_parms->ofdm_symbol_size]);
 
 void nr_pusch_ptrs_processing(PHY_VARS_gNB *gNB,
                               NR_DL_FRAME_PARMS *frame_parms,
@@ -81,8 +82,8 @@ int nr_srs_channel_estimation(int ant,
                               c16_t srs_received_signal[ofdm_symbol_size * N_symb_SRS],
                               c16_t srs_received_noise[ofdm_symbol_size * N_symb_SRS],
                               c16_t srs_estimated_channel_freq[ofdm_symbol_size * N_symb_SRS],
-                              c16_t srs_estimated_channel_time[ofdm_symbol_size],
-                              c16_t srs_estimated_channel_time_shifted[ofdm_symbol_size],
+                              c16_t srs_estimated_channel_time[NR_SRS_IDFT_OVERSAMP_FACTOR * ofdm_symbol_size],
+                              c16_t srs_estimated_channel_time_shifted[NR_SRS_IDFT_OVERSAMP_FACTOR * ofdm_symbol_size],
                               uint32_t *signal_power,
                               uint32_t *noise_power,
                               int16_t *noise_power_per_rb);

openair1/PHY/NR_TRANSPORT/nr_transport_proto.h

@@ -242,7 +242,7 @@ void nr_srs_rx_procedures(PHY_VARS_gNB *gNB,
                           nr_srs_info_t *nr_srs_info,
                           int *srs_est,
                           c16_t srs_estimated_channel_freq[][N_ap][ofdm_symbol_size * N_symb_SRS],
-                          c16_t srs_estimated_channel_time[][N_ap][ofdm_symbol_size],
+                          c16_t srs_estimated_channel_time[][N_ap][NR_SRS_IDFT_OVERSAMP_FACTOR * ofdm_symbol_size],
                           int16_t *snr_per_rb);
 
 int get_nr_prach_duration(uint8_t prach_format);

openair1/PHY/defs_gNB.h

@@ -46,6 +46,7 @@
 
 #define MAX_NUM_RU_PER_gNB 8
 #define MAX_PUCCH0_NID 8
+#define NR_SRS_IDFT_OVERSAMP_FACTOR 2
 
 typedef struct {
   int nb_id;

openair1/SCHED_NR/phy_procedures_nr_gNB.c

@@ -868,14 +868,14 @@ void nr_srs_rx_procedures(PHY_VARS_gNB *gNB,
                           nr_srs_info_t *nr_srs_info,
                           int *srs_est,
                           c16_t srs_estimated_channel_freq[][N_ap][ofdm_symbol_size * N_symb_SRS],
-                          c16_t srs_estimated_channel_time[][N_ap][ofdm_symbol_size],
+                          c16_t srs_estimated_channel_time[][N_ap][NR_SRS_IDFT_OVERSAMP_FACTOR * ofdm_symbol_size],
                           int16_t *snr_per_rb)
 {
   NR_DL_FRAME_PARMS *frame_parms = &gNB->frame_parms;
   nfapi_nr_srs_pdu_t *srs_pdu = &srs->srs_pdu;
   c16_t srs_received_signal[nb_antennas_rx][ofdm_symbol_size * N_symb_SRS];
   c16_t srs_received_noise[nb_antennas_rx][ofdm_symbol_size * N_symb_SRS];
-  c16_t srs_estimated_channel_time_shifted[nb_antennas_rx][N_ap][ofdm_symbol_size];
+  c16_t srs_estimated_channel_time_shifted[nb_antennas_rx][N_ap][NR_SRS_IDFT_OVERSAMP_FACTOR * ofdm_symbol_size];
 
   start_meas(&gNB->generate_srs_stats);
 
@@ -946,7 +946,7 @@ void nr_srs_rx_procedures(PHY_VARS_gNB *gNB,
     T_INT(frame_rx),
     T_INT(0),
     T_INT(0),
-    T_BUFFER(srs_estimated_channel_time_shifted[0][0], ofdm_symbol_size * sizeof(int32_t)));
+    T_BUFFER(srs_estimated_channel_time_shifted[0][0], NR_SRS_IDFT_OVERSAMP_FACTOR * ofdm_symbol_size * sizeof(int32_t)));
 
   T(T_GNB_PHY_UL_SNR_ESTIMATE,
     T_INT(0),
@@ -1171,7 +1171,8 @@ int phy_procedures_gNB_uespec_RX(PHY_VARS_gNB *gNB, int frame_rx, int slot_rx, N
     int srs_est;
 
     c16_t srs_estimated_channel_freq[nb_antennas_rx][N_ap][ofdm_symbol_size * N_symb_SRS] __attribute__((aligned(32)));
-    c16_t srs_estimated_channel_time[nb_antennas_rx][N_ap][ofdm_symbol_size] __attribute__((aligned(32)));
+    c16_t srs_estimated_channel_time[nb_antennas_rx][N_ap][NR_SRS_IDFT_OVERSAMP_FACTOR * ofdm_symbol_size]
+        __attribute__((aligned(32)));
 
     nr_srs_rx_procedures(gNB,
                          frame_rx,