NR_UE: add PI controller to time sync

this allows to compensate larger continuous time drift in DL (e.g. large Doppler from LEO satellite)

executables/nr-ue.c

@@ -947,7 +947,7 @@ void *UE_thread(void *arg)
     if (slot_nr == nb_slot_frame - 1) {
       // we shift of half of measured drift, at each beginning of frame for both rx and tx
       iq_shift_to_apply = shiftForNextFrame;
-      shiftForNextFrame = 0; // We will get a new measured offset if we decode PBCH
+      shiftForNextFrame = -round(UE->max_pos_acc * get_nrUE_params()->time_sync_I);
     }
 
     const int readBlockSize = get_readBlockSize(slot_nr, fp) - iq_shift_to_apply;

executables/nr-uesoftmodem.h

@@ -9,6 +9,8 @@
 #define  CONFIG_HLP_DLSCH_PARA             "number of threads for dlsch processing 0 for no parallelization\n"
 #define  CONFIG_HLP_OFFSET_DIV             "Divisor for computing OFDM symbol offset in Rx chain (num samples in CP/<the value>). Default value is 8. To set the sample offset to 0, set this value ~ 10e6\n"
 #define  CONFIG_HLP_MAX_LDPC_ITERATIONS    "Maximum LDPC decoder iterations\n"
+#define  CONFIG_HLP_TIME_SYNC_P            "coefficient for Proportional part of time sync PI controller\n"
+#define  CONFIG_HLP_TIME_SYNC_I            "coefficient for Integrating part of time sync PI controller\n"
 #define  CONFIG_HLP_NTN_KOFFSET            "NTN cellSpecificKoffset-r17 (number of slots for a given subcarrier spacing of 15 kHz)\n"
 #define  CONFIG_HLP_NTN_TA_COMMON          "NTN ta-Common, but given in ms\n"
 
@@ -59,6 +61,8 @@
   {"ue-timing-correction-disable", CONFIG_HLP_DISABLETIMECORR, PARAMFLAG_BOOL,  .iptr=&(nrUE_params.no_timing_correction),   .defintval=0,      TYPE_INT,      0}, \
   {"SLC",                          CONFIG_HLP_SLF,             0,               .u64ptr=&(sidelink_frequency[0][0]),         .defuintval=2600000000,TYPE_UINT64,0}, \
   {"num-ues",                      NULL,                       0,               .iptr=&(NB_UE_INST),                         .defuintval=1,     TYPE_INT,      0}, \
+  {"time-sync-P",                  CONFIG_HLP_TIME_SYNC_P,     0,               .dblptr=&(nrUE_params.time_sync_P),          .defdblval=0.5,    TYPE_DOUBLE,   0}, \
+  {"time-sync-I",                  CONFIG_HLP_TIME_SYNC_I,     0,               .dblptr=&(nrUE_params.time_sync_I),          .defdblval=0.2,    TYPE_DOUBLE,   0}, \
   {"ntn-koffset",                  CONFIG_HLP_NTN_KOFFSET,     0,               .uptr=&(nrUE_params.ntn_koffset),            .defuintval=0,     TYPE_UINT,     0}, \
   {"ntn-ta-common",                CONFIG_HLP_NTN_TA_COMMON,   0,               .dblptr=&(nrUE_params.ntn_ta_common),        .defdblval=0.0,    TYPE_DOUBLE,   0}, \
   {"agc",                          CONFIG_HLP_AGC,             PARAMFLAG_BOOL,  .iptr=&(nrUE_params.agc),                   .defintval=0,       TYPE_INT,   0}, \
@@ -82,6 +86,8 @@ typedef struct {
   int N_RB_DL;
   int ssb_start_subcarrier;
   int ldpc_offload_flag;
+  double time_sync_P;
+  double time_sync_I;
   unsigned int ntn_koffset;
   double ntn_ta_common;
   int agc;

openair1/PHY/NR_UE_ESTIMATION/nr_adjust_synch_ue.c

@@ -24,7 +24,7 @@
 #include "PHY/NR_UE_ESTIMATION/nr_estimation.h"
 #include "PHY/impl_defs_top.h"
 
-#include "executables/softmodem-common.h"
+#include "executables/nr-uesoftmodem.h"
 #include "common/utils/LOG/vcd_signal_dumper.h"
 
 //#define DEBUG_PHY
@@ -43,12 +43,9 @@ int nr_adjust_synch_ue(NR_DL_FRAME_PARMS *frame_parms,
                        short coef)
 {
   int max_val = 0, max_pos = 0;
-  uint8_t sync_offset = 0;
 
   VCD_SIGNAL_DUMPER_DUMP_FUNCTION_BY_NAME(VCD_SIGNAL_DUMPER_FUNCTIONS_UE_ADJUST_SYNCH, VCD_FUNCTION_IN);
 
-  short ncoef = 32767 - coef;
-
   // search for maximum position within the cyclic prefix
   for (int i = -frame_parms->nb_prefix_samples/2; i < frame_parms->nb_prefix_samples/2; i++) {
     int temp = 0;
@@ -67,32 +64,37 @@ int nr_adjust_synch_ue(NR_DL_FRAME_PARMS *frame_parms,
   }
 
   // filter position to reduce jitter
-  ue->max_pos_avg = ((ue->max_pos_avg * coef) >> 15) + (max_pos * ncoef);
-
-  int diff = ue->max_pos_avg >> 15;
+  const int ncoef = 32767 - coef;
+  ue->max_pos_iir = ((ue->max_pos_iir * coef) >> 15) + (max_pos * ncoef);
+  const int diff = (ue->max_pos_iir + 16384) >> 15;
 
+  // FIXME: Do we really need this hysteresis for FR2?
+  int sampleShift = diff;
   if (frame_parms->freq_range == FR2)
-    sync_offset = 2;
-  else
-    sync_offset = 0;
+    if (abs(diff) <= 2)
+      sampleShift = 0;
 
-  int sampleShift = 0;
-  if (abs(diff) > (NR_SYNCH_HYST + sync_offset))
-    sampleShift = diff;
-
-  const int sample_shift = -(sampleShift / 2);
-  // reset IIR filter for next offset calculation
-  ue->max_pos_avg += sample_shift * 32768;
+  // PI controller
+  const double PID_P = get_nrUE_params()->time_sync_P;
+  const double PID_I = get_nrUE_params()->time_sync_I;
+  int sample_shift = -round(sampleShift * PID_P + ue->max_pos_acc * PID_I);
 
   LOG_D(PHY,
-        "Slot %d: diff = %i, rx_offset (final) = %i : max_pos = %d, max_pos filtered = %ld, max_power = %d\n",
+        "Frame %d, Slot %d: max_pos = %d, max_pos filtered = %f, diff = %i, sampleShift = %i, max_pos_acc = %d, sample_shift (final) = %d, max_power = %d\n",
+        frame,
         slot,
+        max_pos,
+        ue->max_pos_iir / 32768.0,
         diff,
         sampleShift,
-        max_pos,
-        ue->max_pos_avg,
+        ue->max_pos_acc,
+        sample_shift,
         max_val);
 
+  // reset IIR filter for next offset calculation
+  ue->max_pos_iir += -round(sampleShift * PID_P) * 32768;
+  ue->max_pos_acc += max_pos;
+
   VCD_SIGNAL_DUMPER_DUMP_FUNCTION_BY_NAME(VCD_SIGNAL_DUMPER_FUNCTIONS_UE_ADJUST_SYNCH, VCD_FUNCTION_OUT);
   return sample_shift;
 }

openair1/PHY/NR_UE_ESTIMATION/nr_estimation.h

@@ -29,9 +29,6 @@
  * @{
  */
 
-/*!\brief Timing drift hysterisis in samples*/
-#define NR_SYNCH_HYST 1
-
 /* A function to perform the channel estimation of DL PRS signal */
 int nr_prs_channel_estimation(uint8_t gNB_id,
                               uint8_t rsc_id,

openair1/PHY/defs_nr_UE.h

@@ -456,7 +456,8 @@ typedef struct PHY_VARS_NR_UE_s {
   /// temporary offset during cell search prior to MIB decoding
   int              ssb_offset;
   uint16_t symbol_offset; /// offset in terms of symbols for detected ssb in sync
-  int64_t max_pos_avg; /// Timing offset IIR filter
+  int64_t max_pos_iir; /// Timing offset IIR filter
+  int max_pos_acc; /// Timing offset accumuluated error for PI filter
 
   /// Timing Advance updates variables
   /// Timing advance update computed from the TA command signalled from gNB