NR_UE: Perform autonomous timing advance to compensate for fast time drift with relatively large RTT

As we don't have SIB19 with continuously updated ta-Common and ta-CommonDrift values yet, the UE can only adapt it's UL timing according to it's DL drift.
This is required for the UE to keep UL sync with fast time drift and relatively long RTT (e.g. LEO satellite).
We should replace this with the standard compliant way as soon as we have SIB19 with continuously updated contents.

executables/nr-ue.c

@@ -889,6 +889,9 @@ void *UE_thread(void *arg)
       UE->max_pos_acc = get_nrUE_params()->ntn_ta_commondrift * 1e-6 * fp->samples_per_frame / get_nrUE_params()->time_sync_I; // ntn_ta_commondrift is in µs/s, max_pos_acc * time_sync_I is in samples/frame
       shiftForNextFrame = -(UE->init_sync_frame + trashed_frames + 2) * UE->max_pos_acc * get_nrUE_params()->time_sync_I; // compensate for the time drift that happened during initial sync
       LOG_D(PHY, "max_pos_acc = %d, shiftForNextFrame = %d\n", UE->max_pos_acc, shiftForNextFrame);
+      // TODO: remove this autonomous TA and use up-to-date values of ta-Common, ta-CommonDrift and ta-CommonDriftVariant from received SIB19 instead
+      if (get_nrUE_params()->autonomous_ta)
+        UE->timing_advance -= 2 * shiftForNextFrame;
       // read in first symbol
       AssertFatal(fp->ofdm_symbol_size + fp->nb_prefix_samples0
                       == UE->rfdevice.trx_read_func(&UE->rfdevice,
@@ -949,6 +952,9 @@ 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;
+      // TODO: remove this autonomous TA and use up-to-date values of ta-Common, ta-CommonDrift and ta-CommonDriftVariant from received SIB19 instead
+      if (get_nrUE_params()->autonomous_ta)
+        UE->timing_advance -= 2 * shiftForNextFrame;
       shiftForNextFrame = -round(UE->max_pos_acc * get_nrUE_params()->time_sync_I);
     }