NR_UE: make NTN parameter ta_CommonDrift_r17 configurable via command line parameter --ntn-ta-commondrift

While we don't have SIB19 support yet, UE gets this parameter from command line, just like --ntn-koffset and --ntn-ta-common.
We use this value to initialize the UE's time-tracking PI controller accumulator.
This allows to correctly track the time even if the delay is changing significantly (e.g. due to LEO satellite movement).
We also use it to compensate for the time drift that happenes while performing the initial sync.

executables/nr-ue.c

@@ -886,7 +886,9 @@ void *UE_thread(void *arg)
       stream_status = STREAM_STATUS_SYNCING;
       syncInFrame(UE, &sync_timestamp, intialSyncOffset);
       openair0_write_reorder_clear_context(&UE->rfdevice);
-      shiftForNextFrame = 0; // will be used to track clock drift
+      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);
       // read in first symbol
       AssertFatal(fp->ofdm_symbol_size + fp->nb_prefix_samples0
                       == UE->rfdevice.trx_read_func(&UE->rfdevice,