NR_UE: move OFDM symbol cut position handling to nr_slot_fep()

this slightly cleans up nr_initial_sync() and UE_thread()

executables/nr-ue.c

@@ -656,7 +656,7 @@ void *UE_thread(void *arg) {
 
     // use previous timing_advance value to compute writeTimestamp
     writeTimestamp = timestamp + UE->frame_parms.get_samples_slot_timestamp(slot_nr, &UE->frame_parms,DURATION_RX_TO_TX - RX_NB_TH) -
-                     firstSymSamp - openair0_cfg[0].tx_sample_advance - UE->N_TA_offset - timing_advance + UE->frame_parms.nb_prefix_samples / 8;
+                     firstSymSamp - openair0_cfg[0].tx_sample_advance - UE->N_TA_offset - timing_advance;
 
     // but use current UE->timing_advance value to compute writeBlockSize
     if (UE->timing_advance != timing_advance) {

openair1/PHY/MODULATION/slot_fep_nr.c

@@ -106,6 +106,9 @@ int nr_slot_fep(PHY_VARS_NR_UE *ue,
     rx_offset += (idx_symb%(0x7<<frame_parms->numerology_index)) ? nb_prefix_samples : nb_prefix_samples0;
   rx_offset += frame_parms->ofdm_symbol_size * symbol;
 
+  // use OFDM symbol from within 1/8th of the CP to avoid ISI
+  rx_offset -= nb_prefix_samples / 8;
+
 #ifdef DEBUG_FEP
   //  if (ue->frame <100)
   printf("slot_fep: slot %d, symbol %d, nb_prefix_samples %u, nb_prefix_samples0 %u, rx_offset %u\n",

openair1/PHY/NR_UE_TRANSPORT/nr_initial_sync.c

@@ -308,7 +308,6 @@ int nr_initial_sync(UE_nr_rxtx_proc_t *proc, PHY_VARS_NR_UE *ue, int n_frames)
         // every 7*(1<<mu) symbols there is a different prefix length (38.211 5.3.1)
         int n_symb_prefix0 = (ue->symbol_offset/(7*(1<<mu)))+1;
         sync_pos_frame = n_symb_prefix0*(fp->ofdm_symbol_size + fp->nb_prefix_samples0)+(ue->symbol_offset-n_symb_prefix0)*(fp->ofdm_symbol_size + fp->nb_prefix_samples);
-        sync_pos_frame += fp->nb_prefix_samples / 8; // shift sync position ahead by 1/8th of cyclic prefix, same as in nr_adjust_synch_ue()
         if (ue->ssb_offset < sync_pos_frame)
           ue->rx_offset = fp->samples_per_frame - sync_pos_frame + ue->ssb_offset;
         else