gNB: fix MAC TA command scheduling if measurement gaps are configured

In the current implementation, if UE transmission is interrupted via nr_mac_interrupt_ue_transmission(),
UE_sched_ctrl.ta_frame is modified to be the previous frame avoiding TA scheduling in the measurement gap.
So the next scheduled TA is shifted to be in 99 frames. As there are other measurement gaps before that,
the scheduled TA is always shifted, but actually never reached, effectively disabling the MAC TA commands.

To fix this, UE_sched_ctrl.ta_frame is changed to not indicate the last TA command transmission, but the
next one. This allows to check in nr_mac_interrupt_ue_transmission() if the next TA command transmission
is within the measurement gap and if so shift it to just after that gap.

openair2/LAYER2/NR_MAC_gNB/gNB_scheduler_dlsch.c

@@ -649,9 +649,8 @@ static void pf_dl(gNB_MAC_INST *mac,
     stats->current_bytes = 0;
     stats->current_rbs = 0;
 
-    /* Check if this UE should get TA (every 100 frames). If we add the CE,
-     * ta_apply will be reset */
-    if (frame == ((sched_ctrl->ta_frame + 100) % 1024))
+    /* Check if this UE should get TA. If we add the CE, ta_apply will be reset */
+    if (frame == sched_ctrl->ta_frame)
       sched_ctrl->ta_apply = true;
 
     int total_rem_ues = 0;
@@ -690,7 +689,7 @@ static void pf_dl(gNB_MAC_INST *mac,
       update_dlsch_buffer(pp_pdsch->frame, pp_pdsch->slot, UE);
 
       /* Check DL buffer and skip this UE if no bytes and no TA necessary */
-      if (sched_ctrl->num_total_bytes == 0 && frame != (sched_ctrl->ta_frame + 100) % 1024)
+      if (sched_ctrl->num_total_bytes == 0 && sched_ctrl->ta_apply == false)
         continue;
 
       /* Calculate coeff */
@@ -871,8 +870,8 @@ static void pf_dl(gNB_MAC_INST *mac,
     // awaiting. Therefore, for the time being, we put a fixed overhead of 12
     // (for 4 PDUs) and optionally + 2 for TA. Once RLC gives the number of
     // PDUs, we replace with 3 * numPDUs
-    const int oh = 3 * 4 + 2 * (frame == (sched_ctrl->ta_frame + 100) % 1024);
-    //const int oh = 3 * sched_ctrl->dl_pdus_total + 2 * (frame == (sched_ctrl->ta_frame + 100) % 1024);
+    const int oh = 3 * 4 + (sched_ctrl->ta_apply ? 2 : 0);
+    //const int oh = 3 * sched_ctrl->dl_pdus_total + (sched_ctrl->ta_apply ? 2 : 0);
     nr_find_nb_rb(sched_pdsch.Qm,
                   sched_pdsch.R,
                   1, // no transform precoding for DL
@@ -1349,8 +1348,9 @@ void post_process_dlsch(gNB_MAC_INST *nr_mac, post_process_pdsch_t *pdsch, NR_UE
     // ta command is sent, values are reset
     if (sched_ctrl->ta_apply) {
       sched_ctrl->ta_apply = false;
-      sched_ctrl->ta_frame = frame;
-      LOG_D(NR_MAC, "%d.%2d UE %04x TA scheduled, resetting TA frame\n", frame, slot, UE->rnti);
+      sched_ctrl->ta_update = 31;
+      sched_ctrl->ta_frame = (frame + 100) % MAX_FRAME_NUMBER;
+      LOG_D(NR_MAC, "%d.%2d UE %04x TA scheduled, setting next TA frame to %d\n", frame, slot, UE->rnti, sched_ctrl->ta_frame);
     }
 
     T(T_GNB_MAC_DL_PDU_WITH_DATA, T_INT(module_id), T_INT(CC_id), T_INT(rnti),

openair2/LAYER2/NR_MAC_gNB/gNB_scheduler_primitives.c

@@ -125,6 +125,8 @@ static const uint16_t cqi_table3[16][2] = {{0, 0},
 static void determine_aggregation_level_search_order(int agg_level_search_order[NUM_PDCCH_AGG_LEVELS],
                                                      float pdcch_cl_adjust);
 
+static int nr_mac_interrupt_ue_transmission(gNB_MAC_INST *mac, NR_UE_info_t *UE, int slots, int slots_per_frame);
+
 uint8_t get_dl_nrOfLayers(const NR_UE_sched_ctrl_t *sched_ctrl, const nr_dci_format_t dci_format)
 {
   // TODO check this but it should be enough for now
@@ -3435,7 +3437,7 @@ void nr_measgap_scheduling(gNB_MAC_INST *nr_mac, frame_t frame, sub_frame_t slot
       continue;
 
     const int slots_frame = nr_mac->frame_structure.numb_slots_frame;
-    const frame_t f = (frame + (slot + mgc->n_slots_advance) / slots_frame) % 1024;
+    const frame_t f = (frame + (slot + mgc->n_slots_advance) / slots_frame) % MAX_FRAME_NUMBER;
     const slot_t s = (slot + mgc->n_slots_advance) % slots_frame;
 
     // TS 38 331 - Section 5.5.2.9 Measurement gap configuration
@@ -3446,7 +3448,7 @@ void nr_measgap_scheduling(gNB_MAC_INST *nr_mac, frame_t frame, sub_frame_t slot
     // start a timer to stop scheduling UE during MeasGap, or extend timer for
     // duration of measGap with existing follow-up action
     if (!nr_timer_is_active(t) || nr_timer_remaining_time(t) < mgc->mgl_slots) {
-      nr_mac_interrupt_ue_transmission(nr_mac, UE, mgc->mgl_slots);
+      nr_mac_interrupt_ue_transmission(nr_mac, UE, mgc->mgl_slots, slots_frame);
     }
   }
 }
@@ -3530,7 +3532,7 @@ int nr_mac_get_reconfig_delay_slots(NR_SubcarrierSpacing_t scs)
   return (delay_ms << scs) + sl_ahead;
 }
 
-int nr_mac_interrupt_ue_transmission(gNB_MAC_INST *mac, NR_UE_info_t *UE, int slots)
+static int nr_mac_interrupt_ue_transmission(gNB_MAC_INST *mac, NR_UE_info_t *UE, int slots, int slots_per_frame)
 {
   DevAssert(mac != NULL);
   DevAssert(UE != NULL);
@@ -3539,11 +3541,11 @@ int nr_mac_interrupt_ue_transmission(gNB_MAC_INST *mac, NR_UE_info_t *UE, int sl
   nr_timer_setup(&UE->UE_sched_ctrl.transm_interrupt, slots, 1);
   nr_timer_start(&UE->UE_sched_ctrl.transm_interrupt);
 
-  // it might happen that timing advance command should be sent during the UE
-  // inactivity time. To prevent this, set a variable as if we would have just
-  // sent it. This way, another TA command will for sure be sent in some
-  // frames, after the inactivity of the UE.
-  UE->UE_sched_ctrl.ta_frame = (mac->frame - 1 + 1024) % 1024;
+  // it might happen that timing advance command should be sent during the UE inactivity time.
+  // To prevent this, delay next TA command just after the UE inactivity time.
+  const int inactive_frames = slots / slots_per_frame + 1;
+  if ((UE->UE_sched_ctrl.ta_frame - mac->frame + MAX_FRAME_NUMBER) % MAX_FRAME_NUMBER < inactive_frames)
+    UE->UE_sched_ctrl.ta_frame = (mac->frame + inactive_frames) % MAX_FRAME_NUMBER;
 
   LOG_D(NR_MAC, "UE %04x: Interrupt UE transmission (%d slots)\n", UE->rnti, slots);
   return 0;

openair2/LAYER2/NR_MAC_gNB/gNB_scheduler_ulsch.c

@@ -811,7 +811,7 @@ static void nr_rx_ra_sdu(const module_id_t mod_id,
   }
 
   // re-initialize ta update variables after RA procedure completion
-  UE->UE_sched_ctrl.ta_frame = frame;
+  UE->UE_sched_ctrl.ta_frame = (frame + 100) % MAX_FRAME_NUMBER;
 
   LOG_A(NR_MAC, "%4d.%2d PUSCH with TC_RNTI 0x%04x received correctly\n", frame, slot, rnti);
 

openair2/LAYER2/NR_MAC_gNB/mac_proto.h

@@ -65,7 +65,6 @@ void nr_mac_prepare_ra_ue(gNB_MAC_INST *nrmac, NR_UE_info_t *UE);
 bool add_new_UE_RA(gNB_MAC_INST *nr_mac, NR_UE_info_t *UE);
 int nr_mac_get_reconfig_delay_slots(NR_SubcarrierSpacing_t scs);
 
-int nr_mac_interrupt_ue_transmission(gNB_MAC_INST *mac, NR_UE_info_t *UE, int slots);
 int nr_transmission_action_indicator_stop(gNB_MAC_INST *mac, NR_UE_info_t *UE_info);
 
 void clear_nr_nfapi_information(gNB_MAC_INST *gNB,