Add some comments on the code and update documentation

doc/FEATURE_SET.md

@@ -307,8 +307,10 @@ The following features are valid for the gNB and the 5G-NR UE.
 - MAC <-> PHY data interface using FAPI P7 interface for BCH PDU, DCI PDU, PDSCH PDU
 - Scheduler procedures for SIB1
 - Scheduler procedures for RA
-  - Contention free RA procedure
-  - Contention based RA procedure
+  - Contention Free RA procedure
+  - Contention Based RA procedure
+    - Msg3 can transfer uplink CCCH, DTCH or DCCH messages
+    - CBRA can be performed using MAC CE or C-RNTI
 - Scheduler procedures for CSI-RS
 - MAC downlink scheduler
   - phy-test scheduler (fixed allocation and usable also without UE)

openair2/LAYER2/NR_MAC_gNB/gNB_scheduler_ulsch.c

@@ -824,13 +824,17 @@ void nr_rx_sdu(const module_id_t gnb_mod_idP,
             ra->Msg4_slot = 1;
             
             if (ra->msg3_dcch_dtch) {
-              // Check if the C-RNTI still exists in the network
+              // Check if the UE identified by C-RNTI still exists at the gNB
               int UE_id_C = find_nr_UE_id(gnb_mod_idP, ra->crnti);
               if (UE_id_C < 0) {
+                // The UE identified by C-RNTI no longer exists at the gNB
+                // Let's abort the current RA, so the UE will trigger a new RA later but using RRCSetupRequest instead. A better solution may be implemented
                 mac_remove_nr_ue(gnb_mod_idP, ra->rnti);
                 nr_clear_ra_proc(gnb_mod_idP, CC_idP, frameP, ra);
                 return;
               } else {
+                // The UE identified by C-RNTI still exists at the gNB
+                // Reset uplink failure flags/counters/timers at MAC and at RRC so gNB will resume again scheduling resources for this UE
                 UE_info->UE_sched_ctrl[UE_id_C].pusch_consecutive_dtx_cnt = 0;
                 UE_info->UE_sched_ctrl[UE_id_C].ul_failure = 0;
                 nr_mac_gNB_rrc_ul_failure_reset(gnb_mod_idP, frameP, slotP, ra->crnti);