RA procedure fail when connecting multiple UEs simultaneously
The issue is, failing to establish RA procedure for 2 UEs that are being started simultaneously.
The reason for this failure is due to the 2 UEs sending preambles in the same frame and slot leading to an unsuccessful RA procedure, because neither RA process finishes successfully.
As a result, UEs will restart the procedure from the top, resulting in longer time to establish connection.
To visualize this issue, simply start the CN and the gNB using the Docker file provided in this file openairinterface5g/ci-scripts/yaml_files/5g_rfsimulator. Then, start at least 2 UEs at the same time. The greater the number of UEs connected simultaneously, the more this issue will be reproduced, requiring additional time to complete all connections.
[0m17637.561907 [NR_PHY] I [gNB 0][RAPROC] Frame 839, slot 19 Initiating RA procedure with preamble 31, energy 54.9 dB (I0 0, thres 200), delay 0 start symbol 0 freq index 0
[0m17637.561972 [NR_PHY] I [gNB 0][RAPROC] Frame 839, slot 19 Initiating RA procedure with preamble 19, energy 56.4 dB (I0 66, thres 200), delay 0 start symbol 4 freq index 0
[0m17637.562070 [NR_MAC] I [gNB 0][RAPROC] CC_id 0 Frame 839 Activating Msg2 generation in frame 840, slot 7 using RA rnti 10b SSB, new rnti 644b index 0 RA index 0
[0m17637.562203 [NR_MAC] I [gNB 0][RAPROC] CC_id 0 Frame 839 Activating Msg2 generation in frame 840, slot 7 using RA rnti 10b SSB, new rnti a6f8 index 0 RA index 1
[0m[32m17637.562507 [NR_MAC] A [gNB 0][RAPROC] CC_id 0 Frame 840, slotP 7: Generating RA-Msg2 DCI, rnti 0x10b, state 1, CoreSetType 0
[0m17637.562517 [NR_MAC] I [RAPROC] Msg3 slot 17: current slot 7 Msg3 frame 840 k2 7 Msg3_tda_id 3
[0m[1;31m17637.562526 [NR_MAC] E cannot find free CCE for Msg.2 of RA RNTI 0xa6f8!
[0m17637.568245 [NR_MAC] I [gNB 0][RAPROC] Frame 841, Slot 10 : CC_id 0 Scheduling retransmission of Msg3 in (841,17)
[0m17637.573767 [NR_MAC] I [gNB 0][RAPROC] Frame 842, Slot 10 : CC_id 0 Scheduling retransmission of Msg3 in (842,17)
[0m17637.579062 [NR_MAC] I [gNB 0][RAPROC] Frame 843, Slot 10 : CC_id 0 Scheduling retransmission of Msg3 in (843,17)
[0m[93m17637.582664 [NR_MAC] W Random Access 0 failed at state 2 (Reached msg3 max harq rounds)
[0m[1;31m17637.582679 [NR_MAC] E no RA proc for RNTI 0x644b in Msg.3/PUSCHAbove is an example of a gNB log showing 2 UEs trying to connect and failing.