RM520N-GL fails to establish PDU Session on private 5G SA network (OAI) — firmware RM520NGLAAR03A03M4G_A0.301.A0.301

Technical Subjects Driver
1

Hello,

We are running a private 5G Standalone (SA) testbed based on OpenAirInterface (OAI), and we are experiencing a consistent failure with our Quectel RM520N-GL modules that does not occur with other UE models (Quectel RM500Q-GL) on the exact same gNB, same physical setup, and same core network.

Module and firmware

  • Model: RM520N-GL

  • Firmware: RM520NGLAAR03A03M4G_A0.301.A0.301

  • Antenna configuration: 4 antennas connected (no diversity/antenna issue)

  • Interface: MBIM

Symptom The module successfully attaches to the packet service, but fails when establishing the PDU session:

---- mbimcli -p -d /dev/cdc-wdm0 --set-radio-state=on
Hardware radio state: 'on'
Software radio state: 'on'

---- mbimcli -p -d /dev/cdc-wdm0 --attach-packet-service
Packet service status:
  Network error: 'none'
  Packet service state: 'attached'
  Available data classes: 'custom'

---- mbimcli -p -d /dev/cdc-wdm0 --connect=session-id=0,access-string=internet,ip-type=ipv4
error: operation failed: Failure

AT command diagnostics after the failed connection attempt:

AT+CEREG?
+CEREG: 0,0

AT+C5GREG?
+C5GREG: 0,3

AT+CEER
+CEER: No cause information available

AT+C5GREG=2
OK
AT+C5GREG?
+C5GREG: 2,3

AT+CGDCONT?
+CGDCONT: 1,"IP","internet","0.0.0.0",0,0,0,0,,,,,,,,,,"",,,,0

AT+CGATT?
+CGATT: 1

+C5GREG reports status 3 (“registration denied”), but AT+CEER returns no cause information, so we cannot determine whether this is a genuine NAS-level rejection from the network or an internal modem/firmware state.

Network-side observation (gNB logs) On the gNB side (OpenAirInterface, USRP N300, band n78, 106 PRB, 30kHz SCS, TDD 7D2U), the RM520N-GL successfully completes Random Access and RRC Setup (RRC_CONNECTED reached), but then repeatedly fails to deliver RRC/NAS signaling reliably over SRB1:

[RLC] E max RETX reached on SRB 1
[RLC] W UE <RNTI>: RLF detected, but no callable RLF handler registered

This repeats continuously (every ~46ms) until the MAC scheduler declares UL failure after the configured PUSCH DTX threshold and releases the UE context. The module never reaches NAS Registration Complete / PDU Session Establishment, and therefore never obtains a DRB (LCID 4).

Uplink radio statistics for the RM520N-GL show it operating at maximum PRB allocation (NPRB 104, i.e. full channel bandwidth) with a UL SNR plateauing right at the configured target (~20.3 dB) and a very high uplink miss-detection rate (ulsch_DTX ~50/rounds, BLER ~0.7), while other UE models (RM500Q-GL) on the same gNB and same physical position operate normally at NPRB 5 with stable SNR (~23-24 dB) and negligible errors.

What we have ruled out

  • Physical antenna connection and cabling (identical setup and position to the working RM500Q-GL units, 4 antennas connected)

  • gNB scheduler/PDCCH configuration (already tuned for multi-UE robustness; other UE models connect and hold PDU sessions reliably under the same configuration)

  • Core network NSSAI/subscription provisioning (verified correct and working for other UEs on the same PLMN/DNN)

Question Given that other users on the Quectel forum report similar 5G SA connection issues on this same firmware branch (RM520NGLAAR03A03M4G), and that Quectel support has previously provided updated firmware via email in those threads, could you please:

  1. Confirm whether RM520NGLAAR03A03M4G_A0.301.A0.301 has known issues with 5G SA PDU session establishment or uplink link budget in TDD configurations.

  2. Provide the latest available firmware for this module (currently on RM520NGLAAR03A03M4G_A0.301.A0.301).

Thank you for your assistance.

Best regards,

Thierry