NR to LTE Interworking. Handover failure Due to Incomplete CGI Reporting During FTP Test (PLMN 502-153)
Scenario:
During a 5G SA to LTE interworking scenario, the UE failed to hand over from the 5G SA network to a shared LTE MOCN site.
Handover Conditions:
On the 5G SA side, the UE successfully triggered Event B2, meeting all inter-RAT., but no handover command from gNB to UE.
Handover criteria:
Serving 5G SS-RSRP: –118 dBm (B2 Threshold1: –107 dBm)
Neighbor LTE RSRP : –106 dBm (B2 Threshold2: –125 dBm)
Issue Observed:
Although the LTE neighbor’s SIB1 was broadcasting both PLMN IDs — 502-12 (Operator A) and 502-153 (Operator B) — the UE only reported a partial CGI, including only PLMN 502-12.
From analysis, ANR created the LTE neighbor relation under PLMN 502-12 only.
The additional PLMN (502-153) was not automatically added.
Consequently, the gNB was unable to identify a valid LTE target under its own PLMN (502-153), and no handover command sent to UE.
Root Cause:
The failure was due to UE behavior. The UE did not decode and report the completed CGI, specifically PLMN 502-153, even though it was properly broadcasted by the LTE cell in SIB1.
Solution:
The issue was resolved by manually updating the neighbor relation on the gNB to include PLMN 502-153.
If the UE had reported the full CGI, ANR would have automatically updated the neighbor relation for both PLMN.
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