How does SIB24 affect 5G Standalone?

How does SIB24 affect 5G Standalone?

My friend: Hi Ibrahim, I have a big problem.

Me: What is the problem?

My friend: As you know, we started the 5G Standalone (SA) trail on one cluster and we have a strange behavior as our test UE can’t camp on 5G idle mode at all and still kept on 4G idle mode, although all neighbors are defined from both sides, 4G and 5G.

Me: Did you enable SIB24 from 4G side?

My friend: Excuse me, but what is SIB24?

Me: SIB24 (System Information Block 24) is configured at the 4G side to broadcast all the cell reselection info for the 5G UE, so it can perform cell reselection from 4G towards 5G, so SIB24 allows up to 8 5G carriers to be specified using their ARFCN. These ARFCNs correspond to the center frequency of the SSB Blocks.

My friend: But this SIB24 is not enabled in our 5G NSA network?

Me: Yes, that’s right. As in 5G NSA, there is nothing called 5G idle mode and always the UE in idle mode is camped on the 4G network and if 5G service is needed, then B1 measurement should be started followed by SgNB addition process.

My friend: Ok, I got your point, let me check it.

After a few hours….

My friend: You are right, SIB24 was disabled and after enabling it, cell reselection is working properly from 4G towards 5G. Thank you very much.

Me: You are most welcome.

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