How network knows 3G Cell that UE will CS fallback?

4G LTE
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1

Hello Telecom Community.

I have a question about one of the CSFB Features.

In Flash CSFB the Network sends the SI of the 3G Cell with the Release Message to reduce the Call Setup Time.

The question is, how could the network know the 3G Cell that I’d camp on before the Release?

I think it needs some measurements to choose the best 3G Cell and send its SI.

So how could we use Flash CSFB with Blind Redirection which is commonly used?

In Blind Redirection the UE would choose the 3G Cell after the Release.

Any Help?

How network knows 3G Cell that UE will CS fallback

2

In Flash CSFB, RRC Connection release message includes information about a target UTRAN frequency, PSC and their associated system information, In this way, the UE can quickly access the target UTRAN without the need to perform the procedure for acquiring system information of the target UTRAN cell.

In CSFB eNodeB sends an RRC Connection release contains on target UTRAN frequency however procedure of acquiring system information enhanced the time of CSFB.

Flash CSFB uses RIM (RAN Information Management) to exchange the information between the E-UTRAN and UTRAN/GERAN through the core networks.

With this function, SIB can be included into the “RRC Connection Release” during the redirection procedure. This is achieved by the RIM procedure. With RIM procedure, eNodeB can get information from UTRAN/GERAN.

3

Thanks, but I’m still confused about it…

How could the eNodeB know the Target UTRAN Frequency with measurements?

4

Through RIM, 3G-4G exchange information continuously and in the RRC Connection release message includes information about a target UTRAN frequency, PSC and their associated system information.

5

The CSFB can be configured in 2 ways: Measured and Blind.

In case of the Blind, the UE has no prior information about the exact cell the it will camp-on on the UTRAN NW, instead it will have a cell list (configured in the NBR list and sent using the SI to the UE).

The Measured one, the UE will inter the GAP period to search for the better RSCP from the NBR list first, then it will make the CSFB.

The difference is that, the Blind is faster, but no guarantee that the UE will camp on UTRAN cell successfully.

The Measured one is better in maintaining good CSFB rate, because it will know for sure / prepare the resources before the HO takes place.

The signaling in the Blind happens in the A4 event, while in the measured it’s the B1 Event.

6

Thank you so much!

Good explanation thank you!