UE taking too much time to return to LTE after CSFB to 3G

Hello Experts,
In LTE network, when UE performs CSFB to 3G to have a call it is taking some time (more than 20s) to return to LTE.
What are the usual causes?
Other than basic parameters tweakings…
Vendor is Huawei.

Is Fast Return to LTE switch ON?

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Yes, it is.

On one note I am suffering frequent E-RAB failures due to no radio resources (expansion is upcoming).
This should affect the return to LTE, I suppose?

Only for one site or facing problem some other sites as well?

It is on multiple sites.

I see. How many LTE carrier do you have?

Two, both on 1800. (20 MHz + 10 MHz)

Check the CELLNFREQPRIOINFO at 3G end all LTE freq added against the sites.

In Nokia there is a timer in 3G to prevent returning to 4G during the value of the parameter.
Don’t know if exist the same in Huawei or not.

If it takes more than 20s, then it is not a CSFB.
Check the TAC / LAC relation in the Core side.
Maybe that relations was not properly created.

If it is 20s then return to LTE is not working, it’s cell reselection to LTE.
In 3G redirect LTE frequency and paramter mismatch.
Is RRC release with LTE frequency post call release?

Disable PS data in 3G when cause of handover is CSFB.
Sometimes UEs continue background data in 3G which causes issues with return to 4G.

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TAC is mapped with 2G LAC or 3G and also CSFB priority is on which tech?

By Defining LTE cell with 3G cell and same in cell resection too and reduce time parameters less than 20sec