At power on,how&when does UE decides/gets to know which is the Preferred RAT to camp onto? Is this info stored in UE?

Question

When UE is powered on, it first scans for all supported RF channels. And then list the channels whose RSSI is greater than a set threshold. For all cells satisfying this criteria, the PSS, SSS detection-> PCFICH -> MIB detection is done.
I would like to understand the term "preferred Rat" to camp on to a cell. How/when does the UE gets this information? Is this a stored info from USIM .. or from somewhere else?

Answer 1

When the UE powers on, it checks if it is possible to camp on the last registered PLMN (Public Land Mobile Network). If no any stored system on the UE, UE will perform the initial cell selection:
1. Scan all RF frequencies band (ex 900MHz,1800MHz,1900MHz….) and which band are using by network
2. Detect the operating frequency with the strongest RSRP

I am not sure if I have answered your query, please comment -

Comments

Hi Salil .. Thank you for the reply. But I am looking for something else.
 - In a triple mode UE (2G,3G & LTE) the preferred RAT is set to LTE for eg at either USIM or is it hard coded in UE implementation?
 - And at what stage of the power on procedure this information is extracted to decide on the preferred RAT?

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UE will try to search HPLMN or the preferred PLMN. It is stored on USIM, (see 3GPP 31.102 for details)