UE access procedure is as below:
Step 1: UE is Switched ON
Step 2: PLMN Selection
PLMN stands for Public Land Mobile Network, will be established by the network and is used to provide land mobile communication services to public.
PLMN = MCC + MNC
IMSI: International MObile SUbscriber Identification Number.
IMSI = MCC + MNC + MSIN
MCC: Mobile Country Code
MNC: Mobile Network Code
MSIN: Mobile Subscriber Identification Number
Different types of PLMN
RPLMN: Registered PLMN
EPLMN: Equivalent PLMN
HPLMN: Home PLMN
EHPLMN: Equivalent Local PLMN
VPLMN: Visited PLMN
UPLMN: User Controlled PLMN
PLMN Identification Process:
1. When UE is switched on, it will listen to FCCH, Frequency Correction Channel, used to tune to the corresponding frequency.
2. Then it will listen to SCH, Synchronization Channel, used to get the time synchronization.
3. If the selected frequency is not suitable, then it will select the next strongest signal.
PLMN Selection Process:
The selection process is of 2 types.
1. If the UE has no prior information, then UE will perform full band search. Then it will search for the strongest cell on each frequency point and report it to the UE NAS layer. Then NAS layer will decide on PLMN search.
2. If UE has prior information, NAS layer informs AS layer to perform a PLMN search according to previous information stored.
Once the PLMN is selected, cell selection is performed to select the cell with best signal in PLMN.
In cell search, UE will have time and frequency synchronization in downlink direction.
Cell search used SSB. SSB has 2 parts PSSS and SSS. PSS/SSS and PBCH all-together is called as Synchronization Signal Block SSB.
UE achieves downlink synchronization by doing cell search procedure.
UE achieves uplink synchronization by doing Random Access procedure.
UE Attach Procedure