Hello Experts.
What does it mean by cell selection in simple language after power ON?
It means that once UE is powered on starts selecting a suitable cell to camp on.
What does mean by selection?
Let me tell you my doubt.
As we know for cell selection a cell must fulfill criteria of “qRxlevMin” broadcasted in SIB1 in 5G.
But UE has just power ON and doesn’t aware about this threshold for cell selection as it still need to decode SIB1 which is only possible after selecting that cell.
So what criteria comes here for cell selection?
And when that threshold qRxLevMin broadcasted in SIB1 will be use for selection?
Cell selection doesn’t happen just at power ON.
It also happens after each call release.
Ok.
So here S-criteria (SrxLev>0) will not come in picture during initial cell selection?
Selection and Reselection seem practically the same yet somehow different.
The way I remember it is, to connect UE cell select, once connected and to move around it cell reselect.
When the UE is switched on, the UE will select the carrier frequency with highest RSSI because at that point UE has no idea about the RSRP.
It will simply read the PSS and SSS of that carrier and calculate the PCI and then decode Reference signal based PCI%3 or PCI%6.
After that it will read the MIB and SIB1, in SIB1 of that particular carrier it will know about the S criteria parameters.
Fine, thanks.
Now let’s say UE has stored qRxLevMin from SIB1 in serving cell and than a RRC Release happen.
Now just after RRC Release, UE wants to select another cell in idle mode in that case UE will use qRxLevMin which it had stored from SIB1 of previous cell or UE will use it from target cell but still it has to select to get SIB1 of target?
If UE uses stored value of previous cell than what is the use of qRxLevMin available in SIB1 of target cell?
SIB1 is broadcasted every 20 ms.
Yes, periodicity is 160 ms, but assume it has already read SIB1 in previous cell and stored configuration for cell selection.
What if there’s more than one RAT, like 2G, 3G, 4G?
UE will latch on 2G “highest RSSI” then it will re-select to LTE “highest priority”, right?
With Respect to 5G NR - For cell reselection, UE get the information from SIB2 - which contains the cell reselection information related to serving cell, then combination of SIB3 - which contains intra frequency info for cell reselection Or SIB4 - which contains the inter frequency info for cell reselection.
Just for Example (just a tip of iceberg) Say UE is camped on Cell A with S parameter - S(serving) which is calculated with help of Qrxlevmeas and Qrxlevelmin. Now it have variables related to cell reselection criteria obtained in SIB2 and SIB3.
Example For intra frequency cell reselection - UE performs S parameter condition verification - Sserving < SintrasearchP and Squal > SintrasearchQ - if yes It performs measurement for neighbouring cell. Then there is a verification of R parameters - Rn > Rs - If R parameter condition pass Cell reselection happens.
You can read more about SIB2, SIB3, Cell reselection in following links or 3GPP :
- SIB2 - How LTE Stuff Works?: 5G NR: SIB2
- SIB3 -How LTE Stuff Works?: 5G NR: SIB3
- Intra Frequency Cell reselection - Refer to 3GPP TS 38.304 .
- Inter Frequency and Inter RAT - Refer to 3GPP TS 38.304 5.2.4, 38.300 9.2.1.2, 9.3.1.1, and 9.3.2.1.
Thanks dear, great inputs, but all about reselection.
So in Idle mode reselection will happen, your query was related to Idle mode right?
My query was related to cell selection criteria post RRC Release and whose qRxLevMin parameter UE will use for cell selection.
Previous cell where it gets RRC Release or Target cell where it has to select.
After RRC release device goes to Idle, right?
Yes, correct.
qRxLevMin is always used from the SIB 1 of target cell where UE is moving.
Not necessarily it depends upon RAT priority and control is given to user also.
How is it possible that UE will get qRxLevMin of target cell before cell selection?
UE can decode SIB1 only when it selected a cell.