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UMTS Cell Selection / Reselection

An attempt to explain the above procedures in a simpler way...its work in progress..

Anantharaman Iyer

on 1 June 2017

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Transcript of UMTS Cell Selection / Reselection

Selection / Reselection
UE procedures in idle mode
Cell Selection -Background
Cell Selection - climax
The UE (User equipment a.k.a the handset) performs measurements to ensure that it is always served by the most suitable cell
When UE "hears" a cell, the cell has to prove its "worthiness" through something known as "S criteria" (snobbish for Selection criteria)
The criteria is as below -

For FDD Cells: Srxlev > 0 AND Squal >0
For TDD Cells: Srxlev > 0
(hold on , just a handful of equations to go)

WHAT????(I hear you say!)

Does the handset actually do maths???
Surely, you must be joking....
Qqualmeas = "Quality" of the cell measured by the handset
Qrxlevmeas = "Signal Level" of the cell measured by the handset
Qqualmin = Minimum signal "quality" that is required for a cell.
Qrxlevmin = Minimum signal "level" that is required for a cell. (i.e. received signal level from a cell)
Pcompensation =max(UE_TXPWR_MAX_RACH – P_MAX, 0) in dB [In plain English, its the maximum of the 2 quantities - 0 and UE_TXPWR_MAX_RACH – P_MAX] - usually 0 but can be modified.
Story so far....
(a.k.a) What does your handset do while resting on your table/pocket/bag....
So, Yes! the handset does a wee bit of math in the background
Only if
Srxlev (
i.e received signal level)
Squal (
i.e. received signal quality)
are >0
(i.e. positive, for those among us who dont like mathematical inequalities)
the handset selects the cell and starts the ....Cell selection procedure
(fancy word for "checking in")
Is that it???
The handset keeps looking for a "better cell"
Once the handset has camped on a cell, it will
keep “listening” to the network using the SIB (System Information Block) messages and
Perform measurements on the serving cell as well as the surrounding "neighbour" cells to choose the best suitable cell to be connected to.
If the handset finds a better cell, it will initiate re-selection procedure
Well, sort of....however...:)
"Well, hello there!!"
Selection Criteria
Horrendous Equation -1
Qqualmeas = "Quality" of the cell measured by the handset
Selection Criteria
Horrendous Equation -1
Qrxlevmeas = "Signal Level" of the cell measured by the handset
Qqualmin = Minimum quality of a cell to be considered usable. Set as a constant by network operators
Qrxlevmin = Minimum "Signal Level" needed for the cell to be considered suitable.
Cell Re-Selection- Criteria
Cell Reselection is triggered if a UE finds a "better cell"
The trigger-mechanism for a UE to start reselection is governed by the parameters - S-intersearch, S-intrasearch and S-searchRAT
Squal = Qqualmeasured - Qqualmin
where Qqualmeasured = Quality of the serving cell.(CPICH EcNo)
If Squal < =
the UE starts measuring intra-frequency neighbours (i.e. neighbours that are on the same frequncy as the serving cell)
If Squal < =
the UE starts measuring inter-frequency neighbours (i.e. neighbours that are on a different frequency than the serving cell)
If Squal < =
the UE starts measuring inter-RAT neighbours (eg -2G/ LTE)
S-intrasearch, S-intersearch, S-searchRAT
are constants that are set by the macro operators to either “quicken” or “delay” idle mode reselection

If any of the above criteria are satisfied, the UE starts cell reselection procedure
Cell Reselection - Criteria 2
Once the UE starts “looking” for better cell, it performs the measurements and ranks the neighbours using the following equation –
Rs = Qmeas,s + Qhyst,s
Rn = Qmeas,n – Qoffset,n

(Please note that this is a modified version of the actual formula)

Rs and Rn are ranks of the serving cell and the neighbour cell
Qmeas,s = Quality (CPICH Ec/No) of the


cell as measured by the UE
Qmeas,n = Quality (CPICH Ec/No) of the


cell as measured by the UE

Qhyst,s and Qoffset,n are constants that are set by the macro operators as well as by us to bias the ranking process.
That's it for now....
Watch the space for more!
Full transcript