IDLE MODE: How UE reselects a Cell using R-criteria ( Reselection and Ranking Criteria)

What would you do, if you already have a stable ,well paying job. Then suddenly you get a better offer from another company.

Will you jump on to accept the new offer right away ?? Probably not. You may take some time to think about it.

You may take a number of factors into account such as

• Salary
• Benefits
• Insurance
• Commute
• Relocation
• Bonus
• Company Profile
• And the list goes on

You may create your own preference list and compare offered job with your current employment. In other words , you would RANK the new offer with the existing employment.

Just by ranking the two jobs, you would reach to a better conclusion.

In the same way, Shifting gears, when it comes cell Re-selection. It’s the same analogy. If UE is already camping onto one cell in Idle Mode and finds a cell better than the serving cell.

UE will consider a number of factors , before reselecting to a new cell from already existing serving cell.

Initially in Idle mode UE already selects a serving cell using S-criteria. The cell which is selected using S-criteria for camping becomes the serving cell. But when and if a UE finds another cell nearby or in neighborhood better than the serving cell. It will try to reselect the better cell and camp on it.

In order to make the decision for reselection. UE will go through a mathematical equation to identify and verify if the ranking and signal strength of neighboring cell is better than the serving cell for a specific amount of time, only then it will perform cell reselection.

This brief procedure is termed as R-criteria. Call it Ranking criteria or Re-selection criteria or Ranking Reselection Criteria.

Using R-criteria a UE can officially reselect to another cell in Idle mode.

The mathematical equations for R-criteria are given below.

Simply two equations to decide about Cell Reselection

Parameters for R-Criteria:

The parameters which are shown in the above equation need to play their part in R-criteria for cell reselection to take place.

They are given below.

Qoffset is not in this list yet. Look further below to find its value

Parameter Qoffset for Intra-Frequency Cells:

For intra-frequency cells, Qoffset is the optional parameter.

Cell specific Qoffset,n value is broadcast in SIB 4. A value of 0 is assumed if the candidate cell does not have a value broadcast in SIB4.

Parameter Qoffset for Inter-Frequency Cells

For inter-frequency cells, Qoffset is given below

Different Context of Qoffset, depending on Intra-frequency and Inter-Frequency cells

Parameter T reselection

One timing parameter which is not shown above but it is very important in cell reselection is called T reselection.

This T reselection defines the time duration for which a neighboring cells needs to remain better ranked than the serving cell, before it can be selected for Reselection.

Conditions for R-Criteria

1 .Cell re-selection is completed if a neighboring cell is ranked better than the current serving cell according to R-criteria equations.

2. Neighboring cell is better than the serving cell during a time period defined by Treselection, and if more than 1 second has passed since the UE camped on the current serving cell.

Procedure

To elaborate R-criteria briefly in pictorial fashion. I have listed them in three steps as below.

STEP 1

An opportunity for R-criteria : UE camping on serving cell and finds neighboring cells which may be better than serving cell.

STEP 2

UE computes Serving cell and Neighboring cells ranking according to R-criteria equations

STEP 3

Treselection timer parameters has its final say in deciding which neighboring cell can become the highest ranked

FAQs

Q: How Treselection is broadcasted ?

A: Treselection for intra-frequency cells is broadcast in SIB 3, whereas Treselection for inter-frequency cells is broadcast in SIB 5.

Q: What if more than one cell turns out to be better in ranking than the serving cell ?

A: UE always camp on the highest ranked cell to help minimize levels of inter-cell interference, i.e. UE always prefer to get connected to the best cells.

Q: What if I want UE to camp onto a specific cell, even if its not the best in ranking among all cells according to R-criteria ?

A: In order to do so , you can play with measurement offset. If a cell is allocated a measurement offset (i.e., Qoffset ) to increase its ranking then UE can camp on that cell without it being the best cell. However, be cautious in that case, the radio access network can experience an increase in interference. 🙂

Q: Where can I find values for Q offset, T reselection and Q Hyst ?

A: Each telecom operator or carrier configures whatever value they want to use in their network. Usually those specific values are standardized for their network.But they can be configured on need basis. Usually there is a range given for each parameter in which it can be configured.

Q: What if neighboring cells are of different priority than serving cells. How would R-criteria perform in that case ?

A: To take priorities into consideration, while in Idle Mode. Take a look over here. 

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