Priorities: If you haven’t found it yet, keep looking. Don’t settle.

Steve Jobs said, ”  If you haven’t found it yet, keep looking. Don’t settle”

You can interpret this quote in as many contexts as you like.

However, at this point we are going to focus on it with the context of finding a

better cell for your UE in mind. 🙂

How UE selects a cell is already known to you.

Once a UE has already selected a cell for camping. The cell is termed as Serving cell.

Even after UE selects a cells and camps on its. It still keeps on looking for other better neighboring cells.
If other cells offer better coverage than the serving cell, UE will reselect to better cell using R-Criteria ( Reselection Criteria).

If there are more better cells available than the serving cell. Is it possible to give more priority to one cell over the other ??

In order to resolve this issue priorities are assigned to RF carriers. In order to prioritize one cell over the other.

Priorities between RF carriers and different radio access technologies are not used during cell selection. However, during cell reselection. Different RF carriers can be given different priorities.

Cell Re-Selection Scenario Depicted

What is a Priority ?

Priority or Absolute Priority can be allocated to RF carriers belonging to both LTE and other Radio Access Technologies.

This priority can be used to help UE make better decision in terms of which cell to camp on to when doing cell reselection.

We are not going to talk about R-Criteria ( cell reselection criteria ) here. However, we will talk mainly about priorities.

Use of Priorities:

Absolute priorities can be broadcast within System Information Blocks on Broadcast Control Channel (BCCH)

Various Priorities:

Various priorities are used in different SIBs , depending on the specific Radio Access Technology (RATs). Below are the SIBs which contain priority for each RAT

• SIB 3: Contains absolute priority for current LTE RF carrier on which UE is camping
• SIB 5: Contains absolute priorities for other LTE RF carriers
• SIB 6: Contains absolute priorities for UMTS RF carriers
• SIB 7 : Contains absolute priorities for sets of GSM RF carriers
• SIB 8: Contains absolute priorities for CDMA RF carriers
• Absolute priorities can also be inherited from another RAT when completing inter-RAT cell reselection. UE inherits priorities when they have been allocated using dedicated signalling. UE also inherit the remaining validity time when a validity time has been allocated.

SIBs bring Priority values to the UE, when UE enters Idle Mode

How Absolute Priorities are Signaled to UE ?

Absolute priorities can be signaled directly to individual UE within an RRC Connection Release  message i.e., when UE is returning to RRC Idle mode from RRC Connected mode. All absolute priorities within the system information are ignored if priorities are included within an RRC Connection Release message.

Timer 320:

This LTE timer starts as soon as it is received. Upon receipt of T320 or upon cell re- selection to LTE from another RAT with validity time configured for dedicated priorities. The absolute priorities are deleted when T320 expires. T320 can be allocated a values between 5 minutes to 3 hours.

This timer T320 stops, when PLMN selection is performed OR upon cell re-selection to another RAT.   At expiry, it discards the cell re-selection priority info provided by dedicated signaling

Values for Absolute Priorities:

Absolute priorities can be allocated values between 0 and 7, where 0 represents the lowest priority.

Different Re-Selection Priorities can be allocated to different Carriers

FAQs

Q: Does UE perform cell reselection for all RF carriers ?

Ans: UE only perform cell reselection evaluation for LTE RF carriers and inter-RAT RF carriers which are defined within the system information and for which the UE has a priority.

Q: Does blacklisted cells are included in cell reselection

Ans: Blacklisted cells are excluded from the list of candidate cells for cell reselection.

Q: Which RF carrier is considered to have the lowest priority ?

Ans: When a UE is camped on a cell which defines priorities for other network layers but not for the current network layer, the UE assumes the current RF carrier to have the lowest priority (lower than all 8 of the possible signalled values)

Q: How priority is treated in case of Closed Subscriber Group (CSG) ?

Ans: When a UE is camped on a suitable CSG cell, the UE assumes the current RF carrier to have the highest priority (higher than all 8 of the possible signalled values)

Q: Can we assign same priority to different RATs ?

Ans: The allocation of equal priorities to different RAT is not supported.

For any questions, comments or criticism, please comment below.

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