Cell Reselection Procedure
Cell Reselection
The Cell Reselection concept is related to UE moving from one cell site (eNB) to another cell site (eNB) due to better signal strength criteria of target eNB when UE is in idle mode.
To keep it simple, Handover happens from source cell to target cell because target cell signal strength is better than source cell and HO condition met in connected mode. If this happens in UE idle mode, it will be cell reselection.
Cell Reselection Type
There are following two types of Cell reselection depending on two cells operating frequency.
- Intra Frequency Cell reselection
- Inter Frequency Cell reselection
Cell Reselection Criteria
There are following three criteria defined for cell reselection.
- Intra Frequency is High Priority and Inter Frequency is Low priority
INTRA > INTER - Intra Frequency is equal Priority to Inter Frequency
INTRA = INTER - Intra Frequency is Low Priority and Inter Frequency is High priority
INTER > INTRA
Cell reselection Priority ranges from 0 to 7 where 7 means the Highest Priority and 0 means Lowest Priority
From the 3GPP TS 36304 specification
- Intra Frequency is Low Priority and Inter Frequency is High priority
If cell reselection Priority to Inter frequency is High and cell reselection Priority to Intra or same frequency is low, Then UE can directly go for cell reselection to any inter frequency neighbor cell if UE satisfies or fulfills the below criteria.
- Measurement: Neighbor cell priority > Serving cell priority = Always perform inter frequency measurement
- Condition for cell reselection:
RSRP Neighbor or Target cell > target_qrxlevmin (SIB5) + threshXHigh (SIB5)
Example: Inter frequency Priority (7) > Intra frequency Priority (6)
SIB3:
- Non-Intra Search Threshold : 10
- Serving cell Threshold low : 12
- Qrxlev min : -55
- Serving cell Reselection Priority : 6
SIB5:
- Threshold X low : 9
- Threshold X high : 7
- Qrxlevmin : -50
Calculation: Srxlev(neighbor) > [-50 * 2] + [7 * 2]
> -100 + 14
> -86 dBm
Therefore: If UE is reading Neighbor cell RSRP is better than -86 dbm , Then UE directly performs Cell reselection to that neighbor cell in idle mode.
Note: The values those we configure in config file are integer values, so we convert them to dBm by multiplying with 2.
1 & 2 : Intra Frequency is High or equal prioritized with Inter frequency cell reselection Priority:
When UE has to perform Cell reselection in this configuration, UE has to follow and meet certain criteria to achieve cell reselection to Inter or intra frequency cell reselection.
Note: Srxlev = Serving Cell power in dB
Squal = Serving Cell quality
Srxlev = Qrxlevmeas – (Qrxlevmin + Qrxlevminoffset) – Pcompensation
Squal = Qqualmeas – (Qqualmin + Qqualminoffset)
Here Qmeasure = RSRP of the cell and Qrxlevmin = min power level from SIB1
- UE must get permission to read the INTRA frequencies when UE is in idle mode.
- Srxlev > Sintra-search P
- Squal > Sintra-search Q
NO measuring of any intra frequencies & NO reselection as per above conditions.
- Srxlev < Sintra-search P
Squal < Sintra-search Q
UE can read Intra frequencies and go for reselection if reselection criteria met as per above conditions.
UE will start reading Intra frequency neighbors only when UE satisfies the above conditions. UE perform intra frequency cell reselection to any neighbor if neighbor cell satisfies the below condition.
Reselection criteria:
RSRP Neighbor or Target cell > target_qrxlevmin (SIB5) + threshXHigh (SIB5)
- If UE is not getting any intra frequency but other inter frequencies are available to UE to reselect but intra frequency is high priority and Inter frequency is low or equal priority, then,
UE must get permission to read the NON intra frequencies when UE is in idle mode.
- Srxlev > Snon-intra search P
- Squal > Snon-intra search Q
UE will not read any Inter frequencies as per above conditions.
- Srxlev < Snon-intra search P
- Squal < Snon-intra search Q
UE will start reading inter frequencies only when UE satisfies the above conditions. UE perform inter frequency cell reselection to any neighbor if neighbor cell satisfies the below condition.
Cell Reselection Example with UE Logs
Consider the below example for Intra frequency is High priority and Inter frequency is Low Priority: Reselection is here performing only w r t RSRP (Srxlev)
INTRA (7) > INTER (0)
SIB 1: Qrxlevmin = -62
Qrxlevmin offset = 1
SIB 3: Qrxlevmin = -55
Intra-search P = 10
Non-intra search = 8
SIB 5: Qrxlevmin = -50
ThresholdX high = 7
ThresholdX low = 7
Case A
Assume UE is attached and went to idle mode due to inactivity at -85 RSRP and intra freq reselection priority is higher than inter freq reselection priority. Now,
Srxlev = RSRP – (qrxlev + qrxlevmin-offset)
= -85 – (-62*2 + 1)
= -85 + 123
Srxlev = 38 dBm and from above config values, S intra search P = 10*2 = 20 dBm
38 > 20 dBm , so No intra frequency measurements from UE at -85 RSRP.
Case B
UE is in idle mode at -105 dBm RSRP now.
Srxlev = RSRP – (qrxlev + qrxlevmin-offset)
= -105 – (-62*2 + 1)
= -105 + 123
Srxlev = 18 dBm and from above config values, S Intra search P = 10*2 = 20 dBm
18 < 20 dBm, Since Serving cell is satisfies the condition, Srxlev < S intra search P. Now UE can be able to read the Intra frequency measurements and go for Reselection if reselection criteria also meets.
Case C
UE is at -109 dBm RSRP now. Inter frequency measurements and reselection.
Srxlev = RSRP – (qrxlev + qrxlevmin-offset)
= -109 – (-62*2 + 1)
= -109 + 123
Srxlev = 14 dBm and from above config values, S NON-intra search P = 8*2 = 16 dBm
14 < 16 dBm, Since Serving cell is satisfies the condition, Srxlev < S NON-intra search P. Now UE can be able to read the Inter frequency measurements and go for Reselection if reselection criteria also meets.
Important Note
If inter frequency reselection priority is highest UE always measurements all measurements and directly goes for reselection if reselection criteria met.
If Inter frequency is low or equal prioritized with intra frequency reselection priority. Then UE needs permissions to measure or read the intra and inter frequencies of neighbors first. If permission criteria are met, then UE can perform reselection by meeting reselection criteria.
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