Cell Selection:
UE performs cell selection based on RSRP level as per 3GPP Rel 8 or consider both RSRP and RSRQ level as per 3GPP Rel9.
RSRP and RSRQ are key measures of signal level and quality for modern LTE networks. In cellular networks, when a mobile moves from cell to cell and performs cell selection/reselection and handover, it has to measure the signal strength/quality of the neighbor cells.
In LTE network, a UE measures two parameters on reference signal: RSRP (Reference Signal Received Power) and RSRQ (Reference Signal Received Quality).
Cell Reselection:
A procedure initiated by an MS to reselect a cell that provides higher quality services.
When an MS selects a cell as the serving cell ( in cell selection step ) , the MS continues to monitor BCCH carriers of
all neighboring cells in the cell frequency list indicated by the BCCH system message of
the serving cell. The MS then compares the reception level of the serving cell and that of
all BCCH carriers in the cell frequency list. When certain conditions are met,
the MS initiates cell reselection to reselect a cell that provides higher quality services.
Notes:
Scenario 1:
Cell 1 → Cell 2
under Same LAC/TAC:
- idle → idle = Cell Reselection
- connected → connected = HO
Scenario 2:
Cell 1 → Cell 2
under different LAC/TAC:
- idle → idle = Cell Reselection + RAU + TAU
- connected → connected = HO + RAU + TAU
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Cell Selection & Re selection in 5G-NR:
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UE attempts to camp on a ‘suitable’ cell during the cell selection procedure. If the UE fails to camp on a ‘suitable’ cell then the UE will attempt to camp on an ‘acceptable’ cell. When camped on an ‘acceptable’ cell, the UE is restricted to limited services, i.e. emergency calls and notifications.
A ‘suitable’ cell is defined as a cell which:
→ is not barred
→ belongs to the PLMN selected by the NAS layer
–>belongs to at least one Tracking Area which is not forbidden
–>satisfies the below Cell Selection Criteria.
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TS 38.304 specifics the cell selection criteria (‘S’ criteria) as:
Srxlev > 0 AND Squal > 0
Here,
Srxlev = Qrxlevmeas - (Qrxlevmin + Qrxlevmin(ofsset)) - P(comp)- Q(offset)
Squal = Qqualmeas - (Qqualmin + Qqualmin(offset)) - Q(offset)
where:
a. Qrxlevmeas: It is the SS-RSRP measured by the UE.
b. Qrxlevmin: This value defines the Idle Mode coverage area of the cell. Can be configured with a value between -140 and -44 dBm, using a step size of 2dBm.
c. P(comp): It is used to adjust the value of Qrxlevmin according to the UE transmit power capability. It makes the selection criteria more precise to avoid UE camping on the cell at locations where they cannot establish a connection.
d. Q(offset): This temporary offset is applied if the UE experiences repetitive connection setup failures caused by T300 expiring, making it more precise to search for another cell.
e. Qqualmeas: It is the SS-RSRQ measured by the UE.
f. Qqualmin: It defines the minimum RSRQ threshold for the cell. It can be configured with a value between -43 and -12 dB, using a step size of 1dB.
Cell Re selection:
It happens when UE is in the RRC Idle or RRC Inactive state.
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During RRC Idle, cell reselections can happen without informing the network, as long as the UE remains within a Registered Tracking Area(TA).UE completes a NAS Registration procedure with the AMF after moving into an unregistered TA.
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During RRC Inactive, cell reselections also happen without informing the network, as long as UE remains within the allocated RAN Notification Area (RNA). UE completes an RRC Resume procedure after moving outside the allocated RNA.
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