We often see a UE in Cell_FACH/Cell_PCH/URA_PCH state sending Cell update with cause “Re-entering service Area”, In this blog we will try to understand in what all scenarios this can happen.
Before we go ahead it is required to understand below terminology
1. Out Of Service Area
2. In Service Area
Out Of Service
Area detection:
If the UE finds that the serving cell does not fulfil the cell
selection criteria,
UE initiates the measurements of all neighbour cells and if it
does not find any new suitable cell for a predefined period, UE is considered
to be "out of service area".
UE initiates cell selection procedures for the selected PLMN after
this period.
In Cell_PCH/URA_PCH this period is 12 seconds while in Cell_FACH
this period is 4 seconds.
In Service Area detection:
When a suitable cell is found (cell <re>selection criteria
is met ) , the UE considers it as having detected "in service area".
Cell Update with cause “Re-entering service Area” might be sent to
the same cell (on which UE was previously camped on) or to the different cell.
So "In Service Area" detection does not always mean that UE has detected the same cell.
So "In Service Area" detection does not always mean that UE has detected the same cell.
Coming back to the possible scenarios when UE can send Cell
update with cause “Re-entering service Area”
Scenario 1 :
UE enters into Cell_FACH/Cell_PCH/URA_PCH state ----> starts timer T305------>T 305 expires and UE detects “Out Of Service Area”-----> starts timer T307------now if before T307 expiry UE detects "In Service area" -------> UE sends cell update with cause "re entering service Area"
Scenario 1 :
UE enters into Cell_FACH/Cell_PCH/URA_PCH state ----> starts timer T305------>T 305 expires and UE detects “Out Of Service Area”-----> starts timer T307------now if before T307 expiry UE detects "In Service area" -------> UE sends cell update with cause "re entering service Area"
Scenario
2 :
UE in CELL_FACH state and detects out of service Area --->starts timer T317----->before expiry of T 317 UE detects IN Service Area --->UE sends cell update with cause "re entering service Area"
UE in CELL_FACH state and detects out of service Area --->starts timer T317----->before expiry of T 317 UE detects IN Service Area --->UE sends cell update with cause "re entering service Area"
Scenario 3 :
UE in Cell_PCH/URA_PCH state and detects “Out Of Service Area”--->starts timer T316----->Before expiry of T316 OR if T 316 expires and UE detects "In Service Area" ----->UE sends cell update with cause "re entering service Area"
Scenario 4:
UE in Cell_PCH/URA_PCH state and detects “Out Of Service Area”--->start timer T316----->T 316 expires and UE still “Out Of Service Area”----->UE starts T317 and move to CELL_FACH state -----before expiry of T 317 UE detects IN Service Area --->UE sends cell update with cause "reentering service Area"
UE in Cell_PCH/URA_PCH state and detects “Out Of Service Area”--->starts timer T316----->Before expiry of T316 OR if T 316 expires and UE detects "In Service Area" ----->UE sends cell update with cause "re entering service Area"
Scenario 4:
UE in Cell_PCH/URA_PCH state and detects “Out Of Service Area”--->start timer T316----->T 316 expires and UE still “Out Of Service Area”----->UE starts T317 and move to CELL_FACH state -----before expiry of T 317 UE detects IN Service Area --->UE sends cell update with cause "reentering service Area"
I hope this blog is helpful, I would definitely like to
include any suggestion or correction in case needed.
Ref specs : 25.133, 25.331, 25.304
