Paging is initiated upon request
from the CN or triggered in UTRAN. It is used to notify the UE of different
events.
In WCDMA RAN P4 these are:
• UE terminating service request
for PS or CS services (CN initiated). CN initiated paging is applicable to UEs
in idle mode.
• UTRAN initiated broadcast to
inform UEs when System Information is modified. UTRAN initiated paging is used whenever
System Information (e.g. information about cell selection/ reselection,
addition/replacement of neighbors, handover etc.) has been updated. The paging
message has the following characteristics:
• Type of paging message: Paging
Type 1 or Paging Type 2
• UE identity used in the message
(IMSI, TMSI, P-TMSI)
• The physical radio channels and
type of resource required
• The area in which the page will
be broadcasted (LA or globally)
The paging record varies in length depending on
whether it includes the UE identity in terms of IMSI, TMSI, or P-TMSI. A PCH frame can carry one “Paging
Type 1” message of 10 ms and may contain between 3-5 paging records, depending
on whether the paging uses IMSI or TMSI/P-TMSI.
When the UE mode is Cell_FACH or
Cell_DCH common or dedicated physical channels are already in use and the
paging message “Paging Type 2” will be used.
For paging, the capacities of the FACH and the RACH are assumed to be
enough, but there is a risk of congestion in the PCH due to heavy paging load.
Therefore, the probability of congestion in the PCH must be calculated in order
to dimension the LA/RA. If
the
operator wants to check the paging success rate, this should be done on MSC
level. Notice that even if a UE does not response to a paging in a certain
Location Area, a second paging might be sent throughout the whole MSC area
(depending on configuration) and UE can be finally reached. For this reason the most reliable
indicator for paging is the one obtained at MSC level.
