PAGING IN WCDMA

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.