basics of paging in wcdma

BASICS OF PAGING IN WCDMA

Oladunni Egbedeyi

Basics Of Paging In WCDMA | Ericsson Internal | 2013-08-13 |

› Introduction To Concept (valid up to WCDMA RAN W11B)

› Mapping of Pages Into Channels In The Radio Interface

› Paging Message Type 1› Paging Message Type 2

› Paging Areas For Local Pages› Paging Areas For Global Pages

› Timeout Of URA connections

› Conclusion

CONTENT


Paging is used by the CN to inform a UE that it has a terminating service request. These pages originates either from MSC (CS calls) or from SGSN (PS calls), see below

(The paging flow from the core network to the air interface). Paging is also used by UTRAN to initiate a state change from URA_PCH to CELL_FACH or from URA_PCH to IDLE

.

Introduction to concept


› Each attempt by the system to reach a UE is defined as a paging record. These are mapped into paging messages on the PCH and sent into the air at predefined paging occasions.

› These are calculated by the RNC, which inserts the paging record in the paging message corresponding to the calculated 10 ms time slot. The paging message is then sent over Iub to all cells in the paging area.

Introduction to concept


› In the RNC the paging messages are mapped onto radio resources (channels). The paging records are coded into “Paging message type 1” or “Paging message type 2” according to this table (Paging characteristics in different UE states).

Mapping of pages into channels in the radio interface


› A PCH TTI frame is 10 ms and it carries one “Paging message type 1”. Paging messages are sent continuously, and at each TTI different UEs are addressed.

› The UEs are divided into paging groups, dedicated to listen to the PICH at predefined paging occasions.

› The PICH indicates to the UE when it should read the Secondary Common Control Physical Channel (S-CCPCH) that transmits the PCH.



› The time interval deciding how often the UE should listen to the PICH is given by the DRX Cycle length. Thus DRX cycle length defines the time interval for the system to repeat addressing a specific paging group.

› A paging record is queued in NoPagingRecordTransm DRX cycles simultaneously, and sent out regardless if the UE has already responded or not.



› Paging to UEs in states IDLE or URA_PCH is done with “Paging message type 1”. A paging message type 1, which is the vast majority of pages, can fit 3-5 paging records into the Paging occasion, see next slide. The exact number depends on type of UE identity and their size. The UE identities are:

› IMSI: This is used at global pages.

› TMSI: Temporary IMSI for CS calls.

› P-TMSI: Temporary IMSI for PS calls.

› U-RNTI: UTRAN initiated pages to mobiles in URA-PCH

Paging message type 1


› Each paging occasion can handle 240 bits of information. The size of the IMSI is 79 bits while TMSI, P-TMSI and U-RNTI occupy 47 bits. This results in four possible combinations of content for each paging message type 1, and is illustrated below. Each paging message type 1 can be filled with 3-5 paging records. T=TMSI, P=P-TMSI, U=U-RNTI

› .



› Paging to UEs in state CELL_FACH and CELL_DCH is done with “Paging message type 2”.

› These pages are going over SRB and are not loading the PCH. Paging load and paging capacity are defined in relation to the PCH channel, thus “paging message type 2” is not relevant for LA/RA/URA planning.



The paging area for local pages is related to the state of the searched UE. Local pages are usually in vast majority.

›UE State IDLE: The paging area for MSC originated (CS) calls to idle UEs is called an LA, except when pages are global.

The paging area for SGSN originated (PS) calls to idle UEs is called an RA.

Paging areas for local pages


› UE State Cell_FACH or Cell_DCH: The concept paging area is not relevant for these states

since LA/RA/URA planning is based on load on the PCH while UEs in these states are utilizing SRB for paging.

› UE State URA_PCH: The paging area for UEs in state URA_PCH is called a

URA, except when pages are global.

Paging areas for local pages


› Global pages are usually in minority. Those are CN CS initiated and sent over the whole MSC area to UEs in state IDLE or URA_PCH.

› The concept “global pages” does not exist for SGSN and UTRAN initiated pages.

Paging areas for global pages


› The RNC must keep control of UEs in state URA_PCH since they are in connected mode and thus occupying resources. A UE in state URA_PCH that is not possible to contact must be released to avoid hanging resources.

› This is implemented as a connection release if Ue does not respond to paging. A timer is started at UTRAN initiated pages. It is hardcoded and related to DRX cycle length:

utranPagingT=10·noOfPagingRecordsTransm·2UtranDrxCycleLength +1000 [ms]

Timeout of URA connections


› In practice, this means that the utranPagingT elapses after noOfPagingRecordsTransm DRX cycles plus 1000 ms. Thus there is always room for

noOfPagingRecordsTransm repetitions.

› If the UE has not responded with a Cell Update after utranPagingT seconds, another attempt is performed (initiating the timer again), and if still not successful the connection is released by UTRAN, see next slide.



Time supervision of the UTRAN initiated paging procedure. The dotted line represents a cell update that is not received in UTRAN



This document has briefly described:

paging over the air interface,

what pages are sent over LA, RA and URA respectively, and

configuration rules for the areas.

CONCLUSION

basics of paging in wcdma

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