UMTS Paging

1. Paging
The Paging Channel (PCH) is a downlink transport channel. The PCH is always transmitted
over the entire cell. The transmission of the PCH is associated with the transmission of physical-
layer generated Paging Indicators, to support efficient sleep-mode procedures.
Paging Channel selection
System information block type 5 (SIB 5) defines common channels to be employed in Idle mode.
In a cell, a single or several PCHs may be established. Each Secondary Common Control
Physical Channel (SCCPCH) indicated to the UE in system information may carry up to one
PCH. Thus, for each defined PCH there is one uniquely associated PICH also indicated.
In case that more than a single PCH and associated PICH are defined in SIB 5, the UE shall
perform a selection according to the following rule:
 The UE shall select a SCCPCH from the ones listed in SIB 5 based on IMSI as follows:
"Index of selected SCCPCH" = IMSI mod K,
where K is equal to the number of listed SCCPCHs which carry a PCH (i.e. SCCPCHs carrying
FACH only shall not be counted). These SCCPCHs shall be indexed in the order of their
occurrence in SIB 5 from 0 to K-1.
"Index of selected SCCPCH" identifies the selected SCCPCH with the PCH and the uniquely
associated PICH to be used by the UE. If the UE has no IMSI, for instance when making an
emergency call without USIM, the UE shall use as default number IMSI = 0.
The UE may use Discontinuous Reception (DRX) in idle mode in order to reduce power
consumption. When DRX is used the UE needs only to monitor one Page Indicator, PI, in one
Paging Occasion per DRX cycle.
The Paging Indicator Channel (PICH) is a fixed rate (SF=256) physical channel used to carry
the paging indicators. The PICH is always associated with an S-CCPCH to which a PCH
transport channel is mapped. Picture below illustrates the frame structure of the PICH. One PICH
radio frame of length 10 ms consists of 300 bits. Of these, 288 bits are used to carry paging
indicators. The remaining 12 bits are not formally part of the PICH and shall not be transmitted
(DTX). The part of the frame with no transmission is reserved for possible future use.

2. Structure of Paging Indicator Channel (PICH)
Two Paging Procedures:
Paging procedure is used to transmit paging information to selected UEs in idle mode,
CELL_PCH or URA_PCH state using the paging control channel (PCCH). Upper layers in the
network may request paging, to e.g. establish a signalling connection. UTRAN may initiate
paging for UEs in CELL_PCH or URA_PCH state to trigger a cell update procedure. In addition,
UTRAN may initiate paging for UEs in idle mode, CELL_PCH and URA_PCH state to trigger
reading of updated system information. UTRAN initiates the paging procedure by transmitting a
PAGING TYPE 1 message on an appropriate paging occasion on the PCCH.
UE dedicated paging procedure is used to transmit dedicated paging information to one UE in
connected mode in CELL_DCH or CELL_FACH state. Upper layers in the network may request
initiation of paging. For a UE in CELL_DCH or CELL_FACH state, UTRAN initiates the
procedure by transmitting a PAGING TYPE 2 message on the DCCH using AM RLC.
Two Paging Message Types:
PAGING TYPE 1 message is used to send information on the paging channel. One or several
UEs, in idle or connected mode, can be paged in one message, which also can contain other
information
PAGING TYPE 2 message is used to page an UE in connected mode (CELL_DCH or
CELL_FACH state), when using the DCCH for CN originated paging.
PICH / S-CCPCH timing relation
Picture below illustrates the timing between a PICH frame and its associated single S-CCPCH
frame, i.e. the S-CCPCH frame that carries the paging information related to the paging
indicators in the PICH frame. A paging indicator set in a PICH frame means that the paging
message is transmitted on the PCH in the S-CCPCH frame starting tPICH chips after the
transmitted PICH frame.

3. Timing relation between PICH frame and associated S-CCPCH frame
tPICH = 7680 chips (3 slots)
UMTS RCC States
Picture below shows the RRC states in UTRA RRC Connected Mode, including transitions
between UTRA RRC connected mode and GSM connected mode for CS domain services, and
between UTRA RRC connected mode and GSM/GPRS packet modes for PS domain services. It
also shows the transitions between Idle Mode and UTRA RRC Connected Mode and furthermore
the transitions within UTRA RRC connected mode.
RRC States and State Transitions including GSM
CELL_DCH state is characterised by:
 A dedicated physical channel is allocated to the UE in uplink and downlink.
 The UE is known on cell level according to its current active set.
 Dedicated transport channels, downlink and uplink (TDD) shared transport channels, and a
combination of these transport channels can be used by the UE.

4. CELL_FACH state is characterised by:
 No dedicated physical channel is allocated to the UE.
 The UE continuously monitors a FACH in the downlink.
 The UE is assigned a default common or shared transport channel in the uplink (e.g. RACH)
that it can use anytime according to the access procedure for that transport channel.
 The position of the UE is known by UTRAN on cell level according to the cell where the UE
last made a cell update.
 In TDD mode, one or several USCH or DSCH transport channels may have been established.
CELL_PCH state is characterised by:
 No dedicated physical channel is allocated to the UE.
 The UE selects a PCH with the algorithm, and uses DRX for monitoring the selected PCH via
an associated PICH.
 No uplink activity is possible.
 The position of the UE is known by UTRAN on cell level according to the cell where the UE
last made a cell update in CELL_FACH state.
URA_PCH State is characterised by:
 No dedicated channel is allocated to the UE.
 The UE selects a PCH with the algorithm, and uses DRX for monitoring the selected PCH via
an associated PICH.
 No uplink activity is possible.
 The location of the UE is known on UTRAN Registration area level according to the URA
assigned to the UE during the last URA update in CELL_FACH state.
Call reselection procedures:

5. States and procedures in the cell reselection process in connected mode
When a cell reselection is triggered, the UE evaluates the cell reselection criteria based on radio
measurements, and if a better cell is found that cell is selected, procedure Cell reselection. If the
change of cell implies a change of radio access technology, the RRC connection is released, and
the UE enters idle mode of the other RAT. If no suitable cell is found in the cell reselection
procedure, the UE eventually enters idle mode.
When an Initial cell reselection is triggered, the UE shall use the Initial cell reselection procedure
to find a suitable cell. One example where this procedure is triggered is at radio link failure,
where the UE may trigger an initial cell reselection in order to request re-establishment of the
RRC connection. If the UE is unable to find a suitable cell, the UE eventually enters idle mode.