6.2 UE State and Paging Procedure


6.2.1  UE State


The UE may operate in one of two basic modes: Idle mode and connected mode. After being switched on, the UE operates in the idle mode and is identified by a non-access stratum identification such as IMSI, TMSI or P-TMSI. The UTRAN does not save the information of the UE operating in the idle mode. It can only page all the UEs in a cell or all the UEs at one paging time slot.
After establishing an RRC connection, the UE shifts from the idle mode to the connected mode: CELL_FACH or CELL_DCH state. The connected mode of UE is also called the RRC state of UE. It reflects the level of the UE connection and the transport channel that can be used by the UE. When the RRC connection is released, the UE shifts from the connected mode to the idle mode.

In the connected mode, the UE has altogether four states:

1. CELL_DCH state

The CELL_DCH state features the following:
l   A dedicated physical channel is allocated to the UE in both the uplink and the downlink.
l   RNC knows the cell where the UE camps on according the current active set of the UE.
l   The UE can use the dedicated transport channels, downlink/uplink shared transport channels or the combination of these transport channels.
The UE enters the CELL_DCH state in one of the following two ways:
1)      In the idle mode, the UE sets up the RRC connection on the dedicated channel, thus shifting from the idle mode to the CELL_DCH state.
2)      In the CELL_FACH state, the UE uses the common transport channel and then is converted to the dedicated transport channel, thus shifting from the CELL_FACH state to the CELL_DCH state.

2. CELL_FACH state

The CELL_FACH state features the following:
l   No dedicated transport channel is allocated to the UE.
l   The UE continuously monitors a downlink FACH channel.
l   A default uplink common channel (for example, RACH) or an uplink shared transport channel  is allocated to the UE for the UE to use at any time during the access procedure.
l   The cell-level location of the UE is known by the UTRAN, specifically, the cell reported during the last cell update initiated by UE.
The UE performs the following operations in the CELL_FACH state:
l   Monitors an FACH.
l   Monitors the BCH channel of the current serving cell to decode the system messages.
l   Initiates a cell update procedure when the cell becomes another UTRA cell.
l   Uses the C-RNTI allocated in the current cell as the UE identification on the common transport channel unless a new cell is selected.
l   Transmits uplink control signaling and small data packets on the RACH.
In the CELL_FACH state, if the data service is not activated in a certain time period, the UE will enter the CELL_PCH state so as to save the power. In addition, if neither the UE nor the network side has the data transport requirement after the UE temporarily exits the CELL_PCH state and executes the cell update, the UE will return to the CELL_PCH state.

3. CELL_PCH state

The CELL_PCH state features the following:
l   No dedicated channel is allocated to the UE.
l   The DRX (discontinuous reception) technology is adopted for the UE to monitor the information transmitted on the PCH channel at a specific paging time slot.
l   No uplink activity is allowed.
The cell-level location of the UE is known by the UTRAN, specifically, the cell reported during the last cell update initiated by the UE in the CELL_FACH state.
The UE performs the following operations in the CELL_PCH state:
Monitors the paging time slot based on the DRX period and receives the paging messages transmitted on the PCH.
Monitors the BCH channel of the current serving cell to decode the system messages.
Initiates the cell update procedure when the cell changes.
The DCCH logical channel cannot be used in this state. To initiate any activity, the network needs to send a paging request via the PCCH logical channel of the cell where the UE is.
The UE shifts to the CELL_FACH state in one of the following two ways: By paging from the UTRAN and by any uplink access.

4. URA_PCH state

The URA_PCH state features the following:
l   No dedicated channel is allocated to the UE.
l   The DRX technology is adopted for the UE to monitor the information transmitted on the PCH channel at a specific paging time slot.
l   No uplink activity is allowed.
l   The URA-level location of the UE is known by the UTRAN, specifically, the URA reported during the last URA update initiated by the UE in the CELL_FACH state.
The UE performs the following operations in the URA_PCH state:
l   Monitors the paging time slot based on the DRX period and receives the paging messages transmitted on the PCH.
l   Monitors the BCH channel of the current serving cell to decode the system messages.
l   Initiates the URA update procedure when the URA changes.
The DCCH logical channel cannot be used in this state. To initiate any activity, the network needs to send a paging request via the PCCH logical channel of the URA where the UE is.
No resource is allocated for data transport in the URA_PCH state. Therefore, if the UE has the data transport requirement, it needs to first shift to the CELL_FACH state.

6.2.2  Paging Procedure


Different from fixed communications, the location of the communication terminal in mobile communications is not fixed. To establish a call, the CN sends a paging message to the UTRAN via the Iu interface, and the UTRAN forwards the message to the UE through the paging procedure on the Uu interface, so that the UE being paged initiates the signaling connection setup procedure with the CN.
When receiving the paging message from a certain CN domain (CS or PS domain), the UTRAN first needs to decide whether the UE has established a signaling connection with another CN domain. If the UE has not established any signaling connection, the UTRAN can only learn the current service area where the UE is and will send the paging message (PAGING TYPE 1 message) to the UE via the paging control channel. If the UE has already established a signaling connection, the UTRAN can learn the channel where the UE is currently active when the UE is in the CELL_DCH or CELL_FACH state and will send the paging message (PAGING TYPE 2 message) to the UE via the dedicated control channel. In this sense, the paging falls into the following two types based on the UE mode and state:
1)     Paging a UE in the idle mode or the PCH state
In this paging type, the PCCH (Paging Control Channel) is used to send paging messages to the UE in the idle mode or in the CELL_PCH or URA_PCH state to page it. This paging type has the following functions:
l   To establish a call or a signaling connection, the upper layer at the network side initiates the paging procedure.
l   To transit the UE state from CELL_PCH or URA_PCH to CELL_FACH, the UTRAN initiates the paging to trigger the UE state transition.
l   When the system message changes, the UTRAN initiates the paging of the UE in the idle mode or in the CELL_PCH or URA_PCH state so as to trigger the UE to read the updated system information.

The UTRAN initiates the paging procedure by sending a PAGING TYPE 1 message at a proper paging time slot via the PCCH. This paging moment is related to the IMSI of the UE. The UTRAN may page a UE at several paging time slots to let the UE correctly receive the paging message.
2)     Paging a UE in the CELL_DCH or CELL_FACH state
In this paging type, the dedicated paging message is sent to a UE in the CELL_DCH or CELL_FACH state.

For a UE in the CELL_DCH or CELL_FACH state of the connected mode, the UTRAN initiates the paging procedure by sending a PAGING TYPE 2 message via the DCCH (Dedicated Control CHannel). This paging type is also called the dedicated paging procedure. 

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