UE State in 3G

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.

Pict 1 : Operating modes of the UE

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

1. CELL_DCH state

The CELL_DCH state features the following:

•   A dedicated physical channel is allocated to the UE in both the uplink and the downlink.
• RNC knows the cell where the UE camps on according the current active set of the UE.
• 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:

• No dedicated transport channel is allocated to the UE.
• The UE continuously monitors a downlink FACH channel. 
• 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 
• 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: 

•   Monitors an FACH. 
•  Monitors the BCH channel of the current serving cell to decode the system messages. 
•  Initiates a cell update procedure when the cell becomes another UTRA cell. 
•  Uses the C-RNTI allocated in the current cell as the UE identification on the common transport channel unless a new cell is selected. 
•  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 followin : 

•  No dedicated channel is allocated to the UE. 
•  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. 
•  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. CELL_URA_PCH state

  The URA_PCH state features the following:
  
  • No dedicated channel is allocated to the UE.
  •  The DRX technology is adopted for the UE to monitor the information transmitted on the PCH channel at a specific paging time slot.
  • No uplink activity is allowed.
  • 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:
• 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 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
  
  Source : Huawei Documentary
  
  PLUR
  Putri Touor,,si Smith Delvis