Why is CELL_FACH and CELL_PCH not required in LTE?

Before answering that question, lets look at the RRC states in UMTS and what was accomplished by having these.


CELL_DCH state

• A dedicated physical channel is allocated to the UE in uplink and downlink.
• High data rate can be supported because of the dedicated resources

CELL_FACH state

• No dedicated physical channel is allocated to the UE.
• UE is on shared channel in downlink and uplink.
• UE keeps monitoring the FACH channel for transmission destined to its C-RNTI.
• In the Uplink it can access the RACH channel to send small amounts of data.
• UE was put in CELL_FACH to save some dedicated channel resources in network.

CELL_PCH state

• No dedicated or shared 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.
• UE was put in CELL_PCH when there is no activity for a period of time. This saved battery in UE.

URA_PCH:

• 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.
• UE was put in this state to avoid frequent CELL UPDATES

LTE has only two RRC STATES

RRC_IDLE

• there is no RRC CONNECTION and no DTCH/DCCH allocated
• UE can receive Cell Broadcast and monitors paging for incoming call
• UE does cell reselections based on neighbor cell measurements
• UE is not known by the eNodeB

RRC_CONNECTED

• RRC Connection exitsts and UE can receive/transmit data on shared channels
• UE monitors the control channels corresponding to the Shared Data channels
• UE provides channel quality and feedback information
• eNodeB configures DRX based on UE activity.
• UE is known at cell level

In LTE, the RRC_CONNECTED state has the benefits of CELL_FACH and to some extent CELL_PCH incorporated in it. Since in the downlink the scarce resource is the resource blocks, the downlink logical channels are always mapped onto shared transport channels(exhibiting the CELL_FACH properties). While in RRC_CONNECTED, the UE is configured with DRX cycles, conserving the battery in the UE. With just two RRC states, the RRC state machine is simplified to a great extent, saving a lot of signalling.