Enhanced Cell_FACH in DL

Enhanced Cell_FACH in DL:

Enhanced Cell_FACH in DL is introduced in Rel-7 with the following functionalities:

1. HS-DSCH resources can be assigned to UE in Cell_FACH State
2. UE no longer need to monitor FACH - SCCPCH channel for DL data, instead UE can monitor HS-SCCH channel in Cell_FACH State

Why Enhanced Cell_FACH in DL:

In legacy Release Cell_FACH has the following issues:

1. Low data rate on FACH 
2. QoS cannot be maintained on FACH
3. If there is a burst of data to be transmitted to or from UE then NW may first move UE to Cell_DCH State and this involve unnecessary signalling and latency.
4. Frequent switching between Cell_FACH <-> Cell_DCH leads to more battery consuption
5. UE needs to continuously monitor S-CCPCH physical channel to receive data on FACH TrCH

Configuration of Enhanced Cell_FACH in UE

Configuration of HS-DSCH in CELL_FACH State is done as follows:

NW needs to broadcast the following IEs in SIB - 5:

- HS-DSCH Common System Information
- HS-DSCH Paging System Information

HS-DSCH Common System Information:

This IE contains information related to configuration of HS-DSCH in CELL_FACH State

• CCCH Mapping Info

     ccch-MappingInfo                   CommonRBMappingInfo,
   * srb1-MappingInfo                   CommonRBMappingInfo OPTIONAL,
     common-MAC-ehs-ReorderingQueueList Common-MAC-ehs-ReorderingQueueList,
     hs-scch-SystemInfo                 HS-SCCH-SystemInfo,
     harq-SystemInfo                    HARQ-Info,
     common-H-RNTI-information          SEQUENCE (SIZE(1..maxCommonHRNTI)) OF H-RNTI,
     bcchSpecific-H-RNTI                H-RNTI