No DCH’s exist in Uplink or Downlink Direction
The most important difference between CELL_DCH- and CELL_FACH-state is that
there
are no dedicated TrCH’s allocated to the UE in CELL_FACH-state.
DCCH’s are available; DTCH’s may be available
Like in CELL_DCH-state, DCCH exist in CELL_FACH-state and DTCH’s may exist.
However, since no DCH’s are available, these logical channels are linked
to common / shared transport channels.
UE continuously monitors one FACH in Downlink
Since there are no dedicated TrCH’s available in CELL_FACH-state, the UE
shall continuously monitor one FACH-TrCH to receive information on DCCH’s
and, if configured, on DTCH’s which are destined for that UE. The UE can
be addressed through the U-RNTI or the C-RNTI.
If the UE needs to transmit information on DCCH or DTCH in uplink direction,
it shall use the RACH- or CPCH-TrCH. Accordingly, the UE needs to be assigned
a C-RNTI in CELL_FACH-state.
If UTRAN receives a paging message for the UE from the core network, it will
use the PAG_TYPE2-message on DCCH (over FACH) to inform the UE.
No Soft or Hard Handover Scenarios are applicable
There are no handover scenarios possible, since the UE occupies no dedicated
transport channels. It is the responsibility of the UE to select the most suitable
cell and to perform the related cell update scenario. Still, through the CELL_CHAN_UTRAN-message,
also the UTRAN is able to direct the UE to a GSM-cell.
UE performs Cell Updates but no URA Updates
In CELL_FACH-state, the UE will autonomously reselect the serving cell and
perform a cell update scenario consecutively. Cell updates will also performed
periodically
(if configured) upon timer T305 expiry.
UTRAN knows the Location of the UE on Cell Level
Even in CELL_FACH-state, the SRNC (and possible another CRNC) knows the location
of the UE on cell level, because the UE performs cell update scenarios.
UE provides Measurement Reports to the RNC
Which measurements the UE performs depends on the setup information received
from the RNC (ó SIB 11 and 12 / MEAS_CTRL-message).
[3GTS 25.331 (7.2.2.2, Annex B.3.2)]