DL - 4G Americas

This is based on network configuration and deployment option. For 1xRTT CSFB feature, CSFB can be
done with RRC connection release with redirection information or 1xSRVCC based signaling (known as
enhanced 1xCSFB). 1xRTT CSFB UE may also have dual-Rx/dual-Tx or Dual-Rx/Single-Tx capability.
Dual-Rx/dual-Tx 1xRTT CSFB UE can simultaneously transmit and receive on both EPS and 1x at the
same time. This allows the UE to obtain 1x voice service from 1xRTT system while maintaining the data
stream over EPS at the same time. This is also based on network configuration and deployment option,
and UE capability. Dual-Rx/Single-Tx 1xRTT CSFB UE allows simplification in EPS network deployment
because there is no coordination is required between the E UTRAN and 1xRTT network (i.e. S102 is not
required).
After the UE is redirected to GERAN/UTRAN/1xRTT access via one of the above procedures, the existing
CS setup procedure is taken over for the remaining of the call.
In Rel-9, IDLE mode camping mechanism is enhanced in the EPS and GPRS to allow the network to
influence the UE’s RAT camping policy so that a CSFB UE will select GERAN/UTRAN access when it is
in IDLE condition. The intention is to minimize the occurrence of CSFB procedure from EPS to allow the
UE to invoke the CS domain services directly from GERAN/UTRAN as much as possible. On the other
hand, this requires additional intelligence in the cell reselection policy in the GERAN/UTRAN access in
order to move the UE in active state to EPS to enjoy the fast PS access when appropriate. There are also
optimization enhancements to Rel-9 for speeding up the overall CSFB procedure.
As indicated earlier, SMS delivery via CS Domain is also defined as part of the CSFB feature. UE can
utilize this feature after it has successfully attached itself to the CS domain. It should be noted that EPS
has the option to support only the SMS feature and not the CSFB feature which redirect the UE to
another RAT. For GERAN/UTRAN CSFB, MME can indicate this condition by having an SMS-only
indicator to the UE during their combined EPS/IMSI Attach/TAU procedure. For 1xRTT CSFB, this
indication is not specified, as the 1xCS registration procedure is transparent to the EPS. UE receiving the
“SMS-only” indicator will not invoke the CSFB request and should not expect any CS paging coming from
EPS.
When interworking with a 3GPP MSC, SMS is delivered via the SGs interface. For MO-SMS, UE first
establishes a NAS tunnel to transfer the SMS PDU to MME. MME then transfer these SMS PDU over to
MSC via the SGs. MT-SMS works the same way by having the MME establish a NAS tunnel to UE over
E-UTRAN access.
When interworking with 1xMSC, the UE establishes a CDMA2000 tunnel with the 1xCS IWS via EPS and
SMS is delivered via that tunnel. EPS is transparent to this process.
3GPP also defines the CSFB UE in voice-centric and data-centric mode of operation in TS 23.221. Voicecentric
CSFB UE will always attempt to find a RAT where voice services can be supported. In the
example of UE receiving an SMS-only or “CSFB Not Preferred” indication from the network during
combined EPS/IMSI attach procedure, the voice-centric UE will autonomously switch to UTRAN/GERAN
access if coverage is available so voice service is possible to this user. With a data-centric mode of
operation, the CSFB UE will not switch to UTRAN/GERAN given the same scenario with the SMS-only
indication from the network and will forgo the voice services or CS domain services altogether. This is
because the data-centric mode UE wants the best possible PS access and voice is not the determining
factor to move away from EPS.
www.4gamericas.org February 2011 Page 126