2 Optional Features Description
2.23 LOFD-001019 PS Inter-RAT Mobility between E-UTRAN and UTRAN
Availability
This feature is available from eRAN1.0.
Summary
PS inter-RAT Mobility between E-UTRAN and UTRAN provides the function of inter-RAT cell selection and reselection between E-UTRAN and UTRAN, and the function that the UEs can be handed over to an inter-RAT UTRAN cell for the reason of limited cell coverage. If the PS handover is not supported by the current networks, the PS redirection between E-UTRAN and UTRAN is provided to realize the inter-RAT mobility. Moreover, the blind handover is provided if inter-inter-RAT measurements may be omitted (to save time and resources) or can be unavailable.
PS handover between E-UTRAN and UTRAN also supports the function that the UEs can be handed over to an inter-RAT UTRAN cell when there is uplink coverage restriction on E-UTRAN.
PS handover based on uplink power is supported. When UE’s service QoS is limited in uplink, eNodeB can trigger an inter-RAT handover to UTRAN to guarantee the service QoS.
Benefits
The feature provides the following benefits:
Enables the seamless co-existence between E-UTRAN and UTRAN
Guarantees smooth evolution from legacy wireless systems to LTE systems.
Provides supplementary coverage for E-UTRAN in the early phase using the legacy wireless systems to prevent call drop, thus, seamless user coverage
Improves the network performance and end user experience
Description
Handover between E-UTRAN and UTRAN is a critical feature to allow seamless co-existence and a smooth evolution from the legacy wireless communication systems to LTE system. It is one type of inter-Radio Access Technology (RAT) handover. It exists in the early E-UTRAN deployment when a UE moves to an area where E-UTRAN does not have coverage while UTRAN has.
In Huawei eRAN1.0, handover is based on the coverage by evaluating the cells’ DL reference signals that can be RSRP (Reference Signal Received Power), RSRQ (Reference Signal Received Quality) of E-UTRAN, and Received Signal Code Power (RSCP) or Ec/N0 of UTRAN .
When a UE moves out of the area of E-UTRAN, the eNodeB decides whether to handover it from E-UTRAN to UTRAN according to its reported measurement. The UE performs handover to the target UTRAN cell when it receives the handover command from the source eNodeB.
The inter-RAT measurement of the target cell is gap-assisted for the UE with only one RF receiver. In the serving cell, the inter-RAT measurement is triggered by an event A2 that means the DL reference signal quality of E-UTRAN become worse than the absolute threshold, and stopped by an event A1 that means the DL reference signal quality of E-UTRAN becomes better than absolute threshold.
The inter-RAT handover is triggered by an event B1 that means the Common Pilot Channel (CPICH), RSCP and/or Ec/N0 of UTRAN cells is better than absolute threshold. After receiving the measurement report from the UE, the eNodeB decides wether to hand over it to UTRAN.
Huawei eNodeB also supports PS handover from UTRAN to E-UTRAN.
In some specific scenario, inter-RAT measurements may be omitted (to save time and resources) or can be unavailable. In such a scenario, Huawei provides Blind Handover to realize inter-RAT handover from E-UTRAN to UTRAN in eRAN2.0. For example, if an E-UTRAN cell is co-sited with a UTRAN site, and having the same coverage range, operators can configure the UTRAN cell as the E-UTRAN cell’s blind handover target cell. When handover trigger conditions (load, service) are met, the eNodeB can handover the UE to the blind handover target cell without inter-RAT measurement. Blind handover, compared to PS handover, features more reduced handover time.
If the legacy UTRAN or UEs can not support PS handover, Huawei provides PS redirection functionality to realize inter-RAT handover between UTRAN and E-UTRAN in eRAN2.0. There is no update requirement for legacy E-UTRAN and UEs to support PS Redirection.
The procedure of PS redirection is the same as that of RRCConnectionRelease in which the carrier frequency infromation of the target redirection system will be included in the RRCConnectionRelease message. After a RRC connection of a UE is released by the source LTE system, the UE reselects the target system based on the received carrier frequency information during the release procedure and re-establishes
the connection with the target system. In summary, the handover mechanism of PS redirection consists of connection release, carrier-frequency re-selection, and connection re-establishment.
Note that the above description refers to a UE in active mode mobility. In idle mode mobiliy, Cell Selection and Reselection are procedures used for searching a new RAT serving cell. The UE will continually perform this procedure when it moves. Cell selection and Reselection for inter-RAT is usually performed in the following scenarios:
Cell Selection: Procedure of cell selection is invoked when the UE initially turns on. The cell of which technology is selected by the UE is based on the priority setting.
E-UTRAN toUTRAN Cell Reselection: The UE has initially camped on the LTE cell. When the UE moves out of E-UTRAN coverage, the UE needs to reselect UTRAN if available.
UTRAN to E-UTRAN Cell Reselection: The UE has camped initially either on a UTRAN cell. When the UE enters a cell of UTRAN coverage, and if E-UTRAN is configured with higher priority, the UE will reselect E-E-UTRAN. The priority information is broadcasted in cell system information.
When camping on a cell, the UE regularly searches for a better cell according to the cell reselection criteria. If a better cell is found, that cell is selected.
Generally speaking, LTE system is limited in uplink. Sometimes, QoS can be
guaranteed in downlink, but in uplink it is not satisfied even UE has transmitted its full power. To guarantee UE’s service QoS in this scenario, Huawei supports uplink transmission power based inter-RAT handover to UTRAN.
While eNodeB detected UE’s QoS is limited, eNodeB will send measurement control message to UE. When UE reports B1 event to eNodeB, eNodeB decides whether to handover to UTRAN.
Enhancement
In eRAN2.0, PS Redirection and Blind Handover between E-UTRAN and UTRAN are supported.
In eRAN2.1, the Handover based on UL power is supported. It guarantees service continuity in uplink limited power or limited E-UTRAN coverage when a UE moves to the cell edge.
Dependency
The functionality of PS handover between E-UTRAN and UTRAN depends on UTRAN and Core Network’ support.