Load balancing is achieved in NR with handover, redirection mechanisms upon RRC release and through the usage of inter-frequency and inter-RAT absolute priorities and inter-frequency Qoffset parameters.
Measurements to be performed by a UE for connected mode mobility are classified in at least four measurement types:
Intra-frequency NR measurements;
Inter-frequency NR measurements;
Inter-RAT measurements for E-UTRA;
Inter-RAT measurements for UTRA.
For each measurement type one or several measurement objects can be defined (a measurement object defines e.g. the carrier frequency to be monitored).
For each measurement object one or several reporting configurations can be defined (a reporting configuration defines the reporting criteria). Three reporting criteria are used: event triggered reporting, periodic reporting and event triggered periodic reporting.
The association between a measurement object and a reporting configuration is created by a measurement identity (a measurement identity links together one measurement object and one reporting configuration of the same RAT). By using several measurement identities (one for each measurement object, reporting configuration pair) it is then possible to:
Associate several reporting configurations to one measurement object and;
Associate one reporting configuration to several measurement objects.
The measurements identity is used as well when reporting results of the measurements.
Measurement quantities are considered separately for each RAT.
Measurement commands are used by NG-RAN to order the UE to start, modify or stop measurements.
Handover can be performed within the same RAT and/or CN, or it can involve a change of the RAT and/or CN.
Inter system fallback towards E-UTRAN is performed when 5GC does not support emergency services, voice services, for load balancing etc. Depending on factors such as CN interface availability, network configuration and radio conditions, the fallback procedure results in either RRC_CONNECTED state mobility (handover procedure) or RRC_IDLE state mobility (redirection), see TS 23.501
and TS 38.331
SRVCC from 5G to UTRAN, if supported by both the UE and the network, may be performed to handover a UE with an ongoing voice call from NR to UTRAN. The overall procedure is described in TS 23.216
. See also TS 38.331
and TS 38.413
In the NG-C signalling procedure, the AMF based on support for emergency services, voice service, any other services or for load balancing etc, may indicate the target CN type as EPC or 5GC to the gNB node. When the target CN type is received by gNB, the target CN type is also conveyed to the UE in RRCRelease Message.
Inter-gNB CSI-RS based mobility, i.e. handover, is supported between two neighbour gNBs by enabling that neighbour gNBs can exchange and forward their own CSI-RS configurations and on/off status.