In a wireless 5G network, when a mobile device (UE) travelling across different sites, it can be either in connected mobility or Idle mode mobility. Considering the two situation’s 5G mobility scenarios i.e. in Connected Mode Mobility UE does perform handover and in Idle mode mobility UE does cell selection/reselection as shown in below figure.
When UE doing in connected mode mobility and does handovers, these handovers be following types considering a multi frequency deployment.
- Blind Handover
- Coverage Based Intra Frequency Handover
- Coverage Based Inter Frequency Handover
- Frequency Priority Based Inter Frequency Handover
Handover Flow at Top View
In this section we will discuss the top level handover procedure. If you are interested in detailed 5G Handover Call flows are available at following.
Top Level Handover call flow includes, Tigger Phase, Measurement Phase, Target Cell Selection Phase and Handover Phase as shown in following picture.
- Trigger Phase: Trigger phase defines the configuration for neighboring frequencies as meas objects, event report thresholds and neighbor relation at gNB OAM or EMS, without these configuration handover will not trigger.
- Measurement Phase: Measurement phase includes delivery of meas objects, report config delivery to UE from gNB as part of RRC Configuration UE applies it successfully. Based on the configuration UE does measure the neighbor and report measurement report the gNB which includes the potential target cell PCI and signal strength.
- Target Cell Decision Phase: After receiving the measurement serving gNB has to decide the Target Cell. gNB selection for target cell consider following.
- Processing the measurement report (Not required for Blind Handover)
- Determination Handover Policy (Handover or Redirection)
- Identify Target Cell or Target Frequency
- Handover Phase: Once Target Cell and Target Frequency is decided, the Handover Procedure kick-off. Based on the network capability gNB can perform Xn based or N2 Based Handover or perform the redirection.
Coverage Based Intra Frequency Handover
Coverage based Infra Frequency Handover make use of A3 event, When UE detects a neighbor better dB offset than the serving cell gNB can decide the Hanover. Following Layer 3 parameters are important while optimizing Intra Frequency Handover.
- A3 Offset
- A3 Hysteresis
- A3 Time to trigger
Coverage Based Inter Frequency Handover
Coverage based Inter Frequency Handover make use of A2 Event. When UE reports A2 event measurement where serving cell signal goes below than the defined threshold, the gNB determines the UE is in cell edge poor radio condition and configure Event A5 and Event A1 for inter frequency measurement.
When UE reports Event A5 i.e. serving cell signal drops below to Threshold 1 and signal from neighbor cell become better than Threshold 2, the gNB generates the target cells or target frequencies as per the measurement results. Following Layer 3 parameters are important while optimization Inter Frequency Handover.
- A1 Threshold
- A1 Time to trigger
- A1 Hysteresis
- A2 Threshold
- A2 Time to trigger
- A2 Hysteresis
- A5 Threshold 1 and Threshold 2
- A5 Time to trigger
- A5 Hysteresis
Frequency Priority Based Inter Frequency Handover
In multi band co-coverage scenario, the frequency priory-based inter frequency handover is used to allow higher frequency to carry services as much as possible to keep low frequency band less loaded and continue coverage.
It used Event A4 related to inter frequency, where neighbor cell signal becomes better than threshold. When gNB receives Event A4 measurement and reported frequency has the highest traffic priority, gNB choose the frequency as target frequency and initiate handover immediately.
- 3GPP TS 38 300- 5G NR Overall description Stage-2
- 3GPP TS 38.401 – 5G NG-RAN Architecture Description
- 3GPP TS 38.412 – 5G NG-RAN NG signalling transport
- 5G Standalone Mode Registration-Attach Call Flow
- 5G Handovers – Connected Mode Mobiliy
- 5G RAN and 5GC Network Slice Signaling
- 5G SA Handover – Inter gNB-DU and Intra gNB-CU Handover
- 5G EPS Fallback – 5G to 4G Handover
- 5G NR Measurement – Serving Cell and Neighbor Cell
- 5G NR Measurement Events