Conditional Handover

5G will bring substantial performance requirements for wireless networks in order to accommodate the demands of new scenarios and improve the customer experience. Enhanced Mobile Broadband (eMBB) offers uplink and downlink speeds between gigabits per second. Ultra-Reliable and Low Latency Communications (URLLC) requires air interface latency below 1 milliseconds.

Handover performance has significant impact on the capacity to meet these requirements. Interruption of data flow during handover or, even more damagingly a failure in handover performance affects the ability to meet the requirements.

In Release 16, 3GPP Introduced a new Handover procedure that permits UE to decide whether to execute handover after certain conditions are met in the past, the Legacy handover procedure, the handover network was responsible for making the final decision on whether handover should take place or not. It was a reactive procedure and susceptible to the handover failing.

Baseline Handover (BHO)

The Baseline Handover (BHO) procedure employed to hand over data in Long Term Evolution (LTE) is re-used to 5G networks in 3rd Generation Partnership Project (3GPP) release 15. The timing at which you trigger the handover procedure in BHO is crucial. This is due to the signal from the cell serving adequate to get the handover signal, and the signal from the cell that is targeted should be sufficient to access. This is particularly evident in the mm-Wave frequency range due to the fast signal degradations as well as the extensive BS deployment.

Conditional Handover (CHO)

Handover with condition (CHO) was introduced to New Radio (NR) 3GPP release 16 to enhance the robustness to mobility of the BHO. In CHO the connection between the handover preparation and execution is eliminated by introducing a conditional process in which handover preparation is done prior to the serving cell while accessing the cell of the target is completed late.

In the event of a conditional handover the device is set up with a set of potential target cells in advance. This is accomplished by the device receiving the appropriate handover commands for each of the candidates for targets. But, unlike the traditional handover, handover commands are not performed upon the reception.

Instead, the handover commands are saved in the device, and are only performed when specific conditions are fulfilled, such as when the quality of the link to the cell in question falls above a specified threshold. This way it is possible to avoid the possibility that the handover won’t be carried out due to declining channel conditions, which can lead to a lower chance of radio link failure.

Principles for Conditional Handover 

• Conditional handover (CHO) is introduced for 5G NR as part of 3GPP Release 16
• Conditional handover can improve Handover Success rate
• CHO is possible for both 5G NR as well as 4G LTE
• In CHO, Candidate gNB as well as the potential Target gNBs will provide the configuration
• Source gNB provides an Execution condition(s) for the UE regarding the time to trigger the CHO.
• CHO execution condition can comprise one or more trigger conditions.
• It is possible to use at least two trigger quantities e.g. RSRQ and SINR and RSRP and can be configured in parallel to evaluate the CHO execution requirements of a single cell.
• If the UE is configured using CHO Configuration and, before the CHO execution condition has been satisfied , a different HO Command delivered by the gNB then UE will initiate Handover in response to the HO Command that was received and does not have to have to wait to see if any of the conditions to be satisfied.
• The Legacy HO Configuration is taken priority over CHO Configuration (if set).
• While the CHO is running, i.e. beginning at the moment that UE begins synchronization of the targeted cell UE does not keep track of the sour cell. The
• Condition Handover (CHO) does not available for NG-C-based handover until release-16 of the specification.

Conditional Handover call flow

• Step 1 : UE send measurement report informing gNB that handover to an adjacent cell is mandatory. This message, however, has a list of potential neighbor cells.
• Step 2: Assuming there is a possibility that every one of the cells is controlled by a distinct gNB.Thus multiple XnAP Handover Preparation processes are executed and each target gNB will allocate radio resources to the UE and sends the handover command (NR RRCReconfiguration message) which is transmitted to the source gNB
• Step 3. The source gNB creates the conditional handover command that is an message called an NR RRC Reconfiguration message, which includes a list of different reconfiguration options for conditional reconfiguration as well as other RRC measurement options that enable the UE to determine what of the potential cells to target is the best one to suit the requirements.
• Step 4: The UE makes its handover choice and then moves to the cell managed by the this gNB
• Step 5:  UE sends an NR RRC Reconfiguration complete message.
• Step 6: gNB#1 determines that the handover has taken completeness based on the reception from the NR RRC Reconfiguration Complete signal, then executes NGAP Path Switch procedure and initiates the release of the context of the UE in the this gNB source on behalf that it sends the XnAP Context Release message to UE.
• Step 7: The source gNB detects the successful completion of the handover and also the release of radio resources that are provided by the other potential target gNBs, to which it then sends the latest XnAP Conditional Handover cancel message. /li>

CHO in Non Terrestrial Networks

The availability of cell technology via satellites opens up the possibilities to extend coverage across the globe even in areas that were previously devoid of mobile phone communications. But, it comes at certain costs, not least , the need to address the most difficult NTN-specific issues. One such area to consider is mobility, that in Non-Terrestrial Networks can be a challenge, particularly in the case of Low-Earth Orbit (LEO) satellites which typically operate at altitudes of 600 to 1200 km.

This is a huge propagation delays, which can adversely affect the execution on time of the HO. In addition, LEO satellite’s continuous movement causes both UEs and gNBs operating moving, in contrast to Terrestrial Networks (TNs) in which the base stations’ location is fixed. This is the reason for the fact that the coverage on Earth and will shift as satellites move.

LEO satellites’ speed can be around 7.5 km/s and is certainly not a negligible amount for how fast the footprint of Earth will be gone. In the classic method of CHO, the performance criteria are assessed on the basis of the radio conditions (e.g. the strength of the received signal or its quality) of the target cell and the source. For NTN however, new conditions will need to be established to account for the location of the satellite as well as temporary coverage (HO for LEO-based NTN is tested every 5 to 6 seconds).

Relying on the quality/strength of the signal received of the distribution inside the cell might not be sufficient as the variance of the signal measured within the cell’s centre and at the cell’s edge could be too small for the UE to understand that the cell’s signal strength or quality is decreasing and coverage will end soon. Therefore we decided to establish two additional handover execution requirements for NTN

CHO in Integrated Access Backhaul

Integrated Access Backhaul is a feature of New Radio, which allows to transmit wireless communications via Radio Access Network (RAN). A brand new entity, dubbed IAB donor was introduced which is responsible to backhaul traffic and further provide the gNB feature. IAB donors are connected to IAB-nodes. These are the equivalent of the gNB-DU (Distributed Unit) and are also able to support UE capabilities (denoted to this end as IABMT). The full details of the architecture and procedures are described in the technical report

3GPP Release 17 provides further improvements to the IAB framework, such as the CHO feature to increase reliability. Cho is a possible solution for IAB-MTs which will mostly resemble the process described in Release 16 to transfer the UEs. But, CHO can also be a possibility for an IAB-node migration that is, when IAB node relocates to a different parent node in an IAB donor CU (Centralized unit). IAB-node is static and the requirements for carrying out the migration is likely to differ from the criteria set for IAB-MT. It is expected that the 3GPP Release 17 work concerning the enhancement of IAB will be finished around the middle of 2022.

CHO in NR Unlicensed

The mobility of NR-U systems is an additional aspect that needs to be dealt with differently from handovers that are common to systems operating in licensee frequency band. In NR-U, multiple UEs could be competing for spectrum access in a non-coordinated manner. This means that it is not sufficient to reuse the existing Release 16 CHO triggering conditions , which are based solely on the quality of the reference signal as well as power. Instead, it should be considered the best way to incorporate the LBT results into the decision to perform the CHO. The UE must detect the channel of communication and initiate the CHO only in the event that the results of the channel verification is in line with the established standards.

References

• 3GPP 38.300 5G NR and NG-RAN Overall description  Stage-2.

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