1. Source: https://www.etsi.org/deliver/etsi_ts/138400_138499/138401/15.02.00_60/ts_138401v150200p.pdf
5G SA Handover – Inter gNB-DU and Intra gNB-CU Handover
Mobility (Handover) in an important feature in any telecom generation and so it is in 5G. The basic handover procedure
remains same as it was in legacy networks, i.e. UE reports measurement report with neighbor cell PCI and signal
strength to source cell, source cell take the decision to start handover procedure to best target cell and Target
Cell completes the Handover procedure.
In LTE, we use to have different types of Intra System (LTE to LTE) handover like X2 Based Handover, S1 Based
Handover, Inter Frequency, Intra Frequency and Inter sector handovers, similarly 3GPP specification has defined
different Intra system (5G NR to 5G NR) handovers, e.g. Xn based Handover, N2 or NGAP Based Handover, Intra
and Inter Frequency Handovers.
Apart from above mentioned handovers, 5G has defined some handover types based on newly introduced
disaggregated gNB-DU and gNB-CU architecture. Considering gNB-DU and gNB-CU implementation we can have
following types of handovers.
•Intra gNB-DU Handover
•Inter gNB-DU and Intra gNB-CU Handover (Source and Target gNB-DU connected to Common gNB-CU)
•Inter gNB-CU Handover (It can be Xn or N2 based Handover)
3. 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.
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gNB-DU
gNB-DU
gNB-DU
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Performance Measurement
Data collection
Handover Request
Conditional Handover
Execution
𝑡3
FL Training
Initiation
set of users
connected:
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Timeline preceding the FL Model Training in ORAN
6. Principles of Conditional Handover
•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 RRC Reconfiguration 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.
7. Handover Management Between RAN Nodes in ORAN
A RAN needs to be able to hand over connections to a device that is moving between the ranges of RAN base
stations. A seamless handover gives the user an uninterrupted connection as they pass from one base station’s
connection to another.
A specific RAN use case that relies on this ability is maintaining a consistent connection for autonomous
vehicles and user devices within them while driving at high speeds.
The O-RAN Alliance’s architecture, the near real-time RAN intelligent controller (RIC) handles several aspects
of handover management.
It collects and maintains historical data on traffic, navigation, radio, and hand over data. The controller also
monitors traffic and radio conditions in real-time.
The near real-time RIC includes AI and ML as well to detect and predict the irregularities affecting devices
during handover.
Finally, the near real-time RIC is in charge of deploying and maintaining real-time applications capable of
predicting, preventing, and mitigating handover irregularities.
8. Open RAN Use Case: Optimization
To maintain a good user experience, networking resources are provisioned to users and services with the greatest need.
Open RAN interface standards allow artificial intelligence (AI) to more easily communicate with the rest of the network
and optimize use quality of experience (QoE).
AI software can use algorithms to recognize traffic and network health to determine how to provision resources. The
machine learning models that train the AI can be done offline in a Non real-time RIC because the training does not have
to respond to anything in real time.
The near real-time RIC is used to enforce the decisions made by the AI software. Enforcing the decisions has to take
place in the near real-time RIC because timing affects the QoE.
Quality of service (QoS) is similar to QoE; however, QoS maintains the performance of a network service. QoE focuses
more on maintaining a good connection for users.
In a 5G RAN, traffic patterns change quickly and the infrastructure is heterogeneous. This means QoS policies cannot be
generalized and require adaptive capabilities.
Resource allocation controlled by QoS policies are used by both Non real-time and near real-time RICs to optimize how
resources are given to users using the same service. A Non real-time RIC is able to take in information on resource
demand and select users to prioritize for resource allocation. From there, the near real-time RIC can enforce the resource
allocation in the RAN’s central and distributed units.
