The secret of 5G switching is in this 3W1H

1. Introduction to cell switching

When you sit on the high-speed train and are immersed in a hearty team battle, you are about to push to the enemy crystal, and victory is close at hand. At this time, the picture suddenly paused, and the number of "460" was dazzling as if mocking: "If you don't have a signal, you still want to win a team battle?" ”

Don't worry, at this time, the switching technology shines on the stage, shouting "Team battles must not be lost, I will escort you!" ”

In fact, the service community has been silently paying attention to the status of your mobile phone signal, and when the signal begins to "drop the chain", the service community immediately takes action to migrate the mobile phone to the community with a better signal, so as to ensure that your game is not online and the team battle is not left behind. It can be said that switching technology is the "strongest support" behind team battles! So, how exactly does a switch work? Let's uncover its mysteries together!

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2. Principle of cell switching

What is a Toggle?

Definition of cell switching

For a UE in the RRC_CONNECTED state, the base station monitors the air interface status of the UE to determine whether the service cell needs to be changed. This process is called connected mobility management, often referred to simply as handover. The essence of handover refers to the process of migrating the UE from the service area of one base station to the service area of another base station, with the aim of ensuring the continuity of communication.

Toggles are different from redirects and cell reselection. During the switchover process, a connection is established with the target cell, and then the source cell is released. If the redirection condition is not met, the UE carries the information of the target cell through the RRC release message in the RRC_CONNECTED state, releases the source cell, and then connects to the target cell. The first two are initiated by the base station, while the cell re-election is decided by the UE independently, and the UE selects the best cell by monitoring the current cell and neighboring areas in the RRC_IDLE and RRC_INACTIVE states, without the participation of the network in the process.

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The type of cell switching

• The following figure shows the topological relationship between NR and LTE cell connections, which is mainly divided into inter-site handover and intra-site handover.
• According to the system of the cell, it can be divided into intra-system switching and different system switching.
• Taking NR inter-station handover as an example, it can be divided into Xn handover and NG handover according to the different logical ports and links used during handover.

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When switching cells, it is necessary to ensure that the link between the two cell base stations is open to ensure the normal transmission of switching signaling.

Why

Overlay-based switching

When a user device moves to the edge of the current service cell, leaves the effective coverage area of the current cell signal, and enters an area with a weak or no signal, it needs to switch to a cell with a better signal to ensure the continuity of the user's communication services. In high-speed mobile scenarios, such as highways and high-speed railways, users can quickly pass through the coverage areas of different base stations, and the need for switching is particularly obvious.

Load-based switching

In an area with a particularly high traffic density, when there are too many users in a cell and the network load is too high, in order to ensure the quality of user communication services, some users can be switched to adjacent cells with light load, which can optimize the allocation of network resources and improve the overall network performance and user experience. In scenarios such as high-density cells and subway stations, the switching technology enables network resources to be dynamically allocated to users in different regions, avoiding too many users accessing high-load base stations and optimizing the allocation of resources.

Business-based switchover

Based on the specific service type, quality of service requirements (QoS), priority, and network resources running on the user's device, the switchover target is dynamically determined to meet the differentiated network requirements of different services and further improve the user experience. Switch users with high latency requirements, such as online games, Internet of Vehicles, and real-time live broadcasts, to cells that can provide stable connections, intelligently adapt to user business needs, and improve network utilization.

When to switch (When)

Handover triggered by a change in the user's location

During the movement, when the user terminal detects that the signal quality (RSRP/RSRQ/SINR, etc.) of the current service cell drops below the preset threshold, the user will switch to the nearby neighboring area.

Switchover triggered by network load

When the network load (number of users, PRB, traffic, etc.) of a cell exceeds the preset threshold, some users will be switched to the cell with a light load.

The switchover triggered by the user's service

If the current service cell cannot meet the user's high traffic or low latency services, the service will be switched from the current cell to the cell that can guarantee the user's services.

How to switch

The general switchover process is as follows:

• The measurement control delivery
  base station delivers the measurement configuration used for switching to the UE, and the measurement configuration information mainly includes the measurement object, report configuration and other configurations of the measurement task.
• The measurement report is reported to
  the UE to perform the measurement according to the received measurement control message, and after the UE measures and determines that the event trigger condition is reached, the measurement report is reported to the base station.
• The target cell judgment
  base station selects the target cell or the target frequency point, and mainly evaluates and makes decisions on the candidate cell with reference to the information such as the handover mode, measurement report, and handover strategy.
• After the handover is executed
  in the target cell, the base station will execute the handover according to the selected handover policy and initiate a handover request for the best quality cell in the filtered target cell list.
• Check whether the switchover is successful
  , and if the switchover is executed, the switchover will be terminated. In order to reduce invalid handover retries, a handover failure penalty mechanism is added to reduce the number of invalid handover retries in the scenario where the handover fails to be initiated. According to the failures caused by different stages of switchover execution, the penalties for switchover failure are divided into the following penalties: penalties for failover preparation and penalties for failover execution. In the penalty timer, the UE prohibits switching to the target cell.

Switchovers are usually triggered by measurement events, which are commonly seen below:

3. Summary

This time, we introduced the relevant principles of 5G cell switching through 3W1H, which can improve the network experience of users in the mobile process and static scenes. I hope that through the introduction of this article, readers can have an overall understanding of community switching.