MOBILITY MANAGEMENT TODAY

Cellular carriers dispense their services by laying out a blanket of cellular towers around an area. Cellular towers can manage multiple cells (antennas), each of which covers a geographical area. PCI (Physical Cell ID) is the identifier used for cells at the physical layer. For any mobile device, its primary cell is considered to be the backbone of cellular connection. It provides basic control plane signaling (e.g., connection establishment, HO management, and security) along with data services to the user equipment (UE). In addition, a UE (e.g., a smartphone device) can subscribe to multiple secondary cells for higher bandwidths. With the data flowing from a UE via a cellular tower to the 4G/5G core, mobility management procedures (e.g., HOs, MRs, etc.) are employed to switch between cells and continuously report on the signal quality of UE.

移动运营商通过在特定区域布置蜂窝塔网络来提供服务。每个蜂窝塔可以管理多个单元（天线），每个单元覆盖一个地理区域。物理层使用物理单元ID（PCI）来标识这些单元。对于任何移动设备，其主单元被视为蜂窝连接的骨干。它提供基本的控制平面信令（例如连接建立、切换管理和安全性），以及为用户设备（UE）提供数据服务。此外，UE（例如智能手机设备）可以订阅多个辅助单元以获得更高的带宽。随着数据从UE通过蜂窝塔流向4G/5G核心网络，移动性管理程序（例如切换、测量报告等）被用于在单元之间切换，并持续报告UE的信号质量。

HO Procedures. Fig. 1 depicts a basic HO procedure; the detailed description of all steps is in Appendix A.1. Carriers use multiple radio signal quality indicators such as Reference Signal Received Power (RSRP), Reference Signal Received Quality (RSRQ), Signal to Interference & Noise Ratio (SINR), etc. [8] to perform measurements based on the configurations received from the primary cell. We refer to these radio quality indicators as RRS (RSRP, RSRQ, SINR) for the rest of the paper. If any event trigger criterion is met, a measurement event is raised and its report is sent to the primary cell. The primary cell then decides a target cell based on carrierspecific HO logic and directs UE to perform HO with the target cell via an HO command (RRC Connection Reconfiguration [10]). Finally, the UE undergoes HO and performs link synchronization through Random Access Procedure [14].

切换程序。图1展示了基本的切换程序；所有步骤的详细描述见附录A.1。运营商使用多个无线信号质量指标，如接收参考信号功率（RSRP）、接收参考信号质量（RSRQ）和信噪比（SINR）等，根据从主单元接收的配置执行测量。如果满足任何事件触发条件，则会引发测量事件并将其报告发送到主单元。主单元根据特定于运营商的切换逻辑决定目标单元，并通过RRC连接重配置命令指示UE与目标单元执行切换。最后，UE进行切换并通过随机接入程序执行链路同步。

HOs in 5G: A Taxonomy. The classification of HOs has become complex in 5G; Table 2 summarizes the radio access technology change and 4G/5G HO category for each HO type used in the paper. In NSA 5G, all the cells associated with eNB constitute a master cell group (MCG). On the other hand, the group of cells linked to the gNB form a secondary cell group (SCG). A new category of HO procedures was introduced in 3GPP Release-15 [4] for SCG HO management. Fig. 2 provides an overview of SCG HO procedures used to add, modify and release 5G cells. SCG Addition adds 5G-NR cells to the existing LTE connection while SCG Release removes them. SCG Modification is used to switch 5G cells within the same SCG (or gNB). Unlike inter-eNB HO in LTE, NSA 5G does not have an option to perform a direct HO between two gNBs. Hence, the SCG Change procedure (a combination of SCG Release and Addition) is used to move the UE from one gNB to another. A master-eNB (MeNB) HO will change the LTE cell while keeping the gNB the same. In SA 5G, we only observe MCG HO that moves the UE from one 5G-NR cell to another.

5G中的切换：分类。5G中的切换分类变得复杂，表2总结了无线接入技术的变化和4G/5G切换类别。非独立（NSA）5G中，与eNB关联的所有单元构成主单元组（MCG）。另一方面，与gNB关联的单元组构成辅助单元组（SCG）。3GPP Release-15引入了一种新的切换程序类别，用于SCG管理。图2提供了SCG切换程序的概述，用于添加、修改和释放5G单元。SCG添加将5G-NR单元添加到现有的LTE连接中，而SCG释放则删除它们。SCG修改用于在同一SCG（或gNB）内切换5G单元。与LTE中的eNB间切换不同，NSA 5G不支持直接在两个gNB之间执行切换。因此，使用SCG更改程序（SCG释放和添加的组合）来将UE从一个gNB移动到另一个gNB。主eNB（MeNB）切换将更改LTE单元，同时保持gNB不变。在独立（SA）5G中，我们只观察到MCG切换，它将UE从一个5G-NR单元移动到另一个单元。