初入终端公司的modem协议工程师。对于UE PCI的变换，有的同事说是重选，有的同事说是切换。我是傻傻分不清的，完全不知道有什么差异。在做完一个日本项目后，对切换有了一定的了解。对UE切换做下简单的总结
（以下以LTE为例）
1.切换在RRC_CONNECTED
RRC_CONNECTED:
- Network controlled mobility, i.e. handover and cell change order with optional network assistance (NACC) to GERAN (not applicable for NB-IoT);
2.RRCconnectionreconfiguration （配置事件和门限等参数）
3.MeasurementReport（UE测量后满足门限上报）
2和3流程示例如下：
4.Measurement report triggering
为了方便理解事件的含义，根据之前处理问题的经验，画了下面这个图，供大家理解。
（这个图并不是实际场景图）
蓝红为两个LTE小区：蓝为A1小区，红为A2小区
假设服务小区为cell 1
Event A1 (Serving becomes better than threshold
当cell 1 RSRP > - 85 上报A1事件，关闭其它的测量
Event A2 (Serving becomes worse than threshold)
当cell 1 RSRP < - 100 上报A2事件，网络会配置A3事件门限
Event A3 (Neighbour becomes offset better than PCell/ PSCell)
UE起测量，测量cell 2，满足A3门限后上报，并完成切换
Event A4 (Neighbour becomes better than threshold)
Event A5 (PCell/ PSCell becomes worse than threshold1 and neighbour becomes better than threshold2)
流程如下：