在36.211协议中有这么一段描述：
The parameter prach-ConfigurationIndex is given by higher layers. For frame structure type 2 with PRACH configuration 0, 1, 2, 20, 21, 22, 30, 31, 32, 40, 41, 42, 48, 49, 50, or with PRACH configuration 51, 53, 54, 55, 56, 57 in UL/DL configuration 3, 4, 5, the UE may for handover purposes
assume an absolute value of the relative time difference between radio frame iin the current cell and the target cell is less than 153600Ts。
请问下这些配置的随机序列用于切换，原因是什么？无线帧号之间的差异是怎么理解？