LTE随机接入为什么分成reamblesGroupA 和reamblesGrou

请问将随机接入Preamble分成A组和B组的目的是什么？根据什么原则将64个Preaml分成两个组呢？

36.321里面关于随机接入资源选择部分有这么一段描述：

“If the uplink message containing the C-RNTI MAC control element or the uplink message including the CCCH SDU has not yet been transmitted, the UE shall:
- if Random Access Preambles group B exists and if the potential message size (data available for transmission plus MAC header and, where required, MAC control elements) is greater than
MESSAGE_SIZE_GROUP_A and if the pathloss is less than Pmax – PREAMBLE_
INITIAL_RECEIVED_TARGET_POWER – DELTA_PREAMBLE_MSG3 –
messagePowerOffsetGroupB, then:
- select the Random Access Preambles group B;
- else:
- select the Random Access Preambles group A.”

那么我就知道了，当UE的所在路损比较小，而发送的Msg3消息比较大，大于MESSAGE_SIZE_GROUP_A，那么就会选择groupB，当然前提是有groupB存在。因此groupB与A的存在就是用来传送不同大小的Msg3。这个用在基于竞争的随机接入过程。