LTE隨機接入為什麼分成reamblesGroupA 和reamblesGrou

請問將隨機接入Preamble分成A組和B組的目的是什嗎？根據什麼原則將64個Preaml分成兩個組呢？

36.321裡面關於隨機接入資源選擇部分有這麼一段描述：

“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.”

那麼我就知道了，當UE的所在路損比較小，而發送的Msg3訊息比較大，大於MESSAGE_SIZE_GROUP_A，那麼就會選擇groupB，當然前提是有groupB存在。因此groupB與A的存在就是用來傳送不同大小的Msg3。這個用在基於競爭的隨機接入過程。