5G NR 随机接入RACH流程（5）-- Msg2资料下载

终端向基站发送了Msg1，很自然期望得到基站的Msg2（RAR）响应。本文主要针对Msg2讲两个重要的问题：1）终端如何接收Msg2？2）Msg2里面有什么？ 终端如何接收Msg2 In response to a PRACH transmission, a UE attempts to detect a DCI format 1_0 with CRC scrambled by acorresponding RA-RNTI during a window controlled by higher layers   (摘自38213) 这句话其实是对终端如何接收Msg2做了一个总结概括，涉及到以下几个步骤： UE首先需要去解析用于Msg2的下行调度的PDCCH DCI。那么UE在什么时间去监听PDCCH DCI。这里面有个window的概念，UE在这个窗口内去监听PDCCH。window的长度由参数ra-ResponseWindow规定（不能超过10ms），单位是slot。window的起点是距离PRACH发送之后至少间隔1个symbol的用于Typ1-PDCCH CSS的CORESET的首个symbol。听起来有点绕，一般情况都是PRACH发送完之后的第一个下行slot作为起点（下面是协议的描述）。 The window starts at the first symbol of the earliest CORESET the UE is configured to receive PDCCH for Type1-PDCCH CSS set, as defined in Subclause 10.1, that is at least one symbol, after the last symbol of the PRACH occasion corresponding to the PRACH transmission (摘自38213) ra-ResponseWindow: Msg2 (RAR) window length in number of slots. The network configures a value lower than or equal to 10 ms (see TS 38.321 [3], clause 5.1.4). (摘自38331) 要监听的DCI的格式是format 1_0。这个DCI需要去Common Search Space去找，由参数ra-SearchSpace定义。 基站对这个PDCCH DCI的CRC使用了RA-RNTI扰码，所以终端需要在ra-searchspace上面尝试使用RA-RNTI去解析PDCCH DCI。 如果UE成功的使用RA-RNTI解调了DCI。接着它会去解PDSCH payload 如果UE继续成功的解出了PDSCH，UE会检查RAPID（random access preamble identity）是否与自己当时发送Msg1时使用的RACH preamble ID相同。如果相同，UE认为Msg2解调成功 如果UE在ra-ResponseWindow内没有使用RA-RNTI解出DCI，或者没有成功解出PDSCH payload，或者解出了PDSCH但是RAPID不匹配，UE都会认为Msg2接收失败，会触发Msg1重发 Msg2里面有什么 6.1.5 MAC PDU (Random Access Response) A MAC PDU consists of one or more MAC subPDUs and optionally padding. Each MAC subPDU consists one of the following: a MAC subheader with Backoff Indicator only; a MAC subheader with RAPID only (i.e. acknowledgment for SI request); a MAC subheader with RAPID and MAC RAR.