終端向基站發(fā)送了Msg1，很自然期望得到基站的Msg2（RAR）響應(yīng)。本文主要針對Msg2講兩個(gè)重要的問題：1）終端如何接收Msg2？2）Msg2里面有什么？ 終端如何接收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) 這句話其實(shí)是對終端如何接收Msg2做了一個(gè)總結(jié)概括，涉及到以下幾個(gè)步驟： UE首先需要去解析用于Msg2的下行調(diào)度的PDCCH DCI。那么UE在什么時(shí)間去監(jiān)聽PDCCH DCI。這里面有個(gè)window的概念，UE在這個(gè)窗口內(nèi)去監(jiān)聽PDCCH。window的長度由參數(shù)ra-ResponseWindow規(guī)定（不能超過10ms），單位是slot。window的起點(diǎn)是距離PRACH發(fā)送之后至少間隔1個(gè)symbol的用于Typ1-PDCCH CSS的CORESET的首個(gè)symbol。聽起來有點(diǎn)繞，一般情況都是PRACH發(fā)送完之后的第一個(gè)下行slot作為起點(diǎn)（下面是協(xié)議的描述）。 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) 要監(jiān)聽的DCI的格式是format 1_0。這個(gè)DCI需要去Common Search Space去找，由參數(shù)ra-SearchSpace定義。 基站對這個(gè)PDCCH DCI的CRC使用了RA-RNTI擾碼，所以終端需要在ra-searchspace上面嘗試使用RA-RNTI去解析PDCCH DCI。 如果UE成功的使用RA-RNTI解調(diào)了DCI。接著它會(huì)去解PDSCH payload 如果UE繼續(xù)成功的解出了PDSCH，UE會(huì)檢查RAPID（random access preamble identity）是否與自己當(dāng)時(shí)發(fā)送Msg1時(shí)使用的RACH preamble ID相同。如果相同，UE認(rèn)為Msg2解調(diào)成功 如果UE在ra-ResponseWindow內(nèi)沒有使用RA-RNTI解出DCI，或者沒有成功解出PDSCH payload，或者解出了PDSCH但是RAPID不匹配，UE都會(huì)認(rèn)為Msg2接收失敗，會(huì)觸發(fā)Msg1重發(fā) 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.