1.1隨機接入
1.1.1隨機接入的分類
隨機接入發(fā)生的5種場景：
1、 Initial access from RRC_IDLE;
2、 Initial access after radio link failure;
3、 Handover requiring random access procedure;
4、 DL data arrival during RRC_CONNECTED requiring random
access procedure;
5、 UL data arrival during RRC_CONNECTED requiring random
access procedure;

隨機接入的兩種類型
– Contention based (apply to all five events);
– Non-contention based (apply to only handover and
DL data arrival).


Contention based

Non-contention based



1.1.2隨機接入過程的作用
1．獲得CRNTI。用戶接入網(wǎng)絡，給用戶分配CRNTI；上述場景1空閑態(tài)的隨機接入和場景2無線鏈路失步情況下發(fā)起的隨機接入（獲得新的CRNTI）就是為了到達這個目的。
2．上行同步。發(fā)起隨機接入的場景都有獲得上行同步的需求，其中場景3～5發(fā)起的隨機接入完全是為了獲得上行同步。

1.1.3為什么隨機接入過程需要競爭解決
LTE系統(tǒng)中區(qū)分不同終端隨機接入過程的手段：
1、如果不同終端在不同時頻單元發(fā)送公共preamble，終端可以通過RA-RNTI區(qū)別，就是msg2的PDCCH和PDSCH（不同資源）使用不同的RNTI進行加擾。
2、如果不同終端在同一個時頻單元發(fā)送不同公共preamble，終端可以通過RAR subhead中的RAPID區(qū)別，也就是說pdcch和pdsch資源共用，通過mac pdu內(nèi)容區(qū)分。
3、如果不同終端在同一個時頻單元發(fā)送同一個公共preamble，上述手段都無法起作用，這時需要通過后續(xù)競爭解決來完成確認

競爭解決的重點在終端側而不是基站側，基站始終認為是一個用戶，競爭解決是讓終端確認網(wǎng)絡側接納的是自己而不是其它終端。

對于非競爭隨機接入，由于終端使用專用preamble的隨機接入，其它用戶不會用到，且網(wǎng)絡側知道給哪個用戶分配了專用preamble，所以不會出現(xiàn)上面說到的沖突情況，不需要競爭解決，UE收到msg2即認為隨機接入完成（Handover requiring random access procedure(專用preamble) /DL data arrival during RRC_CONNECTED requiring random
access procedure(專用preamble)）

1.1.4如何解決競爭
對于基于競爭的隨機接入，根據(jù)msg3中攜帶的內(nèi)容的不同終端使用不同的方式來確認。
1）Msg3中有CRNTI mac ce
如果是Mac觸發(fā)的隨機接入，終端收到的pdcch用crnti加擾且包含ul grant，競爭解決成功
（Handover requiring random access procedure(公共preamble)/ UL data arrival during RRC_CONNECTED requiring random access procedure）

如果隨機接入由pdcch order觸發(fā)，終端收到的pdcch用crnti加擾，競爭解決成功
(DL data arrival during RRC_CONNECTED requiring random
access procedure)

2）Msg3中有CCCH SDU
pdcch用tempt crnti加擾，pdu中包含競爭解決單元且與msg3中的ccch sdu內(nèi)容一樣，競爭解決成功
（Initial access from RRC_IDLE；Initial access after radio link failure）


1.1.5時序
隨機接入的各個環(huán)節(jié)都有時間范圍約束：
1、Msg1與msg2


Once the Random Access Preamble is transmitted and regardless of the possible occurrence of a measurement gap, the UE shall monitor the PDCCH for Random Access Response(s) identified by the RA-RNTI defined below, in the RA Response window which starts at the subframe that contains the end of the preamble transmission [7] plus three subframes and has length ra-ResponseWindowSize subframes.

2、Msg2與msg3
If a PDCCH with associated RA-RNTI is detected in subframe n, and the corresponding DL-SCH transport block contains a response to the transmitted preamble sequence, the UE shall, according to the information in the response, transmit an UL-SCH transport block in the first subframe,, if the UL delay field in section 6.2 is set to zero. The UE shall postpone the PUSCH transmission to the next available UL subframe if the field is set to 1.

2、Msg3與msg4
如果需要競爭解決的話，終端接收msg4受ContentionResolutionTimer限制。
終端一旦發(fā)出msg3或著進行msg3的重傳，都要啟動或重啟ContentionResolutionTimer，在ContentionResolutionTimer超時前沒有收到msg4，則認為競爭解決不成功。
1.1.6各消息內(nèi)容
Msg1：preamble（物理信道；時頻資源決定RA-RANTI；preamble id標識）
Msg2：pdcch（指示pdsch資源）和pdsch（攜帶隨機接入響應）
Msg2包含的mac pdu內(nèi)容如下（36.321和36.213）
A MAC PDU consists of a MAC header and one or more MAC Random Access Responses (MAC RAR) and optionally padding as described in figure 6.1.5-4.
The MAC header is of variable size.
A MAC PDU header consists of one or more MAC PDU subheaders; each subheader corresponding to a MAC RAR except for the Backoff Indicator subheader. If included, the Backoff Indicator subheader is only included once and is the first subheader included within the MAC PDU header.
A MAC PDU subheader consists of the three header fields E/T/RAPID (as described in figure 6.1.5-1) but for the Backoff Indicator subheader which consists of the five header field E/T/R/R/BI (as described in figure 6.1.5-2).
A MAC RAR consists of the four fields R/Timing Advance Command/UL Grant/Temporary C-RNTI (as described in figure 6.1.5-3).
Padding may occur after the last MAC RAR. Presence and length of padding is implicit based on TB size, size of MAC header and number of RARs.

Figure 6.1.5-1: E/T/RAPID MAC subheader

Figure 6.1.5-2: E/T/R/R/BI MAC subheader

Figure 6.1.5-3: MAC RAR

Figure 6.1.5-4: Example of MAC PDU consisting of a MAC header and MAC RARs
The MAC header is of variable size and consists of the following fields:
-E: The Extension field is a flag indicating if more fields are present in the MAC header or not. The E field is set to "1" to indicate at least another set of E/T/RAPID fields follows. The E field is set to "0" to indicate that a MAC RAR or padding starts at the next byte;
-T: The Type field is a flag indicating whether the MAC subheader contains a Random Access ID or a Backoff Indicator. The T field is set to “0” to indicate the presence of a Backoff Indicator field in the subheader (BI). The T field is set to “1” to indicate the presence of a Random Access Preamble ID field in the subheader (RAPID);
-R: Reserved bit, set to "0";
-BI: The Backoff Indicator field identifies the overload condition in the cell. The size of the BI field is 4 bits;
-RAPID: The Random Access Preamble IDentitfier field identifies the transmitted Random Access Preamble (see subclause 5.1.3). The size of the RAPID field is 6 bits.
The MAC header and subheaders are octet aligned.
The higher layers indicate the 20-bit UL Grant to the physical layer, as defined in [8]. This is referred to the Random Access Response Grant in the physical layer. The content of these 20 bits starting with the MSB and ending with the LSB are as follows:
- Hopping flag – 1 bit
- Fixed size resource block assignment – 10 bits
- Truncated modulation and coding scheme – 4 bits
- TPC command for scheduled PUSCH – 3 bits
- UL delay – 1 bit
- CQI request – 1 bit

Msg3：pusch。根據(jù)隨機接入的原因不同可能是rrc連接建立請求或rrc連接重建請求或BSR或rrc連接重配完成消息。
Msg4：對于空閑態(tài)隨機接入或鏈路失敗后的隨機接入有這個說法，Msg4包含RRC連接建立/重建立消息。