NR随机接入 RA(Random Access)
目的 ：
实现 UE与 gNB之间的上行同步（ TA）
gNB为 UE分配上行资源（ UL_GRANT）
随机接入 类型：
基于竞争随机接入： contention-based random access（ CBRA）
非竞争随机接入： contention-free random access（ CFRA）
Possible cases(接入场景 ) Contention-based (基于竞争 ) Contention-free (基于非竞争 ) Comments(终端或网络关键信息 )
Initial access from RRC Idle state(空闲态初始接入 ) √ - UE:send RRCSetupRequest in MSG3
Transition from RRC Inactive to RRC Connected(从 RRC Inactive到 RRC
Connected) √ - UE:send RRCResumeRequest in MSG3
RRC Connection re-establishment(RRC重建 ) √ - UE:send RRCReestablishmentRequest in MSG3
Handover(切换 ) √ √
BS:provide preamble using RACH-ConfigDedicated within
RRCReconfiguration
UE:send RRCReconfigurationComplete in MSG3
Downlink data arrival while UE is out of sync(终端失步下行数据到达 ) √ √
BS:signal a PDCCH-Order using DCI format 1_0,with(contention-free)
or without (contention-based) preamble infex
UE:for contention resolution,include a C-RNTI MAC CE in MSG3
Uplink data arrival while UE is out of sync(终端失步上行数据到达 ) √ - UE:for contention resolution,include a C-RNTI MAC CE in MSG3
Uplink data arrival for UE without PUSCH resource allocation(上行数
据到达终端无 PUSCH资源分配 ) √ -
UE:for contention resolution,include a C-RNTI MAC CE in MSG3.MSG3
may include a BSR MAC CE to request additional uplink resource
On-demand system information(其它系统消息请求 ) √(MSG3 based) √(MSG1 based)
Contention-based:
BS:do not include si-RequestConfig in SIB1
UE:send rrcSystemInfoRequest in MSG3
Contention-free:
BS:include si-RequestConfig in SIB1
UE:transmit a specific PRACH preamble in MSG1 to request a specific
SB,then monitor PDCCH for SI after MSG2
Beam failure recover(波束失败恢复 ) √ √
Contention-based:UE selects a PRACH preamble corresponding to the
SSB beam for recovery
Contention-free:BS provide UE with one PRACH preamble index for
each beam available for recovery
Scheduling request failure(调度请求失败 ) √ - UE use Random Access if BS does not provide an uplink grant after sr-TransMax(4 to 64)Scheduling Requests
Synchronous reconfiguration(同步信息重配置 ) √ √
BS:trigger synchronous reconfiguration by including
reconfigurationWithSync within RRCReconfiguration
UE:perform contention-based(ra-PreambleIndex excluded from
reconfigurationWithSync) or contention-free(ra-PreambleIndex
included in reconfigurationWithSync)Random Access
Establishing time alignment during Scell addition(添加 Scell期间建立
时间对齐 ) √ √
used to initialize Timing Advance of a newly added Scell belong to a
new Timing Advance Group (TAG).
Contention-based on Conetntion-free depends on If ra-PreambleIndex
is excluded from reconfigurationWithsync
UE gNB UE gNB
Random Access Preamble
Random Access Response
Scheduled Transmission
RRC Setup Request
Contention Resolution
RRC Setup
RRC Setup Completed
Random Access Preamble
Random Access Response
RA Preamble assignment
Contention-Based Contention-Free
从物理层看， NR随机接入过程包括：
? Msg0：高层通知 UE CFRA使用的 Preamble
? Msg1： PRACH，发送随机接入 preamble
? Msg2： PDCCH/PDSCH：发送 RAR
? Msg3： PUSCH，根据 Msg2的调度传输消息
? Msg4： PDCCH/PDSCH，竞争解决
其中 Msg3、 Msg4只在基于竞争的随机接入过程中 存在
NR随机接入信令流程
触发 随机接入事件：
竞争 随机接入场景说明
< Initial Registration – CB >
UE gNB
PRACH Preamble(MSG1)
RACH Response - RAR(MSG2)
RRC Setup Request(MSG3)
RRC Setup(MSG4)
RRC Setup Request Complete
< Initial Reestablishment – CB >
UE gNB
PRACH Preamble(MSG1)
RACH Response - RAR(MSG2)
RRC Reestablishment Request(MSG3)
RRC Reestablishment(MSG4)
RRC Reestablishment Complete
< RRC_INACTIVE – DL Data – CB >
RAR
Window
Contention
Resolution
Timer
RRC_CONNECTED State
OUT OF SYNC
IN SYNC
RA Trig
PRACH Preamble(MSG1)
RACH Response - RAR(MSG2)
RRC Resume Request(MSG3)
RRC Resume(MSG4)
RRC Resume Complete
RRC_INACTIVE State
RRC_CONNECTED State
UE gNB
Paging DL Data
< RRC_INACTIVE – UL Data – CB >
PRACH Preamble(MSG1)
RACH Response - RAR(MSG2)
RRC Resume Request(MSG3)
RRC Resume(MSG4)
RRC Resume Complete
RRC_INACTIVE State
RRC_CONNECTED State
UE gNB
UL Data
< OUT OF SYNC – DL Data – CB >
UE gNB
PRACH Preamble(MSG1)
RACH Response - RAR(MSG2)
CRNTI MAC CE(MSG3)
DCI 0 with CRNTI for RRC Reconfiguration
RRC Reconfiguration Complete
RRC_CONNECTED State
OUT OF SYNC
IN SYNC
PDCCH ORDER(Preamble ID == 0) DL Data
RRC Reconfiguration
< OUT OF SYNC – DL Data – CB >
UE gNB
PRACH Preamble(MSG1)
RACH Response - RAR(MSG2)
CRNTI MAC CE(MSG3)
DCI 0 with CRNTI for RRC Reconfiguration
RRC Reconfiguration Complete
RRC_CONNECTED State
OUT OF SYNC
IN SYNC
RRC Reconfiguration
UL Data
< HO – CB >
UE Tgt-gNB
PRACH Preamble(MSG1)
RACH Response - RAR(MSG2)
CRNTI MAC CE(MSG3)
DCI 0 with CRNTI for RRC Reconfiguration
Complete(MSG4)
RRC Reconfiguration Complete
HO ACK
Src-gNB
RRC Reconfiguration
< ODOSI MSG3方式 – CB >
UE gNB
PRACH Preamble(MSG1)
RACH Response - RAR(MSG2)
RRC SystemInfo Request(MSG3)
MAC CE(CR Identity,MSG4)
SI(broadcast)
基于非竞争的随机接入， gNB为 UE分配
专用的 RACH(Random Access Channel)
资源进行接入，但当专用的 RACH资源不
足时， gNB会指示 UE发起基于竞争的 RA。
非竞争随机接入场景说明
< OUT OF SYNC – DL Data – CF >
UE gNB
PRACH Preamble(MSG1)
RACH Response - RAR(MSG2)
PDCCH ORDER(Preamble ID != 0)
RRC Connection Reconfig
RRC Connection Reconfig Complete
RRC_CONNECTED State
OUT OF SYNC
IN SYNC
DL Data
< HO – CF >
UE Tgt-gNB
PRACH Preamble(MSG1)
RACH Response - RAR(MSG2)
RRC Reconfiguration Complete
HO ACK
(Preamble ID)
Src-gNB
RRC Connection Reconfig
(Preamble ID)
< ODOSI MSG1方式 – CF >
UE gNB
PRACH Preamble(MSG1)
RACH Response - RAR(MSG2)
SI(broadcast)
接入原因 中文解释 典型应用场景
emergency 紧急呼叫 紧急呼叫
highPriorityAccess 高优先级接入 SIM卡接入等级 AC为 11~15的用户初始入网
mt-Access 被叫接入 被叫响应寻呼
mo-Signalling 主叫信令 TAU、初始入网
mo-VoiceCall 主叫语音 VoNR语音业务
mo-VideoCall 主叫视频 视频电话业务
mo-SMS 主叫消息 短信业务
mps-PriorityAccess Mps优先级接入 For Further Study
mcs-PriorityAccess Mcs优先级接入 For Further Study
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