5G / Msg2 of the random access procedure NR

21.6 Msg2

After the UE transmits the preamble, to monitor the PDCCH within a time window RAR (RA Response window), to receive the RAR corresponding to the RA-RNTI (this case is not considered possible measurement gap). If no reply is received gNB RAR RAR within the time window is considered the random access procedure fails.

RAR window start time: RAR window starting at the earliest a first symbol CORESET, which CORESET UE is configured to receive a Type1-PDCCH PDCCH CSS set, and the first PRACH transmission CORESET PRACH corresponding to the occasion interval after the last symbol of at least one symbol, which is a schematic view of RAR window starting time as shown in FIG 21.11 (it is noted that, in a description of the RO 21.11 Although FIG CORESET next, but not a symbol interval, and therefore RAR window and can not be next to a symbol of CORESET first start, but behind that CORESET, and RO b described in the first two CORESET symbol interval, so the ROC and the earliest CORESET a symbol interval, the RAR window it will start in the first CORESET). RAR window length is provided by ra-ResponseWindow, a unit slot, SCS based on the length of Type1-PDCCH CSS set.

FIG 21.11 preamble format PRACH B4 schematic occasion RAR time window starting at different initial symbol

When the preamble index with a preamble index UE transmits the UE simultaneously with the RAR successfully received a RAR (using RA-RNTI described previously decoded), and is considered successfully received RAR, the UE can stop listening to at this time the RAR.

21.6.1 MAC PDU(RAR)

MAC PDU MAC RAR 21.12 composition shown in FIG.

21.12 MAC RAR schematic diagram of a MAC PDU composed of

As can be seen from Fig. 21.12, RAR MAC PDU consists of one or more optional padding and MAC subPDU composition, wherein the MAC subPDU of the following composition:

- BI only a MAC sub-header (can stand alone);

- MAC sub-header having only a RAPID (i.e., SI confirmation request, may be present alone);

- having RAPID MAC sub-header of the MAC RAR.

BI comprise only the MAC subPDU is placed at the beginning of the RAR MAC PDU (if included BI). RAPID having only a MAC subPDU (as request verification SI) and MAC subPDU MAC RAR having a RAPID may be placed on the BI (if present), and anywhere between padding (if present) MAC subPDU.

As can be seen from the structure of the RAR MAC PDU, if gNB detected random access request from a plurality of UE (RA-RNTI as) in the same PRACH resource, using a RAR MAC PDU can respond to the access request , in response to each random access request (corresponding to a preamble index) corresponding to a RAR.

(When the same frequency position, using the same RA-RNTI), if multiple UE transmits in the same PRACH resource Preamble, the corresponding RAR multiplexed in the same RAR MAC PDU.

RAR MAC PDU on a DL-SCH transmission, and RA-RNTI for PDCCH scrambled indicated. If all known UE transmits using the same PRACH resource preamble (preamble need not necessarily identical) listens to the same RA-RNTI scrambled PDCCH, and receives the same RAR MAC PDU, but corresponding to different preamble index different MAC RAR, i.e., RAR MAC PDU in a MAC subPDU.

Since the RAR MAC PDU using only a scrambled RA-RNTI, which means that different PRACH resource (different time-frequency positions) corresponding to RAR Preamble not transmitted multiplexed in the same one RAR MAC PDU.

It is noteworthy that, in NR, triggering a random access procedure increases the number of trigger events, including other SI request triggers King. Other SI request, the random access procedure The UE may notify the two GNB, which carries 1 or 3 message is a request information message of the random access procedure, as described below:

1) seen from 38.331, the UE and a high-layer signaling GNB, namely: RRCSystemInfoRequest, which is included with the signaling message access procedure 3, and can be passed directly gNB, gNB by the confirmation request message 4 (how to verify. ? or do not confirm, but it is not LCID, or directly receive the message received by 4 compete successfully resolved can confirm gNB have received the message 3 (as detailed in doubt part of this chapter) UE; the message 1, it may appear RAR MAC PDU is not the UE itself, even if the received RAR may also be other UE, so be sure);

2) a subset of the SI and PRACH resource if the associated request (based on configuration options rach-OccasionsSI), in this case, the UE needs to indicate the Msg1 SI for the request, and requests the preambles for the SI-specific division , while when used for the Msg1 SI request, the request is a minimum particle size SI (i.e., a set of SIBs), wherein a PRACH preamble or resource may be used to request a plurality of SI messages, and sub-header carried by a SI request the RAR confirmation.

Why and how other SI request UE to determine whether to request SI? In NR, SI ways in addition to broadcast (broadcast and on-demand broadcasts into cycles), but also can be unicast, which include other SI SIB request SIB2 ~ SIB9, not including the MIB and the SIB1 (MIB periodic broadcast, SIB1 broadcast cycle may also be unicast RRC_CONNECTED). If the SI broadcast other, are carried out in the RRC_IDLE state and RRC_INACTIVE state; if the SI unicast other, it requires random access request, and therefore is in RRC_CONNECTED state. For the UE, if in SIB1 SI-SchedulingInfo-> si-BroadcastStatus configured to notBroadcasting, the SI, the UE needs to request additional, wherein the need for a request by the two ways (by competitive or non-contention random access into).

BI MAC sub-header having a header field by five E / T / R / R / BI, as shown in FIG 21.13.

FIG 21.13 E / T / R / R / BI MAC sub-header schematic

If the UE receives a BI sub-header, it is saved a backoff value, which is equal to the value of the subheader BI (which is a backoff index value) corresponding to the value corresponding to the index; otherwise the UE backoff value will be set to 0 .

BI (Backoff Indicator) specifies a time frame UE retransmit the preamble to wait (see the range of 38.321. Section 7.2). If the UE is not received within the RAR RAR time window, or received a preamble RAR does not conform with its own, it is considered the RAR reception failure, this time after the UE needs to wait for some time, once again initiate a random access request . The waiting time is a random value from 0 to BI value specified waiting time interval.

BI values from the reflection of the load cell, if the UE multi-access, this value can be set bigger; if less access to a UE, this value can be set to be smaller, which is achieved by a base station decision.

RAPID is an abbreviation of Random Access Preamble Identifier for detecting preamble index gNB when preamble obtained. If the UE finds the same index of the transmitted preamble value when their use is considered successfully received a corresponding RAR. RAPID MAC sub-header corresponding consists of three fields E / T / RAPID composition. 21.14 shown in FIG.

FIG 21.14 E / T / RAPID MAC sub-header schematic

Wherein each MAC sub-header field meaning shown in Table 21.7.

Table 21.7 RAR MAC PDU in the MAC sub-header of each domain parameter

parameter name	Parameter Description
R(Reserved)域	A value of 0
E (Exension) domain	It indicates whether the current MAC subPDU is the last 0: indicates that the current is the last MAC subPDU 1: this indicates there is at least a MAC subPDU MAC subPDU
T(Type)域	0: BI MAC sub-header indicates 1: RAPID MAC sub-header indicates
BI (Backoff Indicator) area	Indicating the current cell overload condition
RAPID domain	Sleep access preamble transmitted indication

21.6.2 RAR composition

FIG 21.15 fixed RAR FIG.

Composition schematic diagram of 21.15 MAC RAR

RAR UL Grant for PUSCH transmission scheduling the UE. LSB to MSB from the beginning to the end of the RAR UL Grant of content are given in Table 21.8.

表21.8 RAR UL Grant组成大小(38.213 Table 8.2-1)

RAR grant field	Number of bits
Frequency hopping flag	1
PUSCH frequency resource allocation	14
PUSCH time resource allocation	4
MCS	4
TPC command for PUSCH	3
CSI request	1

14比特PUSCH frequency resource allocation用于确定Msg3传输的频域资源位置，该字段解释如下：

Msg3 PUSCH频率资源分配用于上行链路资源分配类型1。在具有跳频的Msg3 PUSCH传输的情况下，Msg3 PUSCH频率资源分配字段的钱一个或两个比特被用作跳频信息比特，且与激活的UL BWP的大小相关，如表21.9所述。

表21.9 Frequency offset for second hop of PUSCH transmission with frequency hopping scheduled by RAR UL grant(38.213 Table 8.3-1)

Number of PRBs in initial UL BWP	Value of $N_{UL,hop}$ Hopping Bits	Frequency offset for 2nd hop
$N_{BWP}^{size} < 50$	0	$\left \lfloor N_{BWP}^{size}/2 \right \rfloor$
$N_{BWP}^{size} < 50$	1	$\left \lfloor N_{BWP}^{size}/4 \right \rfloor$
$N_{BWP}^{size} \geq 50$	00	$\left \lfloor N_{BWP}^{size}/2 \right \rfloor$
	01	$\left \lfloor N_{BWP}^{size}/4 \right \rfloor$
	10	$-\left \lfloor N_{BWP}^{size}/4 \right \rfloor$
	11	Reserved

UE在 $N_{BWP}^{size}$ 个RBs的激活UL BWP中处理频域资源分配如下描述：

如果 $N_{BWP}^{size} \leq 180$ ，则只使用该字段的最低 $\left \lceil log_{2} \left ( N_{BWP}^{size} * \left ( N_{BWP}^{size} + 1 \right ) /2\right )\right \rceil$ 个比特，其解析方式与正常的DCI format 0_0中的频域资源分配字段相同。

如果 $N_{BWP}^{size}\geq 180$ ，14bits会被分为2部分： $N_{UL,hop}$ 个跳频bit和剩余14 – $N_{UL,hop}$ 比特。如果“hopping flag”域设置为1，则个跳频比特的定义如表21.9所示。如果“hopping flag”域设置为0，则 $N_{UL,hop}$ 为0，即此时不存在跳频。由于14比特并不足以表示所有的资源分配情况，因此协议中规定会在 $N_{UL,hop}$ 个跳频比特后插入值为0的 $\left \lceil log_{2} \left ( N_{BWP}^{size} * \left ( N_{BWP}^{size} + 1 \right ) /2\right )\right \rceil - 14$ 特。也就是说，此时认为Msg3 PUSCH frequency resource allocation域包含 $N_{UL,hop}$ + $\left \lceil log_{2} \left ( N_{BWP}^{size} * \left ( N_{BWP}^{size} + 1 \right ) /2\right )\right \rceil - 14$

+ (14 - $N_{UL,hop}$ )比特，然后扩展后的频域资源分配域的解析方式按照DCI format 0_0中的频域资源分配字段进行解析

4比特PUSCH time resource allocation用于确定Msg3传输的时域资源位置，如果UE在slot n收到带有RAR消息的PDSCH，则UE在slot 传输PUSCH(Msg3)，其中k2指的是时隙偏移，由PUSCH time resource allocation确定， Δ指的是第一次Msg3传输附加子载波间隔特定时隙延迟值，其值参见38.214表6.1.2.1.1-5。

1比特Frequency hopping flag，用于判断PUSCH传输是否跳频，如果其值为0，则UE在没有跳频的情况下发送PUSCH(Msg3)；否则，UE通过跳频发送PUSCH(Msg3)。

4比特MCS，取值0~15，说明RAR对应的PUSCH传输(Msg3)的MCS的取值范围为0~15，可参考38.214。

3比特TPC command for PUSCH，用于设置PUSCH传输(Msg3)的功率，如何使用参考38.213的7.1.1，其定义参考38.213的表8.2-2。

1比特CSI request，对于基于非竞争的随机接入而言，RAR中的CSI request字段用于决定在对应的PUSCH传输是否包含非周期CSI上报。而对于基于竞争的随机接入而言，RAR中的CSI request字段是预留的，即没有任何作用。

疑问：

1. 其他SI请求的其中一种请求方式，是通过RRCSystemInfoRequest进行，也就是随机接入过程中的消息3，其相对于另外一种通过Msg1进行请求的方式而言，在Msg2中有RAPID作为SI确认，而该请求方式没有，那么其如何受到SI确认?并且UE在发起消息3时，此时还没有唯一标识，那么UE如何判断SI请求是否成功？

A：从38.331中可得知RRCSystemInfoRequest是48比特，其在CCCH逻辑信道上传输，则此时UE并没有C-RNTI，那么对于UE MAC而言，整个RRCSystemInfoRequest就是一个CCCH SDU，则RRCSystemInfoRequest中requested-SI-List就相当于一个唯一标识，或者说整个RRCSystemInfoRequest相当于一个唯一标识，当UE收到Msg4的时候，则UE会使用保存的CCCH SDU(即RRCSystemInfoRequest)与解码得到的Contention Resolution Identity MAC CE进行匹配，如果匹配成功，并且随机接入过程是由SI请求触发，则MAC会给高层指示一个SI确认。

目前文章逐步移至微信公众号更新，有兴趣可扫下面二维码进行关注，谢谢。

5G加油站

发布了42 篇原创文章 · 获赞 86 · 访问量 9万+

私信关注