前言

5G网络已经开始部署有数年时间了，然而距离大规模应用仍然有一段漫长的旅程。在可预见的未来，4G网络仍将会与5G网络长期共存。在5G网络尚未完全发展成熟、4G网络仍旧使用广泛的今天，出现了ENDC(E-UTRAN New Radio – Dual Connectivity)技术，ENDC以LTE网络为主、NR网络为辅，一方面可以完全利用当前已经发展成熟的4G网络，节约网络部署成本，另一方面也可以降低技术难度，让各大运营商可以尽快平滑过渡到5G。在ENDC技术下，所有的信令都通过LTE网络传输，而数据则既可以通过LTE网络、也可以通过NR网络进行传输。

ENDC技术的出现，带来了一系列新的概念，如MCG/SCG，PCell/SCell/PSCell等，这些概念名字相似，极其容易混淆。本文将会对这些概念进行扫盲。

两种Cell Group之间的区别和联系

abbre	name	properties	primary cell
MCG	Master Cell group	group of serving cells associated with the Master Node comprising of the PCell and optionally one or more SCells	PCell
SCG	Secondary Cell group	SpCell = PCell + PSCell group of serving cells associated with the Secondary Node comprising of the SpCell (PSCell) and optionally one or more SCells.	PSCell

MCG和SCG是双链接（DC，Dual connectivity）下的概念，可以简单理解为UE首先发起随机接入（RACH）的Cell所在的Group就是MCG。如果没有进行双链接，也就没有MCG和SCG的概念。或者也可以理解为，如果没有进行双链接，那么该小区组就对应MCG。

MCG and SCG are concepts under DC (Dual connectivity). It can be simply understood that the group where the initiates random access (RACH) is performed is the MCG. If there is no dual link, there is no concept of MCG and SCG. It can also be understood that when link is performed, the cell group corresponds to the MCG.

两种Node之间的区别和联系

Name	Tags
Master Node	The node (radio access node) that provides the control plane connection to the core network in case of multi radio dual connectivity (MR-DC).
Secondary Node	The radio access node, with no control plane connection to the core network, providing additional resources to the UE in case of MR-DC.

四种 cell 之间的区别和联系

abbre	name	condition	properties
PCell	Primary Cell	under MCG	* Under MCG there may be many Cells. * PCell is used to initiate the initial access. * As the name implies, PCell is the most “main” cell in MCG.
PSCell	Primary Secondary Cell	under SCG	* Most important cell under the SCG. * PSCell Performs the initial access under the SCG
SCell	Secondary Cell	under MCG / SCG	* PCell is the cell which UE initially finds and connects to. * One or more SCells can be configured once the UE is in Connected mode. * SCell can be rapidly activated or deactivated to meet the variations in the traffic Pattern.
SpCell	Special Cell		* SpCell = PCell + PSCell. * a lot of signaling is only sent on PCell and PSCell. * SpCell supports PUCCH transmission and contention based Random access. * It is always activated

假设现在进行了双链接，那么就有了MCG和SCG的概念。不同的cell group上，因为CA(carrier aggregation，载波聚合)的存在，又可能会有多个cell，从而衍生出PCell、SCell、PSCell等概念。假如UE使能了双链接，且在MCG和SCG均使能了CA，那么各个cell group、cell之间就会以上图的拓扑组织到一起。下面对各个cell进行简单阐述。

PCell：Primary Cell，主小区
SCell：Secondary Cell，辅小区
PSCell：Primary Secondary Cell，主辅小区
SpCell: Special Cell，特殊小区
Serving cell: 服务小区

PCell和SCell的概念主要是为了区分在CA情况下，UE初始接入的那个Cell以及其后激活的Cell。在开启CA的情况下，UE会有多个serving cell，但是一定会有一个cell，UE在其上发起初始接入，这个cell就是PCell。在RRC连接建立或重建、handover时，PCell提供NAS mobility信息。在RRC连接重建、handover时，PCell提供security输入。

At RRC connection establishment/reestablishment/handover, one serving cell provides the NAS mobility information (e.g. TAI), and at RRC connection reestablishment/handover, one serving cell provides the security input. This cell is referred to as the Primary Cell (PCell).
–36.300

UE在接入一个Cell之后，UE可能会使能CA(Carrier aggregation)功能，同时接入到多个serving cell，新添加进来的cell就是SCell。MCG下的PCell和MCG下的SCell通过CA(Carrier aggregation)联合在一起。

同样地，在SCG下也会有一个最主要的小区，也就是PSCell，也可以简单理解为在SCG下发起初始接入的小区。SCG下的PSCell和SCG下的SCell也是通过CA技术联合在一起。

因为很多信令只在PCell和PSCell上发送，为了描述方便，协议中还定义了一个概念SpCell（special Cell）, SpCell= PCell + PSCell。

Special Cell: For Dual Connectivity operation the term Special Cell refers to the PCell of the MCG or the PSCell of the SCG, otherwise the term Special Cell refers to the PCell.
----(36.321)

除了上面的Cell概念以外，还有一个概念是serving cell(服务小区) ，按照36.331的定义，上面所介绍的所有的cell(PCell/SCell/PSCell)都属于serving cell。

Serving Cell: For a UE in RRC_CONNECTED not configured with CA there is only one serving cell comprising of the primary cell. For a UE in RRC_CONNECTED configured with CA the term ‘serving cells’ is used to denote the set of one or more cells comprising of the primary cell and all secondary cells.
----(36.331)

两种TAG之间的区别和联系

根据36.321，TAG/pTAG/sTAG定义如下表所示。

abbre	name	definition
`TAG`	Timing Advance Group	A group of Serving Cells that is configured by RRC and that, for the cells with an UL configured, using the same timing reference cell and the same Timing Advance value.
`pTAG`	Primary Timing Advance Group	Timing Advance Group containing the SpCell.
`sTAG`	Secondary Timing Advance Group	Timing Advance Group not containing the SpCell. A Secondary Timing Advance Group contains at least one Serving Cell with an UL configured.

从上表可以看出来，PCell和PSCell(即所有的SpCell)属于pTAG，它们都使用pTAG的上行时钟参考信号和TA值进行上行同步，而其它的serving cell属于sTAG，它们都使用sTAG的上行时钟参考信号和TA值进行上行同步。

两种PUCCH group之间的区别和联系

根据36.300，两种PUCCH group定义如下表所示。

name	definition
`Primary PUCCH group`	a group of serving cells including PCell whose PUCCH signalling is associated with the PUCCH on PCell.
`Secondary PUCCH group`	a group of SCells whose PUCCH signalling is associated with the PUCCH on the `PUCCH SCell`.
`PUCCH SCell`	a Secondary Cell configured with PUCCH.

PUCCH在PCell上发送，在开启了CA(carrier aggregation)且没有DC(Dual connectivity)的情况下，网络可能会再在其中一个SCell配置PUCCH，这个配置了PUCCH的SCell就叫PUCCH SCell，此时不仅PCell能发送PUCCH，PUCCH SCell也能发送PUCCH。在DC(Dual connectivity)的情况下，SCG上的PSCell也能发送PUCCH。

PUCCH is transmitted on PCell, PUCCH SCell (if such is configured in CA) and on PSCell (in DC).

If DC is not configured one additional PUCCH can be configured on an SCell, the PUCCH SCell;

When a PUCCH SCell is configured, RRC configures the mapping of each serving cell to Primary PUCCH group or Secondary PUCCH group, i.e., for each SCell whether the PCell or the PUCCH SCell is used for the transmission of ACK/NAKs and CSI reports.
–36.300

在配置了PUCCH SCell的情况下，网络会将一部分serving cell划入到和PCell同属一个group(Primary PUCCH group)，这个group里所有serving cell的物理层控制信息，比如ACK/NACKs以及CSI report，都会通过PCell的PUCCH发送。而其它的serving cell则划入到和PUCCH SCell同属一个group(Secondary PUCCH group)，这个group里所有serving cell的物理层控制信息，比如ACK/NACKs以及CSI report，都会通过PUCCH SCell的PUCCH发送。

参考文献

[1] 5G NR 中的 PCell / SCell / PScell / SpCell
[2] 5G NR MCG，SCG，PCell，PSCell，SCell，sPCell 概念澄清
[3] 5G abbreviations
[4] Serving Cell
[5] 3GPP 36.321
[6] 3GPP 36.213
[7] 3GPP 36.331
[8] 3GPP 36.300

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