5G abbreviations

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As per readers request this article is based on some 5G abbreviations (PCell,spCell,Scell,PSCell,MCG.SCG,Master-Node,Secondary-Node) for better understanding.

What is Master Node?

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).

For example, it can be master eNB in EN-DC, a Master ng-eNB in NGEN-DC

What is Secondary Node?

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.

For example, It may be an en-gNB in EN-DC or a Secondary ng-eNB (in NE-DC).

What is MCG?

MCG: Master Cell group
It is a group of serving cells associated with the Master Node, comprising of the PCell  and optionally one or more Scells. (see figure shown above)

MCG and SCG are concepts under dual connectivity (DC, Dual connectivity). It can be simply understood that the group where the initiates random access (RACH) is the MCG.

What is SCG?

SCG: Secondary Cell group
It is a group of serving cells associated with the Secondary Node, comprising of the SpCell (PSCell) and optionally one or more Scells. (see figure shown above)

If there is no dual link, there is no concept of MCG and SCG. Or it can also be understood that link is performed, the cell group corresponds to the MCG.

Does UE require Radio link monitoring for MCG and SCG?

3GPP specifies that the UE applies Radio Link Monitoring to the Primary serving Cell (PCell) of the Master Cell Group (MCG). If the UE is configured with a Secondary Cell Group (SCG), then the UE also applies Radio Link Monitoring to the Primary SCG Cell (PSCell).

Does UE  require Radio link monitoring for inactive BWP?

Radio Link Monitoring is only applied to the active Bandwidth Parts, i.e. the UE is not required to monitor Bandwidth Parts which are inactive.

Now clarify the three concepts of PCell, SCell, PScell

PCell: Primary Cell (spCell of a master cell group)
PSCell: Primary Secondary Cell (spCell of a secondary cell group)
SCell: Secondary Cell

Note: Special Cell is abbreviated as spCell.

What is Pcell?

Under MCG, there may be many Cells, one of which is used to initiate initial access. This cell is called PCell. As the name implies, PCell most "main" cell in MCG. The PCell under MCG and the SCell under MCG are united together by Carrier Aggregation (CA) technology. (see figure shown above)

What is PScell?

Similarly, there will also be a most important cell under the SCG, that is, the PSCell, which can also be simply understood as the cell the initial access under the SCG. PSCell under SCG and SCell under SCG are also united by CA technology. PScell is primary cell of the SCG. (see figure shown above)

Because a lot of signaling is only sent on PCell and PSCell, for the convenience of description, a concept spCell (special cell) is also defined in the protocol,
spCell = PCell + PSCell.

Note: spCell supports PUCCH transmission and contention based Random access in addition, it is always activated.

What is Scell?

CA is described using the term cell, CA capable devices is able to receive and transmit to/from multiple cells. Once of these cells is referred to as the PCell. This is the cell which the UE initially finds and connects to, after which one or more Scells can be configured once the UE is in Connected mode. (see figure shown above)

Note: Scell can be rapidly activated or deactivated to meet the variations in the traffic Pattern.

What is CellGroupID?

CellGroupID — The Cell-GroupId IE is used to identify a cell group.
Here 0 (Zero) Value identifies the master cell group (MCG) and 1 values identify secondary cell groups (SCG). (As per 3GPP release 15)

What is spCellConfig?

spCellConfig — it is a special cell for the defined group (For MCG and SCG) in other words, it is primary cell of the mentioned group (PScell of SCG  and Pcell of MCG).

What is ServCellIndex?

ServCellIndex: it is serving cell ID of PScell (see figure shown above). Serving cell ID for master cell group (MCG) is always 0.
ServCellIndex is different from Pcell ID. For every NR cell (PScell & Scell) network gives both ServCellIndex and Pcell ID.  Also, it is just an identifier to bound different IE with a particular cell.

Stay tune for more Update…………………

Pinal Dobariya…


1)3GPP TS 38.331
2)3GPP TS 37.340


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