4G/LTE - Serving Cell

 

 

 

Serving Cell

 

The definition of Serving Cell would be very confusing in many cases. With one carrier, the serving cell is simply the cell the UE is connected to. With CA and DC, it becomes a set of cells, and each procedure refers to a different member of that set. The formal definition of the Serving cell from 36.331 is as follows.

The quotation below is from 36.331 v19.3.0. An older version of this quotation read CA only, and the current text reads CA/ DC because dual connectivity now uses the same definition.

Serving Cell: For a UE in RRC_CONNECTED not configured with CA/ DC there is only one serving cell comprising of the primary cell. For a UE in RRC_CONNECTED configured with CA/ DC the term 'serving cells' is used to denote the set of one or more cells comprising of the primary cell and all secondary cells.

The most confusing parts would be how to define the Serving cell in CA (Carrier Aggregation) situation. With this simple definition of the Serving Cell, refer to following pages to see in which context the definition of the serving cell matters and in what way it would confuse you.

 

The topics covered in this page are as follows.

Which cells make up the serving cells ?

The definition above uses two terms, primary cell and secondary cell, and the specifications build several more names on them. When you read a log or a MAC clause, you need to know which of these names includes which cells. So let's put the definitions from 36.331 and 36.321 side by side.

 

Term

Definition

Primary Cell - PCell

The cell, operating on the primary frequency, in which the UE performs the initial connection establishment procedure or initiates the connection re-establishment procedure, or the cell indicated as the primary cell in the handover procedure. 36.331

Secondary Cell - SCell

A cell, operating on a secondary frequency, which may be configured once an RRC connection is established and which may be used to provide additional radio resources. 36.331

Primary Secondary Cell - PSCell

With DC, the SCG cell in which the UE performs random access, or initial PUSCH transmission if random access is skipped, when it performs the SCG change procedure. Except for (NG)EN-DC, the PSCell is considered to be an SCell. 36.331

Special Cell - SpCell

For DC, the PCell of the MCG or the PSCell of the SCG. Otherwise, the PCell. 36.321

PUCCH SCell

An SCell configured with PUCCH. 36.331

MCG and SCG

Without DC, the MCG comprises all serving cells. With DC, the MCG is the PCell and zero or more SCells, and the SCG is the PSCell and zero or more other SCells. 36.331

 

So the serving cells are always one PCell plus the SCells configured for the UE. Two limits in 36.331 show how large that set can be. The constant maxServCell-r10 is 5, so Release 10 allows at most 5 serving cells, with at most 4 SCells from maxSCell-r10. Release 13 extends the range with maxServCell-r13 = 32 and maxSCell-r13 = 31.

Watch the word SpCell when you read 36.321. Many MAC rules apply to the SpCell, and with DC that means two cells, one in each cell group. Without DC, SpCell and PCell are the same cell.

  • The serving cells are one PCell and all configured SCells : a UE without CA or DC has only the PCell.
  • SpCell means the PCell, or the PSCell of the SCG : with DC, a MAC rule for the SpCell applies to both cell groups.
  • The PSCell is an SCell in the serving cell count : the exception is (NG)EN-DC, where the PSCell is an NR cell.
  • Release 10 allows 5 serving cells and Release 13 allows 32 : maxServCell-r10 and maxServCell-r13 in 36.331 set these limits.

What does the PCell do that an SCell does not ?

All serving cells carry PDSCH for the UE, so the difference between them is not in the data. It is in the control of the connection. With CA, the UE still has only one RRC connection, and 36.300 clause 7.5 gives that connection to a single cell, the PCell. Everything else follows from this choice.

At RRC connection establishment, re-establishment and handover, one serving cell provides the NAS mobility information, such as the TAI. At re-establishment and handover, one serving cell also provides the security input. This cell is the PCell. In the downlink, its carrier is the DL PCC, and in the uplink it is the UL PCC. The SCell carriers are DL SCCs and UL SCCs.

36.300 then lists several rules that separate the PCell from the SCells.

  • The PCell can only be changed with the handover procedure. That means a security key change, and a RACH procedure unless RACH-less HO is configured.
  • The PCell is used for transmission of PUCCH. If DC is not configured, one additional PUCCH can be configured on an SCell, the PUCCH SCell.
  • Unlike SCells, the PCell cannot be deactivated or be in the dormant SCell state. 36.321 states the same for MAC: the SpCell is always activated.
  • Re-establishment is triggered when the PCell experiences RLF, not when SCells experience RLF.
  • NAS information is taken from the PCell.
  • An SCell can carry uplink resources in addition to its downlink ones, but no SCell can be configured for uplink resources only.

SCells also get their system information in a different way. When the eNB adds an SCell, it sends all required system information of that SCell in dedicated RRC signalling. So a UE in connected mode does not need to read the broadcast system information of an SCell. In MAC, a configured SCell is initially deactivated upon addition and after a handover, unless sCellState in the RRC configuration sets it to activated or dormant.

  • The PCell carries the RRC connection : it provides the NAS mobility information and the security input.
  • Only a handover changes the PCell : RRC can add, remove or reconfigure an SCell without a handover.
  • RLF on an SCell does not trigger re-establishment : only RLF on the PCell does.
  • The PCell is always active : SCells are activated, deactivated or made dormant by the network.

Reference

  • 36.331 v19.3.0 : E-UTRA Radio Resource Control - RRC; Protocol specification. Clause 3.1 Definitions, and the constants maxServCell and maxSCell.
  • 36.321 v19.3.0 : E-UTRA Medium Access Control - MAC; Protocol specification. Clause 3.1 Definitions and clause 5.13 Activation/Deactivation of SCells.
  • 36.300 v19.2.0 : E-UTRA and E-UTRAN Overall description; Stage 2. Clause 7.5 Carrier Aggregation.