This might have not been a big deal to most of readers... but this has been confusing me a lot for so long... so I wanted to put down some of my words here.

So let's take the confusion seriously, because it is the useful kind. Two of these three terms name one cell each, and the third names a set. The quotation below is the author's, taken from Release 11. After it I have added what the same clause says today, and what dual connectivity did to two of the three definitions.

• What does 36.331 call each cell?
• How do the counts work in a carrier aggregation example?
• What did dual connectivity add?
• Reference

What does 36.331 call each cell?

The three terms sit inside each other, and that is the whole difficulty. Two of them name a single cell, and the third names a set of cells. Take all three as the same kind of thing, and the third definition reads as a contradiction. That is what the note above describes.

Before I put my own comments, let's see how it is described in 3GPP specification. 36.331 3.1 Definitions (V11.5.0 or any version later than rel 10) defines these Cell Types as follows :

Primary Cell: The cell, operating on the primary frequency, in which the UE either 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.

Secondary Cell: 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.

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.

Most part would be clear and straighforward, but the green part in 'Serving Cell' definition may be a little confusing. At least, it was confusing to me. In stead of describing in general terms, just giving you a couple of example cases would clarify a lot.

One thing to add to that quotation, because the page dates it. The author quotes V11.5.0, and 36.331 is now at v19.3.0. Two of the three definitions have changed since, and both changes are about dual connectivity.

The term that never changed is the one that names a single, unique cell. The two that were extended are the one that names a category and the one that names a set, because dual connectivity added members to both.

• Primary Cell is a role, and only one cell holds it : initial connection establishment, re-establishment, or the target named in a handover. All three are things that happen once.
• Secondary Cell is a category, not a position : any number of cells can be one, and 36.331 now files the PSCell under it as well, outside (NG)EN-DC.
• Serving Cell is a set, which is why it reads oddly : the definition has to say what the set contains in each configuration. So it splits into two sentences rather than giving one property.
• The green half of the quotation is the set case : it is not a different kind of serving cell. It is the same term counted when more than one carrier is configured.
• A Release 11 quotation is still accurate for the PCell : if you are reading an older note, that is the one definition you do not need to re-check.

How do the counts work in a carrier aggregation example?

Counting is the fastest way past the wording. Each example below fixes a carrier layout, then reads the three terms off it. The pattern behind them is one line long, and it holds for every CA configuration LTE allows.

Example 1 > 2 CC CA : 1 PCC + 1 SCC (SCC #1)

In this case, you can get/define following definition

Number of Serving Cell = 2

Serving Cell 1 = PCC

Serving Cell 2 = SCC #1

Example 2 > 3 CC CA : 1 PCC + 2 SCC (SCC #1 + SCC #2)

In this case, you can get/define following definition

Number of Serving Cell = 3

Serving Cell 1 = PCC

Serving Cell 2 = SCC #1

Serving Cell 3 = SCC #2

That pattern is worth stating on its own. The number of serving cells is one PCell plus the number of configured SCells, and the PCC is always serving cell 1. Neither example needs the definition read twice.

The RRC sets how far that count can go, not the examples. 36.331 defines SCellIndex-r10 as INTEGER (1..7), so a Release 10 UE holds at most seven SCells, and therefore eight serving cells. Release 13 widened it. SCellIndex-r13 is INTEGER (1..31), which is thirty-one SCells and thirty-two serving cells.

One distinction is worth keeping apart from the count. A configured SCell is not the same as an SCell carrying data, because SCells are activated and deactivated by MAC. 36.331 carries the timer that governs it, sCellDeactivationTimer, and hands the mechanism to TS 36.321. The serving cell count in both examples is the configured count, not the active one.

• Serving cells = 1 + number of SCells : the PCell is always in the set, so the count never drops below one.
• Serving cell 1 is the PCC in both examples : that ordering is the author's, and it matches the way a tester numbers them. 36.331 itself indexes SCells and leaves the PCell outside the index.
• The ceiling moved from 8 to 32 serving cells : SCellIndex-r10 is INTEGER (1..7) and SCellIndex-r13 is INTEGER (1..31). More than eight carriers in a log means a Release 13 or later UE.
• Configured is not active : sCellDeactivationTimer exists precisely because an SCell can be configured and still switched off. Count carriers in an RRC message, not throughput.
• The PCell has no SCellIndex : it needs none, because there is only ever one of it. That asymmetry is the clearest sign the two terms are different kinds of thing.

What did dual connectivity add?

Everything above assumes one scheduler. Dual connectivity gives the UE two, on two cell groups, and 36.331 had to name the new pieces. These are the definitions the Release 11 text quoted at the top of this page could not have carried.

Start with the definition of the feature itself, which is short. 36.331 says a UE in RRC_CONNECTED is configured with Dual Connectivity when it is configured with a Master and a Secondary Cell Group. Everything else follows from splitting the serving cells into those two groups.

Read the first two rows together and the shape appears. Every serving cell belongs to exactly one group, the MCG always has the PCell, and the SCG always has the PSCell.

The containment is easier to hold as a picture than as four definitions, because the word serving covers every box in both halves. Figure 1 puts the carrier aggregation case beside the dual connectivity one at the same scale.

Figure 1. Dual connectivity does not create a new class of serving cell. It draws a line through the set that already existed, and puts one cell with a special role on each side of that line.

• The outer box is the same term in both halves : Serving Cells covers every inner box on the left and every inner box on the right. That is why the 36.331 definition needed the words CA/ DC rather than a second entry.
• MCG and SCG partition the set : no serving cell sits in both, and none sits outside. The definitions say so directly, one as a subset and the other as the cells not in that subset.
• PCell and PSCell hold the matching role in their own group : both are shaded the same way in the picture. Each is the cell its group does random access on.
• The count does not change : four boxes on the right, four serving cells. Grouping is a division, not an addition.

Two smaller entries complete the set, and both are about exceptions rather than structure. A PUCCH SCell is simply an SCell configured with PUCCH, so uplink control is not a PCell-only function. A Primary Timing Advance Group is the one containing the PCell or the PSCell, so the two special cells anchor timing for their own groups.

Readers arriving from NR expect one more term here, and 36.331 does not have it. 36.331 clause 3.1 carries no entry for Special Cell or SpCell, although a handful of field descriptions in the document use the word. 38.331 has a collective name for the two. LTE's own definitions clause names them separately instead.

• DC is defined by its groups, not by its radios : 36.331 says a UE is in DC when it has a Master and a Secondary Cell Group. EN-DC and NE-DC differ in which access each group uses.
• The PSCell is an SCell, with one exception : the Secondary Cell definition says so, and names (NG)EN-DC as the case where it is not.
• Without DC the MCG is everything : the definition opens with exactly that case, so a plain CA UE has one group and needs none of this vocabulary.
• PUCCH is not PCell-only : the PUCCH SCell entry exists to say that, and sCellDeactivationTimer is explicitly written not to apply to it.
• LTE has no SpCell : if a note uses it for LTE, the term has come from 38.331. 36.331 names the PCell and the PSCell one at a time.

Reference

• 36.331 : 3GPP - E-UTRA; Radio Resource Control (RRC); Protocol specification, v19.3.0. Clause 3.1 Definitions read in full, plus the SCellIndex-r10 and SCellIndex-r13 definitions and the sCellDeactivationTimer field description.
• 36.321 : 3GPP - E-UTRA; Medium Access Control (MAC) protocol specification. Named by 36.331 as the home of SCell activation and deactivation; not read for this page.