4G/LTE - Cell Types

Cell Types - Primary Cell, Secondary Cell, Serving Cell

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.

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.

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