The MCG and the SCG are concepts in dual connectivity (DC, Dual connectivity). It may be simply understood that the MCG is located in a group in which a cell in which the UE first initiates random access (RACH) is located. If there is no dual link, there is no concept of MCG and SCG. Alternatively, it may be understood that if dual link is not performed, the cell group corresponds to the MCG.
If we now have dual links, we have the concept of MCG and SCG. Now clarify the three concepts of PCell, SCell, and PSCell.
PCell: primary cell
SCell: secondary cell
PSCell: primary and secondary cells
There may be many cells under the MCG. One cell is used to initiate initial access. This cell is called a PCell. As the name suggests, PCell is the most important cell in the MCG. The PCell under the MCG and the SCell under the MCG are combined by using a carrier aggregation (CA, CA) technology.
Similarly, there is a primary cell under the SCG, that is, the PSCell, which can also be simply understood as a cell for which initial access is initiated under the SCG. The PSCell and SCell under the SCG are also combined through CA.
Because most signaling messages are sent only on the PCell and PSCell, the protocol also defines a concept sPCell (special cell) for ease of description,
sPCell = PCell + PSCell.