Cell Defining SSB (CD-SSB) in 5G NR

What is Cell Defining SSB (CD-SSB)?

In an NR Carrier Bandwidth, multiple SSBs (Synchronization Signal Blocks) can be transmitted in different frequency locations.

• The PCIs (Physical Cell Identities) of these SSBs do not have to be unique (i.e., different SSBs in the frequency domain can have different PCIs).
• However, when an SSB is associated with an RMSI (Remaining Minimum System Information, i.e., initial bandwidth), the SSB corresponds to an individual cell, which has a unique NCGI (NR Cell Global Identity).

Such an SSB is referred to as a Cell-Defining SSB (CD-SSB).

Key Rule: A PCell is always associated with a CD-SSB located on the synchronization raster.

Role in Cell Selection

In NR, Cell Selection and Cell Re-selection are always based on CD-SSBs located on the synchronization raster.

1. The UE searches the NR frequency bands.
2. For each carrier frequency, it identifies the strongest cell as per the CD-SSB.
3. It then reads cell system information broadcast to identify its PLMN(s).

A UE in RRC_IDLE and RRC_INACTIVE monitors paging only in an initial BWP associated with CD-SSB and performs cell (re-)selection and measurements on the CD-SSB.

• This is also applicable to UEs operating with Reduced Capability (RedCap).

Example Scenario

Consider an NR Cell with a Carrier Bandwidth of 200 MHz and multiple BWPs defined:

• Initial BWP: Associated with SSB#1 (UE can obtain SIB messages associated with this SSB).
• Other SSB: SSB#4 is associated with a different PCI#2 and NCGI#2.

From UE’s point of view:

• It sees two NR Cells.
• However, only SSB#1 is associated with RMSI, so the UE will trigger RACH on this cell.

Flexibility in NR

The gNB has multiple options to utilize the bandwidth:

• Option #1: gNB can assign dedicated BWP#3 to UE, and its initial BWP will be associated with SSB#1.
• Option #2: gNB can assign dedicated BWP#2 to UE, and its initial BWP will be associated with SSB#1 and an SCell pertaining to SSB#4 (which has a unique NCGI#2).

Reference

• 3GPP TS 38.300