Cell Defining SSB (CD-SSB)

What is Cell defining SSB(CD-SSB)?

In an NR Carrier Bandwidth, multiple SSB can be transmitted in different frequency locations and the PCIs of the SSB 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 (i.e initial bandwidth), the SSB corresponds to an individual cell, which has a unique NCGI. Such an SSB is referred to as a Cell-Defining SSB (CD-SSB).

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

In NR Cell selection and Cell re-selection are always based on CD-SSBs located on the synchronization raster. The UE searches the NR frequency bands and for each carrier frequency identifies the strongest cell as per the CD-SSB. 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 UE’s operating with Reduced Capability(RedCap).

In the example shown below an NR Cell has a carrier Bandwidth of  200MHz and has Multiple BWP’s defined

The Initial BWP is associated with SSB#1 i.e UE can obtain SIB messages associated with this SSB SSB#4 is associated with 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 UE will trigger RACH on this cell.

Flexibility in NR Comes here, gNB here 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 a Scell pertaining to SSB#4 which has a unique NCGI#2

Reference: 3GPP TS 38.300

Further reading