Understanding NCD-SSB (Non-Cell Defining SSB) in 5G NR Rel-17

What is NCD-SSB and why is it needed?

In Release-15 the term CD-SSB (Cell-Defining SSB) referred to the SSB that accompanies RMSI. The MIB of the CD-SSB carries information of the scheduled SIB1 which contains initial access information related to the cell.

In contrast, NCD-SSB (Non-Cell Defining SSB) can be an additional SSB broadcasted in the same cell but will not contain any associated RMSI with it (i.e., no SIB1 associated with that SSB burst).

Typically in NR, multiple SSBs may be transmitted within the frequency span of a carrier used by the serving cell to support multiple BWPs. An NR Cell can have multiple BWPs configured, and at least one BWP of the serving cell has one CD-SSB which is transmitted on the sync raster (i.e., on GSCN).

• The PSS and SSS of a CD-SSB carry the PCI.
• The MIB of a CD-SSB includes the scheduling information for SIB1 of the serving cell.

Different from CD-SSB, one or multiple NCD-SSBs can be transmitted on or off the sync raster of a serving cell. These can be configured with different PCIs and are not associated with SIB1 or any other SIB of a cell.

Key Difference: The main difference between a CD-SSB and NCD-SSB is that NCD-SSB is not associated with the SIB1 of the serving cell and it may be transmitted by another cell.

Non-Cell Defining SSB (NCD-SSB) is a new type of synchronization signal introduced in 5G NR Release 17. It is designed to be used by devices that are operating in a RedCap (Reduced Capability) mode. NCD-SSB is a single-frequency signal that does not contain any information about the cell identity or location. This makes it less complex and power-efficient than the cell-defining SSB used by regular 5G devices.

Therefore, in legacy systems, only CD-SSB can be used as the source of DL timing of the serving cell and as the reference signals in RLM/RRM measurements on the serving cell.

NonCellDefiningSSB Configuration

The IE NonCellDefiningSSB is used to configure a NCD-SSB to be used while the UE operates in a RedCap-specific initial BWP or dedicated BWP.

nonCellDefiningSSB-r17    NonCellDefiningSSB-r17    OPTIONAL   -- Need R

If configured, the RedCap UE operating in this BWP uses this SSB for the purposes for which it would otherwise have used the CD-SSB of the serving cell (e.g., obtaining sync, measurements, RLM).

Furthermore, other parts of the BWP configuration that refer to an SSB refer implicitly to this NCD-SSB, such as:

• The "SSB" configured in the QCL-Info IE
• The ssb-Index configured in RadioLinkMonitoringRS
• CFRA-SSB-Resource
• PRACH-ResourceDedicatedBFR

The NCD-SSB has the same values for the properties (e.g., ssb-PositionsInBurst, PCI, ssb-PBCH-BlockPower) as the corresponding CD-SSB, apart from the values explicitly configured in the NonCellDefiningSSB-r17 IE.

NonCellDefiningSSB Information Element

-- ASN1START
-- TAG-NONCELLDEFININGSSB-START

NonCellDefiningSSB-r17 ::=      SEQUENCE {
    absoluteFrequencySSB-r17        ARFCN-ValueNR,
    ssb-Periodicity-r17             ENUMERATED { ms5, ms10, ms20, ms40, ms80, ms160, spare2, spare1 }   
    ssb-TimeOffset-r17              ENUMERATED { ms5, ms10, ms15, ms20, ms40, ms80, spare2, spare1 }      
    ...
}

-- TAG-NONCELLDEFININGSSB-STOP
-- ASN1STOP

Field Descriptions

NCD-SSB Properties in Connected Mode

In connected mode operation, NCD-SSB has the same properties as the corresponding CD-SSB:

• ssb-PositionsInBurst
• PCI
• ssb-periodicity
• ssb-PBCH-BlockPower

Additional properties:

• absoluteFrequencySSB and ssb-periodicity may be provided explicitly by the network.
• Can be used as a QCL source.
• Can be used for RO (RACH Occasion) selection.
• TX power can be same or different from CD-SSB. If different, the UE needs to be informed of the power difference.

Other considerations:

• Periodicity of NCD-SSB shall not be less than the periodicity of the serving cell’s CD-SSB.
• Periodicity of NCD-SSB is not required to be configured the same as CD-SSB.
• Idle/inactive mobility is only based on measurements on the CD-SSB.
• RRM measurement on the serving cell can be based on CD-SSB or NCD-SSB. If configured with NCD-SSB, the UE should perform serving cell measurements based on the SSB that is available within the active BWP (i.e., NCD-SSB).
• Periodicity of NCD-SSB cannot be configured to be greater than 160ms.
• In connected mode, RRM measurement on a neighbor cell can be based on CD-SSB or NCD-SSB.

Measurements with NCD-SSBs

The network can always configure Measurement objects with both NR-ARFCN of both CD-SSB and NCD-SSB. When the network configures MOs with both:

1. Both within Active BWP: If NCD-SSB and CD-SSB of the neighbor cell lie within the UE active BWP, UE should be required to measure only one SSB.
2. Both outside Active BWP: If both lie outside the UE active BWP, UE should be required to measure only CD-SSB.
3. One within Active BWP: If either lies within the UE active BWP, UE should be required to measure the SSB that lies within the UE active BWP.

How a RedCap UE behaves with NCD-SSB

If a RedCap UE is in connected mode and has an active BWP that contains an NCD-SSB, the UE can use the NCD-SSB for:

• RLM, BFD, and serving cell measurements
• QCL source
• RO selection

If the UE needs to perform cell (re-)selection or measurements on a neighbor cell, it will use the CD-SSB.

Key Benefits of NCD-SSB (Rel-17)

1. Reduced Complexity: NCD-SSB is a simpler signal than cell-defining SSB, making it easier to implement in devices.
2. Reduced Power Consumption: Consumes less power, crucial for battery-powered RedCap devices.
3. Support for RedCap Mode: Specifically designed for Reduced Capability devices that are more affordable and power-efficient than regular 5G devices.