What is NCD-SSB and why is it needed and what’s the use case here ?
In Release-15 the term CD-SSB was referred to the SSB that accompanies RMSI i.e. the MIB of the CD-SSB carries information of the scheduled SIB1 which contains initial access information related to the cell. Where as NCD-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 BWP’s. NR Cell can have multiple BWP 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, which can be configured with different PCIs and are not associated with SIB1 or any other SIB of a cell.
Hence 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 the 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 that is used by regular 5G devices.
Therefore, in legacy 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
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| nonCellDefiningSSB 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 (e.g. the “SSB” configured in the QCL-Info IE; the “ssb-Index” configured in the RadioLinkMonitoringRS; CFRA-SSB-Resource; PRACH-ResourceDedicatedBFR) refer implicitily to this NCD-SSB. The NCD-SSB has the same values for the properties (e.g., ssb-PositionsInBurst, PCI, ssb-PBCH-BlockPower) of the corresponding CD-SSB apart from the values of the properties 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| NonCellDefiningSSB field descriptions |
|---|
| absoluteFrequencySSB Frequency of the NCD-SSB. The network configures this field so that the SSB is within the bandwidth of the BWP configured in BWP-DownlinkCommon. |
| ssb-Periodicity The periodicity of this NCD-SSB. The network configures only periodicities that are larger than the periodicity of serving cell’s CD-SSB. If the field is absent, the UE applies the SSB periodicity of the CD-SSB (ssb-periodicityServingCell configured in ServingCellConfigCommon or ServingCellConfigCommonSIB). |
| ssb-TimeOffset The time offset between CD-SSB of the serving cell and this NCD-SSB. Value ms5 means the first burst of NCD-SSB is transmitted 5ms later than the first burst of CD-SSB transmitted after the first symbol of SFN=0 of the serving cell, value ms10 means the first burst of NCD-SSB is transmitted 10ms later than the first burst of CD-SSB transmitted after the first symbol in SFN=0 of the serving cell, and so on. If the field is absent, RedCap UE considers that the time offset between the first burst of CD-SSB transmitted in the serving cell and the first burst of this NCD-SSB transmitted is zero. |
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 selection
- TX power can be same or different from CD-SSB
- If TX power is different, the UE needs to be informed of the power difference
- It may not be always possible to use the same TX power for NCD-SSB as CD-SSB if power boosting is used for CD-SSB
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 (assuming CD-SSB is outside the active BWP).
- 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
- The neighbor cell’s NCD-SSB information (frequency/SCS) shall be provided to the UE if the UE is configured to perform cell identification/measurement on the neighbor cell’s NCD-SSB
Measurements with NCD-SSB’s
The network can always configure Measurement objects with both NR-ARFCN of both CD-SSB and NCD-SSB. When the network configures MO’s with both, and:
- When both, NCD-SSB and CD-SSB of the neighbor cell lie within the UE active BWP, UE should be required to measure only one SSB.
- When both, NCD-SSB and CD-SSB of the neighbor cell lie outside the UE active BWP, UE should be required to measure only CD-SSB.
- When either NCD-SSB or CD-SSB lie within the UE active BWP, UE should be required to measure the SSB that lies within the UE active BWP.
How a RedCap UE will behave in presence of NCD-SSB ?
A RedCap UE in connected mode and if it has an active BWP that contains an NCD-SSB. Then the UE can use the NCD-SSB for the following:
- 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.
Some of the key benefits of using NCD-SSB in 5G NR Release 17:
- Reduced complexity: NCD-SSB is a simpler signal than cell-defining SSB, which makes it easier to implement in devices.
- Reduced power consumption: NCD-SSB consumes less power than cell-defining SSB, which is important for battery-powered devices.
- Support for RedCap mode: NCD-SSB is designed to be used by RedCap devices, which are a new class of devices that are designed to be more affordable and power-efficient than regular 5G devices.