5G/NR  

 

 

 

NCD-SSB(Non Cell Defining SSB)

NCD SSB stands for Non Cell Defining SSB. Just from the word itself, you would guess there should be another comparative term 'CD SSB' meaning 'Cell Defining SSB'. Good guess !!! There IS the term CD SSB. What do these mean ? The definition of CD SSB and NCD SSB are as follows :

  • CD SSB : This is what we know as SSB in most of the situation. In most case, we just call it as SSB. It carries specific information like PCI and the location of SIB1 (meaning the SSB is associated with a SIB1). UE uses this SSB for cell detection and use it for initial attach.
  • NCD SSB : This is specially designed for RedCap application. It carries most of the information as CD SSB except one important piece of information. That missing information is about SIB1. It mean that NCD SSB is not associated with SSB, implying that UE can use this for basic cell detection purpose like timing sync, RLM(Radio Link Measurement), BFD(Beam Failure Detection), but cannot use it for the initial attach because of missing SIB1 information.

Followings are the further details about NCD-SSB.

Why NCD-SSB ?

Is NCD-SSB mandatory for all RedCap deployment ? NO.  Is it possible for RedCap to operate even when there is no SSB at all within the Redcap BWP ?  YES.

There are various scenarios (Cases) of allocating a spectrum for RedCap. In some scenario(case), it would be beneficial to use NCD-SSB for RedCap. It is just matter of benefit of using NCD SSB in some scenario.

Let's look into a few possible scenario (case) of allocating RedCap (these are just a few possible examples, not the exhaustive list of cases) and see how RedCap can operate in each of the scenario (NOTE : These scenario are summarized in illustration based on TDocs : R2-2200401,R2-2200608)

Case 1 : No NCD SSB for RedCap, CD-SSB is within both Initial DL BWP of legacy UE and the initial DL BWP of RedCap UE 

In this case, Initial DL BWP for RedCap is within Initial DL BWP for legacy UE, and there is no NCD-SSB is transmitted. In this scenario, the RedCap UE can use CD-SSB for every purpose (e.g, initial sync, initial attach, measurement (RLC, BFD etc) without any sacrifice of efficiency (e.g, switching back and forth between RedCap and legacy region).  You may transmit NCD-SSB in this BWP allocation, but the benefit would not be that big.

 

Case 2 : No NCD SSB for RedCap, CD-SSB is within both Initial DL BWP of legacy UE but not in the initial DL BWP of RedCap UE 

In this case, Initial DL BWP for RedCap is still within the Initial BWP for legacy UE but CD-SSB is outside of RedCap Initial BL BWP and there is no NCD SSB configured in the RedCap Initial DL BWP. In this scenario, RedCap UE needs to switch the frequency to other spectrum region (i.e, the spectrum where CD-SSB is located) whenever SSB is needed (e.g, time sync and measurement (RLM, BFD etc)).

 

Case 3 :  NCD-SSB for RedCap UE, CD-SSB is within both Initial DL BWP of legacy UE but not in the initial DL BWP of RedCap UE 

NOTE : In this scenario, it is assumed that ActiveBWP of RedCap shares the spectrum where NCD-SSB is transmitted.

In this case, Initial DL BWP for RedCap is still within the Initial BWP for legacy UE but CD-SSB is outside of RedCap Initial BL BWP. However there IS NCD SSB configured in the RedCap Initial DL BWP. In this scenario, for initial attach the RedCap UE should still rely on CD-SSB because NCD-SSB does not carry any SIB1 information, but after the critical phase (i.e, upto Msg4 of RACH process) the RedCap UE utilize NCD-SSB for timing sync, measurement without switching back and forth between RedCap region and legacy region.

How to make an SSB without SIB1 information ?

One of the key characteristics of NCD SSB would be stated as 'SSB that is not associated with SIB1' or 'SSB that does not carry information of SIB1'.  How can we make SSB like this ? All SSB includes PBCH and the PBCH carries MIB. MIB always carries IE(Information Elements) about SIB1 information (i.e, the positioning information about CORESET 0). That being said, how is it possible to make SSB not associated with SIB1.

The simple answer is to set kSSB to out of range value. More specifically, set as described in 38.38.213-chapter 13 as below.

If a UE detects a first SS/PBCH block and determines that a CORESET for Type0-PDCCH CSS set is not present, and for 24 <≤ kSSB <≤ 29 for FR1 or for 12 <≤ kSSB <≤ 13 for FR2, the UE may determine the nearest (in the corresponding frequency direction) global synchronization channel number (GSCN) of a second SS/PBCH block having a CORESET for an associated Type0-PDCCH CSS set as NGSCN + Nsize Noffset Nreference. Here NGSCN is the GSCN of the first SS/PBCH block, NGSCN = 1 in FR1 and FR2-1, NGSCN = 3 in FR2-2, and Noffset is a GSCN offset provided by Table 13-16 for FR1 and Table 13-17 for FR2. If the UE detects an SS/PBCH block and the second SS/PBCH block does not provide a CORESET for Type0-PDCCH CSS as described in 38.213- 4.1, the UE may ignore the information related to GSCN of SS/PBCH block locations for performing cell search.

If a UE detects a SS/PBCH block and determines that a CORESET for Type0-PDCCH CSS set is not present, and for kSSB = 31 for FR1 or kSSB = 15 for FR2, the UE determines that there is no SS/PBCH block having an associated Type0-PDCCH CSS set within a GSCN range [NGSCNreference − Noffset , NGSCNreference + Noffset] where NGSCNreference and Noffset are respectively determined by controlResourceSetZero and searchSpaceZero in pdcch-ConfigSIB1. If the GSCN range is [NGSCNreference, NGSCNreference] the UE determines that there is no information for a second SS/PBCH block with a CORESET for an associated Type0-PDCCH CSS set on the detected SS/PBCH block.

If a UE does not detect any SS/PBCH block providing a CORESET for Type0-PDCCH CSS set, as described in 38.213-4.1, within a time period determined by the UE, the UE may ignore the information related to GSCN of SS/PBCH locations in performing cell search.

< 38.213 - Table 13-16: Mapping between the combination of kSSB and controlResourceSetZero and searchSpaceZero in pdcch-ConfigSIB1 to NGSCNOffsetfor FR1 >

< 38.213 - Table 13-17: Mapping between the combination of kSSB and controlResourceSetZero and searchSpaceZero in pdcch-ConfigSIB1 to NGSCNOffsetfor FR2 >

UE Capability Information

UE can notiffy gNB on whether it support the capability of using NCD-SSB if availabel  via the specific IEs in UE Capability Information message as listed below.

RedCapParameters-r17::= SEQUENCE {
   -- R1 28-1: RedCap UE
   supportOfRedCap-r17 ENUMERATED {supported} OPTIONAL,
   supportOf16DRB-RedCap-r17 ENUMERATED {supported} OPTIONAL
}

                    RedCapParameters-v1740::= SEQUENCE {
   ncd-SSB-ForRedCapInitialBWP-SDT-r17 ENUMERATED {supported} OPTIONAL
}
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      
BandNR ::= SEQUENCE {
   bandNR FreqBandIndicatorNR,
   ...
   bwp-WithoutCD-SSB-OrNCD-SSB-RedCap-r17 ENUMERATED {supported} OPTIONAL,
   -- R1 28-3: Half-duplex FDD operation type A for RedCap UE
   halfDuplexFDD-TypeA-RedCap-r17 ENUMERATED {supported} OPTIONAL,
   -- R1 27-15b: Positioning SRS transmission in RRC_INACTIVE state configured outside initial UL BWP
   ...
}
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      

ncd-SSB-ForRedCapInitialBWP-SDT-r17 : Indicates that the UE supports using RedCap-specific initial DL BWP associated with NCD-SSB for SDT. If absent, the UE only supports SDT in an initial DL BWP that includes the CD-SSB. UE supporting this feature shall indicate support of supportOfRedCap-r17 and ra-SDT-r17 and/or cg-SDT-r17.

bwp-WithoutCD-SSB-OrNCD-SSB-RedCap-r17 : Indicates support of RRC-configured DL BWP without CD-SSB or NCD-SSB. The UE can include this field only if the UE supports supportOfRedCap-r17.

RRC Parameters

BWP-DownlinkDedicated ::= SEQUENCE {
   pdcch-Config SetupRelease { PDCCH-Config } OPTIONAL, -- Need M
   pdsch-Config SetupRelease { PDSCH-Config } OPTIONAL, -- Need M
   sps-Config SetupRelease { SPS-Config } OPTIONAL, -- Need M
   radioLinkMonitoringConfig SetupRelease { RadioLinkMonitoringConfig } OPTIONAL, -- Need M
   ...,
   [[
   sps-ConfigToAddModList-r16 SPS-ConfigToAddModList-r16 OPTIONAL, -- Need N
   sps-ConfigToReleaseList-r16 SPS-ConfigToReleaseList-r16 OPTIONAL, -- Need N
   sps-ConfigDeactivationStateList-r16 SPS-ConfigDeactivationStateList-r16 OPTIONAL, -- Need R
   beamFailureRecoverySCellConfig-r16 SetupRelease {BeamFailureRecoveryRSConfig-r16} OPTIONAL, -- Cond SCellOnly
   sl-PDCCH-Config-r16 SetupRelease { PDCCH-Config } OPTIONAL, -- Need M
   sl-V2X-PDCCH-Config-r16 SetupRelease { PDCCH-Config } OPTIONAL -- Need M
   ]],
   [[
   preConfGapStatus-r17 BIT STRING (SIZE (maxNrofGapId-r17)) OPTIONAL, -- Cond PreConfigMG
   beamFailureRecoverySpCellConfig-r17 SetupRelease { BeamFailureRecoveryRSConfig-r16} OPTIONAL, -- Cond SpCellOnly
   harq-FeedbackEnablingforSPSactive-r17 BOOLEAN OPTIONAL, -- Need R
   cfr-ConfigMulticast-r17 SetupRelease { CFR-ConfigMulticast-r17 } OPTIONAL, -- Need M
   dl-PPW-PreConfigToAddModList-r17 DL-PPW-PreConfigToAddModList-r17 OPTIONAL, -- Need N
   dl-PPW-PreConfigToReleaseList-r17 DL-PPW-PreConfigToReleaseList-r17 OPTIONAL, -- Need N
                    nonCellDefiningSSB-r17 NonCellDefiningSSB-r17 OPTIONAL, -- Need R
   servingCellMO-r17 MeasObjectId OPTIONAL -- Cond MeasObject-NCD-SSB
]]
}
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                
                    NonCellDefiningSSB-r17 ::= SEQUENCE {
   absoluteFrequencySSB-r17 ARFCN-ValueNR,
   ssb-Periodicity-r17 ENUMERATED { ms5, ms10, ms20, ms40, ms80, ms160, spare2, spare1 } OPTIONAL, -- Need S
   ssb-TimeOffset-r17 ENUMERATED { ms5, ms10, ms15, ms20, ms40, ms80, spare2, spare1 } OPTIONAL, -- Need S
   ...
}
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                

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.

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.

SuspendConfig ::= SEQUENCE {
   fullI-RNTI I-RNTI-Value,
   shortI-RNTI ShortI-RNTI-Value,
   ran-PagingCycle PagingCycle,
   ran-NotificationAreaInfo RAN-NotificationAreaInfo OPTIONAL, -- Need M
   t380 PeriodicRNAU-TimerValue OPTIONAL, -- Need R
   nextHopChainingCount NextHopChainingCount,
   ...,
   [[
   sl-UEIdentityRemote-r17 RNTI-Value OPTIONAL, -- Cond L2RemoteUE
   sdt-Config-r17 SetupRelease { SDT-Config-r17 } OPTIONAL, -- Need M
   srs-PosRRC-Inactive-r17 SetupRelease { SRS-PosRRC-Inactive-r17 } OPTIONAL, -- Need M
   ran-ExtendedPagingCycle-r17 ExtendedPagingCycle-r17 OPTIONAL -- Cond RANPaging
   ]],
   [[
   ncd-SSB-RedCapInitialBWP-SDT-r17 SetupRelease {NonCellDefiningSSB-r17} OPTIONAL -- Need M
   ]]
}
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      

Reference

3GPP Reference

  • 38.300 - NR and NG-RAN Overall description
  • 38 306 - User Equipment (UE) radio access capabilities
  • 38.213 - NR;Physical layer procedures for control : 17.1 RedCap UE procedures
  • 38.331 -  NR;Radio Resource Control (RRC) : MIB, B.2 Description of BWP configuration options
  • TDocs
    • R2-2200190 : Discussions on RedCap-specific BWPs - Jan 17th – 25th January 2022
    • R2-2200287 : Open issues on early identification, camping restrictions and NCD-SSB - Jan 17th – 25th January 2022
    • R2-2200401 : BWP configuration for RedCap UE - Jan 17th – 25th January 2022
    • R2-2200554 : Identification and access restriction of RedCap UE, and NCD-SSB related issues - Jan 17th – 25th January 2022
    • R2-2200597 : Remaining issues on NCD SSB, identification and access for RedCap  - Jan 17th – 25th January 2022
    • R2-2200608 : On separate initial BWP and NCD-SSB for RedCap - Jan 17th – 25th January 2022
    • R2-2200830 : Using NCD-SSB or CSI-RS in DL BWPs for RedCap UEs  - Jan 17th – 25th January 2022
    • R2-2200862 : Discussion on use of NCD-SSB or CSI-RS in DL BWPs for RedCap UE  - Jan 17th – 25th January 2022
    • R2-2201113 : RedCap UE power-saving aspects at cell re-selection   - Jan 17th – 25th January 2022
    • R2-2201461 : Aspects related to the use of NCD-SSB   - Jan 17th – 25th January 2022
    • R2-2201732 : Summary of NCD-SSB and Initial BWP aspects  - Jan 17th – 25th January 2022
    • R2-2201738 : NCD-SSB and Initial BWP aspects - Jan 17th – 25th January 2022