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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 :
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 ( 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.
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)).
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 InformationUE 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
}
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
...
}
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
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
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