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5G/NR - Search Space |
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Search Space
PDCCH Search Space refers to the area in the downlink resource grid where PDCCH may be carried. UE perform blind decoding throughout these search space trying to find PDCCH data (i.e, DCI). At high level view, NR Search Space concept is similar to LTE Search Space, but there are many differences in terms of the details.
In order for UE to decode PDCCH(DCI), it has to figure out the exact value for location(CCE index), structure(Aggregation Level, Interleaving etc) and scrambling code(RNTI) etc. But these information is not informed to UE beforehand and in most case those values changes dynamically. The only thing known to UE is the information about a certain range that possibly carries PDCCH(DCI). UE knows about the information about this certain range by a predefined rule or signaling message. Within this range, UE has to try to decode PDCCH/DCI using many, many, many, many different types of parameters (CCE Index, Aggregation Level, RNTI) based on trial and error method. This way of decoding is called Blind Decodinig (See LTE Blind Decoding if you want to know of the concept in more detail. Even though the detailed procedure is a little different, highlevel concept of blind decoding is same in LTE and NR). The predfined region in which UE perform the blind decoding is called Search Space.
There are two types of search space called "UE-specific search space" and "Common Search Space". UE Spacific Search Space is dedicated for each specific UE and informed to UE via RRC signaling message. It means UE need to complete RRC establishment to get the information about UE-specific search space. Common Search Space is the specific search space that every UE need to search for the signals for every UE(e.g, PDCCH for SIB) or signaling message that is applied to every UE before dedicated channel is established for a specific UE(e.g, PDCCH used during RACH process). For example, UE need to detect PDCCH for SIB1 reception or various DCI (PDCCH) during RACH process (e.g, DCI for Msg2 / Msg4 reception).
< Summary of Search Space Type, based on 38.213 - 10.1 >
NOTE : RNTI would give you many additional information. Refer to RNTI page for further information.
< Summary of Search Space Type and RRC Configuration, based on 38.213 - 10.1 >
Following is based on 38.331 v15.3.0
SearchSpace ::= SEQUENCE { searchSpaceId SearchSpaceId, controlResourceSetId ControlResourceSetId OPTIONAL,-- Cond Setup Only monitoringSlotPeriodicityAndOffset CHOICE { sl1 NULL, sl2 INTEGER (0..1), sl4 INTEGER (0..3), sl5 INTEGER (0..4), sl8 INTEGER (0..7), sl10 INTEGER (0..9), sl16 INTEGER (0..15), sl20 INTEGER (0..19), sl40 INTEGER (0..39), sl80 INTEGER (0..79), sl160 INTEGER (0..159), sl320 INTEGER (0..319), sl640 INTEGER (0..639), sl1280 INTEGER (0..1279), sl2560 INTEGER (0..2559) } monitoringSymbolsWithinSlot BIT STRING (SIZE (14)) OPTIONAL, -- Cond Setup nrofCandidates SEQUENCE { aggregationLevel1 ENUMERATED {n0, n1, n2, n3, n4, n5, n6, n8}, aggregationLevel2 ENUMERATED {n0, n1, n2, n3, n4, n5, n6, n8}, aggregationLevel4 ENUMERATED {n0, n1, n2, n3, n4, n5, n6, n8}, aggregationLevel8 ENUMERATED {n0, n1, n2, n3, n4, n5, n6, n8}, aggregationLevel16 ENUMERATED {n0, n1, n2, n3, n4, n5, n6, n8} } OPTIONAL, -- Cond Setup searchSpaceType CHOICE { common SEQUENCE { dci-Format0-0-AndFormat1-0 SEQUENCE { ... } OPTIONAL, -- Need R dci-Format2-0 SEQUENCE { nrofCandidates-SFI SEQUENCE { aggregationLevel1 ENUMERATED {n1, n2} OPTIONAL, -- Need R aggregationLevel2 ENUMERATED {n1, n2} OPTIONAL, -- Need R aggregationLevel4 ENUMERATED {n1, n2} OPTIONAL, -- Need R aggregationLevel8 ENUMERATED {n1, n2} OPTIONAL, -- Need R aggregationLevel16 ENUMERATED {n1, n2} OPTIONAL -- Need R }, ... } OPTIONAL, -- Need R dci-Format2-1 SEQUENCE { ... } OPTIONAL, -- Need R dci-Format2-2 SEQUENCE { ... } OPTIONAL, -- Need R dci-Format2-3 SEQUENCE { monitoringPeriodicity ENUMERATED {n1, n2, n4, n5, n8, n10, n16, n20 } nrofPDCCH-Candidates ENUMERATED {n1, n2}, ... } OPTIONAL -- Need R }, ue-Specific SEQUENCE { dci-Formats ENUMERATED {formats0-0-And-1-0, formats0-1-And-1-1}, ... } } OPTIONAL -- Cond Setup }
searchSpaceId : Identity of the search space. SearchSpaceId = 0 identifies the SearchSpace configured via PBCH (MIB) or ServingCellConfigCommon. The searchSpaceId is unique among the BWPs of a Serving Cell
controlResourceSetId : The CORESET applicable for this SearchSpace. Value 0 identifies the common CORESET configured in MIB and in ServingCellConfigCommon Values 1..maxNrofControlResourceSets-1 identify CORESETs configured by dedicated signalling
monitoringSlotPeriodicityAndOffset : Slots for PDCCH Monitoring configured as periodicity and offset. Corresponds to L1 parameters 'Montoring-periodicity-PDCCH-slot' and 'Montoring-offset-PDCCH-slot'. For example, if the value is sl1, it mean that UE should monitor the SearchSpace at every slot. if the value is sl4, it mean that UE should monitor the SearchSpace in every fourth slot.
monitoringSymbolsWithinSlot : Symbols for PDCCH monitoring in the slots configured for PDCCH monitoring (see monitoringSlotPeriodicityAndOffset).The most significant (left) bit represents the first OFDM in a slot. The least significant (right) bit represents the last symbol. Corresponds to L1 parameter 'Montoring-symbols-PDCCH-within-slot'. This indicates the starting OFDM symbols that UE should search for a SearchSpace. For example, if the value is '1000000000000', it mean that UE should start searching from the first OFDM symbol. if the value is '0100000000000', it mean that UE should start searching from the second OFDM symbol.
nrofCandidates : Number of PDCCH candidates per aggregation level. Corresponds to L1 parameter 'Aggregation-level-1' to 'Aggregation-level-8'. The number of candidates and aggregation levels configured here applies to all formats unless a particular value is specified or a format-specific value is provided (see inside searchSpaceType)
searchSpaceType : Indicates whether this is a common search space (present) or a UE specific search space as well as DCI formats to monitor for
common : Configures this search space as common search space (CSS) and DCI formats to monitor.
dci-Format0-0-AndFormat1-0 : If configured, the UE monitors the DCI formats 0_0 and 1_0 with CRC scrambled by C-RNTI, CS-RNTI (if configured), SP-CSI-RNTI (if configured), RA-RNTI, TC-RNTI, P-RNTI, SI-RNTI
dci-Format2-0 : If configured, UE monitors the DCI format format 2_0 with CRC scrambled by SFI-RNTI
nrofCandidates-SFI : The number of PDCCH candidates specifically for format 2-0 for the configured aggregation level. If an aggregation level is absent, the UE does not search for any candidates with that aggregation level. Corresponds to L1 parameters 'SFI-Num-PDCCH-cand' and 'SFI-Aggregation-Level'
dci-Format2-1 : If configured, UE monitors the DCI format format 2_1 with CRC scrambled by INT-RNTI
dci-Format2-2 : If configured, UE monitors the DCI format 2_2 with CRC scrambled by TPC-PUSCH-RNTI or TPC-PUCCH-RNTI
dci-Format2-3 : If configured, UE monitors the DCI format 2_3 with CRC scrambled by TPC-SRS-RNTI
ue-Specific : Configures this search space as UE specific search space (USS). The UE monitors the DCI format with CRC scrambled by C-RNTI, CS-RNTI (if configured), TC-RNTI (if a certain condition is met), and SP-CSI-RNTI (if configured)
Reference
[1] Type0-PDCCH common search space
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