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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 >

Type

Search Space

RNTI

Use Case

Type0-PDCCH

Common

SI-RNTI for RMSI on a primary cell

SIB Decoding (SIB1)

Type0A-PDCCH

Common

SI-RNTI on a primary cell

SIB Decoding (Other SIBs)

Type1-PDCCH

Common

RA-RNTI,TC-RNTI,C-RNTI on a primary cell

Msg2, Msg4 decoding in RACH

Type2-PDCCH

Common

P-RNTI on a primary cell

Paging Decoding

Type3-PDCCH

Common

INT-RNTI, SFI-RNTI, TPC-PUSCH-RNTI, TPC-PUCCH-RNTI, TPC-SRS-RNTI, C-RNTI, CS-RNTI(s), SP-CSI-RNTI

 
UE-SS

UE Specific

C-RNTI, or CS-RNTI(s), or SP-CSI-RNTI

User specific PDSCH decoding

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 >

Type

Signaling Parameters

Type0-PDCCH

pdcch-ConfigSIB1 in MasterInformationBlock

searchSpaceSIB1 in PDCCH-ConfigCommon

searchSpaceZero in PDCCH-ConfigCommon

Type0A-PDCCH

searchSpaceOtherSystemInformation in PDCCHConfigCommon

Type1-PDCCH

ra-SearchSpace in PDCCH-ConfigCommon

Type2-PDCCH

pagingSearchSpace in PDCCH-ConfigCommon

Type3-PDCCH

SearchSpace in PDCCH-Config with searchSpaceType = common

UE Specific

SearchSpace in PDCCH-Config with searchSpaceType = ue-Specific

 

 

 

RRC Parameters

 

 

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