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5G NR Search Space

Search Space in 5G NR defines the monitoring rule for PDCCH. It tells the UE when to monitor, which symbols within a slot can carry control, which CORESET is being monitored, and how many candidates may exist at each aggregation level.

Read Search Space as the monitoring layer on top of CORESET. CORESET defines the physical control region. Search Space defines the monitoring schedule and candidate pattern inside that region.

Technology 5G NR
Area PHY control monitoring
Main specs 3GPP TS 38.213, 38.331, 38.211
Release Release 18
Main use Defines when and where the UE monitors for PDCCH candidates
Main building blocks CORESET association, periodicity and offset, monitoring symbols, type, and candidate counts
Main types Common Search Space and UE-specific Search Space
Related pages CORESET, PDCCH, DCI Formats, BWP, Frame Structure
5G NR Search Space monitoring timeline showing slot periodicity, offset, monitored symbols, CORESET, and PDCCH candidates
Search Space is a monitoring schedule. The UE checks the associated CORESET only at configured slot occasions and configured symbols within those slots.
5G NR Search Space type map showing common and UE-specific search spaces with SIB1, paging, random access, and dedicated scheduling examples
Common Search Space and UE-specific Search Space serve different control paths. The monitoring rule changes with the procedure and the addressed control type.

Contents

  1. Overview
  2. How the Search Space model works
  3. Operational view
  4. Where Search Space appears in real procedures
  5. Troubleshooting
  6. References
  7. FAQ

Overview

Search Space is not the same thing as CORESET and not the same thing as PDCCH. It is the monitoring rule the UE applies when deciding whether to look for PDCCH candidates in a given slot and symbol set.

  • Each search space is associated with one CORESET.
  • It defines periodicity and offset in slots.
  • It defines monitored symbols within a slot using a symbol bitmap.
  • It defines monitored candidate counts per aggregation level.
  • It defines whether the monitoring is common or UE-specific.

Quick interpretation

Role PDCCH monitoring rule inside an associated CORESET
Configured by SearchSpace RRC signaling, or common initial-access control context
Time control monitoringSlotPeriodicityAndOffset, duration, and monitoringSymbolsWithinSlot
Candidate control nrofCandidates per aggregation level
Type control searchSpaceType common or UE-specific, with procedure-specific common-control flavors
Main impact Grant visibility, paging detection, random-access control, common-control reach, and blind decoding load

How the Search Space model works

The SearchSpace configuration binds monitoring behavior to an associated CORESET through controlResourceSetId. That link tells the UE which control region to monitor. The rest of the fields define when monitoring happens, which symbols can carry candidates, and how many candidates are checked at each aggregation level.

Periodicity and offset

monitoringSlotPeriodicityAndOffset defines the slot periodicity and slot offset. This is the first timing filter. If the slot is not a monitoring occasion, the UE does not search for PDCCH candidates even if the CORESET exists.

Monitoring symbols within a slot

monitoringSymbolsWithinSlot is a 14-bit bitmap that marks which OFDM symbols in the monitoring slot can carry PDCCH candidates for that search space. This is why two search spaces can point to the same CORESET but still monitor different symbol patterns.

Duration

duration sets how many consecutive slots are part of one monitoring occasion. This is separate from the symbol-level CORESET duration and matters for how long the monitoring window stays active.

Candidate counts and aggregation levels

nrofCandidates gives the number of monitored candidates at aggregation levels 1, 2, 4, 8, and 16. Those values drive monitoring effort, blind decoding load, and how likely it is that a valid PDCCH candidate exists at a given robustness level.

Search-space type

searchSpaceType distinguishes common and UE-specific monitoring. Common search spaces support shared control procedures such as SIB1, other SI, paging, random access, MCCH, MTCH, PEI, and small-data or other common-control cases. UE-specific search spaces support dedicated control delivery for one UE.

Field or concept Purpose
searchSpaceId Identifies the search space instance
controlResourceSetId Points the search space to the monitored CORESET
monitoringSlotPeriodicityAndOffset Defines which slots are monitoring occasions
monitoringSymbolsWithinSlot Defines which OFDM symbols within the slot can carry monitored candidates
duration Defines how many consecutive slots belong to one monitoring occasion
nrofCandidates Defines monitored candidate counts for aggregation levels 1, 2, 4, 8, and 16
searchSpaceType Declares common or UE-specific monitoring and the allowed DCI-format family
searchSpaceLinkingId Links search spaces for PDCCH repetition-related monitoring behavior in advanced configurations

Operational view

Read Search Space as a live monitoring schedule. The important question is not only which search spaces are configured, but which one is active for the current procedure, BWP, and DCI format family.

Common Search Space

Common search spaces support shared control procedures. The main practical groups are:

  • Type0 common search space for the initial SIB1 control path with CORESET 0
  • Type0A common search space for other SI-related common control
  • Type1 common search space for random-access related control
  • Type2 common search space for paging-related monitoring
  • Type3 common search space for additional common-control and broadcast-DCI cases

UE-specific Search Space

UE-specific search spaces are used after dedicated control configuration is in place. They are the normal path for per-UE scheduling, uplink grants, downlink assignments, and richer control behavior.

Release 18 reading

Release 18 search-space reading is broader than basic CSS vs USS. It also includes linked search spaces for PDCCH repetition, search-space groups and monitoring adaptation fields, multicast-related DCI-format choices, and the way advanced common-control cases reuse the same monitoring model.

Reading area Why it matters
Associated CORESET Defines which physical control region is actually monitored
Periodicity and offset Defines when the UE expects control opportunities
Monitoring symbols bitmap Defines where inside the slot control can appear
Type and DCI family Defines whether monitoring is for common or dedicated control and which DCI formats matter
Candidate counts Defines monitoring load and practical detection opportunities per aggregation level

Where Search Space appears in real procedures

Initial access and SIB1

SSB -> PBCH / MIB -> CORESET 0 -> Type0 common search space -> PDCCH -> SIB1

This is the main early search-space path. If the Type0 common search space is wrong or not monitored as expected, the UE may find the cell but still miss the first control step after PBCH.

Paging

Paging occasion -> paging search space -> PDCCH -> paging message path

Paging failures are often monitoring failures. The UE may have coverage and a valid cell but still miss paging if the paging search-space assumptions are wrong.

Dedicated scheduling

Active BWP -> UE-specific search space -> PDCCH candidate -> DCI -> PDSCH or PUSCH action

This is the normal scheduled-control path after dedicated configuration.

Troubleshooting

Start with Search Space when PDCCH seems absent only at certain slots, when control works for one procedure but not another, or when the CORESET exists but candidate monitoring still looks empty.

  • Check whether the search space points to the intended CORESET.
  • Check whether periodicity and offset match the expected monitoring slots.
  • Check whether the symbol bitmap matches the CORESET duration and the slot structure.
  • Check whether the right common or UE-specific search-space type is active for the procedure.
  • Check whether candidate counts and aggregation assumptions are realistic for the observed control coverage.
Symptom What to inspect first
Cell found but SIB1 not decoded Type0 common search space, CORESET 0, monitoring symbols, and common-control timing
Paging misses Paging search space, paging occasion timing, active BWP, and monitored symbols
Dedicated grants missed UE-specific search space, periodicity and offset, candidate counts, and the associated CORESET
Control appears only on some slots Monitoring periodicity, slot offset, duration, and whether the slot is actually a monitoring occasion
PDCCH repetition behavior looks inconsistent Linked search spaces, group IDs, monitoring adaptation, and advanced repetition-related settings

Common reading mistakes

  • Treating Search Space as the same thing as CORESET.
  • Looking only at candidate counts while ignoring monitoring slot timing.
  • Ignoring the symbol bitmap and assuming the whole CORESET duration is always monitored the same way.
  • Using the wrong common search-space type when reading SIB1, paging, or random-access control.
  • Explaining missing grants as a data problem before checking whether the UE was monitoring the right search space.

References

FAQ

What is Search Space in 5G NR?

Search Space defines when and where the UE monitors for PDCCH candidates inside an associated CORESET.

What is the difference between CORESET and Search Space?

CORESET defines the physical control region. Search Space defines the monitoring rule inside that region.

What is the difference between common and UE-specific Search Space?

Common Search Space is used for shared control procedures such as SIB1, paging, and random access. UE-specific Search Space is used for dedicated per-UE control delivery.

What controls how many candidates are monitored?

Candidate counts are configured through nrofCandidates for aggregation levels 1, 2, 4, 8, and 16.

Why does Search Space matter in troubleshooting?

Because the UE may be looking at the wrong slots, symbols, or candidate set even when the CORESET and radio quality look fine.

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