5G DCI in a Nutshell

5G DCI in Detail

DCI stands for Downlink Control Information. Simply put, it has several major functionalities as follows :

• Carrying the information to schedule (allocate physical resources) for Downlink Data (PDSCH)
• Carrying the information to schedule (allocate physical resources) for Uplink Data (PUSCH)
• Carrying the information to adjust Uplink Power (PUSCH, PUCCH power) for power control

The main purpose of DCI (Downlink Control Information) is the same as DCI in LTE(I strongly recommend you to read LTE DCI page first if you are not so familiar with what the DCI does). That is, it is a special set of information which schedules downlink data channel (e.g, PDSCH) or uplink data channel (e.g, PUSCH).

Followings are the list of DCI types and its contents defined in 38.212 - 7.3 Downlink control information.

< 38.212 - Table 7.3.1-1: DCI formats >

NOTE : The contents of the table shown here is based on 38.212. This specification is very dry. As any other 3GPP spcecification, it just describes 'what should be done' and does not describe anything on the background story. To get any further technical background, you need to follow through a lot of TDocs. However, unless you are a firmware or DSP engineering implementing the physical channel, it will be very difficult to keep track of all the TDocs. One trick that I would recommend is to go through several latest TDocs not far away from TS (Technical Specification) because those TDocs usually have the summary of past stories and more detailed design concept. I put several TDocs under Reference section that might help you to get some background stories.

The size of DCI (i.e, the bit length of DCI) is determined by the bit length of each field in it and padding bits (if any). See this for the details

• Determining the size of a DCI

Once a DCI data is constructed, it goes to the channel coding process and transmitted over PDCCH. This process is described in followng page.

• PDCCH Transport Process

Format 0_0

This is used for the scheduling of PUSCH in one cell.

Frequency domain resource assignment

The number of bits(bit length) for this field is determined by following formula

 

The meaning of varies depending on the search space where DCI_0_0 is transmitted.

• When stransmitted in common search space,

it indicates the size of the Initial Bandwidth Part

• When stransmitted in UE specific search space and meets following criteria

the total number of different DCI sizes monitored per slot <= 4

the total number of different DCI sizes with C-RNTI monitored per slot <= 3

it indicates the size of the Active Bandwidth Part

The value of this field is determined as follows.

Case 1 : PUSCH hopping = True and Resource allocation type 1

• MSB bits are used to indicate the frequency offset
• = 1 if  Frequency-hopping-offsets-set contains two offset values
• = 2 if  Frequency-hopping-offsets-set contains four offset values
• Remaining Bits indicates PUSCH RIV

 

Case 2 : PUSCH hopping = False and Resource allocation type 1

• the whole bits of this field indicates PUSCH RIV

< 38.214 v15.1 - Table 6.1.4.1-1: MCS index table for PUSCH with transform precoding and 64QAM >

< 38.213 v2.0.0 - Table 7.1.1-1: Mapping of TPC Command Field in DCI format 0_0, DCI format 0_1, or DCI format 2_2, or DCI format 2_3 having CRC parity bits scrambled by TPC-PUSCH-RNTI or TPC-SRS-RNTI, to absolute and accumulated  d_PUSCH,c values >

Format 0_1

This is used for the scheduling of PUSCH in one cell.

< DCI format 0_1 with CRC scrambled by C-RNTI > 

< DCI format 0_1 with CRC scrambled by CS-RNTI > 

< 38.212 v17.3.0 - Table 7.3.1.1.1-1: UL/SUL indicator >

< 38.212 v15.3.0 - Table 7.3.1.1.2-1: Bandwidth part indicator  >

< Frequency domain resource assignment >

The bit length of this field is determined as follows.

The value of this field is determined as follows :

• Resource Allocation Type 0 only : Bitmap determined as described here.
• Resource Allocation Type 1 only : Determined depending on PUSCH Hopping field as follows
• Case 1 : PUSCH hopping = True
• MSB bits are used to indicate the frequency offset
• = 1 if  Frequency-hopping-offsets-set contains two offset values
• = 2 if  Frequency-hopping-offsets-set contains four offset values
• Remaining Bits indicates PUSCH RIV
• Case 2 : PUSCH hopping = False
• the whole bits of this field indicates PUSCH RIV

< 38.214-v15.3.0 Table 6.1.2.2.1-1: Nominal RBG size P >

NOTE : Which RBG(Resource Block Group) size to be used is determined by PUSCH-Config.rbg-Size in RRC message from gNB

< SRS resource indicator >

< 38.212-Table 7.3.1.1.2-24: SRS request >

< Precoding information and number of layers >

NOTE : PUSCH-tp indicates PUSCH Transform Precoding. Refer to Transform Precoding section in Waveform Page if you want to know what this is.

NOTE : txConfig, PUSCH-tp, maxRank, codebook Subset are specified by PUSCH-Config in RRC.

<  Antenna ports >

NOTE : PUSCH-tp indicates PUSCH Transform Precoding. Refer to Transform Precoding section in Waveform Page if you want to know what this is.

<  PTRS - DMRS Association >

Format 1_0

This is used for the scheduling of PDSCH in one cell.

< DCI format 1_0 with CRC scrambled by C-RNTI >  

NOTE 1 : The meaning of varies depending on the search space where DCI_1_0 is transmitted.

When stransmitted in common search space,

it indicates the size of the Initial Bandwidth Part

When stransmitted in UE specific search space and meets following criteria

the total number of different DCI sizes monitored per slot <= 4

the total number of different DCI sizes with C-RNTI monitored per slot <= 3

it indicates the size of the Active Bandwidth Part

NOTE 2 : In case of DCI 1_1, this field indicates the index value of dl-DataToUL-ACK, dl-DataToUL-ACK-r16 configured in RRC, but this field in DCI 1_0 refers to the index of pre-defined set {1,2,3,4,5,6,7,8}. This is based on following specification.

38.213-9.2.3

For DCI format 1_0, the PDSCH-to-HARQ_feedback timing indicator field values map to {1, 2, 3, 4, 5, 6, 7, 8}. For a DCI format, other than DCI format 1_0, scheduling a PDSCH reception or a SPS PDSCH release, the PDSCH-to HARQ_feedback timing indicator field values, if present, map to values for a set of number of slots provided by dl- DataToUL-ACK, dl-DataToUL-ACK-r16, or dl-DataToUL-ACKForDCIFormat1_2

< DCI format 1_0 with CRC scrambled by C-RNTI for PDCCH Order>  

NOTE 1 : If the value of the "Random Access Preamble index" is not all zeros and if the UE is configured with supplementaryUplink in ServingCellConfig in the cell, this field indicates which UL carrier in the cell to transmit the PRACH according to Table 7.3.1.1.1-1( 0 = Non Supplimentary Uplink, 1 = Supplementary Uplink); otherwise, this field is reserved

NOTE 2 : If the value of the "Random Access Preamble index" is not all zeros, this field indicates the SS/PBCH that shall be used to determine the RACH occasion for the PRACH transmission; otherwise, this field is reserved.

NOTE 3 : If the value of the "Random Access Preamble index" is not all zeros, this field indicates the RACH occasion associated with the SS/PBCH indicated by "SS/PBCH index" for the PRACH transmission, according to Clause 5.1.1 of  38.321; otherwise, this field is reserved

NOTE 4 : 12 bits for operation in a cell with shared spectrum channel access; otherwise 10 bits

< DCI format 1_0 with CRC scrambled by RA-RNTI >  

: This is used to schedule RAR (Msg2) in RACH Procedure

The meaning of varies depending on the search space where DCI_1_0 is transmitted.

When stransmitted in common search space in CORESET 0 ,

it indicates the size of the Initial Bandwidth Part

When stransmitted in UE specific search space and meets following criteria

the total number of different DCI sizes monitored per slot <= 4

the total number of different DCI sizes with C-RNTI monitored per slot <= 3

it indicates the size of the Active Bandwidth Part

TB Scaling is special parameters affecting the determination of Transport Block Size. This factor is multiplied to the result of the calculated Ninfo value.

< 38.214 - Table 5.1.3.2-2: Scaling factor of Ninfo for P-RNTI, RA-RNTI and MSGB-RNTI >

< DCI format 1_0 with CRC scrambled by TC-RNTI >  

: This is used to schedule Contention Resolution (Msg4) in RACH Procedure

The meaning of varies depending on the search space where DCI_1_0 is transmitted.

When stransmitted in common search space in CORESET 0 ,

it indicates the size of the Initial Bandwidth Part

When stransmitted in UE specific search space and meets following criteria

the total number of different DCI sizes monitored per slot <= 4

the total number of different DCI sizes with C-RNTI monitored per slot <= 3

it indicates the size of the Active Bandwidth Part

< DCI format 1_0 with CRC scrambled by SI-RNTI >  

: This is used to schedule SIB1 and other SIB messages

[NOTE 1] SIB1 DCI refer to SI-RNTI / Type0Common in 38.214 - Table 5.1.2.1.1-1. As you see in the table, different TimeDomainResourceAllocation table is used depending on SSB/CORESET Multiplexing Pattern as summarized in the following table.

< DCI format 1_0 with CRC scrambled by P-RNTI>  

: This is used to schedule Paging messages

< 38.212 - Table 7.3.1.2.1-1: Short Message indicator >

< 38.331 - Table 6.5-1: Short messages >

Format 1_1

This is used for the scheduling of PDSCH in one cell.

< Antenna port(s) and number of layers >

Format 2_0

This is used for notifying following information to UE. This DCI is scrambled by SFI_RNTI

• Slot format
• COT(Channel Occupancy Time) duration
• Available RB set
• Search space set group switching

If the higher layer parameter slotFormatCombToAddModList is configured

Field (Item)	Bits	Reference
Identifier for DCI formats	1
Slot format indicator	Variable	Bit size is determined by RRC message here

Slot format indicator : a Bit string indicating Slot format indicator 1, Slot format indicator 2, , Slot format indicator N

If the higher layer parameter availableRB-SetsToAddModList is configured

Field (Item)	Bits	Reference
Identifier for DCI formats	1
Available RB set Indicator	Variable	Bit size is determined by RRC message here

Available RB set Indicator : a Bit string indicating Available RB set Indicator 1, Available RB set Indicator 2, , Available RB set Indicator N

If the higher layer parameter co-DurationsPerCellToAddModList is configured

Field (Item)	Bits	Reference
Identifier for DCI formats	1
COT duration indicator	Variable	Bit size is determined by RRC message here

COT duration indicator : a Bit string indicating COT duration indicator 1, COT duration indicator 2, , COT duration indicator N

If the higher layer parameter switchTriggerToAddModList is configured

Field (Item)	Bits	Reference
Identifier for DCI formats	1
Search space set group switching flag	Variable	Bit size is determined by RRC message here

Search space set group switching flag : a Bit string indicating Search space set group switching flag1, Search space set group switching flag 2, , Search space set group switching flag N. (Details are in 38.213 - 11.1.1)

NOTE : Not every UE would support this DCI. That is, supporting DCI 2_0 decoding depends on UE capability. UE Capability Information related to DCI 2_0 decoding are listed below.

Phy-ParametersXDD-Diff ::= SEQUENCE {

    dynamicSFI                                   ENUMERATED {supported} OPTIONAL,

    ...

}

 

Phy-ParametersFRX-Diff ::= SEQUENCE {

    dynamicSFI                                   ENUMERATED {supported} OPTIONAL,

    ...

}

 

SharedSpectrumChAccessParamsPerBand-r16 ::= SEQUENCE {

    ...

    dci-AvailableRB-Set-r16                  ENUMERATED {supported} OPTIONAL,

    dci-ChOccupancyDuration-r16          ENUMERATED {supported} OPTIONAL,

    ...

]

Format 2_1

This is used for notifying the PRB(s) and OFDM symbol(s) where UE may assume no transmission is intended for the UE. This DCI is scrambled by INT-RNTI.

The size of DCI format 2_1 is configurable by higher layers up to 126 bits and each Pre-emtion indication is 14 bits(Details are in 38.213 - 11.2)

Format 2_2

This is used for the transmission of TPC commands for PUCCH, PUSCH. This DCI is scrambled by scrambled by TPC-PUSCH-RNTI or TPC-PUCCH-RNTI.

Following fields are defined for each block

Format 2_3

This is used for the transmission of a group of TPC commands for SRS transmissions by one or more UEs. Along with a TPC command, a SRS request may also be transmitted.

Following fields are defined for each block

If the UE is configured with higher layer parameter srs-TPC-PDCCH-Group = typeA for an UL without PUCCH and

PUSCH or an UL on which the SRS power control is not tied with PUSCH power control

If the UE is configured with higher layer parameter srs-TPC-PDCCH-Group = typeB for an UL without PUCCH and

PUSCH or an UL on which the SRS power control is not tied with PUSCH power control

Format 2_4

This DCI is used for notifying the PRB(s) and OFDM symbol(s) where UE cancels the corresponding UL transmission from the UE (Details are in 38.213-11.2A). This is scrambled by ci-RNTI.

Format 2_5

This DCI is used for notifying the availability of soft resources (Details in 38.473-9.3.1). This is scrambled by AI-RNTI.

Format 2_6

This DCI is used for notifying the power saving information outside DRX Active Time for one or more UEs. This is scrambled by PS-RNTI

Structure of DCI format 2_6 is as follows :

block number 1, block number 2,, block number N

Structure of each block is as follows :

Determining the size of DCI

In 5G NR (New Radio), the DCI (Downlink Control Information) is a critical component of the Physical Downlink Control Channel (PDCCH). It carries scheduling information, resource allocation details, and other control data necessary for the UE (User Equipment) to communicate with the network. The size (bit length) of the DCI is not fixed and is determined dynamically based on several factors, including the DCI format, the configuration of the system, and the use case.

In short, The size of the DCI in 5G NR is determined by:

• The DCI format.
• The fields included.
• The BWP size and resource allocation type.
• RRC-configured parameters.
• Padding for size alignment.

The exact size is calculated by the network and signaled to the UE for efficient PDCCH decoding.

Here's a detailed explanation:

DCI Formats

The size of the DCI depends on the specific DCI format being used. 5G NR defines several DCI formats, each tailored for different purposes:

• DCI Format 0_0: Uplink scheduling (compact, used in fallback scenarios).
• DCI Format 0_1: Uplink scheduling (non-fallback, more flexible and detailed).
• DCI Format 1_0: Downlink scheduling (compact, fallback).
• DCI Format 1_1: Downlink scheduling (non-fallback, more detailed).
• DCI Format 2_x: Group-common DCI (e.g., 2_0 for slot format indication, 2_1 for preemption indication, etc.).

Each format has a baseline set of fields, and the bit length varies depending on the complexity and number of parameters included.

Fields in DCI

The total bit length of a DCI is the sum of the bits required for all fields included in a given format. Common fields include:

• Frequency domain resource assignment: Depends on the bandwidth part (BWP) size and resource allocation type.
• Time domain resource assignment: Typically 4 bits.
• Modulation and Coding Scheme (MCS): Usually 5 bits per transport block.
• HARQ process number: 4 bits.
• New Data Indicator (NDI): 1 bit per transport block.
• Redundancy Version (RV): 2 bits.
• Carrier indicator: 0 or 3 bits (for cross-carrier scheduling).
• BWP indicator: 0–2 bits, depending on the number of configured BWPs.

Bandwidth Part (BWP) Dependency

The size of the frequency domain resource assignment field is a major contributor to DCI size and is calculated based on the active Bandwidth Part (BWP):

• Resource Allocation Type 0: Uses a bitmap for resource block groups (RBGs). The number of bits is determined by:
  Formula: N_RBG = ⌈ (N_BWP + (N_BWP mod P)) / P ⌉, where N_BWP is the number of PRBs in the BWP, and P is the RBG size.
• Resource Allocation Type 1: Uses a contiguous resource allocation. The number of bits is:
  Formula: ⌈ log₂(N_BWP · (N_BWP + 1) / 2) ⌉.

The larger the BWP, the more bits are needed.

Payload Size Alignment

To ensure efficient decoding, the DCI sizes for certain formats are aligned or padded:

• Fallback DCI (0_0 and 1_0): Sizes are aligned within a search space by padding the smaller DCI with zeros.
• Non-fallback DCI (0_1 and 1_1): More flexible, sizes depend on RRC configuration.

Example >

As written in TS 38.212 section 7.3, DCI format 0_0 in common search space is padded with zeros so that it matches the size of DCI format 1_0. So if the DCI format 0_0 carries only 33 bits of contents, it gets padded to get transmitted with the size of 41 (the size of format 1_0

RRC Configuration

The network configures parameters via RRC signaling, affecting DCI size:

• Number of MIMO layers: Impacts precoding information.
• Cross-carrier scheduling: Adds a carrier indicator field.
• Dynamic HARQ-ACK codebook: Adds feedback timing fields.

Search Space and CORESET

The DCI size is influenced by the search space and CORESET configuration:

• In common search space: Simpler formats (e.g., 1_0) with smaller sizes.
• In UE-specific search space: Larger formats (e.g., 1_1).

Reference

[1] 3GPP TS 38.212 NR;Multiplexing and channel coding   

[2] 3GPP TSG RAN WG1 Meeting #92  : R1-1802117 : Discussion on DCI contents

[3] 3GPP TSG-RAN WG1 #92 : R1-1802905 : Remaining issues of the DCI contents and formats  

[4] 3GPP TSG RAN WG1 Meeting #92 : R1-1801976 : DCI Contents and Formats

[5] 3GPP TSG RAN WG1 Meeting #92 : R1-1802097 : Remaining issues on MIMO related DCI design

[6] 3GPP TSG RAN WG1 Meeting #92 : R1-1802482 : DCI contents and formats

[7] 3GPP TSG RAN WG1 Meeting #92 : R1-1802597 : Remaining issues in DCI contents and formats  

[8] 3GPP TSG RAN WG1 Meeting #92 : R1-1802209 : Remaining issues on DCI contents and formats

[9] 3GPP TSG RAN WG1 Meeting #92 : R1-1801730 : Open issues on DCI contents and formats

[10] 3GPP TSG RAN WG1 Meeting #92 : R1-1801624 : Remaining issues on DCI content

[11] 3GPP TSG RAN WG1#100bis-e R1-e-Meeting, April 20th 30th, 2020 : Email discussion/approval on special states/indications in available RB set indication and COT duration indication/determination (NR-U DL Signals and Channels)