mmWave & 5G

DCI Format 1_1

Q: How does DCI format 1_1 differ from DCI format 1_0?

DCI format 1_0 is a fallback message with a fixed, compact field set that can be monitored in both the common and UE-specific search spaces. DCI format 1_1 is monitored only in the UE-specific search space and has a configurable payload whose size depends on RRC parameters such as the number of configured bandwidth parts, the maximum number of MIMO layers, the HARQ codebook type, and whether code-block-group transmission is enabled. As a result 1_0 is typically 40 to 45 bits while 1_1 commonly ranges from 45 to over 70 bits. Format 1_1 adds fields that 1_0 lacks, including a bandwidth-part indicator, an antenna-ports field, a transmission-configuration-indication field for beam selection, and an SRS request, which let the network unlock spatial multiplexing, dynamic BWP switching, and beam management.

Q: Why is the payload size of DCI 1_1 variable?

The 5G NR specification defines several fields of DCI 1_1 with sizes that the gNB sets through RRC configuration rather than fixing them in the standard. The frequency-domain resource-assignment field is ceil(log2(N(N+1)/2)) bits for type 1 allocation over N resource blocks in the active bandwidth part, so it grows with carrier bandwidth. The bandwidth-part indicator is 0, 1, or 2 bits depending on how many BWPs are configured. The antenna-ports field varies with the DMRS configuration, and the rate-matching, ZP-CSI-RS, and CBG-transmission-information fields appear only when those features are enabled. The UE computes the exact size from its active configuration, then applies a size-alignment (budget) procedure so that the number of distinct DCI payload sizes it must blind-decode stays within the limit of four total sizes and three C-RNTI-scrambled sizes per slot.

Q: What does the antenna-ports field in DCI 1_1 control?

The antenna-ports field selects which DMRS ports the PDSCH uses, which in turn defines the number of spatial layers and the demodulation reference-signal pattern. The field indexes a row in the antenna-port tables of 3GPP TS 38.212, and the chosen row also signals the number of DMRS code-division-multiplexing groups without data, so the UE knows which resource elements carry data versus reference signals. Combined with the transmission-configuration-indication field, this lets DCI 1_1 schedule single-user MIMO up to 8 layers or coordinate ports for multi-user MIMO, while 1_0 is restricted to a single layer on a fixed DMRS port.

/D-C-I one-one/

One of the 5G NR downlink control information formats carried on the PDCCH, DCI format 1_1 schedules a PDSCH transmission for a single UE and is monitored only in the UE-specific search space. Unlike the compact fallback format 1_0, its payload is configurable: fields for frequency-domain resource assignment, antenna ports, bandwidth-part selection, transmission-configuration indication, and code-block-group control are sized by RRC, letting the gNB unlock spatial multiplexing up to 8 layers, dynamic BWP switching, and beam management. Typical sizes run from about 45 to more than 70 bits, scrambled by a 16-bit C-RNTI CRC for addressing and error detection.

Category: mmWave & 5G

Search space: UE-specific only

Schedules: PDSCH (DL data)

The Flexible Downlink Scheduling Grant in 5G NR

3GPP TS 38.212 defines a small set of DCI formats, each tailored to a scheduling job. Formats 0_0 and 0_1 grant uplink PUSCH; formats 1_0 and 1_1 grant downlink PDSCH. Within the downlink pair, 1_0 is the fallback grant: a fixed, compact field layout that the UE can decode before it has a full RRC configuration, and which appears in both the common and UE-specific search spaces. Format 1_1 is the workhorse scheduling grant once the connection is established. It lives only in the UE-specific search space, and its field set expands or contracts according to how the network has configured that UE.

That flexibility is the entire point. Format 1_1 carries fields that 1_0 simply does not have: a bandwidth-part indicator so the scheduler can move a UE between configured BWPs slot by slot, an antenna-ports field that selects DMRS ports and the number of MIMO layers, a transmission-configuration-indication (TCI) field that points to a beam for FR2 millimeter-wave operation, an SRS request, and optional code-block-group transmission information for fine-grained HARQ retransmission. Because these fields are present or sized only when the corresponding feature is configured, two UEs on the same cell can be monitoring 1_1 payloads of different lengths in the same slot.

Variable length creates a decoding problem. The UE does not know in advance which PDCCH candidate carries its grant, so it performs blind decoding across an aggregation of control-channel elements. To keep that search tractable, 3GPP caps the number of distinct DCI sizes a UE budgets per slot at four total, of which at most three may be C-RNTI scrambled. When the natural size of 1_1 collides with this budget, a defined padding and alignment procedure either zero-pads the smaller format up to a common size or truncates fields, so the UE never has to test more sizes than the limit allows.

Payload Sizing and Resource Assignment

Type 1 frequency-domain resource assignment (bits):
N_bits = ⌈ log₂( N_RB^BWP × (N_RB^BWP + 1) / 2 ) ⌉

Bandwidth-part indicator (bits):
N_BWP = ⌈ log₂(n_BWP) ⌉, where n_BWP = number of configured DL BWPs (max 4)

Time-domain resource assignment: 0 to 4 bits (indexes the PDSCH-TimeDomainResourceAllocationList)

CRC and addressing: L_CRC = 16 bits, scrambled by the 16-bit C-RNTI

Example: for a 100 MHz FR1 carrier at 30 kHz SCS, N_RB^BWP ≈ 273, so the type 1 field is ⌈ log₂(273 × 274 / 2) ⌉ = ⌈ log₂(37401) ⌉ = 16 bits. With antenna-ports, TCI, and CBG fields enabled the total 1_1 payload reaches roughly 60 to 70 bits before CRC.

DCI 1_1 vs 1_0 Field Comparison

Attribute	DCI Format 1_0	DCI Format 1_1
Role	Fallback PDSCH grant	Full-feature PDSCH grant
Search space	Common & UE-specific	UE-specific only
Payload size	~40 to 45 bits (fixed)	~45 to 70+ bits (RRC-configured)
Resource alloc type	Type 1 only	Type 0, type 1, or dynamic switch
MIMO layers	1 layer (fixed DMRS port)	Up to 8 via antenna-ports field
BWP indicator	Not present	0, 1, or 2 bits
Beam (TCI) field	Not present	0 or 3 bits
CBG retransmission	Not supported	Optional (0, 2, 4, 6, or 8 bits)

Common Questions

Frequently Asked Questions

How does DCI format 1_1 differ from DCI format 1_0?

Format 1_0 is a compact fallback grant with a fixed field set, monitored in both the common and UE-specific search spaces and typically 40 to 45 bits. Format 1_1 is UE-specific only, with an RRC-configured payload of roughly 45 to 70+ bits. It adds a bandwidth-part indicator, an antenna-ports field, a TCI field for beam selection, and an SRS request, unlocking spatial multiplexing, dynamic BWP switching, and beam management that 1_0 cannot signal.

Why is the payload size of DCI 1_1 variable?

Several fields are sized by the gNB through RRC rather than fixed in the spec. The type 1 frequency-assignment field is ⌈log₂(N(N+1)/2)⌉ bits and grows with carrier bandwidth; the BWP indicator is 0 to 2 bits; antenna-ports depends on DMRS configuration; and rate-matching, ZP-CSI-RS, and CBG fields appear only when enabled. The UE computes the exact size, then applies a size-alignment procedure so it blind-decodes no more than four DCI sizes (three C-RNTI scrambled) per slot.

What does the antenna-ports field in DCI 1_1 control?

It selects the DMRS ports for the PDSCH, which sets the number of spatial layers and the reference-signal pattern. The field indexes a row in the antenna-port tables of TS 38.212 and signals how many DMRS CDM groups carry no data, so the UE knows which resource elements hold data versus reference signals. With the TCI field it enables single-user MIMO up to 8 layers or coordinated multi-user MIMO, whereas 1_0 is locked to one layer on a fixed port.

Related Terms

← DCI 1_0 DCI Format →