mmWave & 5G

DCI Format

Q: What is the difference between fallback and non-fallback DCI formats?

Fallback formats (DCI 0_0 for uplink, DCI 1_0 for downlink) have a fixed, compact field set that does not depend on the UE's RRC-configured features, so they can be decoded in the common search space before higher-layer configuration completes. They are used during initial access, random access response, paging, and to deliver system information. Non-fallback formats (DCI 0_1, 1_1, and the wideband 0_2 and 1_2 introduced in Release 16) add fields for carrier aggregation, multi-layer MIMO precoding, bandwidth part switching, and multiple HARQ processes, so their payload size is configurable and typically larger. The gNB chooses the format based on whether the UE has been fully configured and what scheduling flexibility the link requires.

Q: How does a UE know which DCI format it received?

The UE does not get an explicit format identifier in plaintext. Instead it performs blind decoding: for each PDCCH candidate in its search space, at each aggregation level (1, 2, 4, 8, or 16 CCEs), it assumes a candidate DCI payload size, runs polar decoding, then checks the 24-bit CRC after de-scrambling it with one of its assigned RNTIs (C-RNTI, CS-RNTI, SI-RNTI, and so on). A CRC pass means the message was intended for that UE and the assumed size and RNTI were correct, which in turn identifies the format. To bound the number of trials, 3GPP limits the candidate DCI payload sizes a UE must monitor to at most four sizes (three for C-RNTI) per serving cell per slot.

Q: What is the size in bits of a typical DCI 1_1 message?

DCI 1_1 has a configurable payload because its fields scale with the RRC configuration. A baseline single-carrier, single-codeword case is roughly 40 to 45 bits before CRC, but it can exceed 60 to 70 bits when carrier aggregation, two codewords, multi-TRP, and large frequency-domain resource assignments are enabled. After appending the 24-bit CRC, the encoded block is polar-coded and rate-matched onto 1, 2, 4, 8, or 16 control channel elements, where each CCE is 6 resource-element groups of 12 subcarriers each, giving 72 REs (minus DMRS) per CCE. The compact fallback format DCI 1_0 is far smaller, typically around 28 bits plus CRC, which is why it is more robust at cell edge.

/dee-see-eye for-mat/

In 5G New Radio, a Downlink Control Information format is a standardized, fixed-field message structure transmitted on the PDCCH that the gNB uses to grant radio resources and signal control parameters to a user equipment. Each format defines an exact bit layout, covering frequency-domain and time-domain resource assignment, modulation and coding scheme, HARQ process number, redundancy version, and transmit-power control. 3GPP defines fallback formats (0_0 and 1_0) for robust initial access and non-fallback formats (0_1, 1_1, 0_2, 1_2) for fully configured links carrying carrier aggregation and multi-layer 5G NR MIMO scheduling. A 24-bit CRC scrambled with the target UE's RNTI lets the receiver confirm both the recipient and the format through blind decoding.

Category: mmWave & 5G

Carrier: PDCCH

CRC length: 24 bits, RNTI-scrambled

How DCI Formats Schedule the 5G NR Air Interface

Every downlink or uplink data transmission in 5G NR is preceded by a control message that tells the receiver where to look and how to interpret what it finds. That message is the Downlink Control Information, and its on-air structure is selected from a small catalogue of DCI formats defined in 3GPP TS 38.212. A DCI format is essentially a tightly packed binary record: each field has a defined bit width and position so that transmitter and receiver agree on the parse without any self-describing headers. Because the PDCCH is decoded before any data, the format set is kept small and the most robust formats are deliberately compact.

Formats split into two families. Fallback formats, DCI 0_0 for uplink grants and DCI 1_0 for downlink assignments, use a minimal, RRC-independent field set and live in the common search space, which makes them usable during random access, paging, and system-information delivery before the UE is configured. Non-fallback formats (0_1 and 1_1, plus the Release 16 wideband variants 0_2 and 1_2) add fields for bandwidth-part indication, multiple HARQ processes, antenna-port and precoding selection, and carrier-aggregation indexing. The gNB scheduler picks the format per transmission, trading payload size against the scheduling flexibility a given link needs.

Specialized formats round out the set. DCI format 2_0 carries the slot-format indicator for dynamic TDD, 2_1 signals pre-emption for URLLC over eMBB, 2_2 and 2_3 handle group power control for PUCCH, PUSCH, and SRS, and Release 16 added 2_4 through 2_6 for uplink cancellation, sounding, and power-saving wake-up signaling. Each of these is scrambled with a dedicated RNTI so the right group of devices acts on it.

CCE Aggregation and Coded Length

Once assembled, the DCI payload gets a 24-bit CRC appended, that CRC is scrambled by the appropriate RNTI, and the result is polar-coded and rate-matched onto an integer number of control channel elements. A CCE spans 6 resource-element groups, each REG being one resource block (12 subcarriers) over one OFDM symbol, so one CCE carries 72 resource elements before DMRS overhead. Aggregation levels of 1, 2, 4, 8, or 16 CCEs let the network trade coding rate against control-channel reliability: cell-edge UEs receive the same DCI at level 8 or 16 for a far lower effective code rate.

Governing Relationships

Coded PDCCH payload size:
N_bits = (DCI_payload + 24) bits → polar-coded onto L × 108 coded bits

Channel elements available:
REs per CCE = 6 REGs × 12 subcarriers − DMRS ≈ 72 − 18 = 54 RE

CRC scrambling (recipient identification):
CRC_scrambled = CRC₂₄ ⊕ RNTI₁₆ (XOR over the trailing 16 bits)

Effective control code rate:
R_c ≈ (DCI_payload + 24) / (L × 108) where L ∈ {1, 2, 4, 8, 16}

Example: DCI 1_1 of 45 bits + 24 CRC = 69 bits at L = 4 → 432 coded bits, R_c ≈ 0.16, robust to roughly −6 dB SINR.

Common DCI Formats at a Glance

Format	Direction	Family	Typical Payload	RNTI	Primary Use
DCI 0_0	Uplink grant	Fallback	~28 bits	C-RNTI, TC-RNTI	Initial access, robust PUSCH grant
DCI 1_0	Downlink	Fallback	~28 bits	SI-/RA-/P-/C-RNTI	SIB, paging, RAR, fallback PDSCH
DCI 0_1	Uplink grant	Non-fallback	40 to 70 bits	C-RNTI, CS-RNTI	Configured PUSCH, CA, MIMO
DCI 1_1	Downlink	Non-fallback	40 to 70 bits	C-RNTI, CS-RNTI	Configured PDSCH, 2-codeword MIMO
DCI 2_0	Group	Special	Up to 128 bits	SFI-RNTI	Dynamic TDD slot format
DCI 2_1	Group	Special	Up to 126 bits	INT-RNTI	URLLC pre-emption indication

Common Questions

Frequently Asked Questions

What is the difference between fallback and non-fallback DCI formats?

Fallback formats (DCI 0_0 and 1_0) carry a fixed, RRC-independent field set decodable in the common search space before configuration completes, so they serve initial access, RAR, paging, and SIB delivery. Non-fallback formats (0_1, 1_1, and the Release 16 wideband 0_2, 1_2) add fields for carrier aggregation, bandwidth-part switching, multi-layer MIMO precoding, and multiple HARQ processes, giving a larger, configurable payload. The gNB selects the format from the UE's configuration state and the scheduling flexibility the link needs.

How does a UE know which DCI format it received?

There is no plaintext format identifier. The UE blind-decodes every PDCCH candidate in its search space at each aggregation level (1, 2, 4, 8, 16 CCEs), assumes a candidate payload size, runs polar decoding, then checks the 24-bit CRC after de-scrambling it with one of its RNTIs (C-RNTI, CS-RNTI, SI-RNTI, and so on). A CRC pass confirms recipient, size, and therefore format. 3GPP caps the monitored payload sizes at four per cell per slot (three for C-RNTI) to bound decoder load.

What is the size in bits of a typical DCI 1_1 message?

DCI 1_1 is configurable: a single-carrier, single-codeword baseline is roughly 40 to 45 bits before CRC, but it can exceed 60 to 70 bits with carrier aggregation, two codewords, multi-TRP, and large resource assignments. After the 24-bit CRC it is polar-coded and rate-matched onto 1 to 16 CCEs, each CCE being 6 REGs of 12 subcarriers (72 RE before DMRS). The fallback DCI 1_0 is far smaller, near 28 bits plus CRC, so it survives better at cell edge.

Related Terms

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