How does the UE distinguish DCI format 1_0 from format 0_0 when their sizes match?

Format 0_0 (uplink fallback grant) and format 1_0 (downlink fallback grant) are intentionally sized identically so the UE performs a single blind decode for both. The most significant bit of the DCI payload is a 1-bit identifier flag: 0 indicates an uplink DCI (format 0_0) and 1 indicates a downlink DCI (format 1_0). After a successful PDCCH decode and CRC check against the expected RNTI, the UE reads this flag to route the grant to the uplink or downlink scheduler. This size alignment reduces the number of blind decoding attempts and limits PDCCH false-alarm probability.

mmWave & 5G

DCI Format 1_0

Q: How is the DCI format 1_0 payload size calculated?

The DCI format 1_0 size is the sum of fixed and bandwidth-dependent fields. The frequency-domain resource assignment uses ceil(log2(N_RB_DL(N_RB_DL+1)/2)) bits, where N_RB_DL is the size of CORESET 0 when monitored in a common search space, or the active downlink bandwidth part otherwise. Adding the 1-bit format identifier flag, the time-domain assignment (4 bits), VRB-to-PRB mapping (1 bit), MCS (5 bits), new data indicator (1 bit), redundancy version (2 bits), HARQ process number (4 bits), downlink assignment index (2 bits), TPC for PUCCH (2 bits), and PUCCH resource indicator (3 bits) plus PDSCH-to-HARQ feedback timing (3 bits), a typical format 1_0 is on the order of 39 to 44 bits before the 24-bit CRC. When monitored in a UE-specific search space, the size is aligned with format 0_0 by zero-padding the shorter format.

Q: When does a UE monitor DCI format 1_0 versus format 1_1?

Format 1_0 is the fallback format that a UE always monitors in the common search space for system information (SI-RNTI), random access response (RA-RNTI), paging (P-RNTI), and during initial access and reconfiguration with C-RNTI. It uses a fixed field set so it remains decodable even before the UE is fully configured. Format 1_1 is the non-fallback format monitored only in a UE-specific search space once RRC has configured features such as multiple transport blocks, downlink MIMO with up to 8 layers, carrier aggregation, bandwidth part switching, and code block group retransmission. Because 1_1 fields are configurable, its payload is larger and more flexible, but it cannot be used until the UE has the corresponding RRC configuration.

/dee-see-eye for-mat wun zee-roh/

Defined in 3GPP TS 38.212, this is the compact fallback downlink control information that schedules a single PDSCH in one serving cell. Carried on the PDCCH with a 24-bit CRC scrambled by the addressed RNTI, format 1_0 uses a fixed set of fields (frequency and time resource allocation, MCS, HARQ process, redundancy version, and PUCCH feedback control) so it stays decodable before a UE has any RRC configuration. It is the format a device always monitors in the common search space for paging, system information, and random access, making it the workhorse grant during initial access in 5G NR.

Category: mmWave & 5G

Standard: 3GPP TS 38.212

Typical payload: 39 to 44 bits + 24-bit CRC

The Fallback Downlink Grant in 5G NR

In 5G NR the gNB schedules every downlink data transmission by sending a downlink control information message on the PDCCH. Format 1_0 is the simplest of the two downlink scheduling formats: it assigns one PDSCH carrying a single transport block to one UE in one cell. Its defining property is that the field set is fixed and not configurable by RRC. Because nothing about its layout depends on optional features, a UE can decode a format 1_0 grant the moment it acquires a cell, which is precisely why it carries system information, paging, and random access responses long before any UE-specific configuration exists.

Format 1_0 is paired with the uplink fallback format 0_0. The two are deliberately sized to the same number of bits so that a single blind-decode candidate covers both directions, with a 1-bit identifier flag at the most significant position selecting downlink (1) versus uplink (0). When format 1_0 is monitored in a UE-specific search space, its size is budget-aligned against format 0_0 by zero-padding the shorter one, keeping the total number of distinct DCI sizes a UE must blind-decode within the 3GPP limit and holding down PDCCH false-alarm probability.

The frequency-domain resource assignment in format 1_0 always uses contiguous resource allocation type 1, encoded as a resource indicator value over the relevant bandwidth. The reference bandwidth depends on context: when monitored in a common search space the field is sized to CORESET 0, whereas in a UE-specific search space it is sized to the active downlink bandwidth part. This is why two format 1_0 grants on the same carrier can differ in length, and why the search space type must be known before the payload size can be computed.

Payload Size and Field Layout

Frequency-domain resource assignment bits:
N_FDRA = ⌈ log₂( N_RB^DL × (N_RB^DL + 1) / 2 ) ⌉

Total payload (before CRC):
N_DCI = 1 + N_FDRA + 4 + 1 + 5 + 1 + 2 + 4 + 2 + 2 + 3 + 3 bits
(format identifier + time alloc + VRB→PRB + MCS + NDI + RV + HARQ id + DAI + TPC + PUCCH-RI + K₁)

CRC attachment and scrambling:
24-bit CRC computed over the payload and appended, then the CRC bits are ⊕ with the RNTI (Polar-coded on PDCCH)

Example: with CORESET 0 spanning N_RB^DL = 48, N_FDRA = ⌈ log₂(48×49/2) ⌉ = ⌈ log₂(1176) ⌉ = 11 bits, giving N_DCI = 1 + 11 + 27 = 39 bits plus the 24-bit CRC.

DCI Format 1_0 vs Other Scheduling Formats

Format	Direction	Schedules	Search space	Typical RNTI	Configurable fields
1_0	Downlink	1 PDSCH (1 TB)	Common & UE-specific	SI / RA / P / C-RNTI	No (fixed)
1_1	Downlink	1 to 2 TB, up to 8 layers	UE-specific	C-RNTI / CS-RNTI	Yes (RRC)
0_0	Uplink	1 PUSCH (fallback)	Common & UE-specific	C-RNTI / TC-RNTI	No (fixed)
0_1	Uplink	PUSCH, multi-layer	UE-specific	C-RNTI / CS-RNTI	Yes (RRC)
1_2	Downlink	1 PDSCH (low latency)	UE-specific	C-RNTI / MCS-C-RNTI	Yes (RRC)

Common Questions

Frequently Asked Questions

How is the DCI format 1_0 payload size calculated?

Add the fixed fields to the bandwidth-dependent frequency assignment. N_FDRA = ⌈log₂(N_RB^DL(N_RB^DL+1)/2)⌉, sized to CORESET 0 in a common search space or to the active downlink BWP otherwise. Adding the format identifier (1), time allocation (4), VRB-to-PRB (1), MCS (5), NDI (1), RV (2), HARQ id (4), DAI (2), TPC (2), PUCCH resource (3), and K₁ (3) yields roughly 39 to 44 bits before the 24-bit CRC.

When does a UE monitor DCI format 1_0 versus format 1_1?

Format 1_0 is the fallback always monitored in the common search space for SI-RNTI, RA-RNTI, P-RNTI, and C-RNTI during initial access and reconfiguration; its fixed fields keep it decodable before RRC setup. Format 1_1 is monitored only in a UE-specific search space after RRC configures features such as two transport blocks, up to 8 MIMO layers, carrier aggregation, and code block group retransmission, so it is larger and more flexible.

How does the UE tell DCI format 1_0 from format 0_0 when their sizes match?

The two fallback formats are sized identically so one blind-decode candidate covers both. The most significant payload bit is an identifier flag: 1 means a downlink grant (1_0) and 0 means an uplink grant (0_0). After a successful PDCCH decode and CRC check against the expected RNTI, the UE reads that flag to route the grant. Aligning the sizes reduces blind decoding attempts and lowers false-alarm probability.

Related Terms

← DCI 0_1 DCI 1_1 →