mmWave & 5G

DCI Format 0_1

Q: What is the difference between DCI format 0_0 and 0_1?

DCI format 0_0 is the fallback uplink grant: it has a fixed, compact field set, schedules only single-layer PUSCH within the active bandwidth part, and can be monitored in the common search space so it works before RRC configuration completes. DCI format 0_1 is the non-fallback grant whose size and field presence depend on RRC configuration. It adds fields for up to 4 MIMO layers, bandwidth part indication for BWP switching, carrier indication for cross-carrier scheduling, aperiodic CSI and SRS request bits, and up to two configured-grant or HARQ process management fields. Because 0_1 is RRC-configured, it is monitored only in the UE-specific search space. The two formats are padded so that 0_0 and DCI format 1_0 share a common size, simplifying blind decoding on the PDCCH.

Q: How does the UE determine the size of a DCI format 0_1 message?

The payload size of DCI format 0_1 is not fixed. The gNB computes it from the UE's RRC configuration: the frequency-domain resource allocation field width depends on the number of resource blocks in the active uplink bandwidth part and the chosen allocation type (type 0 bitmap or type 1 RIV), the antenna-port and precoding fields scale with the configured maximum rank and codebook, and fields such as carrier indicator, BWP indicator, CSI request, and SRS request are present only when the corresponding features are configured. The UE precomputes the resulting bit length for each configured DCI format and performs blind decoding against those candidate sizes within its monitored PDCCH candidates. A 24-bit CRC scrambled with the C-RNTI confirms a successful decode.

Q: Why does DCI format 0_1 support bandwidth part switching?

A 5G NR carrier can be very wide, up to 100 MHz in FR1 and 400 MHz in FR2. Keeping the full carrier active wastes UE power. Bandwidth parts let the UE operate within a narrower active sub-band and switch to a wider one only when high throughput is needed. The 0 to 2 bit BWP indicator inside DCI format 0_1 tells the UE which uplink BWP to apply for the scheduled PUSCH, and the scheduling itself triggers the switch. Format 0_0 cannot do this because it lacks the BWP indicator field, which is one reason 0_1 is used for the majority of data-phase uplink grants while 0_0 remains the robust fallback.

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Defined in 3GPP TS 38.212, this is the non-fallback uplink scheduling grant of 5G NR: a variable-length Downlink Control Information message carried on the PDCCH that schedules a PUSCH transmission. Unlike the compact fallback format 0_0, format 0_1 carries the full feature set, up to four MIMO layers, bandwidth part switching, carrier aggregation, and aperiodic CSI and SRS request bits, so its payload size is computed from the UE's RRC configuration rather than fixed. It is monitored only in the UE-specific search space and is the workhorse grant for high-throughput data-phase uplink in both FR1 and FR2 deployments.

Category: mmWave & 5G

Specification: 3GPP TS 38.212

Schedules: PUSCH (uplink)

How DCI Format 0_1 Schedules an Uplink Transmission

5G NR uses Downlink Control Information messages to tell a UE exactly when, where, and how to transmit or receive. A 1-bit identifier field inside the DCI separates uplink from downlink: format 0_0 and 0_1 are uplink grants for the PUSCH, while 1_0 and 1_1 are downlink assignments for the PDSCH. The fallback versus non-fallback distinction (the trailing 0 versus 1) is not signaled by a dedicated field; the UE infers it from the search space the candidate was found in and from the configured payload size. Format 0_1 is the non-fallback uplink grant, meaning the gNB only sends it once the UE is RRC-configured and can interpret its optional fields.

The grant is carried in the payload of a PDCCH transmission, encoded with a polar code and protected by a 24-bit CRC scrambled with the UE's C-RNTI. Because the message length is not fixed, the UE precomputes the expected bit length for each configured DCI format and blind-decodes its monitored PDCCH candidates against those candidate sizes. When the descrambled CRC checks, the UE applies the contents: the frequency-domain resource allocation field selects which resource blocks to use, the time-domain field points into a configured table of start symbols and durations, and the modulation-and-coding-scheme field sets the transport block size.

What sets 0_1 apart is the breadth of optional fields. A bandwidth part indicator can trigger a switch to a wider or narrower active uplink BWP. Antenna-port and precoding fields enable codebook-based multi-layer transmission up to rank 4. A carrier indicator supports cross-carrier scheduling in carrier aggregation. CSI-request and SRS-request bits trigger aperiodic channel-state and sounding reference signal reports. These features make 0_1 the high-capacity counterpart to the robust but limited DCI format 0_0.

Payload Size Computation

Type 1 frequency-domain allocation (RIV) field width:
N_FDRA = ⌈log₂( N_RB^BWP × (N_RB^BWP + 1) / 2 ) ⌉ bits

BWP indicator field width:
N_BWP = ⌈log₂(n_BWP)⌉, with 0 ≤ N_BWP ≤ 2 bits

PDCCH CRC scrambling:
CRC_scrambled[8..23] = CRC_24C[8..23] ⊕ C-RNTI (16-bit RNTI XORed onto the 16 LSBs of the 24-bit CRC)

Where N_RB^BWP = resource blocks in the active uplink bandwidth part, n_BWP = number of configured uplink BWPs. Example: a 273-RB BWP (100 MHz, 30 kHz SCS) gives N_FDRA = ⌈log₂(37,401)⌉ = 16 bits.

DCI Uplink and Downlink Format Comparison

DCI Format	Direction	Type	Schedules	Max MIMO Layers	BWP Switch	Search Space
0_0	Uplink	Fallback	PUSCH	1	No	Common / UE-specific
0_1	Uplink	Non-fallback	PUSCH	4	Yes	UE-specific
1_0	Downlink	Fallback	PDSCH	1	No	Common / UE-specific
1_1	Downlink	Non-fallback	PDSCH	8	Yes	UE-specific
2_x	Group	Special	SFI / TPC / preempt	n/a	No	Common

Common Questions

Frequently Asked Questions

What is the difference between DCI format 0_0 and 0_1?

Format 0_0 is the fallback uplink grant: fixed compact fields, single-layer PUSCH within the active BWP, and monitorable in the common search space so it works before RRC configuration. Format 0_1 is non-fallback, with an RRC-dependent size that adds up to 4 MIMO layers, a BWP indicator, carrier indication for cross-carrier scheduling, and CSI and SRS request bits. Format 0_1 is monitored only in the UE-specific search space.

How does the UE determine the size of a DCI format 0_1 message?

The size is not fixed; the gNB derives it from RRC configuration. The frequency-domain allocation width depends on N_RB in the active BWP and allocation type, the antenna-port and precoding fields scale with configured rank and codebook, and optional fields such as carrier indicator, BWP indicator, CSI request, and SRS request appear only when configured. The UE precomputes each candidate length and blind-decodes, confirming with a 24-bit CRC scrambled by the C-RNTI.

Why does DCI format 0_1 support bandwidth part switching?

An NR carrier can reach 100 MHz in FR1 and 400 MHz in FR2; keeping the full carrier active wastes UE power. Bandwidth parts let the UE work in a narrower sub-band and widen only when throughput demands it. The 0 to 2 bit BWP indicator in format 0_1 selects the uplink BWP for the scheduled PUSCH and triggers the switch. Format 0_0 lacks this field, so 0_1 handles most data-phase grants while 0_0 stays the robust fallback.

Related Terms

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