5G PDCCH: Structure, DCI Formats, Resource Scheduling, and Comparison with 4G LTE
- Venkateshu
- Apr 22
- 6 min read
The Physical Downlink Control Channel (PDCCH) is a cornerstone of 5G New Radio (NR) and 4G Long-Term Evolution (LTE) systems, serving as the primary mechanism for delivering Downlink Control Information (DCI) to User Equipment (UE). The PDCCH carries critical scheduling information for downlink (DL) and uplink (UL) resources, power control commands, and other control signaling. While 5G NR PDCCH builds on LTE's foundation, it introduces significant enhancements to support diverse use cases such as enhanced Mobile Broadband (eMBB), Ultra-Reliable Low-Latency Communications (URLLC), and massive Machine-Type Communications (mMTC). This article provides a detailed technical exploration of the 5G PDCCH structure, resource scheduling for DL and UL, DCI formats, and key differences from 4G LTE and insights from UE logs.
1. 5G PDCCH Structure
The 5G PDCCH is designed to be flexible and robust, accommodating the diverse requirements of NR deployments. It carries DCI, which informs the UE about resource allocations, modulation schemes, and other parameters necessary for decoding the Physical Downlink Shared Channel (PDSCH) or transmitting on the Physical Uplink Shared Channel (PUSCH). The PDCCH is transmitted within a Control Resource Set (CORESET) and monitored by the UE in specific Search Spaces.
1.1 Control Resource Set (CORESET)
A CORESET defines the time-frequency resources where the PDCCH is transmitted. A CORESET is configured with:
Frequency Domain Resources: Specified as a set of Physical Resource Blocks (PRBs), typically in multiples of 6 PRBs.
Time Domain Resources: Defined by the number of OFDM symbols (1, 2, or 3) within a slot, configured via the duration parameter in the ControlResourceSet Information Element (IE).
Precoder Granularity: Determines whether the same precoding is applied across all contiguous PRBs (allContiguousRBs) or within a Resource Element Group (REG) bundle (sameAsREG-bundle).
Control Channel Elements (CCEs): Each CORESET is divided into CCEs, where one CCE consists of 6 REGs, and each REG comprises one PRB over one OFDM symbol, containing 9 Resource Elements (REs) for PDCCH payload and 3 REs for Demodulation Reference Signals (DMRS).
A UE can be configured with up to 3 CORESETs per Bandwidth Part (BWP) in a serving cell, CORESET #0 is special, configured via the Master Information Block (MIB) for initial access.
UE Log: CORESET Configuration
ControlResourceSetId: 1
FrequencyDomainResources: 48 PRBs
Duration: 2 OFDM symbols
PrecoderGranularity: allContiguousRBs
This log shows a CORESET with 48 PRBs spanning 2 OFDM symbols, indicating a robust configuration for high-capacity scenarios.
1.2 Search Spaces
Search Spaces define where and when a UE monitors for PDCCH candidates. They are categorized as:
Common Search Space (CSS): Used for broadcast or group-based control information (e.g., paging, system information, or random-access responses).
UE-Specific Search Space (USS): Dedicated to individual UE scheduling.
Search Spaces are configured with:
Monitoring Periodicity: Slots where the UE monitors PDCCH (e.g., every slot or every 10 slots).
Monitoring Offset: The starting slot within the periodicity.
Aggregation Levels (ALs): The number of CCEs used for a PDCCH candidate (1, 2, 4, 8, or 16 in 5G NR, compared to 1, 2, 4, or 8 in LTE).
Number of PDCCH Candidates: The number of possible PDCCH locations per AL.
The UE performs blind decoding to detect PDCCH candidates, limited by the maximum number of monitored candidates and non-overlapped CCEs per slot
UE Log: Search Space Configuration
SearchSpaceId: 2
SearchSpaceType: UE-Specific
MonitoringSlotPeriodicityAndOffset: 1 slot
nrofCandidates: AL1=6, AL2=6, AL4=2, AL8=2, AL16=0
This indicates a USS with frequent monitoring (every slot) and multiple candidates at lower ALs, optimizing for low-latency scenarios.
1.3 PDCCH Processing
The PDCCH processing chain includes:
UE Log: PDCCH Decoding
DCI Format: 1_1
RNTI: C-RNTI
CCE Index: 8
AL: 4
Payload Size: 79 bits
CRC: 24 bits
This log shows a DCI format 1_1 decoded at AL4, indicating a robust transmission for a UE-specific PDSCH allocation.
2. DCI Formats in 5G NR
5G NR defines fewer DCI formats than LTE to reduce complexity. The formats are summarized below:
DCI Format | Usage | RNTI | Description |
0_0 | UL Scheduling (Fallback) | C-RNTI, CS-RNTI, MCS-C-RNTI | Basic PUSCH scheduling for robust operation. |
0_1 | UL Scheduling (Non-Fallback) | C-RNTI, CS-RNTI, MCS-C-RNTI | Advanced PUSCH scheduling with additional fields (e.g., multiple BWPs, CBG). |
1_0 | DL Scheduling (Fallback) | C-RNTI, CS-RNTI, MCS-C-RNTI, P-RNTI, SI-RNTI, RA-RNTI | Basic PDSCH scheduling, paging, or system information. |
1_1 | DL Scheduling (Non-Fallback) | C-RNTI, CS-RNTI, MCS-C-RNTI | Advanced PDSCH scheduling with MIMO, CBG, and multi-BWP support. |
2_0 | Slot Format Indication | SFI-RNTI | Indicates TDD slot format. |
2_1 | Pre-emption Indication | INT-RNTI | Notifies UEs of preempted resources for URLLC. |
2_2 | TPC for PUCCH/PUSCH | TPC-PUCCH-RNTI, TPC-PUSCH-RNTI | Power control commands. |
2_3 | TPC for SRS | TPC-SRS-RNTI | Power control for Sounding Reference Signals. |
Key Features:
Size Alignment: Unlike LTE, NR allows multiple DCI formats to share the same size to reduce blind decoding attempts.
24-bit CRC: Increases reliability compared to LTE’s 16-bit CRC.
Polar Coding: Replaces LTE’s TBCC for better performance at low code rates.
UE Log: DCI Detection
DCI Format: 2_1
RNTI: INT-RNTI
PreemptionIndication: PRBs 10-20, Symbols 5-7
This log indicates a preemption event for URLLC, where the UE is informed to ignore specific PRBs and symbols.
3. Resource Scheduling for Downlink and Uplink
Scheduling in 5G NR is more flexible than LTE, particularly in the time domain, due to the introduction of mini-slots, variable numerologies, and dynamic Time Division Duplexing (TDD). The PDCCH carries DCI formats that dictate DL and UL resource allocations.
3.1 Downlink Scheduling
DL scheduling is primarily handled by DCI formats 1_0 and 1_1, which allocate PDSCH resources. Key parameters include:
Frequency Domain Resource Assignment: Specifies PRBs using Resource Allocation Type 0 (bitmap-based) or Type 1 (contiguous allocation).
Time Domain Resource Assignment: Uses a 4-bit field to index a lookup table (pdsch-TimeDomainAllocationList) in TS 38.214 (Section 5.1.2.1). The table specifies:
K0: The slot offset between the PDCCH and PDSCH (0 to 32 slots).
SLIV: Start and Length Indicator Value, defining the starting OFDM symbol and duration (up to 14 symbols).
Modulation and Coding Scheme (MCS): Determines the modulation (QPSK, 16QAM, 64QAM, or 256QAM) and coding rate.
HARQ Process Number: Identifies the Hybrid Automatic Repeat Request process for retransmissions.
UE Log: DL Scheduling
DCI Format: 1_1
TimeDomainResourceAssignment: Index 3
K0: 0 slots
SLIV: Start=2, Length=12
FrequencyDomainResourceAssignment: Type 1, 50 PRBs
MCS: 16QAM, Index 10
This log indicates a same-slot PDSCH allocation starting at symbol 2, spanning 12 symbols, with a contiguous 50-PRB allocation using 16QAM.
3.2 Uplink Scheduling
UL scheduling is managed by DCI formats 0_0 and 0_1, which allocate PUSCH resources. Key parameters include:
Frequency Domain Resource Assignment: Similar to DL, using Type 0 or Type 1.
Time Domain Resource Assignment: Indexes a pusch-TimeDomainAllocationList, specifying:
K2: The slot offset between the PDCCH and PUSCH (0 to 32 slots).
SLIV: Start and length of the PUSCH allocation.
MCS and HARQ: Similar to DL, with additional fields for UL-specific parameters like transform precoding.
TPC Command: Transmit Power Control for PUSCH or PUCCH.
UE Log: UL Scheduling
DCI Format: 0_1
TimeDomainResourceAssignment: Index 5
K2: 2 slots
SLIV: Start=0, Length=14
FrequencyDomainResourceAssignment: Type 0, RBG bitmap
MCS: QPSK, Index 5
TPC: +1 dB
This log shows a PUSCH allocation scheduled 2 slots after the PDCCH, spanning an entire slot, using a bitmap-based allocation and QPSK modulation.
3.3 Dynamic TDD and Cross-Slot Scheduling
5G NR’s flexible TDD allows dynamic switching between DL and UL slots, signaled via DCI format 2_0 (Slot Format Indication). Cross-slot scheduling (non-zero K0 or K2) enhances power efficiency by allowing the UE to buffer data.
4. Differences Between 4G LTE and 5G NR PDCCH
While 5G NR PDCCH builds on LTE’s framework, it introduces several advancements to support NR’s flexibility and performance requirements. The key differences are:
Feature | 4G LTE | 5G NR |
Channel Coding | Tail-Biting Convolutional Coding (TBCC) | Polar Coding for better error correction. |
CRC Size | 16 bits | 24 bits, reducing false positives. |
Aggregation Levels | 1, 2, 4, 8 | 1, 2, 4, 8, 16, supporting higher robustness. |
Control Region | First 1–4 OFDM symbols, indicated by PCFICH | Configured via CORESET (1–3 symbols), no PCFICH for faster decoding. |
Time Domain Flexibility | Fixed subframe (1 ms) | Mini-slots, variable numerologies, and cross-slot scheduling (K0, K2). |
DCI Formats | More formats (e.g., 0, 1, 1A, 1B, 1C, 1D, 2, 2A) | Fewer formats, with size alignment for efficiency. |
Search Space | Common and UE-specific, dynamic PCFICH decoding | CSS and USS, semi-static CORESET configuration. |
Preemption | Not supported | Supported via DCI format 2_1 for URLLC. |
Beamforming | Limited support | Enhanced with precoder granularity and DMRS-based beamforming. |
UE Log Comparison:
LTE Log:
DCI Format: 1A
RNTI: C-RNTI
CCE Index: 4
AL: 2
CFI: 2 symbols
NR Log:
DCI Format: 1_0
RNTI: C-RNTI
CCE Index: 8
AL: 4
CORESET Duration: 2 symbols
The NR log shows no CFI (Control Format Indicator) due to semi-static CORESET configuration, and a higher AL for robustness.
5. 3GPP References
The following 3GPP specifications provide the foundation for this analysis:
TS 38.211: Physical channels and modulation (PDCCH structure, CORESET).
TS 38.212: Multiplexing and channel coding (DCI formats, Polar coding).
TS 38.213: Physical layer procedures for control (PDCCH monitoring, Search Spaces).
TS 38.214: Physical layer procedures for data (resource allocation, K0/K2).
TS 38.331: Radio Resource Control (RRC) protocol (CORESET and Search Space configuration).
6. Conclusion
The 5G NR PDCCH is a highly flexible and robust control channel, designed to meet the diverse requirements of NR use cases. Its structure, based on CORESETs and Search Spaces, supports fine-grained resource allocation, while Polar coding and a 24-bit CRC enhance reliability. Compared to 4G LTE, 5G NR introduces significant advancements, including mini-slot scheduling, preemption for URLLC, and beamforming support. UE logs reveal practical implementations, such as cross-slot scheduling and dynamic TDD, which optimize performance in real-world deployments. By leveraging 3GPP standards and UE logs, this deep dive underscores the PDCCH’s critical role in enabling 5G’s transformative capabilities.
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