5G NR PDCCH

What is PDCCH?

The Physical Downlink Control Channel (PDCCH) is a critical physical channel in 5G NR (and earlier cellular technologies like LTE) used for transmitting control information from the base station (gNB, or gNodeB) to the User Equipment (UE, such as a smartphone or other device). It operates in the downlink direction (from the network to the device) and is part of the physical layer of the 5G radio interface.

The PDCCH carries Downlink Control Information (DCI), which includes instructions and parameters that the UE needs to receive, decode properly, and process data or perform specific actions on the network. Think of it as a "control panel" or "instruction manual" that tells the UE what to do next in the communication process.

How 5G PDCCH is different from 4G?

• CORESET vs. Fixed Control Region: In 4G, the PDCCH is confined to a fixed control region at the start of each subframe (1–3 symbols), spanning the entire bandwidth. In 5G, the PDCCH uses CORESETs, configurable blocks of time-frequency resources. This allows the 5G PDCCH to be placed anywhere in a slot and tailored to specific needs.
• Flexible Numerology: 4G uses a fixed 15 kHz subcarrier spacing with a 1 ms subframe. 5G introduces flexible numerology (e.g., 15, 30, 60, 120 kHz), enabling shorter slot durations and adapting to different frequency bands (sub-6 GHz and mmWave).
• Beamforming and Reference Signals: 4G relies on CRS, which limits beamforming capabilities. 5G uses DMRS for PDCCH demodulation, enabling advanced beamforming and multi-user MIMO, critical for mmWave bands.
• Bandwidth Parts (BWPs): 5G allows the PDCCH to operate within specific BWPs, subsets of the total bandwidth, improving efficiency for devices with varying capabilities. 4G lacks this concept, always spanning the full bandwidth.
• Latency and Use Case Support: 5G’s flexible slot structure and PDCCH placement reduce latency (e.g., for URLLC), while 4G’s fixed structure is less adaptable.

How Does PDCCH Work?

PDCCH transmits DCI within control resource sets (CORESETs), which are specific areas in the time-frequency grid. Each CORESET is made up of resource blocks (RBs) and OFDM symbols. The DCI is organized into control channel elements (CCEs), with each CCE made of six resource element groups (REGs). The number of CCEs, called the aggregation level, can be 1, 2, 4, 8, or 16, offering flexibility for different channel conditions.

UEs monitor two types of search spaces:

• Common Search Space (CSS) for general information.
• UE-Specific Search Space (USS) for personal scheduling.

The UE tries to decode PDCCH candidates in these spaces, checking for its Radio Network Temporary Identifier (RNTI) in the DCI's CRC. If it matches, the DCI is valid, and the UE acts on it.

Role of PDCCH in 5G NR

In 5G NR, the PDCCH plays several essential roles to ensure efficient and reliable communication between the gNB and UE. Here’s a general overview of its key functions:

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1. Scheduling Information: The PDCCH informs the UE about scheduling downlink data (on the Physical Downlink Shared Channel, PDSCH) and uplink data (on the Physical Uplink Shared Channel, PUSCH). It tells the UE when and where to listen for or transmit data, including resource allocation (time and frequency resources) and modulation/coding schemes.
2. Control Commands: It carries commands for various operations, such as power control, handover instructions, or activation/deactivation of certain features (e.g., carrier aggregation or semi-persistent scheduling).
3. Hybrid Automatic Repeat Request (HARQ) Feedback:?PDCCH provides information about HARQ processes, including acknowledgement (ACK) or negative acknowledgement (NACK) for data transmissions. This enables retransmissions if data is not received correctly.
4. Dynamic Resource Allocation:?5G NR uses dynamic scheduling to adapt to changing network conditions. The PDCCH enables the gNB to allocate resources flexibly in real-time, supporting the low-latency and high-throughput requirements of 5G applications like ultra-reliable low-latency communications (URLLC) and enhanced mobile broadband (eMBB).

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• PDCCH Structure: The PDCCH consists of Control-Channel Elements (CCEs), which are grouped into aggregation levels to improve reliability. These CCEs are mapped to resource elements within Control-Resource Sets (CORESET).
• CORESET Configuration:?A CORESET defines the time-frequency resources for transmitting PDCCH. It consists of resource blocks (RBs) in the frequency domain and OFDM symbols in the time domain.
• Mapping and Aggregation:?CCEs are mapped to Resource-Element Groups (REGs) within a CORESET using interleaved or non-interleaved mapping. The aggregation levels determine the number of CCEs used for a PDCCH transmission (1, 2, 4, 8, 16) to enhance robustness.
• Scrambling and Modulation: PDCCH bits are scrambled to reduce interference and modulated using QPSK (Quadrature Phase Shift Keying) to generate complex-valued symbols for transmission.

Control-Channel Elements (CCEs):

• A CCE is the basic unit of the PDCCH and consists of 6 Resource-Element Groups (REGs). Each REG equals one resource block (RB) in the frequency domain during one OFDM symbol.
• CCEs aggregate PDCCH transmissions at different aggregation levels (1, 2, 4, 8, or 16 CCEs) to provide varying coding gain and reliability depending on channel conditions.
• The aggregation level determines the number of CCEs, as shown in Table 7.3.2.1-1: Level 1: 1 CCE Level 2: 2 CCEs Level 4: 4 CCEs Level 8: 8 CCEs Level 16: 16 CCEs