5G-ORAN Functional Split: An In-Depth Overview

The introduction of 5G has brought a significant evolution in network architecture, especially with the implementation of Open Radio Access Network (ORAN). One of the key features of 5G ORAN is the concept of functional split, which optimizes the deployment and performance of the network. The functional split refers to dividing the network's processing functions between different network entities, such as the Centralized Unit (CU), Distributed Unit (DU), and Radio Unit (RU). This article will provide a detailed explanation of the different functional splits in 5G ORAN, as represented in the provided diagram.

In 5G ORAN, functional splits are categorized into High Layer Splits and Low Layer Splits. Each split option determines how the protocol stack is divided among the CU, DU, and RU, with varying degrees of centralization and processing distribution.

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Split Option 1: IP Layer Processing

In Split Option 1, the CU is responsible for all higher-layer processing, including the PDCP, RLC, and MAC layers. The RU handles only the physical layer tasks. This setup centralizes most processing within the CU, making it suitable for scenarios where latency is not a primary concern.

?The CU manages all layers up to the MAC, with the RU taking charge of the physical layer exclusively. This division allows for centralized management of higher-layer protocols, while the RU handles the more localized physical layer processing.

?Processing Steps:

• Data Packet Reception: Data packets arrive at the CU and are processed through the PDCP layer, which handles tasks like header compression, ciphering, and integrity protection.
• RLC Layer Processing: The RLC layer at the CU manages segmentation and reassembly, error correction, and ARQ (Automatic Repeat Request).
• MAC Layer Scheduling: The MAC layer at the CU takes care of scheduling, multiplexing, and prioritizing data for transmission.
• Physical Layer Processing: Finally, the processed data is sent to the RU, where the physical layer takes over for modulation, coding, and RF transmission.

This split is ideal for Fixed Wireless Access (FWA) and other non-mobile applications where bandwidth efficiency and relaxed latency requirements are prioritized. It is also beneficial in scenarios where the network infrastructure can afford to centralize most processing tasks.

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Split Option 2: PDCP to RLC Split

This split divides processing between the PDCP layer, managed by the CU, and the RLC and MAC layers, handled by the DU. This option balances centralized control with distributed processing, allowing for more flexible and efficient network management.

The CU retains control over the PDCP layer, which performs tasks such as header compression and ciphering. The DU manages the RLC and MAC layers, focusing on error correction, segmentation, and scheduling.

?Processing Steps:

• PDCP Layer: The PDCP layer at the CU handles tasks like ciphering, integrity protection, and header compression before passing the data to the DU.
• RLC Layer Segmentation: At the DU, the RLC layer manages segmentation, reassembly, and error correction, ensuring data integrity before it is scheduled for transmission.
• MAC Layer Scheduling: The MAC layer at the DU handles scheduling and prioritizing data for transmission over the physical layer.
• Physical Layer Transmission: The data is then passed to the RU for physical layer processing and RF transmission.

This split is well-suited for mobile applications where a balance between latency and centralized management is needed. It allows for flexible deployment scenarios, especially in networks that require some degree of centralization for PDCP-related functions.

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Split Option 3: High RLC to Low RLC Split

In Split Option 3, the RLC layer is divided into High RLC (managed by the CU) and Low RLC (handled by the DU). This split allows the CU to centralize error correction and retransmission management, while the DU focuses on lower-layer tasks such as segmentation and scheduling.

The CU manages High RLC, which includes tasks like error correction and retransmission management. The DU takes care of Low RLC and MAC layer processing, including segmentation, reassembly, and scheduling.

?Processing Steps:

• High RLC Processing: The CU processes data through the High RLC layer, which manages error correction and retransmissions, ensuring data integrity.
• Low RLC Segmentation: The DU handles Low RLC tasks such as segmentation, reassembly, and error correction for data packets.
• MAC Layer: The MAC layer at the DU schedules the data for transmission over the physical layer.
• Physical Layer: The data is then passed to the RU for physical layer processing, modulation, coding, and RF transmission.

This split is ideal for scenarios where centralized control of error correction and retransmission is crucial, such as in mobile data networks where data integrity and reliability are paramount.

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Split Option 4: MAC Layer Division

This split divides the MAC layer into two parts: high-level MAC functions (e.g., scheduling, HARQ) managed by the CU, and low-level MAC tasks handled by the DU. This configuration provides precise control over MAC-layer functionalities, essential for latency-sensitive operations.

The CU manages high-level MAC functions, such as scheduling, HARQ (Hybrid Automatic Repeat Request), and flow control, while the DU handles lower-level MAC tasks, including multiplexing and MAC header processing.

?Processing Steps:

• High-Level MAC Scheduling: The CU schedules data packets and manages HARQ, ensuring efficient use of network resources.
• Low-Level MAC Multiplexing: The DU handles tasks like multiplexing and MAC header processing, preparing data for physical layer transmission.
• Physical Layer: The DU passes the processed data to the RU, where it undergoes physical layer processing, including modulation and coding.
• RF Transmission: The RU transmits the data over the airwaves using RF signals.

This split is suitable for applications that demand tight scheduling control and lower latency, such as real-time mobile communications and other critical applications requiring precise timing and resource management.

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Split Option 5: MAC to PHY Split

Split Option 5 places MAC layer functions within the CU, while the DU handles all physical layer tasks. This split significantly reduces latency, making it ideal for time-sensitive applications.

The CU is responsible for all MAC layer functions, including scheduling, HARQ, and flow control. The DU manages the physical layer, handling tasks like modulation, coding, and signal processing.

?Processing Steps:

• MAC Layer Processing: The CU schedules data packets, manages HARQ, and handles flow control, ensuring efficient use of network resources.
• Physical Layer Processing: The DU processes the data through the physical layer, including modulation, coding, and preparing the signal for RF transmission.
• RF Transmission: The RU transmits the processed data using RF signals over the airwaves.

This split is perfect for Ultra-Reliable Low Latency Communication (URLLC) scenarios where minimizing delay is critical. It is also suitable for applications requiring high-speed data transmission with low latency.

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Split Option 6: High PHY to Low PHY Split

This split divides the physical layer into High PHY and Low PHY. High PHY tasks, such as resource mapping and beamforming, are managed by the DU, while Low PHY tasks, including signal processing and modulation, are handled by the RU.

The DU takes responsibility for High PHY functions, while the RU manages Low PHY tasks. This split reduces the processing load on the RU and allows for efficient resource management and signal processing.

?Processing Steps:

• High PHY Processing: The DU handles High PHY tasks, including resource mapping, beamforming, and FFT (Fast Fourier Transform).
• Low PHY Processing: The RU processes the signal through Low PHY tasks, including modulation, coding, and signal conversion.
• RF Transmission: The RU transmits the processed data using RF signals, completing the transmission process.

This split is suitable for scenarios requiring high bandwidth and moderate latency, particularly in multiple antenna (MIMO) deployments where efficient resource management and signal processing are critical.

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Split Option 7: Detailed Physical Layer Splits

• Sub-option 7-1: FFT to Resource Mapping

This sub-option assigns FFT processing to the DU, with resource mapping managed by the RU. This setup simplifies RU complexity and centralizes critical processing functions within the DU.

?The DU handles FFT processing, ensuring efficient frequency domain conversion, while the RU manages resource mapping and subsequent PHY layer tasks.

?Processing Steps:

• FFT Processing: The DU performs FFT, converting time-domain signals to the frequency domain for efficient processing.
• Resource Mapping: The RU manages resource mapping, assigning frequency resources for data transmission.
• Low PHY Processing: The RU handles modulation, coding, and preparing the signal for RF transmission.
• RF Transmission: The processed data is transmitted over the airwaves using RF signals.

Ideal for high-bandwidth applications with stringent latency requirements, such as advanced MIMO setups and other scenarios requiring efficient frequency domain processing.

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• Sub-option 7-2: Resource Mapping to Precoding

In this sub-option, the DU manages resource mapping, while the RU handles precoding and beamforming, providing precise control over these functions.

The DU focuses on resource mapping, ensuring efficient allocation of frequency resources, while the RU handles precoding and beamforming, optimizing signal transmission.

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Processing Steps:

• Resource Mapping: The DU assigns frequency resources for data transmission, optimizing network performance.
• Precoding and Beamforming: The RU handles precoding, adjusting the signal for transmission, and beamforming, directing the signal toward specific users or locations.
• Low PHY Processing: The RU processes the signal through modulation, coding, and prepares it for RF transmission.
• RF Transmission: The processed data is transmitted over the airwaves using RF signals.

Use Case: Optimal for scenarios requiring meticulous beamforming control, such as millimeter-wave (mmWave) deployments and other high-frequency applications.

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• Sub-option 7-3: Precoding to Beamforming

This sub-option splits precoding (handled by the DU) and beamforming (managed by the RU), reducing fronthaul bandwidth requirements and making it suitable for environments with limited capacity.

The DU manages precoding, optimizing the signal for transmission, while the RU handles beamforming, directing the signal toward specific users or locations.

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Processing Steps:

• Precoding: The DU adjusts the signal for transmission, optimizing it for the specific environment.
• Beamforming: The RU directs the signal using beamforming techniques, ensuring it reaches the intended user or location.
• Low PHY Processing: The RU handles modulation, coding, and preparing the signal for RF transmission.
• RF Transmission: The processed data is transmitted over the airwaves using RF signals.

Best for dense urban deployments where fronthaul capacity is constrained, but efficient beamforming is required to ensure signal integrity and performance.

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Split Option 8: RF to Low PHY Split

The most fundamental split, where the RU handles only RF functions, while all other processing tasks are managed by the DU. This split significantly reduces RU complexity, making it easier to deploy and maintain.

The DU manages all PHY processing beyond RF, leaving only RF signal transmission and reception to the RU. This allows the RU to focus solely on RF tasks, simplifying its design and operation.

?Processing Steps:

• Low PHY Processing: The DU handles all PHY tasks beyond RF, including modulation, coding, and signal processing.
• RF Signal Management: The RU manages the RF signal, ensuring it is transmitted and received effectively over the airwaves.
• Signal Transmission: The RU transmits and receives RF signals, handling the final step of data transmission in the network.

Ideal for large-scale deployments with many antenna ports, where minimizing RU complexity is crucial for efficient network management and cost-effective operations.

The functional splits in 5G ORAN provide a flexible, scalable approach to network design, allowing operators to tailor their infrastructure to meet specific needs. By understanding the protocol stack, CU-DU mapping, and the processing steps involved at each layer, network operators can optimize their networks for performance, cost, and scalability.

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Each split option offers unique advantages and challenges, making it essential to carefully consider the specific requirements of the deployment scenario. Whether optimizing for latency, bandwidth, or scalability, the functional splits in 5G ORAN provide the tools necessary to build the next generation of mobile networks.

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