https://www.techplayon.com/5g-nr-gnb-logical-architecture-functional-split-options/

gNB 的逻辑架构如下图所示，包括中央单元 (CU) 和分布式单元 (DU)。Fs-C 和 Fs-U 通过 Fs 接口提供控制平面和用户平面连接。

在这种架构中，中央单元（CU）和分配单元（DU）可以定义如下。

中央单元 (CU)：它是一个逻辑节点，包括 gNB 功能，例如用户数据传输、移动性控制、无线接入网络共享、定位、会话管理等，但不包括专门分配给 DU 的功能。CU 控制 DU 通过前传 (Fs) 接口的运行。中央单元 (CU) 也可以称为 BBU/REC/RCC/C-RAN/V-RAN

分布式单元 (DU)：此逻辑节点包含 gNB 功能的子集，具体取决于功能拆分选项。其操作由 CU 控制。分布式单元 (DU) 也被称为 RRH/RRU/RE/RU 等其他名称。

中央单元 (CU) 和分布式单元功能拆分选项

作为新无线电（NR）研究项目的一部分，3GPP 开始研究中央单元和分布式单元之间的不同功能划分。在初始阶段，3GPP 以 LTE 协议栈作为讨论的基础，直到 RAN2 定义并冻结了新无线电（NR）的协议栈。他们提出了下图所示的大约 8 种可能选项。

• 选项 1（类似RRC/PCDP 1A 的拆分）
• 选项 2（PDCP/RLC 拆分，类似 3C 拆分）
• 选项 3（高 RLC/低 RLC分割，内部 RLC 分割）
• 选项 4（RLC-MAC 分离）
• 选项 5（MAC 内部拆分）
• 选项 6（MAC-PHY 分离）
• 选项 7（PHY 内部拆分）
• 选项 8（PHY-RF 分离）

选项 1（RRC/PDCP，类似 1A 的拆分）：在此拆分选项中，RRC 位于中央单元，而 PDCP、RLC、MAC、物理层和 RF 保留在分布式单元中。因此，整个用户平面都在分布式单元中。

选项 2（PDCP/RLC 分离）：由于 U 平面相似，选项 2 可能是类似 X2 的设计的基础，但某些功能可能不同，例如 C 平面，因为可能需要一些新程序。此选项有两种可能的变体。

选项 2-1 仅分割 U 平面（类似 3C 分割）：在此分割选项中，RRC、PDCP 位于中央单元。RLC、MAC、物理层和 RF 位于分布式单元。
选项 2-2：在此拆分选项中，RRC、PDCP 位于中央单元中。RLC、MAC、物理层和 RF 位于分布式单元中。此外，可以通过将 CP 堆栈的 RRC 和 PDCP 以及 UP 堆栈的 PDCP 分离到不同的中央实体中来实现此选项。
选项 3（高 RLC/低 RLC 分割）：在此选项中，根据实时/非实时功能分割采用两种方法，如下所示：

选项 3-1 基于 ARQ 的拆分
选项 3-2 根据 TX RLC 和 RX RLC 进行拆分
选项 3-1 基于 ARQ 的拆分

低RLC可能由分段功能组成；
高级RLC可能由ARQ和其他RLC功能组成；
此选项将 RLC 子层拆分为高级 RLC 和低级 RLC 子层，这样对于 RLC 确认模式操作，所有 RLC 功能都可以在位于中央单元的高级 RLC 子层上执行，而分段可以在位于分布式单元的低级 RLC 子层上执行。其中，高级 RLC 根据状态报告对 RLC PDU 进行分段，而低级 RLC 将 RLC PDU 分段到可用的 MAC PDU 资源中。

选项 3-2 根据 TX RLC 和 RX RLC 进行拆分

Low RLC可以由发送TM RLC实体、发送UM RLC实体、发送侧AM以及接收侧AM的路由功能组成，与下行传输有关。
高层RLC除了完成与上行传输相关的路由功能和RLC状态报告接收等功能外，还可以由接收TM RLC实体、接收UM RLC实体和接收AM侧实体组成。
选项 4（RLC-MAC 分离）：在此分离选项中，RRC、PDCP 和 RLC 位于中央单元中。MAC、物理层和 RF 位于分布式单元中。

选项 5（MAC 内部拆分）

选项 5 假设以下分布：

RF、物理层和MAC层下部（Low-MAC）位于分布式单元中
MAC 层的较高部分 (High-MAC)、RLC 和 PDCP 位于中央单元
因此，通过将 MAC 层拆分为 2 个实体（例如 High-MAC 和 Low-MAC），MAC 层提供的服务和功能将位于中央单元 (CU)、分布式单元 (DU) 或两者中。下面给出了此类分布的示例。

在 High-MAC 子层中，High-MAC 子层的集中调度将负责多个 Low-MAC 子层的控制。它采取高层集中调度决策。High-MAC 子层中的小区间干扰协调将负责干扰协调方法，例如 JP/CS CoMP。
在 Low-MAC 子层中，Low-MAC 子层中的时间关键功能包括具有严格延迟要求的功能（例如 HARQ）或性能与延迟成比例的功能（例如来自 PHY 的无线信道和信号测量、随机访问控制）。它降低了前传接口的延迟要求。Low-MAC 子层中的无线特定功能可以执行与调度相关的信息处理和报告。它还可以测量/估计配置的操作或服务 UE 的统计数据上的活动，并定期或根据请求向 High-MAC 子层报告。
选项 6（MAC-PHY 分离）： MAC 和上层位于中央单元 (CU)。PHY 层和 RF 位于 DU。CU 和 DU 之间的接口承载数据、配置和调度相关信息（例如 MCS、层映射、波束成形、天线配置、资源块分配等）和测量。

选项 7（PHY 内部分割）：该选项有多种实现方式，包括允许独立获取 UL 和 DL 不同子选项优势的非对称选项。

此选项需要某种压缩技术来减少 DU 和 CU 之间的传输带宽要求。

在 UL 中，FFT 和 CP 移除位于 DU 中，对于两个子变体，7-1 和 7-2 如下所述。其余功能位于 CU 中。
在下行链路中，iFFT 和 CP 加法位于 DU 中，而 PHY 的其余部分位于 CU 中。
考虑到上述情况，此选项有三种子变体可用，如下所述

选项 7-1 在此选项中，UL、FFT、CP 移除和可能的 PRACH 过滤功能驻留在 DU 中，其余 PHY 功能驻留在 CU 中。在 DL 中，iFFT 和 CP 添加功能驻留在 DU 中，其余 PHY 功能驻留在 CU 中。

选项 7-2 在此选项中，UL、FFT、CP 移除、资源解映射和可能的预过滤功能驻留在 DU 中，其余 PHY 功能驻留在 CU 中。在 DL 中，iFFT、CP 添加、资源映射和预编码功能驻留在 DU 中，其余 PHY 功能驻留在 CU 中。

选项 7-3（仅适用于 DL）：只有编码器驻留在 CU 中，其余 PHY 功能驻留在 DU 中。

选项 8（PHY-RF 分离）：此选项允许分离 RF 层和 PHY 层。这种分离允许集中所有协议层级别的流程，从而实现 RAN 的紧密协调。这允许高效支持 CoMP、MIMO、负载平衡、移动性等功能。

RAN 分体架构的优势

具有在中央和分布式单元之间分割和移动新无线电 (NR) 功能的部署灵活性的架构的一些好处如下：

• 灵活的硬件实现允许可扩展且经济高效的解决方案
• 分离式架构（中央单元和分布式单元之间）可以协调性能特性、负载管理、实时性能优化，并支持 NFV/SDN
• 可配置的功能拆分可适应各种用例，例如传输中的可变延迟

在哪里使用哪个拆分函数？

如何划分架构中的新无线电 (NR) 功能取决于与无线电网络部署场景、约束和预期支持的服务相关的一些因素。这些因素的一些示例包括：

• 支持每个提供的服务的特定 QoS（例如低延迟、高吞吐量）
• 支持每个给定地理区域的特定用户密度和负载需求（这可能会影响 RAN 协调水平）
• 具有不同性能水平的可用性传输网络，从理想到非理想
• 应用程序类型，例如实时或非实时
• 无线电网络层的特色要求，例如 CA、eICIC、CoMP 等。

参考：

• 3GPP TR 38.801 无线接入架构和接口版本 14

5G_NR_gNB_Logical_Architecture_and_Its_Functional_Split_Options_95">5G NR gNB Logical Architecture and It’s Functional Split Options

The logical architecture of gNB is shown in figure below with Central Unit (CU) and Distributed Unit (DU). Fs-C and Fs-U provide control plane and user plane connectivity over Fs interface.

In this architecture, Central Unit (CU) and Distribution Unit (DU) can be defined as follows.

Central Unit (CU): It is a logical node that includes the gNB functions like Transfer of user data, Mobility control, Radio access network sharing, Positioning, Session Management etc., except those functions allocated exclusively to the DU. CU controls the operation of DUs over front-haul (Fs) interface. A central unit (CU) may also be known as BBU/REC/RCC/C-RAN/V-RAN

Distributed Unit (DU): This logical node includes a subset of the gNB functions, depending on the functional split option. Its operation is controlled by the CU. Distributed Unit (DU) also known with other names like RRH/RRU/RE/RU.

Central Unit (CU) and Distributed Unit Functional Split Options

As a part of study item for New Radio (NR), 3GPP started studying different functional splits between central and distributed units. For the initial phase, 3GPP has taken LTE protocol stack as a basis for the discussion, until RAN2 defines and freezes the protocol stack for New Radio (NR). They have proposed about 8 possible options shown in below figure.

• Option 1 (RRC/PCDP 1A-like split)
• Option 2 (PDCP/RLC Split 3C-like split)
• Option 3 (High RLC/Low RLC split, Intra RLC split)
• Option 4 (RLC-MAC split)
• Option 5 (Intra MAC split)
• Option 6 (MAC-PHY split)
• Option 7 (Intra PHY split)
• Option 8 (PHY-RF split)

Option 1 (RRC/PDCP, 1A-like split)

In this split option, RRC is in the central unit while PDCP, RLC, MAC, physical layer and RF are kept in the distributed unit. Thus the entire user plane is in the distributed unit.

Option 2 (PDCP/RLC split)

Option 2 may be a base for an X2-like design due to similarity on U-plane but some functionality may be different e.g. C-plane since some new procedures may be needed. There are two possible variants available in this option.

Option 2-1 Split U-plane only (3C like split)

In this split option, RRC, PDCP are in the central unit. RLC, MAC, physical layer and RF are in the distributed unit.

Option 2-2

In this split option, RRC, PDCP are in the central unit. RLC, MAC, physical layer and RF are in the distributed unit. In addition, this option can be achieved by separating the RRC and PDCP for the CP stack and the PDCP for the UP stack into different central entities.

Option 3 (High RLC/Low RLC Split)

In this option, two approaches are taken based on Real time/Non-Real time functions split which are as follows:

• Option 3-1 Split based on ARQ
• Option 3-2 Split based on TX RLC and RX RLC

Option 3-1 Split based on ARQ

• Low RLC may be composed of segmentation functions;
• High RLC may be composed of ARQ and other RLC functions;

This option splits the RLC sublayer into High RLC and Low RLC sublayers such that for RLC Acknowledge Mode operation, all RLC functions may be performed at the High RLC sublayer residing in the central unit, while the segmentation may be performed at the Low RLC sublayer residing in the distributed unit. Here, High RLC segments RLC PDU based on the status reports while Low RLC segments RLC PDU into the available MAC PDU resources.

Option 3-2 Split based on TX RLC and RX RLC

• Low RLC may be composed of transmitting TM RLC entity, transmitting UM RLC entity, a transmitting side of AM and the routing function of a receiving side of AM, which are related to downlink transmission.
• High RLC may be composed of receiving TM RLC entity, receiving UM RLC entity and a receiving side of AM except for the routing function and reception of RLC status reports, which are related to uplink transmission.

Option 4 (RLC-MAC split)

In this split option, RRC, PDCP, and RLC are in the central unit. MAC, physical layer, and RF are in the distributed unit.

Option 5 (Intra MAC split)

Option 5 assumes the following distribution:

• RF, physical layer and lower part of the MAC layer (Low-MAC) are in the Distributed Unit
• Higher part of the MAC layer (High-MAC), RLC and PDCP are in the Central Unit

Therefore, by splitting the MAC layer into 2 entities (e.g. High-MAC and Low-MAC), the services and functions provided by the MAC layer will be located in the Central Unit (CU), in the Distributed Unit (DU), or in both. An example of this kind distribution given below.

• In High-MAC sublayer the centralized scheduling in the High-MAC sublayer will be in charge of the control of multiple Low-MAC sublayers. It takes high-level centralized scheduling decision. The inter-cell interference coordination in the High-MAC sublayer will be in charge of interference coordination methods such as JP/CS CoMP.
• In Low-MAC sublayer the time-critical functions in the Low-MAC sublayer include the functions with stringent delay requirements (e.g. HARQ) or the functions where performance is proportional to latency (e.g. radio channel and signal measurements from PHY, random access control). It reduces the delay requirements on the fronthaul interface. Radio specific functions in the Low-MAC sublayer can for perform scheduling-related information processing and be reporting. It can also measure/estimate the activities on the configured operations or the served UE’s statistics and report periodically or as requested to the High-MAC sublayer.

Option 6 (MAC-PHY split)

The MAC and upper layers are in the central unit (CU). PHY layer and RF are in the DU. The interface between the CU and DUs carries data, configuration, and scheduling-related information (e.g. MCS, Layer Mapping, Beamforming, Antenna Configuration, resource block allocation, etc.) and measurements.

Option 7 (Intra PHY split)

Multiple realizations of this option are possible, including asymmetrical options which allow obtaining benefits of different sub-options for UL and DL independently.

This option requires some kind of compression technique to reduce transport bandwidth requirements between the DU and CU.

• In the UL, FFT, and CP removal reside in the DU and for the two sub-variants, 7-1 and 7-2 are described below. Remaining functions reside in the CU.
• In the downlink, iFFT and CP addition reside in the DU and the rest of the PHY resides in the CU.

Considering above there are three sub-variant available for this option described as below

Option 7-1 In this option the UL, FFT, CP removal and possibly PRACH filtering functions reside in the DU, the rest of PHY functions reside in the CU. In the DL, iFFT and CP addition functions reside in the DU, the rest of PHY functions reside in the CU.

Option 7-2 In this option the UL, FFT, CP removal, resource de-mapping and possibly pre-filtering functions reside in the DU, the rest of PHY functions reside in the CU. In the DL, iFFT, CP addition, resource mapping and precoding functions reside in the DU, the rest of PHY functions reside in the CU.

Option 7-3 (Only for DL): Only the encoder resides in the CU, and the rest of PHY functions reside in the DU.

Option 8 (PHY-RF split)

This option allows to separate the RF and the PHY layer. This split permit centralization of processes at all protocol layer levels, resulting in very tight coordination of the RAN. This allows efficient support of functions such as CoMP, MIMO, load balancing, mobility.

Benefits of RAN Spilt Architecture

Some of the benefits of an architecture with the deployment flexibility to split and move New Radio (NR) functions between central and distributed units are below:

• Flexible HW implementations allows scalable cost-effective solutions
• A split architecture (between central and distributed units) allows for coordination for performance features, load management, real-time performance optimization, and enables NFV/SDN
• Configurable functional splits enables adaptation to various use cases, such as variable latency on transport

Which split function to use where?

The choice of how to split New Radio (NR) functions in the architecture depends on some factors related to radio network deployment scenarios, constraints and intended supported services. Some examples of such factors are:

• Support of specific QoS per offered services (e.g. low latency, high throughput)
• Support of specific user density and load demand per given geographical area (which may influence the level of RAN coordination)
• Availability transport networks with different performance levels, from ideal to non-ideal
• Application type e.g. Real-time or Non- Real Time
• Features requirement at Radio Network level e.g. CA, eICIC, CoMP etc.

Reference:

• 3GPP TR 38.801 Radio Access Architecture and Interfaces Release 14