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RU/DU/CU Architecture

Radio Unit Interface & Architecture :

5G NR radio unit shall be connected to DU through fronthaul RAN low layer split. The DU shall be connected to the CU which connects to the 5G enabled EPC to work along with LTE eNB in Non StandAlone (NSA) mode. At a later time, when the 5G core is ready, the CU will connect to the 5G core as well to support StandAlone (SA) mode Option 2.

For the local display Status Indicator LED in the radio unit, it shall have at least the following status indicators:

• One for fronthaul transport interface (optical fiber or wireless) indicating on/off status

• One for power supply to show on/off status

• One for radio transmission link on/off status

Figure below shows RU Interface & Architecture Requirements

Radio Unit Hardware

The radio unit shall be designed in a modular format that each module is connected through a standard interface such as PCIe. The RU is composed of RF front end, digital front end, Ethernet fronthaul transport, synchronization and lower PHY layer baseband processing. The RU functional module diagram is shown in Figure below

The lower PHY layer processing shall be done by using FPGAs or ASICs. It includes functions of FFT/iFFT, CP addition, PRACH and digital beamforming.

The RF front end is composed of antenna element arrays, bandpass filters, power amplifiers, low noise amplifiers, digital analog converters, and analog digital converters.

The digital front end consists of digital up converter, digital down converter, digital pre-distortion and crest factor reduction.

Distributed Unit:

The Distributed Unit (DU) connects to multiple radio units in the southbound interface, and to the Centralized Unit (CU) in the northbound interface. The DU has a centralized, virtualized baseband pool that performs the functions of high PHY layer, MAC and RLC, synchronization, OAM, Ethernet, as well as F1 interface function. The DU function module diagram is shown in Figure below:

Distributed Unit Capacity and Software Support:

The distributed unit is the baseband processing unit that handles high physical layer, MAC and RLC layer with network function virtualization. The operating system for DU shall support RHEL CentOS 7.x, Ubuntu 16.xx or later release versions. RAN performance KPIs shall be guaranteed under the virtualization implementation. Features such as DPDK, SRIOV are required to enhance the performance. Given the maturity and security, virtualization shall be implemented in VM. Implementation in container will be considered in the future when it becomes mature.

Centralized Unit

The Centralized Unit (CU) shall perform the layer three functions such as the functions of RRC, PDCP, SDAP, X2-U, F1-U, NG-U, S1-U, X2AP (X2-C), F1AP (F1-C), NGAP (NG-C), S1AP (S1-C) and OAM . Figure below is the diagram that shows the functions that the CU will perform:

NG-U/C interface is used to connect with the 5G core network while the S1-U/C interface is used to connect to the 5G enabled EPC. The simultaneous connection with both core networks is required in order to support different UE capabilities. The initial implementation with connecting to EPC only for NSA architecture is accepted.

Ref: Control, User and Synchronization Plane Specifiction, ORAN-WG4.CUS.0-v02.00, Management Plane Specification, ORAN-WG4.MP.0-v02.00.00