RU/DU/CU Architecture in ORAN


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 predistortion 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.

Reference: Control, User and Synchronization Plane Specification, ORANWG4.CUS.0-v02.00
Management Plane Specification, ORAN-WG4.MP.0-v02.00.0

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