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5G NR SCH layer allocates resources in UL and DL for cell and UE based procedures.

O-DU High Architecture

O-DU implements the functional blocks of L2 layer of a 5G NR protocol stack in SA (StandAlone) mode. These layers primarily include NR MAC, NR Scheduler and NR RLC layers.

O-DU modules are developed as shown in the below diagram.

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O-DU High Thread Architecture

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These modules contain platform specific files and support O-DU High functionality and message exchanges.

O1 Module

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 O1 ArchitectureImage Removed

As shown above, in the figure O1 module runs as a thread in O-DU High runs a separate process and communicates with ODU-High process over a TCP interface. Alarm communication happens over a Unix socket between the O1 and O-DU threads. O1 module uses API calls for interacting with the Netconf server(Netopeer) and datastore(sysrepo) for providing the Netconf interface.

O1 architecture has following components:

• Netconf ManagerSession Handler: Subscribe to Netconf YANG modules and events. Register callback handler methods.

• VES Agent : Sends the VES events to SMO

• Alarm Manager: Stores and manages(add/updated/delete) alarms.

• TCP server: Receives the alarm messages sent from O-DU High over TCP socket.

• TCP Client Alarm Interface : Integrates with Provides an interface to O-DU High module and provides an interface threads for sending the alarm messages to O1 module over TCP Unix socket.

• Config Interface : Interface to handle the configurations sent from SMO to the stack

• Netopeer server: Serves the northbound SMO/OAM Netconf requests.

O-DU-High Interfaces

This section describes the other modules that O-DU High interfaces with, as shown in below diagram.

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As shown above, O-DU High interfaces with the following modules:

• O-CU: O-DU High communicates with O-CU on the F1AP interface. The control message exchanges are on F1-C while data message exchanges are on F1-U interfaces. The below F1AP messages on F1-C are implemented, as per 3GPP 38.473-f60 v15.3:

  • Interface Management

    • F1 Setup

    • gNB-DU Configuration Update

    • F1 Reset

  • UE Context Management

    • UE Context Setup

  • RRC Message Transfer

    • Initial UL RRC Message Transfer

    • DL RRC Message Transfer

    • UL RRC Message Transfer

    • RRC Delivery Report

• Near RT RIC: O-DU High communicates with Near RT RIC on the E2 interface. The below E2AP messages are implemented, as per ORAN WG3.E2AP v01.00:

  • Global Procedures

    • E2 Setup

  • Near RT RIC Functional Procedures

    • RIC Subscription

    • RIC Indication

• O-DU Low: O-DU High communicates with O-DU Low on the FAPI interface. The below FAPI messages are supported, as per FAPI interface files shared by Intel:

  • P5 messages - PHY mode control interface

    • PARAM.request/PARAM.response

    • CONFIG.request/CONFIG.response

    • START.request

    • STOP.request

    • STOP.indication

  • P7 messages - Main data path interface

    • DL_TTI.request

    • UL_TTI.request

    • SLOT.indication

    • UL_DCI.request

    • TX_Data.request

    • RX_Data.indication

    • CRC.indication

    • UCI.indication

    • RACH.indication

• OAM: O-DU High communicates with OAM on the O1 interface.

O-DU High functionality

Cell Up and Broadcast Procedure

This section describes the cell-up procedure within O-DU High.

As seen in the above Figure, - The DU APP module of O-DU High sends F1 Setup Request to O-CU. This message contains a list of cells that the O-DU High has been configured with.

• The O-CU responds with F1 Setup Response. This message contains a list of cells which must be activated.

• The O-DU High scans the list of cells received and sends corresponding cell configurations to 5G NR MAC.

• 5G NR MAC, in-turn configures the 5G NR SCH. It also configures the O-DU Low via the Lower MAC module.

• On receiving the cell config response, DU APP sends a gNB DU Config Update towards the O-CU. The O-CU responds with gNB DU Config Update ACK towards the O-DU High.

• The DU APP now exchanges F1 Reset message with the O-CU to initialize the UE contexts.

• DU APP sends Cell Start Req towards 5G NR MAC. This message is translated by the Lower MAC into the FAPI message START.request towards the O-DU Low.

• On receiving START.request, O-DU Low begins to send slot indications towards 5G NR MAC via the lower MAC. The frequency of these slot indications is determined by the numerology(Mu) supported. 5G NR MAC forwards these slot indications to the 5G NR SCH and DU APP modules.

• When the first slot indication reaches the DU APP, cell is marked as up.

• The 5G NR SCH, keeps tracking the SSB and SIB1 ocassions on receiving regular slot indications. On detecting the relevant ocassion, 5G NR SCH schedules SSB/SIB1 and forwards the DL Scheduling Information to 5G NR MAC.

• The 5G NR MAC mutiplexes the PDU and sends SSB/SIB1 packets towards the O-DU Low through the Lower MAC.

UE Related Procedure

The O-DU High supports

• All physical channels - PBCH, PRACH, PDCCH, PDSCH, PUSCH, PUCCH

• All control logical channels - UL CCCH, DL CCCH, UL DCCH, DL DCCH

• All control transport channels - BCH, UL-SCH, DL-SCH, RACH

The above channels are used to achieve the below messages:

• Cell broadcast of System Information which includes SSB and SIB1.

• RACH Procedure

  • RACH Indication

  • Random Access Response

  • RRC Setup Request

  • RRC Setup

• UE attach signalling flow

  • RRC Setup Complete

  • Registraton Request

  • Security Mode Command

  • Security Mode Complete

  • Registraton Accept

  • Registraton Complete

  • Several NAS Message Exchanges

  • RRC Reconfiguration

  • RRC Reconfiguration Complete

Below figure depicts the above call flow, inclusive of all interfaces:

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