The interface between the DU and RU is also known as the fronthaul (FH) interface. It is a high-speed, high-capacity link that carries the IQ samples of the radio signals between the two units. The fronthaul interface is critical to the performance of a 5G network, as it must be able to support the high data rates and low latency requirements of 5G applications.
The fronthaul interface is responsible for transporting the following types of traffic:
- User data: This includes the IQ samples of the radio signals that are transmitted and received by the O-RU.
- Control signalling: This includes messages that are used to configure and control the O-RU.
- Synchronization signals: These are used to ensure that the O-DU and O-RU are synchronized in time and frequency.
The O-RAN Alliance is an industry initiative that is developing open standards for 5G networks. One of the key goals of O-RAN is to enable multi-vendor DU and RU interconnection. This means that operators should be able to choose and deploy the best DU and RU solutions from different vendors, without being locked into a single vendor ecosystem.
To enable multi-vendor DU and RU interconnection, O-RAN has developed a set of signalling formats and control messaging. These are defined in the O-RAN Fronthaul specification.
The O-RAN Fronthaul specification defines a set of standards signalling formats and control messages for the following tasks:
- Radio configuration: This includes parameters such as the carrier frequency, bandwidth, and modulation scheme.
- Antenna configuration: This includes parameters such as the antenna gain and beamforming direction.
- Power control: This includes parameters such as the transmission power and receiver gain.
- Synchronization: This includes parameters such as the time and frequency synchronization between the DU and RU.
- Performance monitoring: This includes metrics such as the signal quality, throughput, and latency.
- Fault detection and reporting: This allows the DU to detect and report faults in the RU.
- Firmware updates: This allows the DU to update the firmware on the RU.
By using the standard signaling formats and control messages defined in the O-RAN Fronthaul specification, vendors can develop DUs and RUs that are interoperable with each other. This gives operators more flexibility and choice when deploying their 5G networks.
The 5G-ORAN C-plane (control plane) is responsible for real-time control between the O-DU (O-RAN Distributed Unit) and the O-RU (O-RAN Radio Unit).
Control Plane defines the procedures:
- Scheduling and beamforming commands transfer procedure.
The scheduling and beamforming commands transfer procedure is the process of transferring scheduling and beamforming commands from the base station to the user equipment (UE). This process is essential for coordinating the transmission of data between the base station and the UE.
The scheduling and beamforming commands transfer procedure is divided into three main steps:
- Symbol numbering and duration: The base station assigns a unique symbol number and duration to each scheduling and beamforming command. This allows the UE to identify and process each command correctly.
- Scheduling command transfer: The base station transfers the scheduling commands to the UE using a dedicated control channel. The scheduling commands inform the UE which symbols it is allocated to transmit and receive data on.
- Beamforming command transfer: The base station transfers the beamforming commands to the UE using a dedicated control channel. The beamforming commands inform the UE how to focus its transmission and reception beams.
- Symbol numbering and duration
The base station assigns a unique symbol number and duration to each scheduling and beamforming command. The symbol number is a unique identifier for the command, while the duration is the length of time that the command is valid.
The symbol numbering and duration scheme is used to ensure that the UE can identify and process each command correctly. The UE uses the symbol number to identify the command, and the duration to determine how long to wait for the command to be executed.
- Mixed numerology and PRACH handling
Mixed numerology refers to the use of different numerologies (i.e., subcarrier spacing and symbol duration) within the same cell. PRACH (physical random-access channel) is a dedicated channel that UEs use to initiate random access to the base station.
- The scheduling and beamforming commands transfer procedure must be able to handle mixed numerology and PRACH. This is because UEs may be using different numerologies and/or PRACH to transmit and receive data.
- To handle mixed numerology, the base station must use a flexible symbol numbering and duration scheme. This scheme must be able to accommodate the different numerologies that are being used within the cell.
To handle PRACH, the base station must use a dedicated control channel to transfer the scheduling and beamforming commands to the UE. This control channel must be separate from the PRACH channel, to avoid interference.
User plane messages in O-RAN are used to efficiently transfer data between the O-DU and O-RU within the strict time limits of 5G numerologies. This is essential to ensure that the RAN can provide high-speed and low-latency mobile data services to users.
User Plane defines the features:
- U-Plane data compression
U-Plane data compression in O-RAN is used to reduce the size of user plane messages before they are transmitted over the fronthaul interface. This helps to improve the efficiency of the data transfer and reduce the latency.
There are a number of different U-Plane data compression algorithms that can be used in O-RAN, including:
- Block floating point (BFP): BFP compression divides the IQ sample data into blocks and then compresses each block using a floating-point format. This algorithm is very efficient and can achieve high compression ratios, but it can also introduce some additional latency.
- Block scaling: Block scaling compression divides the IQ sample data into blocks and then scales each block by a common factor. This algorithm is less efficient than BFP compression, but it also introduces less latency.
- μ-law: μ-law compression is a non-linear compression algorithm that is typically used for voice traffic. This algorithm is very efficient and can achieve high compression ratios, but it can also introduce some distortion to the signal.
The choice of which U-Plane data compression algorithm to use depends on the specific requirements of the RAN deployment. For example, BFP compression is typically used in high-performance deployments where low latency is essential. μ-law compression is typically used in voice deployments.
- IQ data transfer procedure
The IQ data transfer procedure in O-RAN is responsible for transferring the IQ sample data between the O-DU and O-RU. The procedure is designed to be as efficient as possible, while still meeting the strict timing requirements of 5G numerologies.
The IQ data transfer procedure typically involves the following steps:
- The O-DU compresses the IQ sample data using a U-Plane data compression algorithm.
- The O-DU segments the compressed IQ sample data into frames.
- The O-DU transmits the frames to the O-RU over the fronthaul interface.
- The O-RU receives the frames and decompresses the IQ sample data.
- The O-RU uses the IQ sample data to modulate the radio signals that are transmitted and received by the O-RU.
- DL data precoding
DL data precoding in O-RAN is a technique that allows the O-DU to pre-compute the beamforming weights and send them to the O-RU. This helps to improve the performance of the RAN by reducing the amount of processing that needs to be done by the O-RU.
- DL data precoding is typically used in downlink deployments where the O-DU needs to transmit data to multiple users at the same time. By pre-computing the beamforming weights, the O-DU can ensure that the radio signals are transmitted in a way that maximizes the signal quality for each user.
- DL data precoding can be implemented in a number of different ways, but the most common approach is to use a technique called conjugate beamforming. In conjugate beamforming, the O-DU pre-computes the beamforming weights in such a way that the radio signals are transmitted in the direction of the users. This helps to improve the signal quality and reduce the interference between users.
The synchronization plane (S-plane) in O-RAN is responsible for ensuring that the O-DU and O-RU are synchronized in time and frequency. This is essential for the proper operation of many O-RAN features, such as TDD, carrier aggregation, MIMO, and beamforming.
In cloud RAN deployments, the O-DU and O-RUs may be physically separated by a significant distance. This can make it difficult to achieve the high level of synchronization that is required for O-RAN to operate properly.
The S-plane addresses this challenge by using protocols such as PTP (Precision Time Protocol) and SyncE (Synchronous Ethernet) to synchronize the clocks of the O-DU and O-RUs.
- PTP is a protocol that is used to distribute a precise time reference from a grandmaster clock to slave clocks. The grandmaster clock is typically located at the O-DU side. The slave clocks are located at the O-RU side.
- SyncE is a protocol that is used to synchronize the clocks of multiple Ethernet devices. It is typically used to synchronize the clocks of the O-DU and O-RUs, as well as the clocks of other devices in the RAN, such as switches and routers.
The Management Plane (M-Plane) in O-RAN is responsible for managing the Radio Unit (O-RU). It provides a variety of functions to set parameters on the O-RU side, as required by the Control/User Plane (C/U-Plane) and Synchronization Plane (S-Plane). For example, the M-Plane can be used to:
- Manage O-RU software.
- Perform fault management.
- Configure O-RU parameters.
- Monitor O-RU performance.
The M-Plane is an important part of O-RAN because it enables a real multi-vendor Open RAN. This is because the M-Plane provides a standard interface for managing O-RUs from different vendors. This eliminates the need for operators to learn and use different management interfaces for each vendor’s O-RU.
Here is a different way to explain the M-Plane in O-RAN:
Imagine that the O-RAN network is a car. The C/U-Plane is the driver, the S-Plane is the engine, and the M-Plane is the mechanic. The driver tells the car where to go, the engine makes the car move, and the mechanic keeps the car running smoothly.
In the same way, the C/U-Plane tells the O-RAN network what to do, the S-Plane keeps the network synchronized, and the M-Plane manages the O-RUs.
- The M-Plane supports a hierarchical/hybrid model, which means that it can be used to manage O-RUs that are directly connected to the O-DU, as well as O-RUs that are connected to the O-DU through a fronthaul transport network.
- The M-Plane also supports IP and delay management for the C/U Plane. This means that it can be used to configure the IP addresses and delay budgets for the C/U Plane traffic.
- Finally, the M-Plane supports FCAPS (Fault, Configuration, Accounting, Performance, and Security) functions, including sync configuration and status. This means that it can be used to manage the faults, configuration, accounting, performance, and security of the O-RUs.
Overall, the M-Plane is a critical component of O-RAN. It enables a real multi-vendor Open RAN by providing a standard interface for managing O-RUs from different vendors. It also supports a hierarchical/hybrid model, IP and delay management for the C/U Plane, and FCAPS functions.
The C/U-Plane, S-Plane, and M-Plane in O-RAN each use a different protocol stack over Ethernet.
The C/U-Plane supports two protocol stacks:
- eCPRI over Ethernet: This protocol stack is based on the eCPRI standard, which is a high-performance protocol for transporting IQ sample data between the O-DU and O-RU.
- RoE (Radio over Ethernet): This protocol stack is a simpler and more cost-effective alternative to eCPRI over Ethernet. It is based on the standard Ethernet protocol, but it includes some extensions that are specific to O-RAN.
The choice of which protocol stack to use depends on the specific requirements of the RAN deployment. For example, eCPRI over Ethernet is typically used in high-performance deployments where low latency is essential. RoE is typically used in less demanding deployments where cost is a more important factor.
The S-Plane supports a single protocol stack:
- PTP and SyncE over Ethernet: This protocol stack is based on the PTP and SyncE standards, which are used to synchronize the clocks of the O-DU and O-RUs.
This protocol stack is essential for the proper operation of many O-RAN features, such as TDD, carrier aggregation, MIMO, and beamforming.
The M-Plane supports a single protocol stack:
- NETCONF over Ethernet with IP transported using TCP with SSH: This protocol stack is based on the NETCONF standard, which is used to manage network devices.
- SSH is used to encrypt the traffic between the O-DU and O-RU.