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# System prepended metadata

title: Learning Functional Split ORAN system
tags: [Fundamental]

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### :school: TEEP 2024_RT LAB_ORAN DPDK

#### :book: Milestone 5: Learning Functional Split ORAN system

:::success
List the essential information of this chapter.
1. Mention and Explain the Functional Split ORAN
2. Mention advantages and disadvantages of the Functional Split ORAN
:::


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### 1. Mention and Explain the Functional Split ORAN

![image](https://hackmd.io/_uploads/rkGSWR9ra.png)

According to Huber suhner functional split pdf, there is 8 options:

**Option 1** (Split A): This is a high-level split where the PDCP function is separate from the RLC and MAC functions. PDCP is usually placed in the CU, while RLC and MAC in the DU.

**Option 2** (Split B): The PDCP and RLC functions are merged, usually in the CU, and separated from the MAC function which resides in the DU.

**Option 3** (Split C): RLC is separate from MAC, with RLC in the CU and MAC in the DU. This allows data flow control and other functions to be managed centrally.

**Option 4** (Split D): Provides separation within the MAC layer itself, which can allow some MAC functions to be managed centrally and others locally in the DU.

**Option 5** (Split E): A split where only the high MAC functions reside in the CU, and the rest of the MAC functions along with the physical layer (PHY) reside in the DU.

**Option 6** (Split F): This split defines the separation of the low-level MAC function (Low MAC) and the physical layer (PHY), with the Low MAC in the DU and the PHY in the RU.

**Option 7** (Split G & H): This option further splits the PHY function into high and low levels, with High PHY in DU and Low PHY in RU. There are sub-variants such as 7-1, 7-2, and 7-3, each of which offers a different level of separation. For example, 7-2 is often used in an O-RAN (Open Radio Access Network) context.

**Option 8** (Split I): This is the lowest split where almost all processing is done at the RU except for very specific functions such as RF control.

The lines marking "High Layer Split" and "Low Layer Split" indicate that the split options above it focus more on high-level functions (such as control and management) and the split options below it more on low-level functions (such as real-time signal processing).

Each of these splits provides flexibility in designing the network considering factors such as capacity, coverage, latency, and cost efficiency. In general, higher splits reduce the load on network transport but may increase latency, while lower splits may reduce latency but require more transport capacity.

Different standards as mentioned (3GPP, Small Cell Forum, NGMN, and O-RAN) may have their own specific interpretations or implementations of these split options, according to their technological goals and needs.


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**IP (Internet Protocol)**: Located at the OSI network layer (Layer 3), IP is responsible for sending data packets from source to destination using IP addresses.

**PDCP (Packet Data Convergence Protocol)**: This protocol operates at the top of layer 2 (Data Link Layer), and provides functions such as IP header compression, encryption (ciphering), and data integrity.

In the context of 5G mobile networks, the division of protocols into **"high"** and **"low"** often relates to the concept of "split architecture" or "functional splits" in RAN (Radio Access Network) networks. This is a part of network design that allows the separation of signal processing functions and protocols between various network components, such as Central Units (CUs), Distributed Units (DUs), and Radio Units (RUs). The goal is to customize the placement of functions based on criteria such as performance, efficiency, and scalability. The following is a detailed explanation for RLC, MAC, and PHY:

**RLC (Radio Link Control)**

* High RLC: Functions residing in this layer typically include data flow control, data packet merging and segmentation, and handling retransmissions required due to packet loss or errors. "High RLC" may be implemented in the CU, where high-level control over connections is exercised and where decisions requiring an overall view of the network are taken.
* Low RLC: This may include functions that require fast response and closer interaction with the physical layer (PHY), such as fast error detection and data buffer handling. "Low RLC" is usually implemented closer to the physical device, such as in a DU or RU.

**MAC (Medium Access Control)**

* High MAC: This can include functions such as dynamic resource scheduling, access control to the transmission medium, and coordination between multiple inputs and multiple outputs (MIMO). "High MAC" may require a broader view of the network and may be centralized at the CU.
* Low MAC: Includes functions such as frame formation, HARQ (Hybrid Automatic Repeat Request) handling, and precise timing. Since these functions often require direct interaction with the physical layer, "Low MAC" usually resides at the DU or RU.

**PHY (Physical Layer)**

* High PHY: Functions at this physical layer may include signal modulation and demodulation, as well as more complex digital signal processing. "High PHY" can be related to high-level signal processing that does not require very low latency.
* Low PHY: Includes functions such as real-time signal processing, such as Fast Fourier Transforms (FFTs), and frequency upconversion or downconversion. The "Low PHY" requires very low latency and therefore, is placed very close to the radio hardware in the RU.

This division allows for flexibility in network configuration. For example, in scenarios that require very low latency, more functions will be placed on the "low" side (in the DU or RU), whereas for scenarios that are more latency tolerant, some of those functions can be placed on the "high" side (in the CU). This allows optimization based on the specific needs of the applications or services supported by the network.


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Functional View explains:

* **Retransmission buffer**: Stores data that may need to be retransmitted if a transmission error occurs or a data packet is lost in transmission.
* **Numbering**: Numbering of data packet sequences to ensure that they can be sorted and recognized correctly by the receiver.
* **Header compression**: Reducing the size of packet headers to improve bandwidth efficiency by eliminating or compressing redundant information.
* **Ciphering**: The process of encrypting data to keep the transmitted information secure from unauthorized access.
* **Add PDCP header**: Adds a Packet Data Convergence Protocol (PDCP) header that contains important information for sending and receiving data packets.
* **Transmission buffer**: Stores data packets before they are sent, enabling efficient and organized data transmission.
* **Segmentation**: Splits large data packets into smaller segments to fit within the packet size limitations of the network.
* **RLC control**: The Radio Link Control (RLC) control layer is responsible for quality assurance of data transmission such as error detection, resetting, and retransmission.
* **Add RLC header**: Adds an RLC header containing control information for data transmission.
* **Multiplexing**: Combining multiple data streams into a single transmission channel to improve spectrum utilization efficiency.
* **HARQ (Hybrid Automatic Repeat Request)**: A mechanism to request retransmission of data that has encountered errors, combining retransmission and recoding techniques to improve transmission reliability.
* **DL-SCH data transfer**: Data transfer using the Downlink Shared Channel (DL-SCH), which is a common channel used to send data to user devices.
* At the top of the diagram, there is the statement "The lower the split option, the less functions are in the CU (Central Unit)." This indicates that some functions may be split between the Central Unit and other units, such as the Distributed Unit (DU) in the 5G architecture, depending on the network configuration. The split option refers to how these functions are distributed between the CU and DU.
* **CRC attach**: Adds Cyclic Redundancy Check (CRC) for error detection in transmitted data blocks.
* **Coding + block segmentation**: Encodes data for error protection and divides it into smaller blocks if needed.
* **Rate matching**: Adjustment of the encoded data rate to match the available transmission channel capacity.
* **Scrambling**: Scrambling the data bits to avoid repeating patterns that could cause errors in transmission.
* **Modulation**: Transforming data bits into symbols that will be transmitted over radio waves.
* **Layer mapper**: Assigns symbols to different layers in a MIMO (Multiple Input Multiple Output) transmission scheme.
* **Pre-coding**: Customizes symbols for optimal transmission based on channel conditions and MIMO scheme.
* **Resource Element Mapper**: Assigns symbols to specific resource elements in the transmission frequency and time grids.
* **Beamforming**: Uses multiple antennas to direct and shape the radiation pattern of radio waves for more efficient transmission to the user.
* **IFFT (Inverse Fast Fourier Transform)**: Transforms data from the frequency domain to the time domain for radio wave transmission.
* **Cyclic Prefix Insertion**: Adds a cyclic prefix to each symbol block to reduce inter-symbol interference and ease receiver synchronization.
* **Analog Conversion**: Converts the processed digital signal into an analog signal for transmission through an antenna.
* **RF (Radio Frequency) Processing**: Processing the analog signal, including gain and frequency adjustment, to make it ready for transmission through the antenna.
* **Antenna**: The antenna used to send the signal to the user device.
* The statement above the diagram, "The higher the split option, the less functions are in the RU (Radio Unit)," indicates that there are configuration options where some signal processing processes can be executed in different units depending on how high they are "split" between the Central Unit and the Radio Unit.

### 2. Mention advantages and disadvantages of the Functional Split ORAN

### **High layer split**

**Pros:**

-Drastically reduced Bandwidth: By performing higher processing at the Central Unit (CU), the bandwidth required for transport between the CU and Distributed Unit (DU) or Radio Unit (RU) can be significantly reduced as less data is transmitted and is more compact.
-Ideal for non-mobile = FWA (Fixed Wireless Access): High-rate splits are ideal for Fixed Wireless Access applications, where devices are stationary, so higher latency can be tolerated.
-Latency Tolerant = long distances: This architecture is suitable for scenarios where the distance between the CU and RU is long and the additional latency generated by that distance is acceptable.
-Processing in RRH = URLLC (Ultra-Reliable Low-Latency Communications): Processing at the Radio Remote Head (RRH) or RU for URLLC applications suggests that tasks that require fast response and high reliability are executed closer to the end user, possibly reducing the overall latency for those tasks.

**Disadvantages:**

-CoMP is extremely complex or even impossible: CoMP (Coordinated Multi-Point) is a technology used to improve signal quality and throughput, especially in areas with a lot of interference or at the cell edge. In the context of High Layer Split, such coordination becomes extremely difficult as signal processing occurs far away from the end user.
-Complex and expensive RRH (size, heat, cost): RRHs or RUs that handle more processing tend to be larger, generate more heat, and cost more to manufacture and operate. This can be a significant consideration in network design and implementation.

In general, High Layer Split refers to a strategy where processing that normally occurs at the base station (such as PDCP or RLC functions) is moved to the data center or to the cloud. This can reduce the need for bandwidth-intensive communication between the base station and the data center, but it can also add complexity and cost to the network hardware and pose challenges in coordination between various points in the network.

### **Double Split**

**Pros:**

-CU can easily be virtualized: Splitting the CU function allows for virtualization of the CU, which means that the CU can be run on standard computing hardware and can be easily managed as a virtual instance in a data center or cloud.
-Optimal for mobile and URLLC: The architecture supports high mobility and ultra-reliable low-latency communication (URLLC), which is critical for applications such as autonomous vehicles, real-time industrial control, and emergency services.
-Reduced cost: Splitting and virtualizing CUs can reduce costs due to the use of more standard hardware and more efficient resource management.
-Good scalability: Double Split allows the network to scale up or down more easily, adding or reducing computing resources according to service demand.

**Disadvantages:**

-High bandwidth and latency fronthaul requirements: Splitting network functions can increase bandwidth and latency requirements on the fronthaul, which is the link between the CU and DU/RU. This can be an issue in implementations where fronthaul connections are limited in capacity or experience high latency.
In general, Double Split can provide great flexibility and cost efficiency for network operators, but it also requires a robust transport infrastructure to support the increased bandwidth and latency requirements.

### **Low Layer Split**

**Pros:**

-Ideal for CoMP=mobile: CoMP (Coordinated Multi-Point) is a technology designed to improve signal quality and throughput in mobile devices, especially in areas with high interference or at the cell edge. Low Layer Split facilitates CoMP implementation by processing signals locally at the Radio Unit (RU), enabling more effective coordination between multiple points.
-Cost-effective RRH (Remote Radio Head): By placing more processing functions in the RRH, it can reduce data transportation costs between the RRH and other units, and enable simpler and cheaper RRHs.

**Disadvantages:**

-High Bandwidth: Requires high bandwidth for fronthaul, as more data must be sent from the RRH to other units for processing.
Bandwidth scales with antenna ports (8, 7-1): Indicates that bandwidth requirements increase as antenna ports are added, which is characteristic of split options 8 and 7-1 in the 5G standard. These split options separate PHY functions to the RRH or RU, thus requiring greater bandwidth to transmit unfinished signal processing data to the central unit for further processing.
VERY tight latency requirements: Since signal processing occurs close to the antenna, there are very tight latency requirements to ensure that the signal processing and transmission time does not cause delays that could affect the performance of real-time applications.

Overall, Low Layer Split allows for more responsive and effective signal processing at the local level, but requires investment in fronthaul infrastructure capable of handling high bandwidth requirements and meeting stringent latency requirements. It is often used in scenarios that require high performance and fast response times.


### Reference
Huber Suhner Functional Split 
