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HLS (Higher layer split (gNB))

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Introduction:

Higher Layer Split (HLS) refers to the separation of the radio protocol stack between the gNB (gNodeB) and the UPF (User Plane Function) in a 5G network. It enables the gNB to be divided into two separate entities – the CU (Centralized Unit) and the DU (Distributed Unit) – with each unit responsible for a different set of functions. In this article, we will discuss HLS in detail, including its benefits, architecture, and deployment scenarios.

Benefits of Higher Layer Split (HLS):

The Higher Layer Split (HLS) has several benefits over traditional radio protocol stacks, including:

  1. Improved Performance: HLS allows for better resource utilization, which can improve network performance. The separation of the radio protocol stack allows for more efficient use of processing power, reducing the load on the gNB.
  2. Scalability: HLS allows for greater scalability, making it easier to add more nodes to the network. The separation of the radio protocol stack also enables the deployment of smaller, more efficient nodes, reducing the cost and complexity of the network.
  3. Flexibility: HLS provides greater flexibility in network architecture, allowing for the deployment of a wider range of network topologies. This can help service providers optimize their network for different use cases, such as low-latency applications or high-bandwidth applications.
  4. Reduced Latency: HLS can reduce the latency of the network, improving the user experience for applications that require low latency. By separating the radio protocol stack, the DU can be deployed closer to the end user, reducing the distance that data needs to travel.
  5. Improved Security: HLS can improve the security of the network by reducing the attack surface. By separating the radio protocol stack, the CU and DU can be secured independently, reducing the risk of a single point of failure.

Architecture of Higher Layer Split (HLS):

The architecture of Higher Layer Split (HLS) consists of two separate entities – the Centralized Unit (CU) and the Distributed Unit (DU). The CU is responsible for the higher layer functions of the radio protocol stack, while the DU is responsible for the lower layer functions.

The CU is typically deployed in a centralized location, such as a data center, while the DU is deployed closer to the end user, such as on a lamp post or building rooftop. The CU and DU communicate over a standard interface, such as the F1 interface.

The CU performs the following functions:

  1. RRC (Radio Resource Control) protocol stack: The RRC protocol stack is responsible for managing the radio resources of the network, including allocation, release, and configuration.
  2. NAS (Non-Access Stratum) protocol stack: The NAS protocol stack is responsible for managing the connection between the user equipment (UE) and the core network.
  3. PDCP (Packet Data Convergence Protocol) protocol stack: The PDCP protocol stack is responsible for compressing and decompressing data packets, reducing the amount of data that needs to be transmitted over the network.
  4. RLC (Radio Link Control) protocol stack: The RLC protocol stack is responsible for managing the reliability of the radio link, ensuring that data packets are transmitted and received correctly.

The DU performs the following functions:

  1. PHY (Physical) protocol stack: The PHY protocol stack is responsible for the physical layer functions of the radio protocol stack, including modulation and demodulation.
  2. MAC (Medium Access Control) protocol stack: The MAC protocol stack is responsible for managing the medium access of the radio interface, ensuring that multiple users can share the same radio resources.

Deployment Scenarios for Higher Layer Split (HLS):

Higher Layer Split (HLS) can be deployed in several scenarios, depending on the requirements of the network. The following are some of the most common deployment scenarios for HLS:

  1. Urban Deployments: HLS can be used in urban deployments where there are high user densities and limited space for infrastructure. By deploying small, efficient DUs closer to the end user, the network can provide better coverage and capacity while reducing the cost and complexity of the infrastructure.
  2. Rural Deployments: HLS can also be used in rural deployments where there are large distances between the nodes of the network. By deploying a centralized CU and multiple DUs, the network can provide coverage over a larger area while reducing the cost and complexity of the infrastructure.
  3. In-building Deployments: HLS can be used in in-building deployments where there are multiple floors and walls that can interfere with the radio signal. By deploying DUs on each floor and a centralized CU, the network can provide better coverage and capacity throughout the building.
  4. Public Safety Deployments: HLS can be used in public safety deployments where there is a need for low-latency and reliable communication. By deploying DUs closer to the end user, the network can provide better coverage and capacity while reducing the risk of a single point of failure.

Conclusion:

Higher Layer Split (HLS) is a key technology in 5G networks that enables the separation of the radio protocol stack between the gNB and the UPF. By dividing the gNB into a centralized CU and a distributed DU, HLS provides several benefits over traditional radio protocol stacks, including improved performance, scalability, flexibility, reduced latency, and improved security. HLS can be deployed in several scenarios, including urban deployments, rural deployments, in-building deployments, and public safety deployments. As 5G networks continue to evolve, HLS is expected to play an increasingly important role in improving network performance and user experience.