5G NR NSA Architecture Overview

The 5G Non-Standalone (NSA) architecture represents an essential transitional phase in the global shift from 4G LTE to 5G networks. It enables mobile network operators to leverage existing 4G infrastructure to support 5G services, facilitating a faster and cost-effective deployment. In this article, we explore the fundamentals of the 5G NSA architecture, its deployment options (including Options 3, 3a, and 3x), and how it integrates with existing LTE systems.

The 5G NSA architecture is designed to accelerate the deployment of 5G services by using the existing LTE infrastructure. Unlike standalone (SA) 5G, which requires a new 5G core network (5GC), NSA relies on the existing 4G LTE core network (Evolved Packet Core or EPC). In this architecture, LTE and 5G technologies work together, with LTE serving as the anchor for control signalling while 5G NR (New Radio) is used to provide additional user data bandwidth.

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The 5G NSA architecture integrates existing 4G LTE components with 5G NR to provide improved data capabilities while using the existing LTE infrastructure for control and signalling. Here's a detailed explanation of the interaction:

Core Network (Evolved Packet Core - EPC):

• The EPC serves as the central part of the architecture, managing both control (S1-MME) and user plane data (S1-U) for LTE and 5G nodes.
• Components like the Mobility Management Entity (MME) and Serving Gateway (S-GW) manage mobility, session setup, and data routing.

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LTE Base Stations (eNB):

• LTE eNBs are the anchor points, managing control signalling and connecting to the EPC via the S1 interface for control and data paths.
• They coordinate with 5G NR nodes for seamless integration.

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5G New Radio (NR) Base Stations (en-gNB):

• en-gNBs are 5G nodes designed to boost capacity and provide faster data speeds.
• They operate with LTE eNBs, relying on LTE for control signaling but interacting directly with the EPC for user plane data.
• These nodes connect with LTE eNBs through the X2 interface to facilitate coordination and manage seamless user mobility.

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Interfaces:

• S1 Interface: Connects LTE and 5G nodes to the EPC for signaling (S1-MME) and user data (S1-U).
• X2 Interface: Links LTE eNBs with en-gNBs, allowing them to work together and manage transitions between LTE and 5G coverage.
• X2-U Interface: Specifically handles user plane data exchange between LTE and 5G components.

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Why Choose NSA Architecture?

• Faster Deployment: NSA allows mobile operators to deploy 5G services rapidly without the need for a full 5G core upgrade.
• Cost Efficiency: Leveraging existing LTE infrastructure reduces the initial investment required for full 5G deployment.
• Interoperability: NSA ensures seamless interoperability between 4G LTE and 5G NR, allowing devices to move between these technologies smoothly.

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Deployment Options for 5G NSA

To better understand the 5G NSA architecture, it’s crucial to explore its deployment options, which define how LTE and 5G NR integrate and operate together. The 3GPP standard outlines several deployment options, among which Options 3, 3a, and 3x are the most common for NSA.

Option 3 (Dual Connectivity)

Option 3 is also known as LTE-NR Dual Connectivity (EN-DC). In this setup, the LTE network acts as the master node (MeNB), while the 5G NR cell functions as the secondary node (SgNB). Data flows through both the LTE and NR nodes, with the LTE anchor providing control and signalling functions.

Key Characteristics:

• The EPC serves as the core network.
• LTE handles both signalling and data anchor, while 5G NR provides additional bandwidth.
• No changes are required in the core network, making this option attractive for quick deployments.

Advantages:

• Minimal changes to the existing LTE infrastructure.
• Efficient use of existing LTE spectrum while leveraging new 5G NR spectrum for enhanced data speeds.

Disadvantages:

• LTE remains the anchor for control signalling, which may limit the full capabilities of 5G NR.

Option 3a

Option 3a is a variation of Option 3, where the user plane data directly flows between the EPC and both the LTE and NR nodes. In this option, LTE continues to serve as the anchor for control plane functions, while 5G NR provides additional user plane bandwidth. The primary difference is that the data path does not have to pass through the LTE node.

Key Characteristics:

• The EPC is still used as the core network.
• Direct data paths are established between the EPC and the NR cell, optimizing throughput.
• Reduced dependency on LTE nodes for user data flow, leading to lower latency.

Advantages:

• More efficient use of resources as data paths are optimized for throughput.
• Reduces the load on LTE nodes, improving network performance.

Disadvantages:

• Complexity in managing and configuring direct data paths between EPC and NR nodes.

Option 3x

Option 3x is another variant of Option 3. In this option, the control plane is anchored in LTE (like other Option 3 variants), but the user plane bypasses the LTE anchor entirely and flows directly between the 5G NR cell and the EPC. This option maximizes the performance of 5G NR in terms of data throughput and latency.

Key Characteristics:

• EPC remains the core network, but the user plane data flows directly through the 5G NR node.
• LTE provides only the control plane functionality.

Advantages:

• Significantly improved user plane latency as the data bypasses the LTE node.
• Maximizes the potential of 5G NR capabilities in terms of speed and efficiency.

Disadvantages:

• Increased complexity in managing the control and user plane separation.
• Requires advanced coordination between LTE and 5G NR nodes.

Challenges of 5G NSA Architecture

• Limited 5G Capabilities

While NSA enables 5G deployment, it limits the full potential of 5G NR since it relies on the LTE core for control signalling. Features like ultra-reliable low latency communication (URLLC) may not be fully realized in an NSA setup.

• Increased Complexity

The dual connectivity approach in NSA introduces complexity in network management. Coordinating data flow and signalling between LTE and NR cells requires advanced algorithms and optimization.

• Performance Limitations

Although NSA enhances network performance, it may not reach the full potential of standalone 5G networks. The reliance on LTE for control functions can become a bottleneck, affecting latency and overall throughput.

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The 5G NSA architecture plays a vital role in the transition from 4G LTE to 5G. By leveraging existing infrastructure, it offers a cost-effective and rapid deployment solution for operators looking to provide 5G services. However, it comes with challenges, such as increased complexity and performance limitations. Understanding these aspects helps in making informed decisions on deployment strategies and network planning.

The deployment options (3, 3a, and 3x) provide flexibility and various performance benefits, ensuring operators can choose the most suitable approach for their needs. As the industry moves towards standalone 5G, the NSA architecture remains a crucial foundation, enabling the gradual evolution and expansion of 5G services globally.

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References :

• 3GPP TS 38.300: "NR and NG-RAN Overall Description"
• 3GPP TS 23.401: "General Packet Radio Service (GPRS) enhancements for Evolved Universal Terrestrial Radio Access Network (E-UTRAN) access"
• 3GPP TS 37.340: "Evolved Universal Terrestrial Radio Access Network (E-UTRAN); Dual Connectivity (DC)"
• Dahlman, E., Parkvall, S., & Skold, J. (2018). 5G NR: The Next Generation Wireless Access Technology. Academic Press.
• Andrews, J. G., Buzzi, S., Choi, W., Hanly, S. V., Lozano, A., Soong, A. C., & Zhang, J. C. (2014). "What Will 5G Be?" IEEE Journal on Selected Areas in Communications, 32(6), 1065-1082.
• GSMA: "5G Implementation Guidelines." Available at: https://www.gsma.com
• ETSI (European Telecommunications Standards Institute): "5G Specifications and Standards." Available at: https://www.etsi.org
• ITU (International Telecommunication Union): "IMT-2020: 5G System Overview." Available at: https://www.itu.int

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