From Earth to Space: A First Deployment of 5G Core Network on Satellite

Recent developments in the aerospace industry have led to a dramatic reduction in the manufacturing and launch costs of low Earth orbit satellites. The new trend enables the paradigm shift of satelliteterrestrial integrated networks with global coverage. In particular, the integration of 5G communication systems and satellites has the potential to restructure nextgeneration mobile networks. By leveraging the network function virtualization and network slicing, the satellite 5G core networks will facilitate the coordination and management of network functions in satellite-terrestrial integrated networks. We are the first to deploy a 5G core network on a real-world satellite to investigate its feasibility. We conducted experiments to validate the satellite 5G core network functions. The validated procedures include registration and session setup procedures. The results show that the satellite 5G core network can function normally and generate correct signaling.

近年来，航空航天工业的发展已导致近地轨道（LEO）卫星的制造成本和发射成本急剧下降。这一新趋势推动了实现全球覆盖的星地一体化网络的范式转变。特别是，5G通信系统与卫星的融合具有重构下一代移动网络的潜力。通过利用 网络功能虚拟化（NFV）和网络切片技术， 卫星5G核心网将促进星地一体化网络中网络功能的协调与管理。我们首次 在真实世界的卫星上部署了5G核心网，以研究其可行性。我们进行了实验以验证卫星5G核心网的功能。验证的流程包括注册和会话建立流程。 结果表明，卫星5G核心网能够正常运行并产生正确的信令

Introduction

Traditional terrestrial communication systems have flourished in many ways. Especially, the 5G networks have enabled much higher data rate, ultra low latency, and massive network capacity [1]. However, there are still some inherent disadvantages that are difficult to resolve [2]. Mobile networks have poor coverage in remote areas. They suffer long-distance transmission delays. Besides, vulnerability to natural disasters makes terrestrial communication systems unavailable in extreme cases. Meanwhile, the advancements in aerospace technology have brought the satellite industry a resurgence, from serving vertical fields to providing more universal services. Previously, operators provided limited communication services to dedicated users through geostationary satellites. Recent projects like Starlink have used low Earth orbit (LEO) constellations to deliver global broadband Internet access. The urgent needs and the rapid developments boost the integration of satellites and 5G systems. In addition, the significance of incorporating satellites is not merely enhancing terrestrial networks. The dominant class of services is content delivering, which takes about 90% of the total traffic in telecommunication networks. The satellites will have better cost effectiveness in broadcasting/multicasting connectivity and makes the services more convenient.

传统的地面通信系统已在多方面蓬勃发展。特别是5G网络，实现了更高的数据速率、超低延迟和海量的网络容量[1]。然而，仍存在一些难以解决的固有缺陷[2]:

1. 移动网络在偏远地区的覆盖范围很差。它们存在长距离传输延迟
2. 对自然灾害的脆弱性使得地面通信系统在极端情况下无法使用

与此同时，航空航天技术的进步为卫星产业带来了复兴，使其从服务垂直领域转向提供更普惠的服务。此前，运营商通过地球静止（GEO）卫星向特定用户提供有限的通信服务。

近期的项目（如Starlink）已利用近地轨道（LEO）星座来提供全球宽带互联网接入。迫切的需求和快速的发展推动了卫星与5G系统的融合。此外，融合卫星的意义不仅在于增强地面网络。内容分发是主导的服务类别，占电信网络总流量的约90%。卫星在广播/组播连接方面将具有更高的成本效益，并使服务更加便捷。

There have been numerous efforts devoted to the integration of 5G and satellites [3, 4]. The 3rd generation partnership project (3GPP) initiated activities towards non-terrestrial networks that study the role of satellites in 5G and acknowledge long-term research within B5G and 6G [5–7]. Besides, prior works [8, 9] investigated the standardization of non-terrestrial networks. Reference [10] further summarized the networking challenges for non-terrestrial networks in the 5G ecosystem. Studies [11, 12] focused on the satellites which were used as a complement for radio access networks in 5G communication systems. Technology companies like Lynk, Omnispace, Vulcan Wireless, Lockheed Martin, and AST SpaceMobile have already combined 5G and satellite to enable direct connections between user equipment and satellites. While researchers explored the advantages brought by the satellites being a part of transmission networks [13, 14]. In addition, satellite networks can bring extra use cases for the 5G network slicing, which has been studied in [15, 16].

目前已有大量工作致力于5G与卫星的融合[3, 4]:

1. 第三代合作伙伴计划（3GPP）启动了针对非地面网络（NTN）的活动，研究卫星在5G中的作用，并确认了B5G和6G的长期研究[5–7]
2. 此外，先前的工作[8, 9]研究了非地面网络的标准化问题。参考文献[10]进一步总结了5G生态系统中非地面网络的组网挑战
3. 研究[11, 12]关注卫星在5G通信系统中作为无线接入网（RAN）补充的应用
4. 像Lynk、Omnispace、Vulcan Wireless、Lockheed Martin和AST SpaceMobile这样的科技公司已经将5G与卫星相结合，以实现用户设备（UE）与卫星之间的直接连接
5. 同时，研究人员探讨了卫星作为传输网络一部分所带来的优势[13, 14]

此外，卫星网络可以为5G网络切片带来额外的用例, [15, 16]已对此进行了研究

作为 RAN/transmission Network的一部分

其实很简单, 也很直观. 只是乍一看transmission net这个名词, 感觉很震撼, 还以为是什么新概念...

RAN:

• 负责用户设备（UE）的无线接入，并管理无线资源
• 组件: Base Station (e.g. gNB)

Transmission Net:

• 负责在网络各组件之间传输数据和信号
• 组件: 链路, 回传路径 (如地面光纤、ISLs)

5G core networks have evolved into software-defined communication systems based on virtualization technology [1]. With the explosive growth in scale and business, the management, operation, and maintenance of satellite networks will be more complex. Benefited from software-defined networking and network function virtualization, 5G core networks can achieve distributed deployment and elastic scaling[17]. Migrating some functions of 5G core networks to satellites can connect users and data networks more flexibly. With these techniques, 5G core networks have the potential to be used to implement a satellite-terrestrial integrated network[12]. There are also studies on the role of core networks in the 6G era [18–20]. Some core network functions can be moved onboard to gain more flexibility and efficiency [21]. Satellite core networks may suffer from signaling storms and the data session migrations. To address these challenges, [22] has proposed a prototype of satellite core networks, which adopted a stateless design.

5G核心网（5GC）已经演变为基于虚拟化技术的软件定义通信系统[1]

随着规模和业务的爆炸性增长，卫星网络的管理、运营和维护（OAM）将变得更加复杂。

受益于软件定义网络（SDN）和网络功能虚拟化（NFV），5G核心网可以实现分布式部署和弹性伸缩[17]。将5G核心网的部分功能迁移到卫星上，可以更灵活地连接用户和数据网络。

借助这些技术，5G核心网有潜力用于实现星地一体化网络[12]。也有关于核心网在6G时代角色的研究[18–20]。一些核心网功能可以被转移到星上，以获得更高的灵活性和效率[21]。卫星核心网可能会遭受信令风暴和数据会话迁移的困扰。为应对这些挑战，[22]提出了一个卫星核心网原型，该原型采用了无状态设计。

This paper aims to study the convergence of satellites and 5G core networks. In this direction, we introduce the motivations for deploying the 5G core network functions onto satellites. Next, we provide an overview of the capabilities with satellite 5G core networks. We then propose an architecture of a satellite-terrestrial integrated network. The architecture shows the advantages of the satellite 5G core network. Finally, we present the deployment and experiment procedures of the satellite 5G core network. It is the first attempt to setup a satellite-terrestrial integrated network with the 5G core network deployed on a real-world satellite.

本文旨在研究卫星与5G核心网的融合。在这个方向上，我们介绍了将5G核心网功能部署到卫星上的动机。接着，我们概述了卫星5G核心网所具备的能力。然后，我们提出了一个星地一体化网络的架构。该架构展示了卫星5G核心网的优势。最后，我们展示了卫星5G核心网的部署和实验流程。这是首次尝试在真实世界的卫星上部署5G核心网，以构建星地一体化网络。

Motivations

2.1 Main Aspects

2.1.1 Demand Perspective

The costs of launching and manufacturing satellites continue to decrease. The LEO constellations are expected to expand exponentially in the near future [23]. Onboard computing, communication, and storage resources will also usher in a period of explosive growth. Based on that, deploying core networks on satellites becomes practical. Meanwhile, traditional satellite services like remote sensing, navigation, and communications have great needs for computing power. The demand for in-orbit computing will drive the deployment of additional satellite services. With satellite 5G core networks, mobile users can access the satellite services more conveniently.

卫星的发射成本与制造成本持续降低。LEO（近地轨道）星座预计在不久的将来将呈爆发式增长[23]。星上计算、通信和存储资源也将迎来一个爆发式增长期。基于此，将核心网部署在卫星上变得切实可行。同时，遥感、导航和通信等传统卫星服务对计算能力有着巨大需求。在轨计算的需求将推动附加卫星服务的部署。借助卫星5G核心网，移动用户可以更便捷地接入卫星服务。

2.1.2 Performance Perspective

The terrestrial mobile core networks tend to sink to the edge. The purpose is to reduce the transmission delay and improve the quality of user experience. The satellite core networks will benefit from the low-latency and wide-coverage links of LEO satellites. The terrestrial fiber paths are generally long-winded, in which the light travels at roughly 2c/3 [24]. While most of LEO satellites are orbiting at 500km to 1,000km above the Earth’s surface. Previous study [25] showed that the LEO satellite constellation can achieve a 50% improvement in latency over today’s terrestrial networks. For typical end-to-end connections, even the relatively small constellation can (almost always) achieve latencies better than the best possible with fiber. Thus, the satellite core networks can get a considerable performance gain compared to the terrestrial ones. Additionally, satellite core networks eliminate unnecessary backhauling which takes a large fraction of the network latency. As a result, satellite core networks will reduce the control plane signaling interaction delay and speed up the user access procedures.

地面移动核心网正呈现出向边缘下沉的趋势。其目的是减少传输延迟并提升用户体验质量。卫星核心网将受益于LEO卫星的低延迟和广覆盖链路。

地面光纤路径通常较为迂回，光在其中的传播速度约为 2c/3 [24]。而大多数LEO卫星在距离地球表面500至1000公里的轨道上运行。先前的研究[25]表明，LEO卫星星座相比当今的地面网络，在延迟方面可实现50%的改善。

对于典型的端到端连接，即使是规模相对较小的星座也（几乎总是）能实现优于最佳地面光纤路径的延迟。因此，与地面核心网相比，卫星核心网可以获得可观的性能增益。

此外，卫星核心网避免了不必要的回传（backhauling），而回传延迟占网络总延迟的很大一部分。因此，卫星核心网将减少控制面的信令交互延迟，并加速用户接入流程。

TLDR (性能层面的优势):

1. 低延迟+覆盖率广
2. 避免不必要的 backhauling

2.1.3 Functional Perspective

According to the 3GPP technical report, satellites will have base station functions and be a part of access networks[6, 26]. Access networks rely on core networks to provide mobility management, session management, and other functions. Based on that, the satellite 5G core networks can benefit in two ways. Firstly, the satellite 5G core networks will make users be able to get the satellite services without backhauling. Secondly, the satellites with base station can be managed more conveniently by the satellite core networks. Due to the continuous change in relative position of LEO satellites, the connections between satellites and ground stations are usually unavailable. It means that satellites are out of the control of the terrestrial core networks most of the time. Satellite core networks can integrate with future access networks which are made up of largescale LEO satellite constellations.

根据3GPP技术报告，卫星将具有基站功能并成为接入网的一部分[6, 26]。接入网依赖核心网提供移动性管理、会话管理等功能。

基于此，卫星5G核心网可从两方面受益:

1. 卫星5G核心网将使用户无需回传即可获取卫星服务
2. 通过卫星核心网，可以更便捷地管理具有基站功能的卫星
   • 由于LEO卫星相对位置的不断变化，卫星与地面站之间的连接通常会中断。这意味着在大多数时间内，卫星都处于地面核心网的控制范围之外
   • 卫星核心网可以与 由大规模LEO卫星星座组成的RAN 集成

2.2 Use Cases

2.2.1 Orbital Edge Computing

Edge computing on LEO satellites is gaining popularity recently [23, 27]. Traditional satellites generally adopt the bent-pipe architecture which does no modifications to the downloaded data. The alternative is using the onboard payloads to do specific processing tasks. Typical applications include remote sensing, in-orbit AI inferencing, and space storage. The satellite core network can help to realize in-orbit edge computing services for mobile users. Additionally, the inter-satellite links (ISLs) can reduce the dependence of constellations on ground stations. Co-orbiting satellites can achieve large-scale and uninterrupted services through collaboration. Deploying satellite core networks in such constellations can ensure higher availability of edge computing services.

LEO卫星上的边缘计算近来正日益普及[23, 27]。传统卫星通常采用“弯管”（bent-pipe）架构，不对下行数据进行任何修改。另一种方案是利用星上载荷来执行特定的处理任务。典型应用包括遥感、在轨人工智能（AI）推理和空间存储。卫星核心网有助于为移动用户实现在轨边缘计算服务。此外，星间链路（ISL）可以降低星座对地面站的依赖性。共轨卫星可通过协同工作实现大规模、不间断的服务。在此类星座中部署卫星核心网，可以确保边缘计算服务具有更高的可用性。

2.2.2 Emergency Communication

The satellite 5G core network provides a better option for emergency communications. On the one hand, satellite communication can solve the problem of emergency communication when the ground infrastructure is damaged. The main benefits of satellite communication are wide coverage and large transmission capacity. when an emergency occurs, the satellite can act as a base station and core network facility to route traffic to any data network. On the other hand, the deployment of satellite core networks can be regarded as an upgrade of satellite communication capabilities. The original remote sensing and experimental satellites will be transformed into satellites with emergency communication capabilities after the deployment.

卫星5G核心网为应急通信提供了更优的选项:

一方面，当地面基础设施受损时，卫星通信可以解决应急通信问题。卫星通信的主要优势在于其广覆盖范围和高传输容量。当紧急情况发生时，卫星可充当基站和核心网设施，将流量路由至任意数据网络。

另一方面，卫星核心网的部署可被视为对卫星通信能力的一次升级。部署后，原有的遥感卫星和实验卫星将转变为具备应急通信能力的卫星。

Overview of The Satellite 5G Core Network

核心内容梳理一下, 同时和 SpaceX D2C 目前的进度进行对比, 以免概念混淆

卫星5G核心网的能力结构分为多个层级:

• 虚拟化层（实线框所示）直接抽象于硬件平台之上, 为核心网中的功能提供支持
• 卫星5G核心网的能力则展示在虚线框中

梳理当前 RAN + CoreNet 上天的系统与dataflow:

(1) 多个子系统协同合作

• 通信子系统负责设置无线链路传输数据或命令。由于下行链路通常具有更高的带宽，卫星核心网的用户平面设计需要考虑这一点
• 卫星5G核心网可以部署在载荷子系统中

(2) 虚拟化

• 作用:
  • 虚拟化层在底层的异构通信, 计算和存储资源之上提供逻辑封装, 方便卫星开发者调用
  • 有助于实现敏捷控制、灵活管理和减少维护成本
• 分层: SDN + NFV
  • NFV构建了可编程的 数据平面, 提供基础网络能力
  • 控制平面 遵循SDN的一般规则, 但 控制逻辑可以分布式部署以适应卫星的高度动态特性
  • 上层管理平面应采用 cloud-native 设计, 利用 serverless 和 功能粒度的服务 来最大限度地利用稀缺的星上资源并消除冗余

(3) 5G核心网的基础功能 && 架构设计

• UPF
  • 作为移动基础设施和数据网络之间的互连点, 充当不同移动性会话的anchor
  • 星上UPF可以处理数据包路由和转发. 可以扮演上行链路分类器的角色, 将流量分流到不同的数据网络, 实现将用户流量导向星上服务
• AMF
  • 问题1: 卫星与用户终端之间的高速相对运动可能导致信令风暴和频繁注册
    • 解法: 状态-功能-位置解耦方法, 消除了星上AMF作为移动性锚点的需求, 将移动性管理交给UE处理 (IEEE Internet Computing 24)
  • 问题2: 妥善处理星上AMF的基础设施移动性
    • 解法: 需要新的切换和注册机制
• SMF
  • 问题: LEO 有高度动态的拓扑结构/负载波动, 如何给用户规划合理可靠的 data path

架构设计:

尤其注意, 这篇文章的设计架构是 RAN 和 CoreNet 都上天!!!

本文 与 目前SpaceX-D2C 的架构差异

二者采用了完全不同的架构设计:

• SpaceX: 将 RAN (gNB/eNodeB) 搬到了卫星上，但将 CoreNet 留在了地面
• 本文: RAN 和 CoreNet 都上星

SpaceX-D2C 的 Dataflow:

1. 手机 -> 卫星: 用户（UE）使用一部未经修改的标准 4G (LTE) 或 5G 手机，直接向LEO Starlink D2C 卫星发送信号
2. 卫星 (作为基站): D2C 卫星接收信号。卫星上搭载了先进的 "eNodeB / gNB 调制解调器", 使其能够充当一个“太空中的移动基站”
3. 卫星 -> 卫星: 卫星通过其搭载的 "激光回程" (Laser Backhaul) 功能, 将数据在 Starlink 卫星星座之间进行高速传输 by ISL
4. 卫星 -> 地面站: 数据流最终从卫星星座下行传输到地球上的 "Starlink 地面站" (Ground Station / Gateway)
5. 地面站 -> MNO 核心网: 地面站通过地面网络, 连接到 SpaceX 的合作伙伴移动网络运营商（MNO）—— 例如: T-Mobile
6. MNO 核心网 -> 互联网: 数据进入 T-Mobile 的地面核心网 (Core Network), 然后像处理标准漫游数据一样, 将其连接到互联网

(4) 5G核心网拓展功能

虚拟化技术通过提供标准接口以可编程方式操作资源, 支持实现这些高级特性, 使卫星在同一硬件基础上提供多种服务. 如: 为终端用户和业务场景提供差异化能力

1. 策略控制功能 (PCF): 保障QoS. 本质上就是保障数据流, 因此需要保障“内容分发流量”
2. 网络切片 (Net Slicing):
   • 通过端到端划分，网络可以定制化以支持专用服务
   • Challenge: 切片必须保持卫星移动性, 因此需要“虚拟静止性” [来自ETH HotNets'20]
3. 网络能力开放功能 (NEF):
   • 第三方企业可以按需组合或定制应用功能
   • NEF将为不同类型的卫星服务提供多种解决方案

System Design

Background:

3GPP架构分类: 3GPP针对非地面网络定义了两种基于有效 payload type 的架构

• 透明有效载荷 (Transparent Payload): 卫星充当信号中继器，在用户设备（UE）和基站（BS）之间转发信号
  • "无情的转发机器" / "bent-pipe架构中唯一的relay"
• 再生有效载荷 (Regenerative Payload): 卫星可以再生从地面接收到的信号
  • 这种信号再生能力由星上处理能力支持，从而使 gNB卫星 (具备基站功能的卫星) 和 星间链路（ISL） 的应用成为可能

系统集成与部署架构:

• 未来网络需求: 系统设计不仅要参考3GPP标准化活动，还需考虑未来 "空-天-地" 集成网络的需求
• 功能下沉至边缘:
  • 为了实现泛在接入, 卫星核心网的部分功能必须被拆分并下沉到边缘
  • 以满足时间敏感控制、自主管理，和便捷能力开放的要求
• 混合部署: 5G核心网可同时部署在 卫星 和 地面数据中心
  • 部署灵活性: 通过控制平面和用户平面的解耦，5G核心网支持灵活部署，使其部署位置不依赖于特定的轨道
• 功能角色分离:
  • 部署在低轨道（LEO）卫星上的5GC: 可提供移动性管理、接入控制，以及简单的路由和会话管理协调
  • 部署在地球静止卫星（GEO）上的5GC: 可为星座范围的服务协调提供服务注册和发现

[1] 架构设计:

[2] 5GC核心操作 DataFlow:

Conclusion

5G technology and satellite systems are complementary in function and demand. The integration of the two will bring huge economic and social benefits. As the control center of 5G systems, satellite 5G core networks will play an important role in the satellite-terrestrial integrated network. This article looks forward to the cutting-edge development direction of the satellite-terrestrial integrated network. It puts forward key points and a feasible architecture for the implementation of the satellite 5G core network. Finally, we carried out the deployment and experimental verification of the satellite 5G core network on an orbiting satellite. According to the results, the satellite 5G core network can operate normally and perform the coordination between the satellite and the ground infrastructure. Our experiment also serves as a reference to future research on B5G and 6G satellite-terrestrial integrated communication networks.

5G技术与卫星系统在功能和需求上具有互补性，两者的融合将带来巨大的经济效益和社会效益。作为5G系统的控制中心，卫星5G核心网将在星地融合网络中发挥重要作用。

本文展望了星地融合网络的前沿发展方向，提出了卫星5G核心网的实施要点和可行的架构。最后，我们在一颗在轨卫星上进行了卫星5G核心网的部署和实验验证。

结果表明，卫星5G核心网能够正常运行，并完成星地协同工作。实验也为未来B5G和6G星地融合通信网络的研究提供了参考。