Accelerating Handover in Mobile Satellite Network

Abstract

The construction of Low Earth Orbit (LEO) satellite constellations has recently spurred tremendous attention from academia and industry. 5G and 6G standards have specified LEO satellite network as a key component of 5G and 6G networks. However, ground terminals experience frequent, high-latency handover incurred by satellites’ fast travelling speed, which deteriorates the performance of latency-sensitive applications. To address this challenge, we propose a novel handover flowchart for mobile satellite networks, which can considerably reduce the handover latency. The innovation behind this scheme is to mitigate the interaction between the access and core networks that occupy the majority of time overhead by leveraging the predictable travelling trajectory and spatial distribution inherent in mobile satellite networks. Specifically, we design a fine-grained synchronized algorithm to address the synchronization problem due to the lack of control signalling delivery between the access and core networks. Moreover, we minimize the computational complexity of the core network using information such as the satellite access strategy and unique spatial distribution, which is caused by frequent prediction operations. We have built a prototype for a mobile satellite network using modified Open5GS and UERANSIM, which is driven by actual LEO satellite constellations such as Starlink and Kuiper. We have conducted extensive experiments, and the results demonstrate that our proposed handover scheme can considerably reduce the handover latency compared to the 3GPP Non-terrestrial Networks (NTN) and two other existing handover schemes.

低轨道（LEO）卫星星座的构建近年来在学术界和工业界引发了极大关注。5G 与 6G 标准已将 LEO 卫星网络指定为下一代通信网络的重要组成部分。然而，由于卫星高速运行，地面终端频繁经历高延迟的切换过程，严重影响了对时延敏感应用的性能。为应对此挑战，本文提出了一种适用于移动卫星网络的创新切换流程图，能够显著降低切换时延。该方案的核心创新在于 利用移动卫星网络中可预测的运行轨迹与空间分布特性，削弱接入网与核心网之间的交互开销，而这部分通常占据了切换过程的大部分时间开销。

具体而言，本文设计了一种细粒度同步算法，用于解决接入网与核心网之间因控制信令无法及时传递而导致的同步问题。此外，结合卫星接入策略及其所具有的独特空间分布规律（源于频繁的轨迹预测操作），本文进一步降低了核心网侧的计算复杂度。我们基于实际的 LEO 星座（如 Starlink 和 Kuiper），对 Open5GS 和 UERANSIM 进行了修改，构建了一个移动卫星网络原型系统。大量实验结果表明，所提出的切换方案相较于 3GPP 的非地面网络（NTN）方案以及其他两种现有切换方案，在降低切换时延方面表现出显著优势。

Introduction

The Internet service provided by Low earth orbit (LEO) satellites such as Starlink has been emerged rapidly [1]. The satellite internet can provide global coverage and thus is a valuable complement to traditional terrestrial network. To this end, 3GPP 5G and 6G standards have specified that satellite communication is a key component of the whole system to establish the mobile satellite network [2]–[4], where several well-known telecommunication operators like T-Mobile collaborate with satellite network providers to provide world-wide communication services to users by directly building the link between satellites and mobile phones [5], [6].

低轨道（LEO）卫星（如 Starlink）所提供的互联网服务近年来快速发展［1］。卫星互联网具备全球覆盖能力，因此可作为传统地面网络的重要补充。为此，3GPP 在其 5G 和 6G 标准中已将卫星通信明确列为构建移动卫星网络的关键组成部分［2］–［4］。当前，已有多家知名通信运营商（如 T-Mobile）与卫星网络服务商展开合作，尝试通过在卫星与移动终端之间直接建立连接，为全球用户提供通信服务［5］、［6］。

There exist two operating modes in mobile satellite network, respectively as the transparent mode and the regenerative mode [7]. Initially, most satellites adopt the transparent mode, as shown in Fig. 1a. In this case, satellites serve as transparent physical nodes between ground terminals. However, operating in this mode suffers from several obvious shortcomings such as limited coverage, single-point bottleneck, and relatively high latency [8]. Recently, the regenerative mode, where LEO satellites act as base stations and provide worldwide coverage by leveraging inter-satellite links (ISLs), has been proposed and attracts extensive attention, as described in Fig. 1b. Meanwhile, mobile satellite networks operating in the regenerative mode have been in test [9]. In the following discussion, we focus on the regenerative mode and refer to these satellites as S-gNB (Satellite next Generation NodeB).

移动卫星网络主要存在两种运行模式：透明转发模式（transparent mode）与再生模式（regenerative mode）［7］。最初，大多数卫星采用透明转发模式，如图 1a 所示，在该模式下，卫星仅作为地面终端之间的透明物理转发节点。然而，该模式存在显著局限，如覆盖范围有限、存在单点瓶颈以及相对较高的通信时延［8］。近年来，再生模式逐渐受到关注。在该模式下，LEO 卫星充当基站，通过星间链路（ISL）实现全球覆盖，如图 1b 所示。目前，基于该模式的移动卫星网络也已开始进入测试阶段［9］。本文后续内容将聚焦于再生模式，并将这些卫星称为 S-gNB（Satellite next Generation NodeB）。

However, there exists a critical challenge in the mobile satellite network, i.e., handovers are triggered frequently due to the fast travelling speed of satellites. Different from the terrestrial network, the S-gNB, as a crucial infrastructure for user access located at LEO satellite, travels at a high speed and is usually far away from the core network. Consequently, ground terminals experience handover between two satellites every 2-5 minutes, resulting in an average handover latency of around 400 ms. This high-latency handover declines the user experience, especially for the latency-sensitive applications.

然而，移动卫星网络面临一个关键挑战： 由于卫星高速运行，用户频繁触发切换。与地面网络不同，S-gNB 作为驻留于 LEO 卫星上的关键接入基础设施，与核心网之间存在物理距离远、移动速度快的特性。

因此，地面终端平均每 2–5 分钟便需在两颗卫星间完成一次切换，平均切换时延约为 400 毫秒。这一高时延切换严重影响了用户体验，尤其是对于对时延敏感的应用场景。

Naturally, to deal with this challenge, one may wonder why not adopt the seamless handover strategy named soft handover designed in 3G [10] and dual active protocol stack (DAPS) in 5G [11]. The core idea of above handover schemes is to support multiple parallel links with different base stations, which brings multiple times of hardware overhead [12]. With regard to this factor, seamless handover is inappropriate and thus not applied in 5G communications. In mobile satellite network, maintaining multiple parallel links between the ground terminal and several satellites exacerbates the hardware overhead in order to mitigate the high channel loss (up to 160 dB) [13], thus substantially increasing the system complexity.

自然地，有人可能会提出是否可以采用 3G 中的软切换（soft handover）［10］或 5G 中的双活协议栈（DAPS, Dual Active Protocol Stack）［11］来解决此问题。这类无缝切换策略的核心思想是在多个基站间建立并维护并行连接，但这一机制会带来额外的硬件开销［12］。因此，在 5G 中并未实际应用无缝切换策略。在移动卫星网络中，为了抵抗高达 160 dB 的信道损耗［13］，在地面终端与多颗卫星间维持并行连接会进一步放大硬件负担，并显著增加系统复杂性。

In this paper, inspired by the concept of computing in the network [4], [14], we design a novel handover flowchart for mobile satellite network, which reduces handover latency through adding additional computation in the core network. The innovations behind the proposed scheme are to achieve handover only involving the interaction between the UE and access network (i.e., LEO satellites) based on the predictable travelling trajectory and unique spatial distribution of LEO satellites. In this way, interaction between the access and core networks that accounts for the majority of time overhead in the handover procedure is avoided since handover signaling transmission from the access network to core network should pass through multiple satellites and then to the ground.

受网络内计算（computing in the network）理念的启发［4］、［14］，本文设计了一种适用于移动卫星网络的全新切换流程图，通过 在核心网引入附加计算以降低切换时延。

该方案的创新之处在于： 基于 LEO 卫星可预测的运行轨迹与空间分布特性，将切换操作限制在用户设备（UE）与接入网（即卫星）之间 ，从而避免了传统方案中需经由多颗卫星转发至地面核心网的控制信令交互，而这一交互通常是造成高切换时延的主要来源。

The implementation of the proposed handover scheme entails two main challenges. First, without control signaling interaction, it is non-trivial to maintain strict synchronization between the access and core networks because the core network has no access to the handover triggering time. Secondly, the proposed handover scheme requires predicting access satellites for all UEs based on the predicted trajectories, which imposes substantial computation overhead on the core network. To address the first challenge, we propose a fine-grained synchronized algorithm, where two specific time points are set to determine the access satellite of the UE. Meanwhile, we leverage the access strategy and spatial distribution features in LEO satellite networks to reduce the number of UEs and satellites required for prediction, thus considerably alleviating the computing pressure.

Finally, we have built a prototype for achieving handover in mobile satellite network. This prototype mainly consists of modified UERANSIM and Open5GS and is driven by real LEO satellite traces including Starlink and Kuiper [15]. Based on this prototype, we have conducted extensive experiments and results verify that the proposed handover scheme can considerably reduce the handover latency (around 10×) and improve the user-level performance like TCP compared to three existing handover strategies. The detailed implementation of our built prototype for mobile satellite network will be published later.

该切换方案的实现面临两项主要挑战。首先，在缺乏控制信令交互的前提下，如何确保接入网与核心网之间的时间同步较为复杂，因核心网无法获知切换触发的具体时间点。其次，该方案需基于卫星预测轨迹对所有用户设备的接入卫星进行预测，从而给核心网带来显著的计算负载。为解决第一个挑战，我们提出了一种细粒度同步算法，通过设置两个具体时间点来确定 UE 的接入卫星。而针对第二个挑战，我们利用卫星接入策略与空间分布特性，减少需预测的 UE 与卫星数量，从而有效缓解了计算压力。

最后，我们构建了一个用于移动卫星网络切换的原型系统。该原型系统基于 UERANSIM 和 Open5GS 进行修改，并采用了 Starlink 和 Kuiper 等真实 LEO 卫星轨迹数据作为驱动［15］。基于此系统，我们开展了大量实验。实验结果表明，所提出的切换方案可将切换时延降低约一个数量级（约 10 倍），并在用户层性能（如 TCP）上显著优于现有三种切换策略。该原型的具体实现将在后续工作中详细发布。

Additionally, we have also evaluated the performance of the prediction algorithm and the impact of user mobility. The results validate the feasibility of the proposed handover scheme in large-scale mobile satellite network.

此外，我们还评估了所用预测算法的性能及用户移动性对方案的影响。实验结果验证了该方案在大规模移动卫星网络中的可行性。

The contributions of this paper can be summarized as:

• To the best of our knowledge, this work represents the first research efforts to address the high latency problem of handover in mobile satellite network, which is an integral part for 5G and beyond NTN.

• We for the first time demonstrate how to decouple the interaction with the core network from the handover procedure by leveraging several intrinsic features in LEO satellite network such as predictable satellite trajectory and unique spatial distribution.

• We have built an experimental prototype and conduct extensive experiments which is driven by real satellite traces including Starlink and Kuiper. Results verify that the proposed handover scheme can reduce the handover latency by around 10× compared to three existing handover strategies.

The rest of this paper is structured as follows. Section II introduce the background of the problem and our motivation. Section III gives an overview of our design. Section IV provide detailed explanations of the two aspects of our design. Section V describes our experimental setup and result. Section VI presents a review of related work in the field. Section VII discusses additional considerations and issues related to our work. Finally, Section VIII briefly concludes this work.

本文的主要贡献如下：

1. 据我们所知，这是首个专注于解决移动卫星网络中切换高时延问题的研究工作，而该问题是 5G 及未来非地面网络（NTN）中的关键挑战之一
2. 本文首次展示了如何通过利用 LEO 卫星网络中可预测的轨迹与空间分布等固有特性， 实现切换流程中与核心网解耦的设计
3. 我们搭建了一个由真实卫星轨迹（包括 Starlink 与 Kuiper）驱动的实验原型，并进行了系统评估。结果表明，该方案相较于现有三种切换策略可将切换时延降低约 10 倍。

本文的结构如下：第二节介绍问题背景与研究动机；第三节概述我们的设计方案；第四节详细解释设计的两个关键部分；第五节描述实验设置与结果；第六节回顾相关领域的研究工作；第七节讨论额外的设计考量；第八节对全文进行总结。

Note

核心网间切换成本太高, 现在将“切换”这个动作变成 UE-RAN 的, 而非传统的 RAN-CoreNet