Preliminary for LEO Satellite Networks

A. Integrated satellite and terrestrial network (ISTN)

Typically, a ISTN contains a space segment together with a ground segment [42]. In the space segment, several companies have been eager to file plans to build inclined orbit mega-constellations with thousands of LEO satellites, such as Starlink [2], Kuiper [1], OneWeb [3], etc. The space segment includes ISLs that interconnect satellites. In the ground segment, many companies are accelerating the construction of geo-distributed GSes to support ISTN. SpaceX has currently built more than 50 GSes [6] and announced their cooperation with Google to install GSes at Google data centers [18]. The ground segment also includes user terminals, such as Starlink Kit [7], especially for users in remote areas.

一个典型的天地一体化网络（ISTN）包含一个空间段和一个地面段 [42]。在空间段，多家公司已积极提交计划，旨在构建由数千颗低轨卫星组成的倾斜轨道巨型星座，例如Starlink [2]、Kuiper [1]、OneWeb [3]等。

• 空间段包含用于互联卫星的星间链路（ISLs）
• 在地面段，许多公司正在加速建设地理上分布式的地面站（GSes）以支撑天地一体化网络

SpaceX目前已建成超过50个地面站[6]，并宣布与谷歌合作，在谷歌数据中心安装地面站[18]。地面段还包括用户终端，例如为偏远地区用户特供的星链套件（Starlink Kit）

B. Current satellite-ground topology design in ISTN

While currently most mega-constellations are still under heavy construction and it is still unclear how they will interconnect LEO satellites in their final stage, it is undoubted that the network topology design will play a vital role in the attainable network performance (e.g. latency, throughput) of the ISTN [11], [13], [26], [27]. Recent efforts [11], [13] have studied the design of the inter-satellite topology of ISTN to enable low latency data transmission. However, in the early stage of ISTN, few existing works focus on the design of satellite-ground topology due to three facts as described below.

尽管目前大多数巨型星座仍处于大规模建设阶段，其最终阶段将如何互联低轨卫星尚不明确，但毫无疑问，网络拓扑设计将对天地一体化网络可达的网络性能（如时延、吞吐量）起到至关重要的作用[11], [13], [26], [27]。近期的工作[11], [13]已对天地一体化网络的星间拓扑设计进行了研究，以实现低时延数据传输。然而，在天地一体化网络的早期阶段，鲜有工作关注星地拓扑的设计，这可归因于以下三个事实。

Geostationary satellite orbit. The first generation of SNs exploits geostationary (GEO) satellites to provide Internet services, like ViaSat [8] and Inmarsat [5]. GEO satellites can remain stationary with the terrestrial networks, making the topology design static and similar to terrestrial networks.

LEO constellations in limited scale. LEO constellations, represented by Iridium [38] and Globalstar [20], ushered its first wave of development before 2000s. However, LEO constellations in the early years, consisting of no more than 100 satellites, are small in scale. Only a very limited number of satellites are in line-of-sight for every GS, making satelliteground topology design space very small.

Bent-pipe space-ground architecture. Satellites used to communicate using a bent-pipe architecture, where satellites relay a data stream [22] and do not participate in network routing calculation. The influence of satellite-ground topology is limited to one hop between the satellite and GS, making the design criteria in existing works very unitary.

1. 地球静止轨道卫星：第一代卫星网络利用地球静止轨道（GEO）卫星来提供互联网服务，如 ViaSat [8] 和 Inmarsat [5]
   • GEO卫星能与地面网络保持相对静止，这使得拓扑设计是静态的，且与地面网络相似
2. 规模有限的低轨星座：以铱星（Iridium）[38]和全球星（Globalstar）[20]为代表的低轨星座，在21世纪前迎来了其第一波发展浪潮
   • 然而，早期的低轨星座规模较小，卫星数量不超过100颗
   • 对于每一个地面站而言，仅有极其有限的卫星处于其视线范围内，这使得星地拓扑的设计空间非常狭小
3. “弯管”式空地架构：过去，卫星常使用“弯管”（bent-pipe）架构进行通信，即卫星仅作为数据流的中继[22]，而不参与网络路由计算
   • 在这种模式下，星地拓扑的影响仅限于卫星与地面站之间的一跳，导致现有工作中的设计准则非常单一