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LEO Satellite Constellations for 5G and Beyond:How Will They Reshape Vertical Domains?

Shicong Liu, Zhen Gao*, Yongpeng Wu, Derrick Wing Kwan Ng, Xiqi Gao, Kai-Kit Wong, Symeon Chatzinotas,and Bjorn Ottersten

Abstract—The rapid development of communication technolo-gies in the past decades has provided immense vertical oppor-tunities for individuals and enterprises. However, conventionalterrestrial cellular networks have unfortunately neglected thehuge geographical digital divide, since high bandwidth wirelesscoverage is concentrated to urban areas. To meet the goalof “connecting the unconnected”, integrating low Earth orbit(LEO) satellites with the terrestrial cellular networks has beenwidely considered as a promising solution. In this article, wefirst introduce the development roadmap of LEO satellite con-stellations (SatCons), including early attempts in LEO satelliteswith the emerging LEO constellations. Further, we discuss theunique opportunities of employing LEO SatCons for the deliveryof integrating 5G networks. Specifically, we present their keyperformance indicators, which offer important guidelines forthe design of associated enabling techniques, and then discussthe potential impact of integrating LEO SatCons with typical5G use cases, where we engrave our vision of various verticaldomains reshaped by LEO SatCons. Technical challenges arefinally provided to specify future research directions.

I. INTRODUCTION

THE unprecedented development of communication tech-nology in the past few decades has demanded an expo-

nential growth in the overall transmission rate [1], [2]. Unlikeprevious mobile communications, from 1G to 4G, 5G networksaim for ubiquitous connections to help digitize the economyand contribute towards the global digital transformation, ratherthan barely improving the overall data rate. The driving forcefor further beyond 5G (B5G) even 6G research is the demand-intensive vertical industries, which can be classified as oneor a mix of the following usage scenarios: enhanced mo-bile broadband (eMBB), massive machine type communica-tions (mMTC), and ultra reliable low latency communications(URLLC) [3].

Affected by the COVID-19 pandemic, vertical applicationssuch as media, public safety, and eHealth have attractedwidespread attention, and are expected to become a newnormality in the future. According to the report of internationaltelecommunication union (ITU), nearly half of the populationstill remains unconnected [4], whereas eliminating the blindareas by deploying much more terrestrial base stations (BSs)will impose a heavy burden on the telecom operators. Onthe other hand, the coverage of non-deployable areas suchas aero and maritime lanes remains to be solved. Againstthe aforementioned constraints, space-based satellite constel-lations (SatCons) emerge as the times require.

*: Corresponding author.

With the emerging concept referred to as mega-constellations [5], the era of seamless connectivity is more re-alistic than ever. Tremendous amount of satellites in low Earthorbit (LEO), medium Earth orbit (MEO), and geostationaryEarth orbit (GEO) can collaborate together forming SatCons,which completely changes the static topology in conventionalterrestrial networks and enables flexible network deployment.Despite the fact that 3 GEO satellites are enough to provideseamless coverage across the globe, the associated latency andlow area spectral efficiency are generally intolerable in most5G application scenarios. Hence, exploiting LEO satellitesbecomes a better option as operating on a lower orbit provideslower latency and higher service density.

In recent years, new launching and propulsion technologieshave drastically reduced the cost, and advanced volume manu-facturing technologies have paved the way for the deploymentof mega constellations, especially for LEO satellites. For LEO,the lower orbital altitude leads to a lower latency for delay-sensitive tasks such as video calls and streaming, while thehigh density constellation guarantees service capacity globally.Therefore, the amalgamation of satellite access and conven-tional terrestrial networks is an inevitable convergence for B5Gand even 6G in meeting the increasing demands.

After this introduction, the article first introduces theroadmap of LEO SatCons development, after which the oppor-tunities of employing LEO SatCons are discussed. We furtherdiscuss the technical challenges and highlight the researchdirections, and finally summarize our conclusion.

II. DEVELOPMENT ROADMAP OF LEO SATCONS

The idea of communicating via commercial LEO satellitesdates back to the early 1990s. Since the earliest launch in 1957,satellites used for data transmission are mainly GEO satellitesand the majority of them served as data broadcasting satellites,whose applications include satellite TV programs, positioning,etc. With the increasing demands for higher system capacityand lower latency, the concept of constellation composed ofLEO satellites has been proposed [5]. Early designs of LEOsatellites typically included a small number of satellites toensure that every part of Earth is covered by at least onesatellite. In this section, we provide a comprehensive overviewand comparison on the existing SatCons. Table II gives abrief summary of the most well-known commercial satellitecommunication systems that utilize constellations to supportwide-area coverage.

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TABLE ICOMPARISON OF SELECTED CONSTELLATIONS

Constellation Regime Orbital height Quantity Bands Services Est. data rateIridium Gen. 1 LEO 781 km 66 L Voice, data 2.4 Kbps

Globalstar LEO 1414 km 48 S, L Voice, data ∼9.6 KbpsOrbcomm Gen. 1 LEO 700 ∼ 800 km 36 VHF IoT & M2M∗ communication 2.4 Kbps

Skybridge LEO 1457 km 64 Ku Broadband Internet 60 MbpsTeledesic LEO 1375 km 288(840) Ka Broadband Internet 64 Mbps

Iridium NEXT LEO 781 km 66 L, Ka Voice, data 1.5 Mbps, 8 MbpsOrbcomm Gen. 2 LEO 700 ∼ 800 km 18 VHF IoT & M2M communication 4.8 Kbps

O3b MEO 8063 km 20 Ka Broadband Internet 500 MbpsOneWeb LEO 1200 km 648 Ku Broadband Internet 400 Mbps

Starlink Gen. 1LEO

335.9 ∼ 570 km 11926 VBroadband Internet 100 Mbps

Starlink Gen. 2 328 ∼ 614 km 30000 Ku, Ka, ETelesat Phase. 1

LEO 1015 ∼ 1325 km298

Ku, Ka Broadband Internet -Telesat Phase. 2 1373

Hongyan LEO 1100 km 320 L, Ka Voice, broadband Internet 100 MbpsKuiper LEO 590 ∼ 630 km 3236 Ka Broadband Internet -

* : Internet of Things and machine to machine.

A. Overview of Early LEO Satellites

In the late 20th century, network operators and companieshave made their initial attempts to deploy LEO constellations.Although most of the early attempts ended with bankruptcy,their experiences provide valuable lessons for the ventures.

Iridium went into operation by Motorola in 1998 as oneof the earliest LEO constellations adopting the Walker con-stellation pattern. 11 satellites are uniformly distributed on 6orbital planes with a height of 780 km, and are able to workin both time division multiple access (TDMA) and frequencydivision multiple access (FDMA) modes.

Different from Iridium, Globalstar adopted the WalkerDelta pattern, which is also known as Rosette, achieved thebest coverage diversity at mid-latitudes, but no coverage atpoles, and became commercially available in late 1998. 48LEO satellites are designed at the altitude of 1414 km adoptingcode division multiple access (CDMA) for satellite phones andnarrow band data communication services.

Teledesic also started its operation in 1998 and initiallyattempted to build the largest constellation with 840 satel-lites at the altitude of 700 km supporting the space-basedbroadband Internet access. After several reorganizations andmerging, Teledesic progressively scaled down the number ofits satellites to no more than 288.

The past verticals were underdeveloped to nurture a matureLEO commercial operation in the last century. Therefore, inthe beginning of 21st century, the aforementioned early trialseventually lost their business in competition with the relativelymature terrestrial networks.

B. State-of-the-Art Emerging Constellations

Current flourishing verticals are booming a huge LEOmarket, which can further fuel the growth of the verticals.Particularly, with the development of terrestrial networksgradually approaching the bottleneck period, the unresolvedvertical issues are left to the emerging SatCons.

Starlink intends to launch around 42000 LEO satellites,which make up the largest constellation ever, on severaldifferent altitudes of orbits. At the initial stage of arrange-ment, around 12000 satellites with 2 groups of orbital height(335.9 ∼ 345.6 km for 7518 satellites and 540 ∼ 570 kmfor 4408 satellites) will be deployed, whereas 30000 moresatellites awaiting approval will be deployed at the finalstage. As an emerging SatCon, it is the first constellationthat has applied for the use of V-band and E-band to supportits advanced services in broadband Internet access globally,including commercial, residential, and governmental scenarios.

Apart from Starlink, several developing or designing LEOSatCons are in progress. OneWeb1 targets at seamless globalcoverage with customer demonstrations and commercial ser-vices in the upcoming future, and has launched 74 of 648planned satellites to date. It is expected to provide up to400 Mbps downlink and 30 Mbps uplink rate through specialuser terminals (UTs) or gateways. On the other hand, Telesatalso seeks to provide global coverage and high throughputbroadband capacity on LEO constellation in polar and inclinedorbits. Its first LEO satellite was launched in 2018, and isnow supporting live demonstrations of satellite services acrossa variety of markets and applications. The project Kuiperis an initiative that proposes to build an LEO constellationwith 3236 satellites for providing broadband connectivity tothe uncovered users. The related detail of other constellationcompetitors can also be found in Table II. Moreover, wepresent the typical network architecture of LEO SatCons andcommon UT-SatCon access modes in Fig. 1.

In recent years, integrating 5G with LEO SatCons hasbecome a trending topic. The low altitude characteristic ofLEO SatCons reduces latency, and the massive deploymentachieves global coverage with high service density, whichcan be an innovative component in building future wireless

1OneWeb also went bankrupt in March 2020 due to the high cost of man-ufaturing and launching satellites, while seeking to expand the constellationto up to 48000 satellites after being taken over by the new owners.

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Optical fiber

Service link

Inter-satellite link

Feeder link

Optical fiber

Service link

Inter-satellite link

Feeder link

UTs

Terrestrial BS

Satellite Receiver

GatewayCore Network

LEO SatCon

Satellite Receiver

Fig. 1. A schematic diagram of the network architecture of LEO SatConsand the common UT-SatCon access modes.

networks.

III. OPPORTUNITIES OF LEO SATCONS FOR 5G ANDBEYOND

In this section, we will provide the motivation of adoptingLEO constellations, and discuss the impact on typical usecases as well as how they will reshape the vertical domains.

A. Motivation of Integrating LEO SatCons

To explore the unique advantages of applying LEO Sat-Cons over terrestrial 5G networks, we first discuss the keyperformance indicators (KPIs) of space-based wireless net-work. These unique advantages motivate us to integrate LEOSatCons with B5G systems to achieve better performances.Specifically, these KPIs are discussed as follows.

1) Latency: The orbital altitude determines the end-to-endcommunication latency. Extremely high altitude brings widecoverage, while the round-trip propagation latency (e.g., upto 240 ms via GEO satellites) becomes intolerable under therequirements of current 5G standard. By contrast, the altitudeof some of the latest constellations such as Starlink is as low as320 km. As a result, the corresponding round-trip air interfacelatency is reduced to around 3 ms, which well meets thecontrol plane round-trip latency requirements for most eMBBand mMTC usage, especially the IoT-based vertical scenarios.

2) Reliability: Reliability is usually measured by the errorrate of communication links, and is crucial to various verticalapplications (e.g., eHealth and finance). In addition to errorrate, outage time is another manifestation of reliability ineMBB and URLLC scenarios. As the scale of constellationsincreases, users can be served by more satellites simultane-ously, which significantly improve the reliability thanks to thespatial diversity gains.

3) Data rate: In current standards, data rate for digitalvideo broadcasting is around 30 Mbps, while the customizedstreaming rate is far limited, which can be seen in earlyattempts of satellite voice services shown in Table. II. In recentyears, the data rate of customized streaming supported byconstellations is increasingly required, hence it is important toreconsider the position of data rate in SatCon constructions.

4) Availability: Availability is one of the most importantKPIs for satellite communications, which refers to the proba-bility that at least one satellite is visible to the UT. Deployingmore LEO satellites implies an improved availability, whichwill significantly contribute to the quality of service (QoS).

5) Cost: Cost is arguably the biggest challenge to prof-itability and long-term viability. Due to the financial burdenfrom manufacturing, launching, operating, and so on, severalcompanies have gone bankrupt in the early 21st century.Fortunately, the recently developed satellite technologies andthe soaring demand for ubiquitous broadband Internet accesswith promising market profits have fueled the development ofaffordable and healthy operation of LEO constellations.

6) Mobility: Due to higher frequency bands and harshterrain, smaller cellular coverage and frequent migration viapublic transportation have caused mobility issues. The problemof mobility management in SatCons is more complicated thanthat of terrestrial networks, since the mobile entities includenot only the UTs but also the high-speed moving satellites,which will pose the paging and handover issues as well asdynamic varying Doppler shifts. Therefore, effective mobilitymanagement is another important KPI for LEO SatCons.

7) Connectivity density: An unprecedented number ofsatellites can be exploited to support high density of services,especially in rural areas. In recent year, the massive number ofmachine-type nodes has been increasing rapidly, which makesthe connection density as a KPI. However, the terrestrial BSsare insufficient in rural areas and pathless areas, where thelow-cost satellite receivers can be deployed to accommodatemassive connectivity.

8) Survivability: Fragile optical cables in terrestrial cellularnetworks can be easily destroyed by natural disasters, leadingto frequent network paralysis. By contrast, the SatCons havethe superiority of survivability against natural disasters owingto the highly flexible topology and wireless access, whichplays a vital role in disaster recovery.

B. Impact on B5G Use Cases and Future Visions

As mentioned before, the huge geographical digital divideissue [6] caused by harsh terrain environments in conventionalterrestrial cellular networks needs to be addressed. B5G oreven 6G integrated with the space-based LEO constellationsis envisioned to reduce the digital divide. Benefited from thelower latency, higher reliability and data rate, as well as denserconnectivity and vast coverage, LEO SatCons are anticipatedto reshape the traditional vertical domains and promise amyriad of opportunities for rising industries. Below we discussthe potential impact of LEO SatCons on typical use cases asshown in Fig. 2.

1) Industrial IoT: Industrial IoT (IIoT), also known asIndustry 4.0, is the combination of massive IoT nodes withindustry applications. Basically, IIoT can be summarized assensor-equipped industrial machines connected via privatenetwork with other supporting devices, and is suffering frompiles of issues such as network topology, security, distributedmanufacturing, personalized assembling, and cognitive supplynetworks. Overcoming these problems relies on fine-grained

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Manufacturing

Logistics

Vehicular

Communication

eHealth

Smart City

Energy

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Fig. 2. Ambitious vision of various vertical domains reshaped by LEO SatCons.

connectivity and high-accuracy positioning services, whichhave to require massively deployed terrestrial BSs. By contrast,LEO SatCons is a viable approach to solve these challengesand an enabling technology for business-to-business (B2B)promotion.

2) Agriculture: Compared with industry, agriculture reliesmore on the convenience brought by the LEO SatCons. Thecultivated land is typically located in the vast and sparselypopulated areas, which brings the burden of cost for opticalcables deployment. As more machine-type nodes are utilizedin agricultural practice, deploying LEO satellites for crops andlivestock monitoring and unmanned operations is an inevitabletrend, and will significantly contribute to the productivity inagriculture.

3) eHealth: Public heath emergencies have stimulatedglobal attention on medical services, which along with dig-italization, virtualization, and decentralisation constitute themajor trend towards eHealth. Among the most frequentlycited issues being standardizing progressively in the roadmap,ubiquitous access and lifestyle evolution can be settled by LEOSatCons. With the intensification of decentralization, medicalresources can be accessed from trusted entities such as SatConoperators. On the other hand, as the wearable devices becomepopularized, SatCons are still one of the most competitivesolution for seamless health data monitoring and analysis inreal time.

4) Energy: The newly discovered fossil energy is generallydistributed in remote areas and maritime areas, which are usu-ally far from the 5G terrestrial cellular coverage. Nevertheless,LEO SatCon is the most suitable solution to support intelligentand remote operations. Most of the operation environment,such as the offshore plaform and the radioactive environment,is not compatible for long-term onsite in person operations.Therefore, the full information and necessary control via LEOconstellations will greatly promote the development of energyfields.

5) Intelligent Vehicular Networks: As the LEO SatConsbecome popularized thanks to the advanced manufacturing and

low-cost launching techniques, elimination of coverage deadzones in air lines, sea lanes, and high-speed rails becomes areality. Employing new antennas tailored for LEO SatCons, theairplanes, vessels, and high-speed trains can instantaneouslybecome new dynamic nodes in the satellite network topology.Network coverage can be unprecedentedly wide, whereas thelink performance such as reliability, capacity, and latency willbe much better than before. The enhanced vehicular networkcan fulfill the requirements raised by eMBB, and further makesit possible to deal with trading, streaming, and monitoring.

6) Remote Interactions: Remote interactions including ed-ucation, online-work, online-entertainment and shopping forisolated and rural areas is not a novel idea, but it is notuntil the pandemic that these remote applications have gainedmost public attention. Global coverage via LEO SatCons withlow latency and wide bandwidth paves the way for people inrural areas to interact remotely. Through allocating satelliteresources, we can effectively promote the remote interactionsas a safe and efficient solution in emergencies, a daily lifestyle,and even driving forces which conform to the unmannedfashion for labor-intensive industries.

7) Finance: Financial applications such as asset tracking(AT) and high-frequency trading (HFT) are typical scenariosfor LEO satellites. AT requires one-way communication toachieve physical tracking, which is implemented by terrestrialnetworks and global positioning system (GPS). LEO satel-lites will improve the unreliability of terrestrial network andthe positioning accuracy of GPS, therefore achieving betterperformances. HFT can be processed by computer algorithmsrapidly in fractions of seconds and is sensitive to latencies.Space-based transmissions are expected to offer lower latencythan undersea fiber, which may promote financial profit in thelong term [7].

8) Smart City: The dominant issue for creating a smartcity is the obsolete infrastructure, imperfect coordinationmechanism, insufficient resources, and etc. In a typical smartcity, massive IoT sensors distributed in cities can be directlyconnected to the “City Brain”, where the cross-system multi-

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Mobile Host

Spotbeam handover

Router

Satellite handover

ISL handover

Fig. 3. Typical link layer handover schemes.

mode data are processed in real time. LEO satellites willplay an important role in synchronous information exchangeand dynamic resource allocation with fast response and highconcurrency, and is envisioned to play a part in traffic man-agement, large-scale activity flow monitoring, smart tourism,and cloud-edge integrated infrastructure management.

IV. TECHNICAL CHALLENGES AND RESEARCHDIRECTIONS IN LEO SATCONS

In this section, we introduce some key technologies thatsupport various vertical applications mentioned above. Thechallenges are presented from the following three aspects.

A. Network Architecture

As shown in Fig. 1, there are 2 types of methodologies thatestablish the constellation-based network architecture:

• UT directly communicates with LEO satellites, orthrough portable receivers.

• UT communicates via terrestrial BSs, and the satellite-ground link serves as backhaul for data exchange.

The former strategy is adopted by Iridium, Globalstar, andsome other operators by promoting their satellite UTs in theearly stages, which has caused inconvenience and financialburden on both subscribers and operators. In contrast, thelatter strategy still depends highly on the terrestrial stationsto complete the network topology.

In the past few years, the integration of spaceborne, air-borne, and terrestrial networks has aroused widespread inter-est, where high altitude platforms and unmaned aerial vehiclescan effectively enhance the SatCons’ service coverage andQoS to accommodate various vertical scenarios. However,the network structure and routing protocols among differentlayers, as well as the heterogeneous network access problemsfor UTs among diversified devices still need to be furtherdiscussed.

B. Mobility Management

Mobility management was first considered in cellular net-works for supporting continuous communication applications.Cells in conventional terrestrial cellular networks are staticallytied to a radio access network (RAN). By contrast, spotbeamsof LEO satellites are moving at high speed with a fixed

trajectory. The cell has to be covered by different beams fromdifferent satellites in every time instant, since it is difficult forsatellite based RAN to provide service continuously after theinitial registrations. Besides, paging and handover proceduresshould be adapted to take into account the relative motionof the cell pattern with respect to the tracking area [8].Particularly, handover techniques for link layer and networklayer are urgently required for LEO satellite communicationsystems [9], [10].

1) Link layer: Link layer handover occurs when one ormore links between the communication nodes have changeddue to the dynamic connectivity patterns caused by highmobility of LEO satellites. This can be further classified intospotbeam handover, satellite handover, and inter-satellite link(ISL) handover as illustrated in Fig. 3:

• Spotbeam handover happens when terrestrial nodes ac-tively or passively cross the boundary between 2 adjacentspotbeams of one satellite.

• Satellite handover happens when a node is no longerserved by the previous satellite, and is transferred toanother satellite.

• ISL handover happens when an ISL is interrupted dueto the change in distance or field-of-view angle, wherenew ISL establishes and causes rerouting issues.

2) Network layer: Network layer handover occurs whena satellite or terrestrial node needs to change its IP address.When satellites operate as mobile hosts that exchange datawith terrestrial stations for different communication terminals,the IP address of the satellite is bonded with the terrestrialstations. When the satellite leaves the coverage area of theprevious terrestrial station and starts to bond with anotherterrestrial station, the IP address of the satellite has to beupdated, which requires network layer handover. Additionally,when the satellite operates as a router, users covered by thesesatellites may require a handover in the case of switchingbetween spotbeams or satellite routers.

For handover and paging to be efficiently operated in LEOSatCons, future UT may be required to report its accurate loca-tion information periodically, and the ephemeris informationof the LEO SatCons should be exploited to determine theirfootprints of each beam. Therefore, SatCons can select thebest beams from each satellite to cover the users in demand.Moreover, the duration that UT would remain to be coveredcan also be estimated, which is convenient for SatCons tocalculate the best candidate to handover. The adaptation ofterrestrial protocols also needs further study to possibly utilizethe location and ephemeris information in fixed trajectory LEOSatCons.

C. Physical Layer TransmissionExperiments for LEO SatCons are underway [5], while the

integration of satellite access with terrestrial 5G networks stillremains unsettled. Commercial solutions for mobile satellitecommunication include setting up several terrestrial gateways,densely deploying tailored antennas, and using specified UTs.

According to the current 5G technology and non-terrestrialnetwork status, several incompatibilities still need to be re-solved in future integrations.

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mMTC URLLC

Key Techniques

Latency

Data rate

Industry

Agriculture

Finance

Smart City

Remote Interaction

SpectrumPlanning

Multi-Beam Technique

Service Requirement (KPIs)

Vertical Domains

Fig. 4. Relationship between vertical domains, service requirements, and key techniques.

1) Spectrum planning and interference management:5G has scheduled dozens of new frequency bands, whichfurther squeeze the idle bands in sub-6G frequencies. Theavailability of tens of thousands of satellites in the near futurewill accelerate the exhaustion of limited spectrum, thereforepursuing higher frequency such as Ka bands has become abroad consensus as shown in Table II. Considering multipleverticals require large amount of spectrum resources, theinterferences caused by frequency reuse and spectrum sharingamong different orbital planes and terrestrial stations remainopen issues, since the available radio spectrum is still costlyfor SatCon operators [11]. To aviod the undesired interference,flexible dynamic spectrum resource sharing schemes shouldbe standardized to ensure robust interference management,and the advanced beamforming algorithms and digital signalprocessing techniques [12], [13] are demanded to handleinterferences.

2) Advanced array and multi-beam technique: Massivemultiple-input multiple-output (MIMO) has been applied interrestrial cellular wireless networks, and recognized as oneof the enabling technologies in 5G [14]. As the spacebornestations are facing the increasing demand for availability,data rate, and reliability, massive MIMO with phased-array isexpected to be exploited on SatCons to support flexible beam-forming. On the other hand, active phased array and multi-beam antennas also require further research to be employedon UTs to make highly effcient use of spectrum resourcesand effectively track the beams from space-based stations withhigh mobility.

3) Terahertz and free space optical based ISL: Theemerging data-intensive verticals promote the application ofISLs to accommodate unprecedented data traffic. Terahertz(THz) and free space optical (FSO) are two of the mostpromising techniques to support ISL due to their extremelyhigh directivity, ultra-wide band, and security characteristics.In a turbulence-free outer space scenario, THz or FSO basedISLs can be established for routing or data exchange withoutdistortion, therefore enhance the performance of SatCons.Note that optical frequencies can provide extremely highantenna gain at a relatively small antenna size, thereby effec-tively reduce the size and power consumption [7]. Therefore,trending is that FSO based ISLs have the potential to ulteriorlyenhance the SatCon capacity, however, acquisition, tracking,and pointing of the extremely narrow laser beams are stillchallenging in practice. Issues such as routing under time-varying network topology and creating multiple co-existinglinks among different orbital planes also require further re-search.

4) Fast time-varing effect mitigation: Unlike terrestrialcellular systems, wireless channel in satellite communicationis generally a Rician distributed LoS channel, and the satellitestations are in a state of high speed motion. Explicit distribu-tion is conducive to channel modeling and algorithm design,but highly mobile satellites with fixed antenna inclinationwill introduce intractable issues, in which Doppler effectmitigation and synchronization are the most chanllenging tasksin broadband verticals.

5) Access protocols for low latency: Now that the propa-gation latency is inevitable due to the round-trip delay between

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satellite and terrestrial nodes, novel mechanism should bedeveloped to address the challenges of low-latency process-ing, particularly in massive IoT access networks. Grant-freerandom access (GFRA) allows active nodes to transmit theirpilots and data to the BS without waiting for permissions [15],which cuts down the overall latency, however, the correspond-ing spectrum resources consumption increases as the numberof users increases, and the requisite accurate channel stateinformation is difficult to obtain.

V. CONCLUSIONS

In this article, we have given the development roadmapof LEO SatCons, and discussed the opportunities and keytechnologies of integrating LEO constellations with futurecellular networks, where we have highlighted the verticaldomains reshaped by LEO SatCons. In summary, as shownin Fig. 4, the key techniques involved in LEO SatCons are thepillars for achieving the superior KPIs and supporting variousservice requirements. Superior KPIs are the foundations forbreeding and reshaping the various vertical applications, andrevolutionary vertical applications will further bring profits forpromoting the technical innovations of LEO constellations. Itis highly anticipated that LEO SatCons will play a criticalrole in the reshaping of various vertical domains and furthertransform our society.

VI. ACKNOWLEDGEMENT

This work was supported by NSFC 62071044, BeijingNatural Science Foundation L182024.

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Shicong Liu received the B.S. degree in School of Information and Electronicsin Beijing Institute of Technology, Beijing, China, in 2020, and is currentlypursuing a M.S. degree in Beijing Institute of Technology, Beijing, China.

Zhen Gao (Member, IEEE) received the Ph.D. degree in communicationand signal processing from the Tsinghua National Laboratory for InformationScience and Technology, Department of Electronic Engineering, TsinghuaUniversity, China, in July 2016. He is currently an Assistant Professor withthe Beijing Institute of Technology.

Yongpeng Wu (Senior Member, IEEE) received the Ph.D. degree in communi-cation and signal processing from National Mobile Communications ResearchLaboratory, Southeast University, Nanjing, China, in 2013. He is currentlya Tenure-Track Associate Professor with Shanghai Jiao Tong University,Shanghai, China.

Derrick Wing Kwan Ng (Fellow, IEEE) received the Ph.D. degree from TheUniversity of British Columbia (UBC) in 2012. He is currently working as aSenior Lecturer and a Scientia Fellow at the University of New South Wales,Sydney, Australia.

Xiqi Gao (Fellow, IEEE) received the Ph.D. degree in electrical engineeringfrom Southeast University, Nanjing, China, in 1997. He joined the Departmentof Radio Engineering, Southeast University, in 1992, where he has been aProfessor of information systems and communications, since 2001.

Kai-Kit Wong (Fellow, IEEE) received the Ph.D. degree in electrical andelectronic engineering from The Hong Kong University of Science andTechnology, Hong Kong, 2001. He is currently a Chair in Wireless Communi-cations at the Department of Electronic and Electrical Engineering, UniversityCollege London, U.K.

Symeon Chatzinotas (Senior Member, IEEE) received the Ph.D. degree inelectronic engineering from the University of Surrey, Surrey, U.K., 2009. He iscurrently a Full Professor/Chief Scientist I and the Co-Head of the SIGCOMResearch Group, SnT, University of Luxembourg.

Bjorn Ottersten (Fellow, IEEE) received the Ph.D. degree in electricalengineering from Stanford University, Stanford, CA, USA, in 1989. He heldresearch positions with Linkoping University, Stanford University, and theUniversity of Luxembourg.

8

Optical fiber

Service link

Inter-satellite link

Feeder link

Optical fiber

Service link

Inter-satellite link

Feeder link

UTs

Terrestrial BS

Satellite Receiver

GatewayCore Network

LEO SatCon

Satellite Receiver

Fig. 5. A schematic diagram of the network architecture of LEO SatConsand the common UT-SatCon access modes.

9

TABLE IICOMPARISON OF SELECTED CONSTELLATIONS

Constellation Regime Orbital height Quantity Bands Services Est. data rateIridium Gen. 1 LEO 781 km 66 L Voice, data 2.4 Kbps

Globalstar LEO 1414 km 48 S, L Voice, data ∼9.6 KbpsOrbcomm Gen. 1 LEO 700 ∼ 800 km 36 VHF IoT & M2M∗ communication 2.4 Kbps

Skybridge LEO 1457 km 64 Ku Broadband Internet 60 MbpsTeledesic LEO 1375 km 288(840) Ka Broadband Internet 64 Mbps

Iridium NEXT LEO 781 km 66 L, Ka Voice, data 1.5 Mbps, 8 MbpsOrbcomm Gen. 2 LEO 700 ∼ 800 km 18 VHF IoT & M2M communication 4.8 Kbps

O3b MEO 8063 km 20 Ka Broadband Internet 500 MbpsOneWeb LEO 1200 km 648 Ku Broadband Internet 400 Mbps

Starlink Gen. 1LEO

335.9 ∼ 570 11926 VBroadband Internet 100 Mbps

Starlink Gen. 2 328 ∼ 614 km km 30000 Ku, Ka, ETelesat Phase. 1

LEO 1015 ∼ 1325 km298

Ku, Ka Broadband Internet -Telesat Phase. 2 1373

Hongyan LEO 1100 km 320 L, Ka Voice, broadband Internet 100 MbpsKuiper LEO 590 ∼ 630 km 3236 Ka Broadband Internet -

* : Internet of Things and machine to machine.

10

Stock Exchange

Ground Station

Manufacturing

Logistics

Vehicular

Communication

eHealth

Smart City

Energy

aGVhbH

RoIGluZ

m9ybWF

0aW9u

YW5vdG

hlciBoZW

FsdGgga

W5mbw

ISLFinance

Remote

Interactions

Fig. 6. Ambitious vision of various vertical domains reshaped by LEO SatCons.

11

Mobile Host

Spotbeam handover

Router

Satellite handover

ISL handover

Fig. 7. Typical link layer handover schemes.

12

Intelligen

t A

gricultu

reS

mar

t B

usi

nes

s

eMBB

mMTC URLLC

Key Techniques

Latency

Data rate

Industry

Agriculture

Finance

Smart City

Remote Interaction

SpectrumPlanning

Multi-Beam Technique

Service Requirement (KPIs)

Vertical Domains

Fig. 8. Relationship between vertical domains, service requirements, and key techniques.