Reflection on the Development of Satellite Communications and the Integration with Terrestrial Mobile Communications

XU Xiaofan,WANG Niwei,WANG Chunting

China Academic of Electronics and Information Technology,Beijing 100041

Abstract:With the further reduction in cost and the increase in bandwidth,as well as the increase in internet applications,satellite communications are gradually shifting from a complementary role to becoming a fully integrated component of terrestrial communications networks.This paper firstly introduces the development of satellite communications,mobile communications and the global space-terrestrial integrated network.We then propose the functional architecture and network architecture for the integration of satellite communications and terrestrial mobile communications based on 5G core networks.Finally,in order to support the network of the future,four key technologies are presented,a space-terrestrial integrated air interface design,a multi-band space-terrestrial integrated transmission waveform design,space-terrestrial integrated switching and routing technology,along with spectrum sharing and interference coordination technology,all necessary for the development of space-terrestrial integrated networks.

Key words:satellite communications,mobile communications,space-terrestrial integrated network,functional architecture,network architecture

1 INTRODUCTION

Satellite communications has the characteristics of wide coverage,broad frequency bandwidth,large capacity,with little influence from geographical factors.It is widely used in military,emergency,aviation,navigation and other communications fields.For a long time,satellite communications and terrestrial mobile communications were developed in parallel,competing and complementary in the application market.In recent years,with the further reduction of the cost and the increase in bandwidth,as well as the increase of Internet applications,satellite communications is gradually shifting from a complementary role to be fully integrated with terrestrial communications networks.In particular,the current research on the sixth-generation mobile networks (6G) has clearly proposed the need to integrate satellite communications and terrestrial mobile communications to build a space-terrestrial integrated network.For this reason,the key technology of the integration has been attracting more and more attention,and its integration will also bring new application paradigms.

2 BACKGOUND

2.1 Development of Satellite Communications

The concept of satellite communications can be traced back to October 1945,when Arthur C.Clarke,a radar specialist in the Royal Air Force,proposed the idea to achieve global communications with three geosynchronous Earth-orbit (GEO)satellites in the

Wireless World

magazine.From that point onwards,the development of satellite communications has successively passed through six stages,as shown in Table 1.

In the 1990s,the satellite communications constellation composed of multiple low Earth-orbit (LEO) satellites enabling the first development climax,typified by the Iridium satellite system.The Iridium satellite system was proposed by Motorola in June 1990 to overcome the shortcomings of the first generation of ground analog mobile communications systems,including non-uniform standards,difficulty in international roaming,and poor signal quality.The system consisted of 66 satellites operating on 6 orbital planes at 778 km.Each satellite used 48 L-band beams for ground coverage,employing GSM technology,and used Ka-band inter-satellite links and on-board processing technology to achieve space-based networking.The Iridium systemset up 12 gateways around the world,and the network control center located in Washington D.C..However,due to the rapid development of terrestrial mobile communications,as well as the unsatisfactory quality of the initial telephone service,and insufficient users to meet the needs of continuous operations,Iridium was forced into bankruptcy in March 2000.However,commercial failure cannot conceal its technical success.After Chapter 11 reorganization,Iridium changed their business model to rely more on the military,government and other major customers to achieve steady business,and subsequently successfully completed its second-generation Iridium Next system upgrade.

Table 1 The development stages of satellite communications

After entering the 21st century,high-throughput satellites(HTSs) have become a major focus in the development of satellite communications.By using Ku,Ka and other high-frequency transmission technologies,dense multi-spot beams,large-aperture satellite antennas and other technologies,the communications capacity can reach hundreds of Gbps.Since then,the bit cost has been greatly reduced,and it is gradually approaching that of terrestrial networks,thus significantly improving the competitiveness of satellite communications.In 2004,the world’s first HTS,IPSTAR 1,was launched into orbit with a capacity of about 45 Gbps.The ViaSat-3 satellite,which is expected to be launched in 2022,will reach a capacity of 1 Tbps.

Although GEO-HTS has significantly decreased bandwidth costs,they have long transmission delays and are not available for high-latitude areas.Meanwhile,the global average Internet penetration rate as of January 2020 was only 59%,and the growth has been weak.In order to provide the remaining half of the population with an Internet service,the satellite Internet constellation has created another development peak from around 2010.The most representative constellations are One-Web,Starlink,and Telesat.

In June 2017,the FCC approved a constellation plan proposed by the satellite Internet startup OneWeb.It has planned three generations of constellations,in total 1980 satellites.The first-generation constellation adopts a polar orbit configuration including 882 LEO satellites (648 in orbit,234 backup) at an orbital height of 1200 km.It adopts simple transparent transponders and fixed beam antennas.Each satellite provides 16 Kuband user beams and reaches a capacity of 8 Gbps.The system does not employ any inter-satellite link or on-board processing,the signal has to be transferred to the nearest gateway for processing.So far,OneWeb has launched 6 prototype satellites in February 2019,and 104 satellites in 2020.

Starlink is the next-generation satellite Internet proposed by SpaceX in 2015,which consists of satellites in various orbits.The construction can be divided into three phases:firstly,1600 satellites will be launched to reach global coverage;secondly,2825 satellites are used to achieve global networking using Ku and Ka bands;thirdly,7518 satellites with Q/V-band will form a very low Earth-orbit (VLEO) constellation.The Starlink system adopts key technologies such as active phased array antennas,digital processing and forwarding,and all-electric propulsion with krypton.It also supports inter-satellite links and space networking.The capacity of a single satellite is designed to about 20 Gbps.As of the end of 2020,SpaceX has completed 16 batches of launching using Falcon rockets,in total 955 satellites.

China also proposed its own satellite communications plan in 1958,and proposed a three-step strategy,namely successful launch,small satellite in orbit,and GEO satellite in orbit.Following this plan,DFH-1 satellite and China’s first GEO communications satellite DFH-2 were successfully launched in 1970 and 1984,respectively.

Since entering the 21st century,the development of China’s satellite communications has entered a new period in its history.In August 2016,China’s first self-made mobile communications satellite Tiantong 1 was successfully launched,which uses 109 S-band beams to cover the mainland and coastal areas of China,supporting voice,messaging,and low speed data services.In April 2017,China’s first high-throughput communications satellite Chinasat-16 was launched,with a total capacity of 20 Gbps,which exceeded the total capacity of all previous communications satellites of China.

With the upsurge in the development of communications constellations in the world,a series of constellation development plans have been proposed by the China Aerospace Science &Industry Corporation,China Aerospace Science and Technology Corporation,China Electronics Technology Group Corporation Limited,and other companies,of which have launched several LEO test satellites and conducted inter-satellite linking and space-based networking tests since 2018.

2.2 Development of Mobile Communications

In the 1970s,Bell Laboratories invented the cellular concept and proposed to use frequency reuse and cell splitting technologies.Since then,terrestrial mobile communications have been evolving from 1G to the current 5G.The generations of mo-bile communications are shown in Table 2.

Table 2 The generations of mobile communications

5G utilizes higher data rates,has lower latency,energy saving,reduced cost,improved system capacity,and provides connection support for large-scale networks as its main characteristics,supporting enhanced Mobile Broadband (eMBB),massive Machine Type Communication (mMTC) and Ultra-Reliable and Low Latency Communication (URLLC) concepts.In terms of network architecture,5G uses Software Defined Networking(SDN) to separate the data plane and control plane,and Network Function Visualisation (NFV) to realize the decoupling of software and hardware.From the wireless transmission point of view,5G uses massive Multiple-input and Multiple-output(MIMO),Polar coding,non-orthogonal multiple access (NOMA),millimeter wave communications and other key technologies to further increase the transmission rate to multiples of Gbps.In the 5G standardization process,the 3rd Generation Partnership Project (3GPP) has proposed four preliminary models of space-terrestrial integration under the scenario of non-terrestrial networks (NTN),including the use of transparent transponders and the deployment of base stations on satellites,which will extend 5G into the field of satellite communications.

6G,which is expected to be commercially deployed in 2030,will not only increase traffic density and connection density by 10 to 1000 times compared with 5G,but also will support connections for user mobile at speeds greater than 1000 km/h (commercial aircraft) and provide peak rates of the order of Tbps,while also achieving space-terrestrial integrated coverage.In July 2018,the ITU established a Focus Group on Technologies for Network 2030 (FG NET-2030),and emphasized satellite access as an important feature for future networks.

2.3 Development of Space-Terrestrial Integrated Network

In order to integrate the advantages of space-based networks and terrestrial networks,and realize larger-scale and richer applications,efforts have been put into the exploration of integration technologies for 20 years.

At the beginning of this century,in order to meet the requirements for network-centric warfare,the U.S.military proposed a Transformational Communications Architecture (TCA)to provide a protected Internet-like secure communications system that combines the space,air,ground,and sea-based networks.The space segment of TCA is called Transformational Satellite Communications Systems (TSAT),which consists of 5 GEO satellites,and uses a series of advanced technologies such as laser communications,IP based space-borne routing,and large-aperture space-borne antennas to form a space highspeed data backbone network.It will help to obtain data from space-based intelligence,reconnaissance,and surveillance information sources,and achieve high-capacity information sharing,thereby extending the US military’s Global Information Grid(GIG) to areas formerly with less functional ground infrastructure.Due to technology and funding problems,TSAT project was put on the shelf in 2009.

In 2005,the European Integral Satcom Initiative Technology Platform proposed the concept of an Integrated Space Infrastructure for global Communication (ISICOM).It not only aims at the integration with future global communication networks,but also enhances the solution with value-added services such as the Galileo navigation system and Global Monitoring for Environment and Security (GMES) system.The space segment of ISICOM consists of 3 GEO satellites,combined with medium Earth-orbit (MEO) and LEO satellites,high-altitude platforms(HAPs),and unmanned aerial vehicles (UAVs).By adopting a series of key technologies such as multiple and reconfigurable orbit system design,space-based laser communications,multiband radio frequency access,and virtual beam forming,it promotes the development of an integrated network.

Since the development of 5G,the terrestrial mobile communications groups have also deepened their exploration of the area of space-terrestrial integration.In June 2017,16 companies and research institutions including BT,Avanti,SES,University of Surrey jointly established the Satellite and Terrestrial Network for 5G (SaT5G) organization to study space-terrestrial 5G integration technologies,including the implementation of SDN and NFV technologies,multi-link and heterogeneous transmission technologies,integration of satellite and 5G network control and data planes,and integration of satellite and 5G network management and operation.In May 2019,Telesat,the University of Surrey and Newtec jointly tested the 5G backhaul scenario with a LEO satellite.The 3GPP organization also began to study the advantages and feasibility of integrating satellites into 5G since R14,and further studied in R15 and R16.

Although TSAT,ISICOM,SaT5G,3GPP have carried out some early studies on integration,it is still far away from commercial adoption.The integrated network architecture,air interface,transmission waveform,switching and routing methods,frequency sharing and management technologies are all key issues that have to be studied in depth.

3 INTEGRATED ARCHITECTURE DESIGN

Due to the limited on-board resources of satellites,to study the integration of satellite communications and terrestrial mobile communications,one must first consider two modes of using satellite resources,namely the transparent mode and on-board processing mode.In the transparent mode,the satellite only undertakes the retransmission roll,the processing of signals is mainly carried out on the ground.In the on-board processing mode,this requires more resources on the satellites to perform the processing.With the advancement of technology,the latter approach will bring more flexibility for the application of the network,however,several aspects have to be studied in order to realize an efficient architecture.First is an optimal division of network element functions between the space segment and ground segment.Second is a solution to the relative motion between satellites and between the satellite and ground-based users.Third is a way to support multiple services such as forwarding via ground and without gateways.

At present,the 5G core network adopts a service based architecture (SBA),which transforms the network element functions of hardware devices into relatively independent,reusable,loosely coupled,and flexibly callable micro-services,which are scheduled by cloud computing facilities to separate the control plane from the service plane.In addition,the positions of LEO and MEO satellites relative to the gateway,ground users,as well as other satellites are dynamically changing,leading to difficulty to disperse the functions of a base station Next Generation Node B (gNB) according to the concept of Central Unit -Distributed Unit (CU-DU) separation.Based on the above considerations,we propose a function architecture that integrates satellite communications and terrestrial mobile communications as shown in Figure 1.

The design of the system takes the idea of on-board processing,and a fully functioned 5G base station gNB is deployed on the satellite.The computing and storage resources of satellites and ground gateways together form a unified resource pool.The satellite can support on-demand partial core network service scheduling functions,such as Access and Mobility Function (AMF),Session Management Function (SMF),and User Plane Function (UPF),while Authentication Server Function(AUSF),Unified Data Management (UDM),Policy Control Function (PCF),Application Function (AF),and Data Network(DN) are deployed on the gateways.

The relative motion problem between the satellites and the ground terminals can be solved by the terminal handover process between the base stations,and the problem of the dynamic interaction between the on-board base station and the on-board core network service,as well as the ground gateway service can be tackled by the network architecture as shown in Figure 2.

Figure 1 Function architecture for the integration of satellite communications and terrestrial mobile communications

Figure 2 Network architecture for the integration of satellite communications and terrestrial mobile communications

The satellite here would be equipped with multi-protocol label switching (MPLS) protocol label edge router (LER) and label switch router (LSR).According to the pre-calculated satellite orbits,the ground SDN server would adopt constellation routing algorithms such as sequential snapshots to periodically establish,remove and maintain the label switch path (LSP) between LERs.This centralized management and control architecture that combines MPLS and SDN is in line with the current mainstream SD-WAN architecture.It not only realizes the decoupling between the problem of the interconnection between the on-board base station gNB and the core network and the problem of the dynamic topology of the constellation,but also can construct a virtual private network (VPN) or perform network slicing as required,which is very useful for government,enterprise,and military users.

4 KEY TECHNOLOGIES

4.1 Space-Terrestrial Integrated Air Interface Design

Satellite mobile communications usually adpots L and S frequency bands,which actually have achieved partial integration with terrestrial networks in terms of the air interface design,such as Tiantong 1,Thuraya (GMR-3G standard),Inmarsat-4(IAI-2 standard),which adopt the 3GPP-R4/R6 air interface layering scheme,where most of the design of the upper layer protocol (NAS layer) is retained.However,the physical layer waveform,special coded voice carrier,frame structure of MAC layer,and resource allocation algorithm of RRC layer are mainly designed according to the characteristics of the satellite-ground transmission link.

Satellite fixed communications using Ku,Ka and other frequency bands typically follows DVB standards,such as DVBS2X and DVB-RCS2.The waveforms and protocol structures are quite different from the 3GPP standard framework.Due to the lack of a handover mechanism,mobility management is not supported by DVB standards and other issues,hence it is suggested to adopt the protocol structure of the 3GPP standard framework and the NAS layer protocol design for future integration network.

4.2 Multi-Band Space-Terrestrial Integrated Transmission Waveform Design

The transmission waveform design lies at the center of air interface design.Especially,under the conditions of inconsistent high and low frequency transmission channel models,communication elevation angles,available bandwidth,terminal radio frequencies,and antenna characteristics,the core issue is to design waveforms to meet the requirements of satellite scenarios,multi-frequency bands,taking into account peak-to-average ratio,spectrum efficiency,and communication rate.The selection of the physical layer waveform in the satellite access scenario requires comprehensive consideration of the channel model and the characteristics of the on-board transponders.If 5G multi-carrier technology is adopted,it is necessary to solve a series problems,including the Doppler frequency shift in the LEO downlink,the low peak-to-average ratio requirement caused by the power back-off of the satellite amplifier,the extremely limited spectrum resource of the low frequency band,the reduction of Hybrid-ARQ (HARQ) efficiency caused by the long satellite-ground delay,and the applicability of MIMO.

At present,the ITU suggests that the satellite-ground link transmission model in the frequency bands below 20 GHz can adopt the“two-state Markov+Loo multipath”model,and the strengths of multipath components exhibit significant differences under different available elevation angles and frequency bands.For the L and S frequency bands,even if the available bandwidth constraints are not considered,due to the limited link margin,it is still difficult to adopt the downlink Orthogonal Frequency Division Multiplexing (OFDM)/uplink DFT-S-OFDM scheme,hence support for large amounts of carriers which is typical for 5G is not possible.For high frequency bands such as Ku and Ka,the Doppler frequency shift can be compensated by the frequency offset based on orbit prediction,but it is still necessary to design related mechanisms to solve the problem of residual frequency offsets due to orbit prediction error and the estimation error from the moving terminals.The design of a variable OFDM subcarrier bandwidth solution can effectively resist the impact of residual frequency offset on system performance,but the configuration of large amounts of subcarriers reduces the number of permitted users and the flexibility of allocation.Therefore,it is necessary to select appropriate waveforms according to the scenarios.Nowadays,there is basically a consensus on the use of DFT-OFDM for the uplink,but there are still some who believe that the use of the DVB-S2X standard single-carrier time division multiplexing (SC-TDM) waveform design for the downlink can achieve better Peak to Average Power Ratio (PAPR)performance.

Recently,a new type of Orthogonal Time Frequency &Space (OTFS) technology has been proposed for 6G,which can be regarded as a spread spectrum technology in the time-frequency domain.This technology can be easily combined with high-order modulation and MIMO,and has good robustness to narrow-band interference.In addition,by supporting a mechanism called Doppler transversal allocation,it is expected to achieve a PAPR performance at the same level compared with single-carrier,which will provide solutions for the future integration of satellite-ground waveform problems.

4.3 Space-Terrestrial Integrated Switching and Routing Technology

Due to the movement of LEO and MEO satellites,network topology is continuously changing,and the inter-satellite links,as well as the satellite-ground links have to be switched frequently,which poses a challenge to the design of routing protocols.At present,a comparably mature satellite constellation routing algorithm is based on the sequential snapshots.It divides the satellite network in a cycle into independent topology snapshots.The constellation topology in a snapshot is stable and predictable,therefore,a routing table in each snapshot can be calculated in advance for link switching.

In most of the cases,the change of the satellite constellation topology is predictable.Therefore,in the routing mechanism design,a centralized control scheme based on SDN technology can be used.The routing scheme is calculated in the ground SDN controller,and then uploaded by a TT&C station to the satellites to achieve control.In order to simplify the design and support special services such as the CCSDS protocol relay,the on-board switch of the future integrated network can combine the transmission frame structures of inter-satellite links,satellite-ground user links,and feeder links to design a private MPLS protocol to carry and switch packets of a variety of network layer protocols,and the ground SDN controller is responsible for processing routing protocols such as OSPF,and configures labels and network addresses mapping table,label forwarding tables on the satellite and terminals,and establish,remove,and maintain MPLS label distribution protocol (LDP) switching paths.

4.4 Spectrum Sharing and Interference Coordination Technology

Frequency resources have always been scarce both in satellite communications and terrestrial mobile communications.The main objective of resource management in 5G is the allocation of time-frequency resources blocks (RBs) and MIMO resources.However,the main issue is how to share frequency and power resources between beams efficiently in satellite communications.

Currently,the satellites of the Ka-band constellations such as Starlink and Telesat are equipped with multi-beam phased array antennas.The beams can be hopped and scanned between multiple beam spots to satisfy multi-user services.It is also worthy to further study how to allocate broadcast channels and dedicated control channels in free beams and ensure a balance of fairness between beams.In addition,satellites are typical power-constrained systems.In order to avoid premature saturation of some beams due to the unbalanced services in the coverage area,a multiple feed per beam (MFB) scheme is exploited,in which a space-borne power amplifier is shared between beams.In order to match limited power resources and processing resources with actual demands,beam hopping (BH)technology is often used for time slicing,in which only some spot beams are functional at the same time.For the future integrated networks,both the power allocation mechanism and the resource scheduling strategy have to be considered to promote the efficient use of power resources.

Interference is another important problem in resource allocation for integrated networks.As the positions of LEO constellation systems are changing with time,the interference to other systems should be considered both in time and power aspects.The current effective scheme for interference coordination is to use the progressive pitch separation angle generated by the difference of orbital height between NGSO and GSO satellites.In recent years,NGSO and GSO interference coordination technologies based on spectrum sensing have also been gathered great attention.As the numbers of satellites in Starlink and Kuiper constellations are increasing to tens of thousands,the network becomes more and more dynamic and complex.Thus,it is crucial for the application of artificial intelligence in interference coordination,such as for spectrum sensing and spectrum sharing based on deep reinforcement learning.

5 CONCLUSIONS AND OUTLOOK

Whether from the perspective of satellite communications or terrestrial mobile communications,to develop space-terrestrial integrated networks has gained broad consensus.With the advancement of basic science and technology components,the capabilities of satellites will be further improved and will carry more on-board network management and control functions.In the near future,the integration will penetrate all demands,visions,frequency usage,R&D,networking,services,and operation and maintenance,then we will truly realize unified planning,unified design,unified construction and unified management of space-based and terrestrial networks,which will further contribute to the coordinated development of the entire industrial chain from innovation,products,engineering to application services.