Hybrid Satellite-Aerial-Terrestrial Networks in Emergency Scenarios: A Survey

Ying Wang , Yichun Xu, Yuan Zhang, Ping Zhang

State Key Laboratory of Networking and Switching Technology, Beijing University of Posts and Telecommunications, Beijing 100876, China

* The corresponding author, email： wangying@bupt.edu.cn

I. INTRODUCTION

When large-scale disaster occurs in some area, the communication infrastructures are generally severely damaged. And the basic communication abilities are highly possible not be supported such as commanding and dispatching. A so called Information Island is thereafter formed in such disaster area. In addition even some communication facilities remain in working condition, too many users use phones at the same time which will cause the entire communication networks overload.Thus emergency communication should provide an efficient and effective alternative for the damaged communication infrastructures.Moreover undamaged communication resources should be cleverly used, especially for the network congestion problems. Currently Public Protect and Disaster Relief (PPDR) are pursued to maintain the ability of radio communications possessed by responsible agencies and organizations to keep social stability.

There are several key characteristics of emergency communication including paroxysm in time, indetermination in place, urgency in operations and multiplicity in information.The whole operations of emergency communication systems are necessary to be concise and highly extendable with high security and low overhead [1]. The whole system should have the high mobility and can easily set up, which would also provide efficient and reliable information transmission in the entire disaster area [2]. Only in this way can command and communication be effective in response to the situation when the infrastructures suffer significant destruction.

The research on the development of hybrid satellite-aerial-terrestrial(HSAT) networks in emergency scenarios is presented in this paper. The challenges of the HSAT networks are also outlined.

The main research works focused on the emergency communication are concentrated in cluster, ad-hoc self-organized network and satellite communication. Private networks are used to guarantee the timeliness and reliability of emergency communications, and trunked communication systems represent the development direction of the private mobile communication systems. Most of the existing private mobile communication systems are based on Terrestrial Trunked Radio (TETRA)[3] or Project 25 [4]. These trunked communication systems are based on narrow band technology, which can only provide limited secure voice and data services. In China, the broadband wireless trunking (BWT) project[5] is launched to offer enhanced broadband services containing voice, messaging, e-mail,web browsing and other wideband services.The architecture of BWT has three main features, including supporting multi-access modes, having a uniform standard for the network interfaces and providing flexible multimedia trunking services.

Nevertheless, trunked communication systems rely on the pre-existing communication infrastructure, they cannot work with severe damages of the infrastructural facilities. Hence building up an emergency network without relying on the existing infrastructures is critical. For example, ad-hoc networks can be deployed quickly with strong invulnerability,and the failure of any node would not affect the operation of the whole network. Of course routing, as one of the difficult problems in the area of ad-hoc networks, is also the key point of the research with respect to the emergency communications [6].

The development of satellite technology makes satellite communication to be a promising solution for emergency communication.Satellite communication networks have advantages of long communication distance, large coverage, no communication environment restriction and flexibility in grouping, which is shown in European project REMSAT [7].The advantage of satellite communications as a solution for the emergency communication,are verified in China since 2008 Wenchuan earthquake [8, 9]. Of course the satellite communications face challenges to provide higher bandwidth with Limited transmission capacity,high deployment/utility cost and difficult technical support. It is clearly that the mobile satellite communication systems are not suitable for the large deployment.

In the existing communication systems,traditional terrestrial communication technologies provide data transmission with high speed for large amount of users, but the terrestrial infrastructures are vulnerable in large scale disasters. Mobile satellite networks demonstrate the superiority of the robust and independent of surroundings when the existing communication facilities paralyzed.However, mobile satellite networks cannot ignore the problems of transmission capacity and construction costs. How to establish a broadband, stable and flexible communication network, which can be settled quickly,is an important problem. Multi-functional,multi-scheme and multi-service user-oriented network architectures provide a new effective direction, the idea of “Uniting multi network as one” appears. In this paper, we focus on the aerial platforms and combine aerial networks with satellite-terrestrial networks to establish a hybrid satellite-aerial-terrestrial (HSAT) network.

The rest of this paper is organized as follows. Section II introduces aerial platforms and the hybrid HSAT network architecture,followed by the key technologies of the HSAT network in Section III. Finally, Section IV analyzes the challenges for HSAT systems and concludes the paper.

II. AERIAL PLATFORM AND HYBRID SATELLITE-AERIAL-TERRESTRIAL NETWORK ARCHITECTURE

2.1 Hybrid satellite-terrestrial network

Considering the different advantages of mobile satellite networks and terrestrial networks in the aspect of coverage, bandwidth, cost and flexibility, it is preferable to combine the satellite networks with terrestrial networks. In the European SUITED project [10], multiple networks including mobile satellite networks,terrestrial cellular networks and wireless local area networks (WLAN) etc are integrated together. However, it needs to switch to mobile satellite networks manually when the terrestrial networks are unavailable. Japanese STICS project [11] however proposes only using two kinds of networks namely the satellite systems and terrestrial systems, which share the same spectral bandwidth. The mobile terminals have the ability to connect to both of satellite and terrestrial segments.

In the foundation of autonomous working and mutual integration, hybrid satellite-terrestrial networks integrate the advantages of satellite networks which could provide long transmission distance, wide coverage without the limitation of surroundings with terrestrial networks which have large network capacity and high rate. ITU-R S.2222 standard [12] is proposed to a cross layer design based on the IP hybrid satellite-terrestrial networks to provide multimedia services and ensure the user quality of service (QoS) requirements. Hybrid satellite-terrestrial systems combined with relay are also being studied constantly [13]. In China, a hybrid information network architecture with a central space-based network, contains one or any two fusion of communication,remote sensing and navigation longitudinally,integration of user segment and space segment of communication networks, remote sensing networks and navigation networks transversely[14]. Hybrid satellite-terrestrial systems have been more and more applied to the fields of public safety networks in recent years because of their superior performance. A typical hybrid satellite-terrestrial system is shown in Fig. 1.

2.2 The aerial platforms

Although the hybrid satellite-terrestrial system is superior to the traditional single system, it is hard to achieve full deployment due to its high cost, especially on the part of satellite. Aerial communications based on aerial platforms absorb the advantages of satellite communi-cations and terrestrial communications so that they are applicable to emergency communication in complex environment.

Fig. 1 The hybrid satellite-terrestrial communication system

The aerial platforms generally can be divided into two primary categories： high aerial platforms (HAPs) and low-medium aerial platforms (LMAPs) according to the height.The typical HAPs work in the stratosphere at an altitude of 17-23km. The research on HAP can be traced back to 1783. HAPs with broader coverage, lower latency, better channel conditions and lower cost become the best supplement to the terrestrial and/or satellite infrastructure. Hence, researches on HAPs have never been interrupted, especially have developed fast in the aspect of broadband communication [15].

LMAPs work on the height from tens of meters to several kilometers in the lower troposphere. LMAPs are usually used as a kind of temporary blocked communication [16],which previously do not attract many research works. However, with the frequent occurrence of large-scale disasters, people turn their attentions to LMAPs again, especially when it comes to emergency communication [17].LMAPs equipped with a reasonable communication system is considered to be a rapidly deploy solution which is ideally suited in providing basic communication service for affected areas in a short period of time. Balloons have become the most commonly used carrier of low-medium aerial base station for their low cost and simple implementation.The LMAPs, in particular, as represented by low aerial platforms (LAPs), are starting to be used to realize emergency communication in recent years. Table 1 shows the comparison in characteristics of satellite, terrestrial wireless and aerial platform systems, which is updated from [18].

2.3 Hybrid satellite-aerial-terrestrial networks

Based on the hybrid satellite-terrestrial networks and aerial networks, the HSAT networksare built containing satellite communication networks, aerial communication networks and terrestrial communication networks. In the European FP7 ABSOLUTE program [19], HSAT is pursued to satisfy the requirement of the network capacity through the combination of satellite, aerial and terrestrial communication links. The seamless reconfigurable network environment will provide mobility support and high energy efficiency. With the purpose of achieving information sharing and random access among users of different networks,Chinese 863 program, key technologies and demonstration of future multi integrated networks, puts forward to design a heterogeneous integrated network architecture in different dimensions of network.

Table I Comparison in characteristics of satellite, terrestrial wireless and aerial platform systems

As shown in Fig. 2, the HSAT communication system relates to a three-layer heterogeneous network structure. The upper layer is the satellite network. Satellites with different communication system conduct a mesh network and form multiple heterogeneous subnets. Satellites create remote relayed connections between terrestrial nodes and aerial platforms, which enable the overlay network access backbone switching network. The middle layer is the aerial network based on aerial platforms. Multiple aerial platforms connect to each other in Mesh network for a larger regional coverage. The bottom layer is the terrestrial network based on long-term evolution(LTE) technologies. The communication mode between aerial platforms and terrestrial nodes is designed by Point to Multi-Point (PMP),which have strong survivability. And even if an aerial platform is damaged, communication equipment is also accessible with either terrestrial network or other aerial platforms in the Mesh network. If some terrestrial base stations are damaged, other terrestrial Mesh nodes can be temporarily used as dynamic base stations,and users can also establish connection with base stations via aerial networks. Three layers not only work independently, but also cooperate coherently. In general, the aerial platform is in a sleeping state, communication is based on the LTE network. If the terrestrial network is intact or slightly damaged when natural disasters happen, the cellular mobile communication and other public network is still supported. If the facilities are seriously damaged,communication can be established by the hybrid communication system with satellites and aerial platforms.

Fig. 2 The hybrid satellite-aerial-terrestrial (HSAT) communication system

III. KEY TECHNOLOGIES AND TRENDS OF THE HSAT NETWORK

As far as we know, only a few research works are focused on the HSAT network. Three aspects are studied on the HSAT network including radio resource management, transparent handover and promising technologies.

3.1 Radio resource management technology

With the growing user demand and limited spectrum resources, a reasonable radio resource management technology is important to improve the overall system performance. The shortage of available radio resources is quite critical issue in the emergency situations due to the instantaneous increased user demand.An effective radio resource management technology is becoming more and more important in the scene of emergency communication[20]. Moreover, the HSAT network pay attention to not only the internal resource allocation problem within either terrestrial network or satellite network respectively, but also trigger a set of new problem including the systemic fusion problem between aerial network and terrestrial network, the fusion problem of satellite backhaul links etc. The main challenges include interference management and call admission control.

3.1.1 Interference management

Usually terrestrial networks deploy dense cells to meet the mobile data traffic demand of the area. However, as shown in Fig. 3, there will be an overlapping between the temporary erection of new cell covered by aerial network and the existing macro cell, small cell, which would lead to interference when emergency happens. The interference between the aerial communication networks and the terrestrial communication networks, as well as the internal interference among the terrestrial communication networks, will affect the network performance. It is essential to deal with the interference generated from aerial and traditional ground base stations in the case of coexistence of multiple communication systems.

The inter-cell interference between aerial network and terrestrial network can be effectively controlled by reducing the height of platform and limiting the power, the potential interference can be further reduced by adjusting the affected carrier frequency [21]. Felletti et al. [22] analyze the interference of the joint communication system combined with HAP and universal mobile communication system(UTMS) in different scenarios. Although the cellular system based on HAP is proved to be superior to both the terrestrial communication system and satellite communication system according to the channel multiplexing interference, the inter-cell interference is still necessary to be considered. [23] believe that the key factor to improve the channel interference is the performance of beamforming, which will refrain the co-channel interference by reducing the signal overflow quantity in neighboring cells.

Fig. 3 Illustration of overlapping of HSAT network

In the terrestrial network, the inter-cell interference is also an important issue in mixed networking between macro-cellular, remote wireless and micro-cellular low-power nodes.Inter cell interference coordination (ICIC)[24], proposed by 3GPP R8/9, is limited in the application of solving the interference problem in heterogeneous network scenarios. On such a basis, 3GPP R10 raises enhanced inter cell interference coordination (eICIC) [25,26]. 3GPP R11/R12 proposes enhanced Interference Management and Traffic Adaptation(eIMTA) [27]. Its core idea is to suppress inter cell interference in heterogeneous networks by means of time domain solution, frequency domain solution or power control scheme.For example, almost blank subframe (ABS),muting in time domain, and carrier aggregation (CA), fraction frequency reuse (FFR) in frequency domain. These technologies have been proved to be effective in suppressing downlink interference and improving network capacity and spectrum utilization. Unlike time and frequency domain solution, power control scheme is particularly suitable for suppressing uplink interference.

Similarly, the inter cell interference in the aerial network can be solved by adapting methods in the terrestrial network. Dirty paper coding (DPC) [28] has been proved to be the best method in obtaining multi user broadcast channel capacity. However, DPC method has a high complexity, which is not appropriate especially for the aerial base station. Instead,some pre coding techniques, such as zero-forcing beam forming (ZFBF) technology [29],are widely used due to their effectiveness and low complexity in precoding at the transmitter,and can also be applicable to reduce the interference between aerial base stations.

3.1.2 Call admission control (CAC)

In the HSAT network, several new calls with different resource requirements are proposed including horizontal handoff calls and vertical handoff calls of different coverage areas. According to the difference in available bandwidth, resource allocation and QoS negotiation mechanism of aerial network and terrestrial network for the multi user and multi service scene, an efficient CAC scheme should be proposed to avoid call blocking and QoS degradation caused by imbalanced resource allocation.

Wireless access technologies mainly contain High-Level Data Link Control (HDLC),Asynchronous Transfer Mode (ATM) and Consultative Committee for Space Data Systems (CCSDS). There are two ways for wireless communication network to implement admission control and management： distributed control method without center node and centralized control method. The distributed control method is flexible and robust, where any two nodes in a certain range can communicate with each other. But it also accompanied by incomprehensive coverage and it is difficult to obtain the exact position in high speed movement. The centralized control method is easy to be settled with large communication coverage, but the broken of the center node would result in the paralysis of whole network.

Fig. 4 (a) and (b) illustrate the HSAT network whether have the heterogeneous admission control of satellite network and aerial network or not. The heterogeneous admission control is used to reorder the data packets forwarded from satellites and aerial platforms.The key issue about using a complete centralized admission control method in the HSAT network is to solve excessive call blocking,which is easily produced in the coverage areas of aerial platforms. Pace et al. [30] propose an access policy based on dynamic power sharing to ensure the fairness of all cellular users where the call blocking occurs. Foo et al. [31]point out that different from the terrestrial network, a speed and direction adaptive CAC scheme make the hybrid network simpler instead. Luglio et al. [32] begin with TCP transmission in the HSAT network and present a cross layer CAC framework. The research on the CAC in the HSAT network is still premature currently so that the related achievements are limited. However, the HSAT network is a kind of heterogeneous wireless network in essence, many research works on CAC can be borrowed from the terrestrial heterogeneous network area [33, 34].

Fig. 4 (a) HSAT network without heterogeneous admission control

Fig. 4 (b) HSAT network with heterogeneous admission control

3.2 Transparent handover

Handover occurs when a mobile terminal moves from one cell to another in the communication process. Under the general circumstance, terrestrial communication system is in the active status and the aerial base station is closed instead. When emergency events occur, the aerial platform is activated. And it is important to select the appropriate network according to the current situation of each network and the user personal requirements.

The ultimate goal of HSAT network is to provide users with seamless and transparent communication coverage. A transparent handover is necessary for an end user to switch from the aerial network to the terrestrial network and vice versa. There is no pause in the service period. Compared with satellite communication, aerial communication can greatly reduce the transmission delay and path loss, the aerial platforms provide services for terminal equipments as relays. However, the resources in aerial base station are generally limited, it is essential to adopt an efficient and effective handover scheme to support more users and ensure QoS.

The design of a reliable handover mechanism should consider the difference among satellite communication systems, aerial communication systems and terrestrial communication systems in power and delay. Sadek et al. [35] put forward an adaptive handover algorithm by calculating the probability that the received signal strength is below the set threshold. Park et al. [36] proposed an efficient coverage decision algorithm aiming at providing seamless connectivity and establishing a fully connected aerial network. A large number of literature have studied the hando-ver algorithms in hybrid satellite-terrestrial network [37-39]. The addition of the aerial platforms makes it necessary to consider the frequent handover between aerial network and terrestrial network.

3.3 Promising Technologies for a HSAT Network

Within the traditional terrestrial networks, new communication technologies have been proposed for the evolution of PPDR. Some novel technologies, such as LTE, software-defined network (SDN), device to device (D2D), software-de fined radio (SDR) and cognitive radio(CR) is worth to be involved into the HSAT network.

3.3.1 LTE

LTE is one of the mainstream technologies in the future commercial network by providing packet transmission, high data rate, low latency and wide coverage. LTE could provide more applications and services at a lower price for users and operators in a commercial environment. In the field of public safety, LTE has a strong advantage in the excellent interoperability with other networks and high performance in terms of delay, data rate and spectral efficiency [40]. Brokar et al. [41] discuss the coexistence of public safety network and the existing commercial LTE network, the public safety network has a priority in an emergency environment and the simulation results show that this shared mode could provide broader coverage. The HSAT network based on LTE is now well studied. Gomez et al. [42] verify the applicability of the HSAT network in different environments. Some achievements have also been made in the aspects of resource allocation [43] and system capacity estimation[44]. The combination of LTE technology and HSAT system architecture will be one of the most important ways in public safety network area.

3.3.2 SDN

Traditional network architecture has been unable to meet the growing demand of the network. SDN provides a new network framework which separate control plane from data plane. It introduces uniform data exchange standards and programming interfaces so that it can manage the whole network equipment uniformly to keep the substrate network infrastructure transparent to the application. SDN has many applications in the wired network,but it has not been widely applied in the wireless environment due to the lack of standard programmed wireless network data plane [45].One way is to use SDR, which processes the signals by using radio element together with the general processor as the embedded system software. Chen et al. [46] introduce the concept of control, forwarding and separation,and network virtualization. They put forward a new HSAT network with polymerized SDN control system to uniformly manage the whole network equipment in the aspect of network maintenance and resource allocation with the OpenFlow protocol.

3.3.3 D2D

D2D which transfers data directly between two terminals has a great potential in improving the spectrum efficiency, enhancing the user experience and expanding the communication application. As one of the key technologies in 5G, the characteristics of D2D should bene fit the public safety network. Al-Hourani et al. [47] confirm that D2D technology can eliminate the influence of disaster on the communication range. D2D can also be used in combination with other technologies. Software-de fined D2D communication framework is suitable to resolve the problems existing in energy consumption, damage of infrastructures and hot spot traffic condition [48]. Simulation results show that the D2D-based LTE network could get 3.8 times as much system throughput as ordinary LTE network [49]. Mozaffari et al. [50] study static unmanned aerial vehicles (UAV) and take it as aerial base station combined with D2D. The maximum system data rate can be obtained when the UAV flies at the appropriate height, and the appropriate reduction of the communication range could shorten the duration of covering the entire target area.

3.3.4 SDR and CR

SDR adopt the software reconfiguration method on fixed hardware platform to realize the flexible radio system. CR based on SDR is proposed regarding to the low frequency spectrum utilization in unlicensed bands [51].It can automatically sense the surrounding electromagnetic environment and allocate the spectrum effectively. ETSI TR 102 745 de fines the user requirement of SDR and CR for the public safety [52]. SDR could also improve the flexibility and con figurability of the aerial platform. SALICE project [53] establishes a HSAT communication architecture based on SDR, aimed at supporting enhanced communication between satellite, aerial platform and terrestrial components. From a technical point of view, the main challenge is therefore about the coexistence and combination of different communication systems and different network architectures. The research on the integration of CR and HSAT network is scarce until now,the main reason is that the spectrum allocation of CR in emergency communication has not been standardized.

IV. CHALLENGES AND CONCLUSIONS

4.1 Challenges

Although the HSAT network has a wide range of advantages, it still faces some practical challenges. From the technical perspective, the major challenges may include interoperability,QoS assurance and security issues.

4.1.1 Interoperability

Both traditional satellite communication networks and terrestrial communication networks have their own system architectures. The emergence of the aerial platform brings about the development of the aerial communication network. It is urgent to solve the interoperability among the three systems. The HSAT communication system is trying to establish a single system to ensure effective operation and reliable transmission through aerial platform.Thus the main challenges arise in the conformity of platform [54].

4.1.2 QoS assurance

QoS refers to the degree on the user satisfaction with services provided by the communication systems. QoS usually is measured in terms of delay, delay jitter, bandwidth and packet loss rate, etc. QoS is easy to be guaranteed in the case of wired network, but for the wireless network, especially the multi-layer HSAT network, QoS guarantee schemes need to be carefully designed and studied. The channel state information may not be accurately obtained due to the lossy channel characteristics between nodes in HSAT network,and network bandwidth, battery capacity and storage space are limited, all of these will affect QoS. It is essential to design a stable and reliable resource allocation scheme to meet QoS requirements.

4.1.3 Security

In traditional communication networks, the overall network architectures are usually stable and the link connections are fixed. In order to ensure a variety of services, some security measures have been added to the network,such as encryption, authentication, privilege management and firewall, etc. However, the traditional security mechanisms are not suitable for the temporary aerial base station and the connection with the terminals in the HSAT network. Limited energy, storage and power of aerial platforms and the dynamic connection among mobile users make it difficult to implement the complex security technology.Nevertheless, the much higher priority on the communication connection than the security in emergency situation leads to a decrease in the importance of the security issue currently.

4.2 Conclusions

Much achievement has been obtained in aerial platforms. Different from HAPs mainly applied in the field of military communications,LAPs demonstrate tremendous application prospects in the emergency communications with its low cost and simple deployment ways.Aerial communications develop toward the direction of high reliability, high data rate and miniaturization. The HSAT communication network consisting of terrestrial segments,satellite and aerial segments, compensates for some inherent defects of traditional terrestrial network with respect to the emergency events.It also reduces the pressure on the transmission capacity and construction costs of satellite communications. Radio resource management,transparent handover and the combination of promising technologies are the focus of research in the field of HSAT network. Interoperability, QoS guarantee and security issues as well add additional challenges to this HSAT network.

ACKNOWLEDGEMENTS

This material is supported by the National 863 Project under Grant No. 2015AA015701,National Nature Science Foundation of China under Grant No. 61421061).

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