The rapid growing data traffic brings more and more pressure to the wireless networks, which is predicted to increase by over 10,000 times in the next 20 years.However, currently, there is still large number of population without coverage of mobile service. In addition to the issue of coverage, future wireless networks also need to guarantee the service continuity for emerging services such as Machine-to-Machine and Internet of Things.Many emerging scenarios, such as Unmanned Aerial Vehicle (UAV) control, pipeline monitoring, or global container tracking, require the aid of wireless networks.However, existing networks are severely hindering these services from achieving their true potentials. Considering the deficiency in existing wireless networks nowadays,ubiquitous on-demand coverage is the key to ensure service continuity in future communication paradigm.

On the other hand, the cost of providing ubiquitous converge has also to be taken into account for mobile networks operators. The concept of a ``Long tail’ distribution is embodied in many innovative Internet business models. It indicates that the revenues may be made from infrequent usage if one can effectively cater to various individual demands. It is applicable to wireless networks operation as well, where the usages far from urban centers can also lead to considerable revenue. However, the cost of pure terrestrial coverage quickly becomes unbearable with the increa sing network requirements for those rural, remote and other areas.

Satellite network is an ideal solution to address such a dilemma. While the terrestrial networks can achieve high-speed data service at low cost, satellites may cover an area of thousands of kilometers in radius, providing coverage to otherwise inaccessible locations. Therefore,the satellite and terrestrial networks should be deeply integrated to realize the full benefits of each of them.The coexistence and cooperation between terrestrial and satellite networks are of great potential in future communication networks. As a matter of fact, the satellite radio access network has already been considered in thefifth-generation (5G) networks to be supported for Phase 2. Thus, investigations of innovative technologies in terrestrial-satellite networks are in great need to bring out its true potential.

Notable research groups are focusing on satellite applications in 5G, such as the 5G CHAMPION program (8 European and 13 Korean partners), mainly investigating integrated 5G radio-access, core-network and satellites.Meanwhile, FCC, ITU will discuss the Ka-band spectrum sharing between satellites and 5G in WRC 2019. Furthermore, China launched the Space Integrated Ground Network Program in 2016, as a national major project.On the other hand, in the satellite mobile communication field, integration, miniaturization and multifunction are the developing trends. In the past, the satellite stations are usually large scale with very low mobility, and the satellite can only realize the point to point communication. Recently, the Sat-Fi was developed by Globalstar,in which the mobile antenna and modem are integrated into a hotspot box that can establish a bridge between the satellite and smart phones. Meanwhile, the users’ terminals are also becoming increasingly miniaturized, compared with the past heavy and big satellite terminals with less functionality. As technology continues to develop,the Thuraya XT-Pro was produced by Thuraya, on which the satellite modem was designed as a phone companion,providing quite convenient experience to users.

A new vision of next generation networks (NGN) has been proposed by International telecommunication union(ITU), in which the integrated and/or hybrid satellite and terrestrial network is supposed to play important roles. Against this background, this special issue calls for papers on the topic of “Integrated Terrestrial-Satellite Networks” to explore the key technologies and possible applications in future terrestrial-satellite networks. In the second part, we have accepted three papers as follows.

The first paper, “Satellite Constellation Design with Multi-Objective Genetic Algorithm For Regional Terrestrial-Satellite Network”, investigated the problem of constellations design for regional terrestrial-satellite network to strengthen the coverage for incomplete terrestrial cellular network. In this paper, a regional satellite constellation design scheme with multiple feature points and multiple optimization indicators is proposed by comprehensively considering multi-objective optimization and genetic algorithm, and “the Belt and Road” model is presented in the way of dividing over 70 nations into three regular target areas. Following this, the authors formulate the optimization model and devise a multi-objective genetic algorithm suited for the regional area with the coverage rate under simulating, computing and determining. Meanwhile, the total number of satellites in the constellation is reduced by calculating the ratio of actual coverage of a single-orbit constellation and the area of targets. Moreover, the constellations’ performances of the proposed scheme are investigated with the connection of C++ and Satellite Tool Kit (STK). Simulation results show that the designed satellite constellations can achieve a good coverage of the target areas.

Satellite communication networks have been evolving from standalone networks with ad-hoc infrastructures to possibly interconnected portions of a wider Future Internet architecture. Experts belonging to thefifth-generation (5G) standardization committees are considering satellites as a technology to integrate in the 5G environment. Software Defined Networking (SDN) is one of the paradigms of the next generation of mobile and fixed communications. It can be employed to perform different control functionalities, such as routing, because it allows traffic flow identification based on different parameters and traffic flow management in a centralized way. The second paper, “The Impact of Delay in Software-Defined Integrated Terrestrial-Satellite Networks”, investigates the employment of the SDN paradigm in an integrated terrestrial-satellite network where three GEO satellites act as SDN controllers and the Data Plane is embedded in a LEO satellite constellation. A model to estimate the mean time required to complete the SDN control actions and to deliver the first packet of a new traffic flow is also proposed. The obtained results allow understanding which is the lower bound of the required mean delivery time. This value should be used by a user/application tofix the performance requirements in case the satellite component is used or by the network management to decide if traffic may be forwarded through satellite in a software-defined integrated terrestrial-satellite network.

Finally, the third paper “Satellite Communications Integration with Terrestrial Networks” is accepted for the contribution on the feasibility of seamless and efficient integration of terrestrial communication systems with satellite networks. A comprehensive system is designed and implemented in an emulation prototype, including standard 3GPP LTE core network functionality with its different layers: networking, data forwarding, control,management and monitoring and is validated through performance measurements. This work is a technical feasibility study of extending terrestrial communication systems with satellite networks as backhaul, increasing the energy efficiency, network robustness during natural disasters as well as being an alternative for peak-time data forwarding of the terrestrial communication services.Due to its global coverage property, terrestrial-satellite integration provides an obvious extension of communication services towards isolated and remote areas and an alternative for rural or highly distributed/highly mobile enterprise networks.

The editors would like to thank all the authors for their excellent research activity and also for having chosen this Special Issue for disseminating the outcomes of their work. We also appreciate the valuable time and carefully review work of the reviewers. Their expert knowledge of thefield has helped to strengthen the quality significantly. We hope this special issue can give an overview of the key issues of future terrestrial-satellite networks, and more works are expected to further explore this area.