Te Wei’s research while affiliated with Tsinghua University and other places

Terrestrial communication networks mainly focus on users in urban areas but have poor coverage performance in harsh environments, such as mountains, deserts, and oceans. Satellites can be exploited to extend the coverage of terrestrial fifth-generation (5G) networks. However, satellites are restricted by their high latency and relatively low data rate. Consequently, the integration of terrestrial and satellite components has been widely studied, to take advantage of both sides and enable the seamless broadband coverage. Due to the significant differences between satellite communications (SatComs) and terrestrial communications (TerComs) in terms of channel fading, transmission delay, mobility, and coverage performance, the establishment of an efficient hybrid satellite-terrestrial network (HSTN) still faces many challenges. In general, it is difficult to decompose a HSTN into a sum of separate satellite and terrestrial links due to the complicated coupling relationships therein. To uncover the complete picture of HSTNs, we regard the HSTN as a combination of basic cooperative models that contain the main traits of satellite-terrestrial integration but are much simpler and thus more tractable than the large-scale heterogeneous HSTNs. In particular, we present three basic cooperative models, i.e., model X, model L, and model V, and provide a survey of the state-of-the-art technologies for each of them. We discuss future research directions towards establishing a cell-free, hierarchical, decoupled HSTN. We also outline open issues to envision an agile, smart, and secure HSTN for the sixth-generation (6G) ubiquitous Internet of Things (IoT).

With the rapid development of marine activities, there has been an increasing number of Internet of Things (IoT) devices on the ocean. This leads to a growing demand for high-speed and ultra-reliable maritime communications. It has been reported that a large performance loss is often inevitable if the existing fourth-generation (4G), fifth-generation (5G) or satellite communication technologies are used directly on the ocean. Hence, conventional theories and methods need to be tailored to this maritime scenario to match its unique characteristics, such as dynamic electromagnetic propagation environments, geometrically limited available base station (BS) sites and rigorous service demands from mission-critical applications. Towards this end, we provide a survey on the demand for maritime communications enabled by state-of-the-art hybrid satellite-terrestrial maritime communication networks (MCNs). We categorize the enabling technologies into three types based on their aims: enhancing transmission efficiency, extending network coverage, and provisioning maritime-specific services. Future developments and open issues are also discussed. Based on this discussion, we envision the use of external auxiliary information, such as sea state and atmosphere conditions, to build up an environment-aware, service-driven, and integrated satellite-air-ground MCN.

Terrestrial communication networks can provide high-speed and ultra-reliable services for users in urban areas but have poor coverage performance for the ubiquitous Internet of Things (IoT) in harsh environments, such as mountains, deserts, and oceans. Satellites can be exploited to extend the coverage of terrestrial fifth-generation (5G) and beyond networks. However, satellites are restricted by their high latency and relatively low data rate. Hence, the integration of terrestrial and satellite components, taking advantage of both networks, has been widely studied to enable seamless broadband coverage. Due to the significant difference between satellite communications (SatComs) and terrestrial communications (TerComs) in terms of channel fading, transmission delay, mobility, and coverage performance, the establishment of an efficient hybrid satellite-terrestrial network (HSTN) still faces many challenges. In general, it is difficult to decompose a HSTN into the sum of separated satellite and terrestrial links, due to complicated coupling relationships therein. To uncover the complete picture of HSTNs, we regard the HSTN as a combination of basic cooperative models, which contain the main traits of satellite-terrestrial integration, but are much simpler and thus more tractable than the whole network. Particularly, we present three basic cooperative models for HSTNs and provide a survey of the state-of-the-art technologies for each of them. We investigate some main problems and their solutions, including cooperative pattern, performance analysis and resource management issues. We also discuss open issues to envision an agile, smart, and secure HSTN for the sixth-generation (6G) ubiquitous IoT.

To satisfy the growing demand for broadband maritime communications, space-ground integrated maritime communication networks (MCNs) arise, which are envisioned to take full advantage of both satellites and terrestrial shore-based networks. In practice, due to the frequent beam hopping of satellites and the limited number of geographically available onshore base-station sites, the space-ground integrated MCN usually presents a highly non-cellular network structure, leading to both challenging blind zones and coverage areas with severe interference. In this paper, we optimize the coverage performance by exploiting the marine environment information. Particularly, the transmit antenna correlation is estimated using the location and mobility information of scatterers on the sea, such as lighthouses, reefs, islands, and vessels. The position and attitude information of satellites are also utilized for interference estimation. Based on that, we optimize the input covariance and precoding matrix to maximize the ergodic sum capacity for all mobile terminals within the coverage. It is a complicated non-convex problem, especially because the ergodic sum capacity is difficult to be expressed straightly without the expectation operator.We introduce an upper bound of the ergodic sum capacity using the path loss and the transmit antenna correlation estimated from the environment information, and then propose an iterative algorithm to solve the problem by solving a set of convex subproblems. The proposed environment-aware scheme is evaluated using the real-world geographic information of a coastal area of China. Simulation results demonstrate that the proposed scheme can greatly improve the ergodic sum capacity and the energy efficiency compared with existing approaches, and achieve dynamic coverage to match the non-cellular network structure.

Energy efficiency is a crucial issue for maritime communications, due to the high path loss and limited number of geographically available base station sites. Different from previous studies, we promote the energy efficiency by exploiting the specific characteristics of maritime channels and user mobility. Particularly, we utilize the shipping lane information of marine users to estimate their large-scale channel state information. Based on that, the resource allocation is jointly optimized for all users over all time slots during the voyage, which is a mixed 0-1 non-convex programming problem. We transform this problem into a convex one by means of variable substitution and time-sharing relaxation, and propose an iterative algorithm to solve it based on the Lagrangian dual decomposition method. Simulation results demonstrate that the proposed scheme can significantly reduce the power consumption compared with existing approaches, due to the global optimization over a much larger time span by utilizing the shipping lane information.

With the rapid development of marine activities, there has been an increasing number of maritime mobile terminals, as well as a growing demand for high-speed and ultra-reliable maritime communications to keep them connected. Traditionally, the maritime Internet of Things (IoT) is enabled by maritime satellites. However, satellites are seriously restricted by their high latency and relatively low data rate. As an alternative, shore & island-based base stations (BSs) can be built to extend the coverage of terrestrial networks using fourth-generation (4G), fifth-generation (5G), and beyond 5G services. Unmanned aerial vehicles can also be exploited to serve as aerial maritime BSs. Despite of all these approaches, there are still open issues for an efficient maritime communication network (MCN). For example, due to the complicated electromagnetic propagation environment, the limited geometrically available BS sites, and rigorous service demands from mission-critical applications, conventional communication and networking theories and methods should be tailored for maritime scenarios. Towards this end, we provide a survey on the demand for maritime communications, the state-of-the-art MCNs, and key technologies for enhancing transmission efficiency, extending network coverage, and provisioning maritime-specific services. Future challenges in developing an environment-aware, service-driven, and integrated satellite-air-ground MCN to be smart enough to utilize external auxiliary information, e.g., sea state and atmosphere conditions, are also discussed.

Energy efficiency is a crucial issue for maritime communications, due to the limitation of geographically available base station sites. Different from previous studies, we promote the energy efficiency by exploiting the specific characteristics of maritime channels and user mobility. Particularly, we utilize the shipping lane information to obtain the long-term position information of marine users, from which the large-scale channel state information is estimated. Based on that, the resource allocation is jointly optimized for all users and all time slots during the voyage, leading to a mixed 0-1 non-convex programming problem. We transform this problem into a convex one by means of variable substitution and time-sharing relaxation, and propose an iterative algorithm to solve it based on the Lagrangian dual decomposition method. Simulation results demonstrate that the proposed scheme can significantly reduce the power consumption compared with existing approaches, due to the global optimization over a much larger time span by utilizing the shipping lane information.

Different from conventional cellular networks, a maritime communication base station (BS) has to cover a much wider area due to the limitation of available BS sites. Accordingly the performance of users far away from the BS is poor in general. This renders the fairness among users a challenging issue for maritime communications. In this paper, we consider a practical massive MIMO maritime BS with hybrid digital and analog precoding. Only the large-scale channel state information at the transmitter (CSIT) is considered so as to reduce the implementation complexity and overhead of the system. On this basis, we address the problem of fairness-oriented pre-coding design. A max-min optimization problem is formulated and solved in an iterative way. Simulation results demonstrate that the proposed scheme performs much better than conventional hybrid precoding algorithms in terms of minimum achievable rate of all the users, for the typical three-ray maritime channel model.