Near-space communications: The last piece of 6G space–air–ground–sea integrated network puzzle

The advent of the fifth-generation (5G) technology has revolutionized the way we think about communication networks, enabling a plethora of new use cases and scenarios that were not possible with previous generations of wireless technology. The existing 5G communication networks predominantly rely on terrestrial communication infrastructure, which, however, exhibits several notable limitations such as capacity constraints, latency issues, and energy inefficiencies, necessitating this next-generation leap. The 6G aims to provide a comprehensive solution for a hyperconnected, intelligent, and immersive digital ecosystem, seamlessly integrating terrestrial, airborne, and spaceborne networks which is referred to as the space-air-ground-sea integrated network (SAGSIN). A traditional SAGSIN scenario mainly includes 3 segments: spaceborne network, airborne network, and terrestrial network. The classical SAGSIN architecture has shown remarkable capabilities in providing massive connectivity by integrating spaceborne, airborne, and terrestrial cellular networks, which, however, encounters inherent technical challenges against their respective practical implementation. On the other hand, the philosophy of near-space communications (NS-COM) is gradually emerging. The distinctive location of near space which is 20 ~ 100 km above Earth’s surface positiones at the core of the SAGSIN, thus allowing the NS-COM network to address the shortcomings of traditional spaceborne, terrestrial, and airborne networks. NS-COM present a promising addition to SAGSIN, making them the final piece of the SAGSIN puzzle. In a review article recently published in Space: Science & Technology, a group of scholars present a detailed review of NS-COM in order to provide researchers with a more comprehensive understanding of NS-COM and inspire further advances.

First, a comprehensive investigation on characteristics of NS-COM is provided in terms of deployment, coverage, channel characteristics, and its unique problems in comparison with SAGSIN (see Fig. 1). As for the deployment, terrestrial, airborne, NS-COM, and spaceborne networks rely on cables and cell towers, unmanned aerial vehicles (UAVs), high-altitude balloon or aircraft, and rocket launch, respectively. The deployment costs are high for terrestrial and spaceborne networks and moderate for airborne and NS-COM networks. Besides, terrestrial and airborne networks need frequent inspections and repairs while the NS-COM network needs less frequent maintenance and the spaceborne network maintenance are limited. As for temporal and spatial coverage, terrestrial network offers good continuity of service as they can operate continuously once established. Airborne network is suitable for temporary or emergency coverage. NS-COM network is not be as flexible and convenient as airborne network but offers longer endurance. Spaceborne network offer extensive coverage but may suffer from intermittent interruptions. Terrestrial network excels in providing localized coverage in urban and densely populated areas. Airborne network offers a unique advantage in extending coverage to remote and hard-to-reach regions but the coverage area remains limited. NS-COM provides extended coverage over larger geographic regions compared to airborne network. Spaceborne network can achieve global coverage. As for channel characteristics, terrestrial and airborne network primarily operates in sub-6-GHz bands, benefiting from moderate path loss and low transmission delay. StratoSats, on the other hand, utilize millimeter wave (mmWave) frequencies to achieve high data rates, while traditional satellites use multiple bands. In terrestrial and near-space network, interference is generally low, ensuring reliable communication. However, it is essential to consider environmental conditions, such as weather and attenuation, which may affect airborne and stratospheric layers. More characteristics are summarized in Table 2. NS-COM possesses distinct characteristics that render existing technologies used in spaceborne, airborne, and terrestrial networks unsuitable for near-space environments. The unique properties of NS-COM can be manifested in terms of channel conditions and beam reliability of the platforms. The atmospheric environment undergoes complex variations with altitude. All these factors that increase the complexity of the channel render traditional channel modeling impractical as a result. StratoSats can use large-scale phased antenna arrays at mmWave or higher frequencies for highly directional data transmission. However, narrow beamwidth requires precise beam alignment, leading to severe fading due to antenna misalignment from the slight rotations or wobbles of StratoSats under windy conditions.

Second, the key technology of NS-COM network is underscored.

(1) Channel modeling is a crucial aspect of communication system design as it involves creating mathematical representations of the transmission medium to predict signal behavior. The near-space environment introduces factors such as variable atmospheric conditions, high altitudes, and platform mobility which make accurate channel modeling in NS-COM network particularly challenging. Enabling multiple-input multiple-output (MIMO) technology in NS-COM network is a promising solution for improving spectral efficiency. Investigating the 3-dimensional (3D) StratoSat-MIMO channel model has become an indispensable aspect in NS-COM communication research.

(2) The application of random access techniques is crucial to efficiently manage the simultaneous access of multiple users competing for limited resources, especially in the NS-COM-network-assisted SAGSIN. The necessity arises from the challenges posed by a massive number of users attempting to access the network concurrently. Many scholars have put forward different schemes utilizing grant-free unsourced random access technique to meet the requirement of massive URLLC.

(3) Channel estimation is a crucial process in communication networks that involves predicting the characteristics of the transmission medium. The unique dynamics of the near-space environment, including variable atmospheric conditions and platform mobility, necessitate accurate channel estimation for the subsequent signal transmission. Several channel estimation and tracking schemes have been proposed to obtain accurate estimates of fast time-varying channels while reducing the training overhead caused by frequent channel estimation.

(4) In the NS-COM network, array-based beam management plays a vital role, particularly with the use of high frequencies like millimeter wave for ground-to-air and terahertz for air-to-air links. Regular monitoring and real-time adjustments of the narrow beams are necessary to maintain precise alignment, compensating for platform movements. This capability ensures reliable communication links, vital for NS-COM network stability in near-space environments.

(5) The introduction of introducing NS-COM network into SAGSIN presents challenges in fostering collaboration among heterogeneous networks and within the NS-COM network itself, particularly concerning joint beamforming and edge computing. Collaborative beamforming across diverse network layers enhances optimization flexibility, thereby elevating network throughput. In addition, a globally supported edge computing approach can effectively assist in joint decision-making for beam deployment, resource allocation, and interference mitigation.

Third, typical application scenarios of NS-COM network based on StratoSats are presented. First, structural expansion in SAGSIN communication (Fig. 4). StratoSats, acting as floating BSs, play a crucial role in providing diverse communication services to users in disaster and underserved areas (Fig. 4 (A)). StratoSats can also serve as valuable backhaul links in the network architecture (Fig. 4 (B)). StratoSats can act as reliable relay nodes, establishing connections among core network hubs and data centers (Fig. 4 (C)). StratoSats can play in future networks is serving as a relay for LEO satellites (Fig. 4 (D)). Second, civil aviation communication. StratoSats can extend communication services to aircraft over maritime routes and polar routes. By facilitating seamless communication between aircraft and ground-based operations, StratoSats can improve flight coordination, enhance safety measures, and optimize airspace utilization. Third, remote and urgent communication. StratoSats can provide service for users with obstructed ground signals. Emergency service based on StratoSat in disaster-prone regions, such as those affected by tsunamis, hurricanes, and earthquakes, can be achieved with relative ease and cost-effectiveness. StratoSats can improve high-speed rail communication since they serve a much larger area than the terrestrial network avoiding frequent cell switching. Fourth, weather monitoring. StratoSat’s maneuverable control and strong payload capacity make it well suited for carrying equipment such as radars and radio sondes (Fig. 6). Fifth, carbon neutrality. One promising approach to achieving high-spatiotemporal-resolution carbon emission maps is through the deployment of floating sensing node balloons at lower altitude compared to StratoSats, as shown in Fig. 7.

Finally, ten open issues and future prospects (Fig. 8) are put forward.

(1) StratoSat-to-ground direct links for mobile terminals. The hardware limitations of mobile phone antennas result in a lower-frequency band for direct communication with StratoSats, which can easily cause interference with existing ground-based mobile communication networks. In addition, limited antenna gain and a restricted number of RF chains in mobile devices constrain communication performance, indicating that the hardware capabilities need further improvement.

(2) Emerging antenna design: Reconfigurable MIMO and holographic MIMO. Reconfigurable MIMO can be introduced as terrestrial BS antennas to communicate with StratoSats. Since this technique has just been introduced in NS-COM, the pattern reconfiguration that aims to actively manage the transmission environment for NS-COM still needs to be modified and trained.

A holographic MIMO involves incorporating a vast quantity of small and cost-effective antennas or reconfigurable elements into a confined area, achieving a holographic array with a continuously spatial aperture, which leads to extremely high-performance computing requirements. Thus, it is imperative to investigate methodologies for integrating high-performance computing devices within the constrained payload capacity of StratoSats.

(3) Federated learning in NS-COM networks. There is a critical need for efficient resource distribution and scheduling techniques to oversee federated learning operations. Also, it is necessary to develop a gradient transmission policy with higher energy efficiency and lower latency.

(4) Maritime communication. Acting as a communication relay, StratoSats establish seamless links between ships, submarines, onshore facilities, and aircrafts, especially in areas lacking terrestrial BS service. This enables reliable and efficient communication for maritime operations and safety, addressing challenges posed by vast distances and natural barriers in sea regions. However, the maritime communication through NS-COM also encounters several challenges, including substantial interuser interference, insufficient bandwidth resources, limited downlink transmission rates for maritime users, and limited capacity of air–sea communication links.

(5) Electromagnetic spectrum sensing and adversarial game. Efficient algorithms and data processing techniques to manage large spectrum data volumes, electromagnetic compatibility to prevent interference with legitimate communications, and energy-efficient systems for extended operational endurance are still required.

(6) Integrated sensing and communications. The limited space onboard the StratoSat may lead to densely packed communication equipment and radar sensing devices, increasing the risk of electromagnetic interference. Moreover, integrated sensing and communications system requires addressing compatibility and coordination issues between different technologies to ensure overall performance and efficiency.

(7) StratoSat-based radar detection and imaging. Energy-efficient radar systems need to be designed, and the energy management of the StratoSat must be optimized to extend mission duration.

(8) NS-COM-assisted enhanced global navigation system. There is a need for effective integration of navigation and communication systems, ensuring stability and reliability.

(9) NS-COM-assisted intelligent unmanned system. However, adverse weather conditions, such as cloud cover, can affect the stability of communication links. Therefore, contingency plans for controlling communication link disruptions and strategies for link restoration after disruptions need to be devised.

(10) Free-space optical (FSO) communication. Although the direct LoS transmission inherent in FSO signals can effectively prevent eavesdropping in contrast to RF signals, FSO technology also grapples with sensitivity to atmospheric obstacles like clouds and fog.