Satellite communication plays a crucial role in supplementing blind areas and enhancing ubiquitous coverage, to meet the ubiquitous connectivity demands of the future 6G Internet of Things (IoT). As a vital means for integrated space-ground and the intelligent interconnectivity of everything in the future 6G system, Direct-to-Smartphone technology has emerged as a global development hotspot and got widespread attention. This paper analyzes various technical routes and numerous technological challenges of Direct-to-Smartphone technology, and innovatively proposes key technical solutions, such as air interface adapting to high-dynamics, spaceborne ultra-large array multi-beam antennas, on-demand scheduling of extremely narrow beams for random access (RA), and high-integration smartphone design. These solutions support to realize Direct-to-Smartphone technology to adapt the high-dynamic characteristic of Low Earth Orbit (LEO) satellites, provide all-area broadband service, satisfy high-performance and miniaturized. Finally, this paper outlines three potential development directions for Direct-to-Smartphone technology.

The space-air-ground integrated network (SAGIN) has received extensive attention due to its wide footprint, high communication capacity, and flexible networking modes. As every coin has two sides, wireless security in such a huge and heterogeneous network becomes more vulnerable than other traditional network architectures. Although the traditional encryption mechanism has guaranteed communication security to a certain extent, it brings many issues such as high computational cost, difficulty in key generation and distribution, compatibility of different encryption standards in heterogeneous networks, and consuming additional resources to guarantee the security, thus degrading the performance of ubiquitous access. Wireless endogenous security (WES) utilizes the dynamic, heterogeneous, and redundant (DHR) inherent security attributes of physical layer resources, such as channel, noise and interference, to achieve wireless security with autonomous defense. In this article, we firstly provide an overview of SAGIN and analyze its two critical requirements: ubiquitous access and secure communications. Then, we thoroughly investigate the concept, application, and potential solutions of WES to achieve symbiotic massive access and secure transmission in SAGIN. As a case study, a hybrid frequency-power-spatial domains (HFPSD) WES scheme is proposed to demonstrate the potential of enhancing efficiency and security of SAGIN with WES solutions. Finally, some future research directions of SAGIN and WES are discussed.

The collision of ranging signals in the process of massive concurrent access ranging seriously affects the networking speed of satellite communication networks. Random access schemes are usually used to reduce the collision probability, but their effects are limited due to the limitation of ranging access rules and the network size. To further improve the performance of access ranging, the idea of optimal access ranging parameters is introduced in this article. A multiobjective anti-collision algorithm (MOACA) is proposed to optimize the concurrent access ranging process of high-capacity nodes of Internet of Things (IoT) to multifrequency time-division multiple-access (MF-TDMA) satellite communication systems. The model of the ranging measurement process is put forward and the expressions of ranging measurement time, collision probability, and the number of ranging channels are derived. On this basis, a multiobjective optimization problem (MOP) of the access ranging process is established. In MOACA, an ideal point database is formed based on the solutions of the MOP problem with typical inputs, and the negotiation curves of objectives are introduced to obtain the most appropriate ideal point from the database under certain requirements and system states. Besides, MOACA also contains a single-objective problem (SOP) for achieving the ideal point in the situation where no matching pattern can be found in the database. The simulation result shows that MOACA can improve the performance of the concurrent access ranging process.

The deployment of low earth orbit (LEO) satellites megaconstellations presents a promising way for achieving global coverage and service, attributed to their comparatively low round-trip latency and launch costs. However, this surge in LEO satellite launches exacerbates the scarcity of the limited spectrum resources. Spectrum sharing between satellite constellations and terrestrial networks and beam hopping (BH) technology emerge as viable strategies to mitigate this spectrum shortage. To enhance spectrum efficiency and avoid serious inter-system interference, we investigate the beam hopping scheduling of satellites for interference avoidance. The beam hopping scheduling of the integrated satellite-terrestrial wireless networks system is formulated as throughput-driven beam hopping (TDBH) problem and satisfaction-rate-driven beam hopping (SDBH) problem, respectively. In particular, we decompose the TDBH problem into two sub-problems by relaxation, and a genetic algorithm (GA) is introduced to handle the SDBH problem. The impact of channel conditions and traffic load intensity on the satellite system throughput is analyzed in TDBH simulation. As for SDBH optimization problem, the simulation results show that the proposed GA algorithm improves the average traffic satisfaction rate by 16.96% at least, compared with other benchmarks and suits to scenarios with different traffic demands and fading channel conditions.

The co-frequency interference (CFI) of the nongeostationary orbit (NGSO) constellation to the geostationary Earth orbit (GEO) satellite Internet of Things (IoT) system is an nonnegligible problem. In this article, based on the calculation method of the NGSO constellation satellite distribution state period, we propose a global characteristic method to analyze the CFI of the NGSO constellation to the GEO satellite IoT system by spatio-temporal slices. In order to analyze and evaluate the global characteristic and overall situation of interference, the temporal and spatial cut set of interference is established by fragmenting time and gridding space. The correctness of the proposed NGSO constellation satellite distribution state period is verified by taking the OneWeb constellation’s interference to the SINOSAT-5 satellite communication system as the simulation scenario. Furthermore, the global interference characteristic of the OneWeb constellation to the SINOSAT-5 satellite communication system is analyzed with the method mentioned above in the range of 80 °E–140 °E and 0 °N–50 °N. The results show that the analysis method can reflect the overall situation of the CFI of the NGSO constellation to the GEO satellite IoT system.