Mega-constellations have the potential for providing 6G Internet owing to the unique advantage of global coverage. However, current satellite technologies are not omnipotent. There are still many challenging problems that need to be solved for mega-constellations to support 6G, e.g., efficient resource allocation, gratifying mobility management, and large-scale full-time TT&C (tracking, telemetry, and command). This paper starts with a novel definition of LEO mega-constellations and a brief review regarding the current typical mega-constellations, discussing the development direction of the mega-constellation air interface. Then, the key technologies development status of satellite networks is illustrated and analyzed from five aspects: network protocol, multiple access, satellite handover, TT&C, and interference mitigation, especially their adaptability in mega-constellations for 6G global coverage. Finally, considering the features and requirements of 6G, future challenges for mega-constellations and some potential solutions are proposed.

The performance of low earth orbit (LEO) satellite Doppler positioning is fundamentally dominated by the precision with which the Doppler frequency is estimated, generally using an explicit pilot from the received signal. This letter proposes an algorithm for the precise estimation of the LEO satellite downlink Doppler frequency based on the joint use of the explicit pilot signal and an implicit pilot, which is a signal segment opportunistically available in an LEO downlink frame with all zero bits, all one bits, or interleaved zero and one bits. The theoretical performance is derived, and simulation results show that the proposed algorithm reaches the Cramer-Rao lower bound (CRLB). Verifications are conducted on signals from the Iridium mobile satellite communications system. The results show that 58.41% of Iridium frames in the ring alert and paging channels have implicit pilots and that for frames with implicit pilots, the proposed algorithm reduces the Doppler frequency estimation error by 70% compared to algorithms that use only the explicit pilot.

In the upcoming 6G communication era, the satellite Internet based on mega constellations will become an indispensable extension of the terrestrial communication network. However, it is difficult for the traditional ground-based and geostationary earth orbit (GEO)-based telemetry systems to satisfy the requirements on monitoring the mega constellation. This paper designs the networked telemetry system, where data is transmitted through the inter-satellite-links (ISL) of the low earth orbit (LEO) and medium earth orbit (MEO) satellites. Furthermore, the resource allocation problem for the networked telemetry system is decomposed using the block coordinate descent method into the access scheduling and the subchannel-power coordinate allocation subproblems, which are iteratively solved to optimize the resource allocation scheme. Finally, the simulation results shows that the proposed resource allocation algorithm effectively increases the transmitted data amount of the system and approximates the upper bound performance.

Network slicing is one the key features of software-defined networks (SDNs) and can be used in next-generation communication networks. Admission control of network slices is the basis of providing the heterogeneous quality-of-service performance guarantee and maximizing the optimization objectives of the network operator. Various admission control mechanisms have been proposed in the literature, including those based on traffic forecasting. However, recurrent neural network-based probabilistic forecasting models have not been given thorough consideration for slice admission control. In this study, the network slicing scheme design problem is formulated mathematically, with an equivalent formulation of the constrained bandwidth-sharing scheme. Then, a DeepAR-based slice admission control mechanism is proposed for sequential decision making for network slice requests in SDN, with the support of the SDN controller. An improved variant is further proposed with a closed-loop parameter update mechanism. The experiments based on real-world historical traffic data validate the effectiveness of the proposed mechanisms, with metrics, including revenue, resource reservation and utilization ratios, and service admission ratio.

Recently, numerous mega LEO satellite constellation plans have emerged as an indispensable part supporting the future 6G satellite communications. Since the traditional telemetry systems are inappropriate for monitoring the operation status of all satellites, a networked telemetry system is adopted to achieve full time, low delay telemetry for mega LEO satellite constellations, which is a significant scenario of satellite Internet of Things. Furthermore, this article formulates a channel allocation problem to maximize the overall transmitted data amount. Adopting the dual decomposition method, this problem can be decomposed and transformed into a dual problem and multiple path scheduling subproblems for each LEO satellite. This article develops an optimal dynamic programming algorithm to solve the path scheduling subproblems and an iterative algorithm to solve the channel allocation problem. Numerical simulations show that the proposed channel allocation algorithm improves the transmitted data amount and approximates the upper bound performance.

In recent years, in order to provide worldwide broadband Internet access services, many mega-constellation projects have been proposed. Traditional ground-based and space-based telecommand systems only rely on a single path for data transmission, which is vulnerable once the path fails. This paper utilizes inter-satellite-links (ISLs) in the constellation network to reduce the risk of data transmission failure, and proposes the failure-tolerant and low-latency redundant multi-path routing algorithm (RMPR). RMPR transmits multiple data copies on multiple shortest edge-disjoint paths according to the optimal proportions with the minimum delay. Compared with traditional single-path routing and backup multi-path routing, RMPR ensures both reliability and timeliness in telecommand system.