With the rapid development of satellite-enabled Internet of Things (IoT), malicious interference has become an inevitable issue, especially in the uplink due to the exposed satellite location. To mitigate uplink interference, this paper investigates the beam nulling technique for multi-beam satellite communication systems equipped with hybrid analog and digital (HAD) arrays. Unlike fully digital arrays, the beamforming for HAD architectures consists of a high-dimensional analog part implemented exclusively with phase shifters and a low-dimensional digital part with adjustable amplitude and phase, introducing the challenge of limited degrees of freedom. Despite extensive research on HAD beamforming, there is no consensus on the efficient paradigm to implement beam nulling against extremely strong malicious interference. In this paper, we first verify that phase-only beam nulling is an achievable paradigm that fully exploits the beam utilization for multi-beam satellites while effectively mitigating interference. Additionally, we design two normalized phase-only beam nulling approaches based on majorization minimization and per-antenna optimization respectively. Simulation results substantiate our analysis concerning the beam nulling paradigm for HAD arrays, and validate the superiority of our proposed approaches which require significantly lower computational complexity. Furthermore, a prototype is developed to demonstrate the practical effectiveness of the proposed phase-only beam nulling approaches.

This paper proposes a novel angle-of-arrival (AoA) estimation approach for planar antenna arrays with hybrid analog and digital architectures. Although such array structure can significantly reduce system cost and hardware complexity compared to the traditional fully digital arrays, in terms of AoA estimation, the available degree-of-freedom (DoF) is also reduced since the received signals are combined in the analog domain before digital processing, introducing fundamental challenges including angle ambiguity and limited multi-source resolution capability. To overcome these limitations, we propose a two-stage coprime matching estimator that systematically eliminates angle ambiguity through strategic analog beamforming and effectively extracts the exact AoA information even in challenging multi-source scenarios. Furthermore, we derive the closed-form expression of the Cramér-Rao lower bound (CRLB) for two-dimensional AoA estimation in planar hybrid arrays, providing a theoretical performance benchmark that accounts for both azimuth and elevation angles. Numerical simulations demonstrate that our proposed approach achieves superior multi-source resolution and estimation accuracy compared to state-of-the-art methods, while asymptotically approaching the CRLB.

Nongeostationary orbit (NGSO) constellations characterized by seamless coverage and high throughput have been envisioned as a promising solution to 6G, where beam hopping (BH) technique plays an indispensable role to achieve on-demand service. However, owing to the dynamic topology and uneven terrestrial traffic, how to effectively manage multisatellite time-frequency-spatial beam resources to match the heterogeneous demands is challenging for NGSO satellite operators. This article proposes a novel return-link BH scheduling framework for NGSO mega-constellations, where the ground resource management center periodically executes BH scheduling for users in Earth-fixed cells to improve the long-term average service satisfaction (ASS) while reducing the intersatellite handover. Specifically, we decouple the NP-hard scheduling problem into three subproblems, namely, user-satellite association problem, BH pattern design problem and resource block (RB) allocation problem. User-satellite association problem is solved by a low-complexity heuristic algorithm to reduce the handover and avoid intersatellite interference. Furthermore, a graph-based maximum weight method and an iterative scheme based on successive convex approximation are developed to obtain the BH patterns and RB allocations, respectively. Simulation results demonstrate that compared with other benchmarks, such as the best channel association and round-robin BH schemes, the proposed method can reduce the satellite load unevenness by 60% and improve the ASS by 35% while maintaining a low handover overhead.

Mobile user equipment (UE) which can connect to satellites directly attracts increasing attention. Power consumption is one of the most significant challenges for mobile UEs. In existing satellite communication systems, beam hopping technology has been utilized to improve the satellite efficiency, but the power consumption of UEs is ignored. Discontinuous reception (DRX) mechanism, wherein UEs turn off their receivers intermittently to save power, has been researched and utilized in terrestrial communication systems. However, the satellite efficiency will decrease if the DRX mechanism is applied to beam hopping satellite systems directly. In this paper, we propose a novel DRX mechanism for beam hopping satellite systems (DRX-BHS) and a new beam hopping operation strategy combining traffic-driven strategy and pre-scheduled strategy. Besides, a new pre-scheduled beam hopping strategy based on UE grouping is proposed, which can overcome the limitation of the satellite efficiency in traditional pre-scheduled beam hopping strategy. A semi-Markov model is established to evaluate the performance of the proposed DRX-BHS mechanism. The numerical results reveal the relationship between the performance and the number of UEs in each group, and show that our proposed DRX-BHS mechanism can balance the satellite efficiency and UE power saving.

Flexible spot beams have been widely utilized in satellite communication systems to project high power and reuse frequency spectrum. To improve the system efficiency, beams share the same frequency channel and are only steered to the user terminals (UTs) which need to be served. Satellites with numerous beams can serve large numbers of UTs simultaneously, which may lead to severe inter-beam interference because of the non-uniform distributed UTs. Therefore, it is significant to study the effect of beam configuration parameters on the coverage performance to design an efficient satellite communication system. In this paper, we derive analytical expressions for the coverage probability and the average data rate of a flexible beam using stochastic geometry tools. We model the locations of beam centers as a nonhomogeneous spherical binomial point process whose intensity depends on the density of UTs. Meanwhile, the average sum rate of a satellite is derived using two different models of the locations of satellites. From the numerical results, the average sum rate of a satellite first increases but then levels off as the number of beams increases, which may provide insightful design guidelines on satellite communication systems.

In nongeostationary orbit (NGSO) satellite networks, multicast transmission is increasingly important to improve the system performance. Making full use of the beams to cover numerous nonuniform distributed users is a new challenge. User subgrouping techniques are utilized to optimize the coverage of beams and maximize the data rate. However, due to the high-speed movement of NGSO satellites, frequent handovers occur and the user subgroups should be updated frequently, aggravating network control overheads. This article proposes an Earth-fixed multicast user subgrouping scheme for NGSO satellite constellations to reduce network control overheads and improve network throughput (NT). In our proposed scheme, users are partitioned into multiple subgroups according to their geographical locations, and these subgroups remain the same while satellites keep moving along the orbit. First, we formulate a joint user subgrouping and beam optimization problem and decompose it into three subproblems by the approximation of models. An iterative greedy Earth-fixed user subgrouping algorithm is presented to obtain the user subgroups. Each user is chosen into a user subgroup which can maximize the NT, and then the position of each user subgroup is updated according to the positions of users in this subgroup. Next, the subgroup-satellite association relationship is optimized according to the maximum elevation angle criterion. Finally, a simple algorithm is proposed to optimize the direction and beamwidth of the beam serving each subgroup in each satellite. Simulation results demonstrate that our proposed scheme can reduce the update frequency significantly compared with the existing satellite-fixed user subgrouping scheme, and the NT achieved by our proposed scheme is improved by 1~3 times than conventional beam coverage scheme.

The communication satellite equipped with phased array antennas can produce high power density by narrow spotbeams and thus can achieve high data rates. A large number of narrow spotbeams are required when the satellite provides multicast service for a group of distributed ground users, which limits the system capacity. In this paper, we propose a new satellite multicast scheme as taking advantage of beams with flexible direction and beamwidth generated by phased array antennas. Users are partitioned into multiple groups and an appropriate beam is allocated to each group, where all users in the same group are located in the mainlobe of the beam. It is formulated to jointly optimize user grouping and beamwidth for maximizing the average data rate of satellite and guaranteeing the quality-of-service (QoS) for users. Moreover, we develop an iterative algorithm to solve the problem with low complexity. Finally, simulation results show that our proposed method is superior to other schemes in terms of average data rate.