To achieve global network coverage and the need for high-speed communication, the idea of providing Internet access from space has made a strong comeback in recent years. The low earth orbit (LEO) communication satellite constellation is once again on the stage of the world with its unique features and new technology. In order to provide faster and more affordable communication resources, low-orbit satellites need be customized to design satellites. The beam coverage design is essential to the user-customized design. This paper combines the user traffic demand model and the low-orbit satellite beam coverage model to analyze the impact of beam coverage characteristics on the performance of low-orbit satellite systems. The user traffic model bases on the user simulative distribution (uniform, normal) and the user geographic distribution (according to the AIS and ADS-B historical data acquired by STU-2B and STU-2C which are the LEO satellites launched in Sep, 2015, Jiuquan, China). The beam coverage model compares the OneWeb system to the SpaceX system. The beam coverage model takes the variability in performance induced by atmospheric conditions for the user links into account. Follow that this paper proposes a system method to simulate the two satellite system which described by the throughput, delay, access probability. Finally, the sensitivity of beam coverage to user diversification is summarized and discussed.
Laser diodes (LDs) are used in a wide range of applications, such as optical wireless communications and LIDAR. To meet the demanding requirements of LDs for high accuracy and stability of the injection current, a high-precision, high stability LD driver with overvoltage protection is proposed. A novel structure based on enhanced Howland current source is described: composite topology enhanced Howland current source (CTEHCS), which has the advantages of high precision, high stability, and extensive regulation range. A 20-bit DAC and high-precision reference source are used to form a front-stage DAC circuit for precise and stable voltage reference. A closed-loop feedback calibration loop is applied to eliminate significantly the absolute errors and auxiliary calibrating of the effect of power operational amplifier on the temperature rise of critical devices. An innovative overvoltage protection circuit is designed for the load side of the CTEHCS, and the protection range can be flexibly set to 4/5/6 V to avoid damage to loads such as LDs. The noise performance, accuracy and stability, modulation bandwidth, nonlinear error, overvoltage protection performance, and turn-on and turn-off time of the experimental prototype are described in detail.
As the number of non-geostationary orbit (NGSO) satellites continues to grow, interference with other communication systems, including radio astronomy systems (RASs), is becoming increasingly critical. In this study, an interference simulation framework was developed to analyse the potential impact of NGSO systems on RAS in accordance with the relevant International Telecommunication Union (ITU) regulations and recommendations. In addition to the simulation of interference generated by individual NGSO satellite systems, the framework also supports the analysis of aggregate interference from multiple NGSO satellite systems. By inputting satellite system parameters, including constellation configuration, user distribution and beam scheduling strategy, the framework is able to obtain interference probability distributions for a typical RAS ground station at different latitudes and observation directions. The simulation results provide a reference for the analysis of interference from NGSO satellite systems to RASs, and can also be used to guide the development of strategies to mitigate harmful interference to RASs.
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