Small-and medium-sized unmanned aerial vehicles (UAVs) can fly for a short distance (<2 km) from a control station in a nonsegregated air space (altitudes < 100 m). It is of great interest to model the propagation channel under such condition, where there is an important influence from the environment. This paper presents multiple measurements carried out in low altitudes with a medium-sized UAV flying over a semiurban environment. Path loss exponent is given based on the measurements done at different altitudes and a height-dependent Rician factor model is proposed. The results clearly reveal the existence of two propagation zones with very distinct channel characteristics. The breakpoint indicates the height where the condition of the channel changes rapidly. At low altitudes, the obstacles generate a large amount of multipath and the propagation is greatly affected, while at higher altitudes the influence mitigates. Our results are useful for the modeling of low altitude air-to-ground (AG) propagation channels and the performance analysis of UAV-enabling AG communication systems, such as the channel capacity and the throughput.
The harmonic currents generated by the line side converter of high speed trains may cause the time varying resonances when the harmonic frequency is close to the resonance frequency of the power supply line. It may destroy the facilities in the power supply range and affect the operation of trains. To avoid the affection, the characteristic of resonance impedance is calculated and simulated with a chain network topology in AT-fed power supply system. To take the coupled relation between the system and trains, a fundamental power source and harmonic voltage source traction drive system combining model is proposed instead of the harmonic current source model. Take the CRH380A and Beijing-Shanghai high speed railway as illustration. In this paper, the regularity of resonance frequencies and the characteristic of resonance voltage are expounded and analyzed in details.
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