Multicell Edge Coverage Enhancement Using Mobile UAV-Relay

Unmanned aerial vehicle (UAV)-assisted communication is a promising technology in future wireless communication networks. UAVs can not only help offload data traffic from ground base stations (GBSs) but also improve the Quality of Service (QoS) of cell-edge users (CEUs). In this article, we consider the enhancement of cell-edge communications through a mobile relay, i.e., UAV, in multicell networks. During each transmission period, GBSs first send data to the UAV, and then the UAV forwards its received data to CEUs according to a certain association strategy. In order to maximize the sum rate of all CEUs, we jointly optimize the UAV mobility management, including trajectory, velocity, and acceleration, and association strategy of CEUs to the UAV, subject to minimum rate requirements of CEUs, mobility constraints of the UAV, and causal buffer constraints in practice. To address the mixed-integer nonconvex problem, we transform it into two convex subproblems by applying tight bounds and relaxations. An iterative algorithm is proposed to solve the two subproblems in an alternating manner. Numerical results show that the proposed algorithm achieves higher rates of CEUs as compared with the existing benchmark schemes

Confirming the 115.5-day periodicity in the X-ray light curve of ULX NGC 5408 X-1

The Swift/XRT light curve of the ultraluminous X-ray (ULX) source NGC 5408 X-1 was re-analyzed with two new numerical approaches, Weighted Wavelet $Z$-transform (WWZ) and CLEANest, that are different from previous studies. Both techniques detected a prominent periodicity with a time scale of $115.5\pm1.5$ days, in excellent agreement with the detection of the same periodicity first reported by Strohmayer (2009). Monte Carlo simulation was employed to test the statisiticak confidence of the 115.5-day periodicity, yielding a statistical significance of $> 99.98$ (or $>3.8\sigma$). The robust detection of the 115.5-day quasi-periodic oscillations (QPOs), if it is due to the orbital motion of the binary, would infer a mass of a few thousand $M_\odot$ for the central black hole, implying an intermediate-mass black hole in NGC 5408 X-1.Comment: 6 pages, 2 figures, submitted to Research in Astronomy and Astrophysics (RAA

Cosmological investigation of multi-frequency VLBI observations of ultra-compact structure in z∼3z\sim 3 radio quasars

In this paper, we use multi-frequency angular size measurements of 58 intermediate-luminosity quasars reaching the redshifts $z\sim 3$ and demonstrate that they can be used as standard rulers for cosmological inference. Our results indicate that, for the majority of radio-sources in our sample their angular sizes are inversely proportional to the observing frequency. From the physical point of view it means that opacity of the jet is governed by pure synchrotron self-absorption, i.e. external absorption does not play any significant role in the observed angular sizes at least up to 43 GHz. Therefore, we use the value of the intrinsic metric size of compact milliarcsecond radio quasars derived in a cosmology independent manner from survey conducted at 2 GHz and rescale it properly according to predictions of the conical jet model. This approach turns out to work well and produce quite stringent constraints on the matter density parameter $\Omega_m$ in the flat $\Lambda$CDM model and Dvali-Gabadadze-Porrati braneworld model. The results presented in this paper pave the way for the follow up engaging multi-frequency VLBI observations of more compact radio quasars with higher sensitivity and angular resolution.Comment: 10 pages, 5 figures, 2 table, accepted for publication in European Physical Journal

Calculation analysis of yaw bearings with a hardened raceway

The yaw bearing is a key support structure of wind turbines and is often exposed to substantial complex loads that cause damage and fatigue failure. Raceway surfaces accommodate high contact stress and require a hardening treatment. The hardened depth has a great influence on both the carrying capacity and fatigue life. We establish a whole finite element model of a yaw bearing and use non-linear springs instead of a ball to obtain the maximum contact load. The results of a strain gauge experiment and an empirical formula are compared to verify the spring model results. A local finite element model of a ball and raceway with different hardened depths is established to analyse the stress distribution and fatigue life. The raceway is divided into a hardened layer, transition layer, and core layer. An indentation experiment verifies the raceway model with different layers. The stress results are compared with Hertz contact theory, and the fatigue life results are compared with yaw bearing fatigue life theory. The influence of different hardened depths on the stress and lifetime of yaw bearings is analysed

Aqua­(4-nitro­phthalato-κO 1)bis­[2-(1H-pyrazol-3-yl-κN 2)pyridine-κN]­mangan­ese(II) hemihydrate

In the title compound, [Mn(C8H3NO6)(C8H7N3)2(H2O)]·0.5H2O, the Mn2+ ion is octa­hedrally coordinated by two 2-(1H-pyrazol-3-yl)pyridine ligands, one 4-nitro­phthalate ligand and one coordinated water mol­ecule leading to an overall MnN4O2 coordination environment. The two 2-(1H-pyrazol-3-yl)pyridine ligands, which deviate from planarity by 0.0187 (2) and 0.0601 (2) Å, make a dihedral angle of 81.90 (6)°. An intra­molecular N—H⋯O hydrogen bond occurs. Inter­molecular π–π stacking inter­actions with a face-to-face separation of 3.61 (1) Å between the 2-(1H-pyrazol-3-yl)pyridine ligands is observed. Additionally, O—H⋯O hydrogen bonding involving the uncoordinated water (which is situated on an inversion center), coordinated water mol­ecules and 2-(1H-pyrazol-3-yl)pyridine ligands leads to a three-dimensional network in the crystal structure