References of "Zhang, J"
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See detailOn the Application of Directional Antennas in Multi-Tier Unmanned Aerial Vehicle Networks
Zhang, J.; Xu, H.; Xiang, Lin UL et al

in IEEE Access (2019), 7

This paper evaluates the performance of downlink information transmission in three-dimensional (3D) unmanned aerial vehicle (UAV) networks, where multi-tier UAVs of different types and flying altitudes ... [more ▼]

This paper evaluates the performance of downlink information transmission in three-dimensional (3D) unmanned aerial vehicle (UAV) networks, where multi-tier UAVs of different types and flying altitudes employ directional antennas for communication with ground user equipments (UEs). We introduce a novel tractable antenna gain model, which is a nonlinear function of the elevation angle and the directivity factor, for directional antenna-based UAV communication. Since the transmission range of a UAV is limited by its antenna gain and the receiving threshold of the UEs, only UAVs located in a finite region in each tier can successfully communicate with the UEs. The communication connectivity, association probability as well as coverage probability of the considered multi-tier UAV networks are derived for both line-of-sight (LoS) and non-line-of-sight (NLoS) propagation scenarios. Our analytical results unveil that, for UAV networks employing directional antennas, a necessary tradeoff between connectivity and coverage probability exists. Consequently, UAVs flying at low altitudes require a large elevation angle in order to successfully serve the ground UEs. Moreover, by employing directional antennas an optimal directivity factor exists for maximizing the coverage probability of the multi-tier UAV networks. Simulation results validate the analytical derivations and suggest the application of high-gain directional antennas to improve downlink transmission in the multi-tier UAV networks. [less ▲]

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See detailPer-antenna constant envelope precoding and antenna subset selection: A geometric approach
Zhang, J.; Huang, Y.; Wang, J. et al

in IEEE Transactions on Signal Processing (2016), 64(23), 6089-6104

Constant envelope (CE) precoding can efficiently control the peak-to-average power ratio (PAPR) and improve the power efficiency of power amplifiers in large-scale antenna array systems. Antenna subset ... [more ▼]

Constant envelope (CE) precoding can efficiently control the peak-to-average power ratio (PAPR) and improve the power efficiency of power amplifiers in large-scale antenna array systems. Antenna subset selection (ASS), combined with CE precoding, can further improve power efficiency by using a part of antennas to combine the desired signal. However, due to the inherent nonlinearity, the joint optimization of CE precoding and ASS is very challenging and satisfactory solutions are yet not available. In this paper, we present new methods for CE precoding and ASS optimization from a geometric perspective. First, we show the equivalence between the CE precoder design and a polygon construction problem in the complex plane, thus transforming the algebraic problem into a geometric problem. Aiming to minimize the computational complexity, we further transform the CE precoder design into a triangle construction problem, and propose a novel algorithm to achieve the optimal CE precoder with only linear complexity in the number of used antennas. Then, we investigate the joint optimization of ASS and CE precoding to minimize the total transmit power while satisfying the QoS requirement. Based on the geometric interpretation, we develop an efficient ASS algorithm, which, using only addition and comparison operations, is guaranteed to find the globally optimal solution and provides robustness to channel uncertainty. The complexity of the proposed ASS algorithm is at most quadratic in the number of antennas in the worst case. The optimality and superiority of the proposed geometric methods are demonstrated via numerical results. [less ▲]

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See detailImpact of thermal annealing on nonequilibrium carrier dynamics in single-crystal, freestanding GaAs mesostructures
Mikulics, Martin; Hardtdegen, Hilde; Adam, Roman et al

in Semiconductor Science & Technology (2014), 29

We report on the impact of thermal annealing to carrier transport and transient, subpicosecond photoresponse of freestanding GaAs mesostructures. Our measurements included micro-photoluminescence and dark ... [more ▼]

We report on the impact of thermal annealing to carrier transport and transient, subpicosecond photoresponse of freestanding GaAs mesostructures. Our measurements included micro-photoluminescence and dark current and responsivity studies as well as optical femtosecond characterization. The fabricated GaAs mesostructures consisted of both mesowires and platelets that were integrated into coplanar striplines to form a photoconductive switch. We demonstrate that an optimized annealing process of our mesostructures, performed at 600 ◦C for 20 min, led to restoring bulklike properties of our freestanding devices. They exhibited dark currents below 600 pA at 10 V bias, responsivity of 0.2 A W−1 at 30 V, and mobility as high as 7300 cm2 V s−1. The annealed freestanding GaAs photodetectors were characterized by subpicosecond carrier relaxation dynamics with negligible trapping and a cutoff frequency of 1.3 THz. The latter characteristics make them excellent candidates for THz-bandwidth optoelectronics. [less ▲]

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See detailGaAs nanowhiskers for femtosecond photodetectors and THz emitters
Mikulics, Martin; Zhang, J.; Sobolewski, Roman et al

in ASDAM 2012, The Ninth International Conference on Advanced Semiconductor Devices and Microsystems (2012)

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See detailMonolithic Integration of Ultrafast Photodetector and MESFET in the GaN Material System
Mikulics, M.; Kordoš, P.; Gregušová, D. et al

in IEEE Photonics Technology Letters (2011), 23(17), 1189-1191

We have fabricated and characterized ultrafast metal–semiconductor–metal (MSM) photodetectors integrated with metal–semiconductor–field-effect-transistors (MESFETs) integrated in coplanar strip lines in ... [more ▼]

We have fabricated and characterized ultrafast metal–semiconductor–metal (MSM) photodetectors integrated with metal–semiconductor–field-effect-transistors (MESFETs) integrated in coplanar strip lines in the GaN/AlN/SiC material system. We recorded electrical transients of the single photodetector as short as 0.9 ps wide by optoelectric pump–probe measurements using 360-nm-wavelength and 100-fs-duration laser pulses. Electric photoresponse transients of the photodetector with 6-mV peak amplitude were amplified by the MESFET, resulting in 4-ps-wide and 35-mV peak amplitude signals. This monolithically integrated optoelectronic circuit is presented as a potential candidate for high-speed ultraviolet optoelectronics [less ▲]

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