Angular Spread Research Articles

Spray-assisted alignment of Layer-by-Layer assembled silver nanowires: a general approach for the preparation of highly anisotropic nano-composite films.

The present article focuses on the build-up and the properties of oriented silver nanowire monolayer films and Layer-by-Layer assembled multilayer films. We describe the template-free oriented spray-assisted assembly of silver nanowires at solid/air-interfaces using Grazing Incidence Spraying, a simple and versatile approach that allows the formation of highly oriented thin films with a tunable density and in-plane orientation. Depending on the spraying conditions the nematic order parameter, which describes the angular spread of misaligned nanowires, can be as high as 0.98 (a value of 1.00 corresponding to a perfectly parallel alignment). The combination with the Layer-by-Layer assembly allows building multilayer thin films possessing in-plane anisotropy. In order to demonstrate that the local alignment does not cancel out on the macroscopic scale but leads to direction-dependent properties, we use linearly polarized UV-Vis-NIR spectroscopy to probe the selective excitation of the transverse and longitudinal localized plasmon resonances of the nanowires. The polarization efficiency of the thin films increases strongly with the in-plane density, the degree of orientation, and the number of silver nanowire layers. Multilayer films containing 4 layers of nanowires oriented in the same direction reach a polarization efficiency of up to 97% in the near-infrared region.

Two-dimensional direction-of-arrival estimation of coherently distributed noncircular signals via symmetric shift invariance

In practical applications such as mobile communication, radar and sonar, the effect of angular spread on the source energy can no longer be ignored due to multipath phenomena. Therefore, a spatially distributed source model is more realistic than the point source mode in these complex cases. A lot of direction-of-arrival (DOA) estimation methods for distributed sources have been published. Whereas researches concentrated on the complex circular signal case, the noncircular property of signal can be employed to further improve the estimation performance, which has received extensive attention recently. To date, several low-complexity DOA estimation algorithms for two-dimensional (2D) coherently distributed (CD) noncircular sources have been proposed. However, all these algorithms need obtain the approximate shift invariance relationship between the sub-arrays by applying the one-order Taylor series approximation to the generalized steering vectors, which may introduce additional errors and affect the estimation accuracy. In this paper, a novel 2D DOA estimation algorithm based on the symmetric shift invariance relationship is proposed using the centro-symmetric three-dimensional (3D) linear arrays. Firstly, the extended array model is established by exploiting the noncircularity of the signal. Then, it is proved that the deterministic angular distribution function vector of the CD source has a symmetrical property for arbitrary centro-symmetric array, based on which the symmetric shift invariance relationships of extended generalized steering vectors are established in the three sub-arrays of 3D linear arrays. On the premise of such relationships, the center azimuth and elevation DOAs are obtained by the polynomial rooting method without spectral peak searching. Finally, the cost function implementing the parameter matching is constructed by the symmetric shift invariance relationship of the generalized steering vector of the whole array. Theoretical analysis and simulation experiment show that compared with the existing low-complexity algorithms, the proposed algorithm avoids the additional errors introduced by the Taylor series approximation, which allows it to achieve higher estimation accuracy with the small complexity cost. Moreover, the proposed algorithm can achieve omnidirectional angle estimation in the three-dimensional space.

3-D Spatial Modeling of Network Interference in Two-Tier Heterogeneous Networks

The multi-tier heterogeneous network (HetNet) architecture can potentially address the massive connectivity and high throughput demands of the emerging fifth generation (5G) of wireless networks. However, the inter-tier interference in HetNets is considered to be a major performance bottleneck. This paper proposes a geometry-based three-dimensional (3-D) stochastic channel model for the spatial characterization of the sum interference in a two-tier HetNet with small cells in tier-1 overlaid with massive multiple-input-multiple-output equipped macro-cell base stations in tier-2. The angular spreads of the interference and the desired signals are analyzed by using the theory of 3-D multipath shape-factors and analytical expressions are derived for their second-order fading statistics, viz: level-crossing-rate (LCR), average-fade-duration (AFD), spatial autocovariance, and the coherence distance. Further, analytical expressions to investigate the second-order fading statistics against signal-to-interference ratio are also derived. The validation of the derived analytical expressions is established through a comparison with computer-based simulations. To provide insights into the network sum interference mechanism, the LCR and AFD expressions are derived for the special case when the rate of fluctuation of the desired signal is much higher than that of the interference signal and vice versa. Furthermore, the impact of the model’s physical parameters, such as the link distance and the receiver’s direction of motion as well as the fading distribution parameters such as its intensity and shape factors on the fading statistics of the interference are evaluated. These results demonstrate that the elevation angle has a significant impact on the interference characterization in HetNet architectures such that it cannot be ignored in modeling emerging 5G communication scenarios.

THREE-DIMENSIONAL SHOCK STRUCTURE OF THE ORION KL OUTFLOW WITH IGRINS*

ABSTRACT We report a study of the three-dimensional (3D) outflow structure of a 15″ × 13″ area around the H2 peak 1 in Orion KL with slit-scan observations (13 slits) using the Immersion Grating Infrared Spectrograph. The datacubes have a high-velocity resolution (∼7.5 km s−1), provide high-contrast imaging within ultra-narrow bands, and enable the detection of the main stream of the previously reported H2 outflow fingers. We identified 31 distinct fingers in the H2 1−0 S(1) λ2.122 μm emission. The line profile at each finger shows multiple-velocity peaks with a strong low-velocity component around the systemic velocity at = +8 km s−1 and high-velocity emission ( = 45–135 km s−1), indicating a typical bow-shock. The observed radial velocity gradients of ∼4 km s−1 arcsec−1 agree well with the velocities inferred from large-scale proper motions, where the projected motion is proportional to the distance from a common origin. We construct a conceptual 3D map of the fingers with estimated inclination angles of 57°–74°. The extinction difference (ΔA v > 10 mag) between blueshifted and redshifted fingers indicates high internal extinction. The extinction, the overall angular spread, and the scale of the flow argue for an ambient medium with a very high density (105–106 cm−3), consistent with molecular line observations of the Orion Molecular Cloud core. The radial velocity gradients and the 3D distributions of the fingers together support the hypothesis of a simultaneous radial explosion of the Orion KL outflow.

Multiplexing efficiency for MIMO antenna‐channel impairment characterisation in realistic multipath environments

Multiple-input multiple-output (MIMO) systems in multipath environments suffer from high correlations (in the presence of small angular spreads of propagation waves and similar antenna patterns) and power imbalances (due to cross-polarisation discrimination of the propagation channel and possibly different antenna efficiencies). A simple performance metric that takes into account of the antenna-channel impairments on the MIMO system will be of great value for MIMO antenna characterisations. In this study, a generalised multiplexing efficiency (ME) is derived, which takes into account antenna pattern and polarisation mismatches in arbitrary multipath environments. In addition, it is shown that the generalised ME can characterise not only the throughput (or coded error rate) degradation but also the degradation of uncoded error rate. For verifying the generalised ME, the authors use both the correlation-based channel model (where correlations and power imbalances can be readily specified) and the more realistic WINNER+channel model (where correlations and power imbalances have to be indirectly controlled by changing the antenna setup).

Status of the ELIMED Beamline at the ELIMAIA facility

Laser-target acceleration represents a very promising alternative to conventional accelerators for several potential applications, from the nuclear physics to the medical ones. However, some extreme features, not suitable for multidisciplinary applications, as the wide energy and angular spreads, characterize optically accelerated ion beams. Therefore, beyond the improvements at the laser-target interaction level, a lot of efforts have been recently devoted to the development of specific beam-transport devices in order to obtain controlled and reproducible output beams. In this framework, a three years contract has been signed between the INFN-LNS (IT) and Eli-Beamlines-IoP (CZ) to provide the design and the realization of a complete transport beam-line, named ELIMED, dedicated to the transport, diagnostics and dosimetry of laser-driven ion beams. The transport devices will be composed by a set of super-strong permanent magnet quadrupoles able to collect and focus laser driven ions up to 70 MeV/u, and a magnetic chicane made of conventional electromagnetic dipoles to select particles within a narrow energy range. Here, the actual status of the design and development of these magnetic systems is described.

Spatial degrees-of-freedom in large-array full-duplex: the impact of backscattering

The key challenge for in-band full-duplex wireless communication is managing self-interference. Many designs have employed spatial isolation mechanisms, such as shielding or multi-antenna beamforming, to isolate the self-interference waveform from the receiver. Because such spatial isolation methods confine the transmit and receive signals to a subset of the available space, the full spatial resources of the channel may be under-utilized, expending a cost that may nullify the net benefit of operating in full-duplex mode. In this paper, we leverage an antenna-theory-based channel model to analyze the spatial degrees of freedom available to a full-duplex capable base station. We observe that whether or not spatial isolation out-performs time-division (i.e., half-duplex) depends heavily on the geometric distribution of scatterers. Unless the angular spread of the objects that scatter to the intended users is overlapped by the spread of objects that backscatter to the base station, then spatial isolation outperforms time division, otherwise time division may be optimal.

Surface plasmon polariton beams from an electrically excited plasmonic crystal.

Surface plasmon polariton (SPP) beams with an in-plane angular spread of 8° are produced by electrically exciting a 2D plasmonic crystal using a scanning tunneling microscope (STM). The plasmonic crystal consists of a gold nanoparticle (NP) array on a thin gold film on a glass substrate and it is the inelastic tunnel electrons (IET) from the STM that provide a localized and spectrally broadband SPP source. Surface waves on the gold film are shown to be essential for the coupling of the local, electrical excitation to the extended NP array, thus leading to the creation of SPP beams. A simple model of the scattering of SPPs by the array is used to explain the origin and direction of the generated SPP beams under certain conditions. In order to take into account the broadband spectrum of the source, calculations realized using finite-difference time-domain (FDTD) methods are obtained, showing that bandgaps for SPP propagation exist for certain wavelengths and indicating how changing the pitch of the NP array may enhance the SPP beaming effect.

Feasibility study and spatial–temporal characteristics analysis for 28 GHz outdoor wireless channel modelling

This study presents analyses of the feasibility for the 28 GHz band with a comprehensive overview of important channel parameters. A channel measurement system with a time synchronisation scheme is also introduced which is a sliding correlator channel sounder using 250 Mcps (Mega-chips-per-second) pseudo-random noise sequences. The system enables the observation of the behaviour of 28 GHz wave propagation not only to use of forming narrow beams in certain directions (classical narrow beam beamforming) but also omni-likely. This novel synchronised system makes it possible to analyse the wideband channel characteristics. The outdoor channel measurements are conducted considering various features such as foliage effects, signal outage, path loss and outdoor-to-indoor conditions. Furthermore, the spatial channel modelling for omni-like observations with temporal characteristics is firstly interpreted through analysing the essential channel modelling parameters, including the number of clusters, excess delay, root-mean-square (RMS) delay spread, power delay spectrum, and angular spread. In the authors’ measurements, the average number of clusters was 2.41 with standard deviation of 1.44. The average RMS delay spread was 36.06 ns with standard deviation of 46.73 ns. Furthermore, the mean values of the RMS azimuth spreads of the angle of departure and arrival were 6.65° and 24.04°, respectively.

Measurements and analyses of 28 GHz indoor channel propagation based on a synchronized channel sounder using directional antennas

This paper presents measurements and analyses of channel propagation to investigate the channel characteristics of the millimeter wave for the indoor mobile communication system. The candidate for the promising carrier frequency is 28 GHz. We conducted the measurements considering conditions for the line of sight and the obstructed line of sight. The measurement points are located in a typical office room that is one of the main scenarios for the channel model. For channel propagation analysis, a novel channel sounder with a time synchronization scheme is introduced to combine channel measurements from different directions using an automatic scanning measurement system. With this system, omni-like power delay profiles of 28 GHz communication was successfully obtained. Furthermore, we analyze the spatial-temporal channel characteristics of an indoor environment such as path loss, delay and angular distributions, delay and angular spreads, and cross-correlation between the large-scale channel parameters.

3-D Spatial Spread Quantifiers for Multipath Fading Wireless Channels

This letter proposes a novel generalized method to quantify the angular dispersion of radiated energy in realistic 3-D space. Based on the first three coefficients of the 2-D Fourier transform, simple mathematical expressions are derived to independently or jointly quantify the angular dispersion of energy in a reduced 2-D or realistic 3-D space, respectively. The proposed quantifiers include mean direction of arrival, circular variance, true standard deviation, angular spreads, angular constriction, and direction of maximum fading. The proposed quantifiers are helpful in establishing an appropriate mechanism to measure the angular dispersion in 3-D space. These quantifiers provide results in radians with true physical meaning unlike their counterparts in the literature. Moreover, an analysis on the impact of distribution truncation/distortion on the degree of accuracy in measuring the angular dispersion is presented which signifies the importance of true angular spread quantification. The proposed quantifiers can thus be considered as an appropriate goodness-of-fit criterion in the characterization of spatial statistics of multipath fading channels.

Channel Measurement and Packet-Level Modeling for V2I Spatial Multiplexing Uplinks Using Massive MIMO

Massive multiple-input multiple-output (MIMO) is a key technology in fifth-generation (5G) networks. By using a large number of antenna elements placed in a 2-D array, an evolved Node B (eNB) can form multiple vertical beams in the elevation domain and adaptively track moving vehicles. Thus, the throughput and spatial multiplexing efficiency in vehicle-to-infrastructure (V2I) access can be significantly increased. However, multipath spatial propagation in wireless channels results in the angular spread of the power arrival profile, which causes intersector interference in massive-MIMO beamforming and becomes a limiting factor for the system capacity. In this paper, we investigate the channel propagation characteristics and the intervehicle interference in the elevation-domain spatial multiplexing for V2I mobile access and develop a packet-level channel model that can be used for system design and network protocol simulations. First, a measurement campaign for the urban macrocell street uplink channels was performed using a 3-D MIMO channel sounder, and the elevation power spectra (EPSs) on the eNB side were measured. Then, the intersector interference in the vertical beamforming is evaluated based on the antenna array pattern and channel EPS. Finally, a packet-level channel model in the form of a finite-state Markov chain (FSMC) is proposed for the V2I elevation multiplexing uplinks. The model describes the received signal-to-interference ratio (SIR) variation when a vehicle is moving toward an eNB. The steady-state distribution of the FSMC is analytically derived and verified by simulations. Based on the stochastic process of the SIR, the concepts of “large/small-scale multiplexing fading” are proposed. The field measurement results in this work have revealed the spatial power arrival profiles of the V2I street channels that are critical for the MIMO system design, and the proposed packet-level channel model provides an enabling tool for the analysis and simulation of the V2I elevation multiplexing links in the 5G networks.

Grain Boundary Induced Bias Instability in Soluble Acene-Based Thin-Film Transistors.

Since the grain boundaries (GBs) within the semiconductor layer of organic field-effect transistors (OFETs) have a strong influence on device performance, a substantial number of studies have been devoted to controlling the crystallization characteristics of organic semiconductors. We studied the intrinsic effects of GBs within 5,11-bis(triethylsilylethynyl) anthradithiophene (TES-ADT) thin films on the electrical properties of OFETs. The GB density was easily changed by controlling nulceation event in TES-ADT thin films. When the mixing time was increased, the number of aggregates in as-spun TES-ADT thin films were increased and subsequent exposure of the films to 1,2-dichloroethane vapor led to a significant increase in the number of nuleation sites, thereby increasing the GB density of TES-ADT spherulites. The density of GBs strongly influences the angular spread and crystallographic orientation of TES-ADT spherulites. Accordingly, the FETs with higher GB densities showed much poorer electrical characteristics than devices with lower GB density. Especially, GBs provide charge trapping sites which are responsible for bias-stress driven electrical instability. Dielectric surface treatment with a polystyrene brush layer clarified the GB-induced charge trapping by reducing charge trapping at the semiconductor-dielectric interface. Our study provides an understanding on GB induced bias instability for the development of high performance OFETs.

An Angular Parameter Estimation Method for Incoherently Distributed Sources Via Generalized Shift Invariance

This paper proposes a new algorithm to estimate the nominal direction-of-arrival (DOA) and the angular spread of multiple incoherently distributed (ID) sources. With the general array manifold (GAM) model, the nominal DOAs are first separated from the original array manifold. Then, a generalized shift invariance property inside the array manifold is identified, based on which the nominal DOAs are obtained when the rank of a trickily formulated matrix drops. Next, the angular spreads are estimated from the central moments of the angular distribution. We also derive a polynomial rooting based search-free method for nominal DOA estimation. This method can greatly reduce the computational complexity. As compared with the popular ESPRIT-ID algorithm, the proposed algorithm can achieve higher accuracy, can handle more sources, and applies to a more general array structure. Extensive simulations are provided to demonstrate the superior performance of the proposed algorithm over the existing works.

A method to achieve rapid localised deep heating in a laser irradiated solid density target

Rapid heating of small buried regions by laser generated fast electrons may be useful for applications such as extreme ultraviolet (XUV) radiation sources or as drivers for shock experiments. In non-structured targets, the heating profile possesses a global maximum near the front surface. This paper presents a new target design that uses resistive guiding to concentrate the fast electron current density at a finite depth inside the target. The choice of geometry uses principles of non-imaging optics. A global temperature maximum at depths up to 50 μm into the target is achieved. Although theoretical calculations suggest that small source sizes should perform better than large ones, simulations show that a large angular spread at high intensities results in significant losses of the fast electrons to the sides. A systematic parameter scan suggests an optimal laser intensity. A ratio of 1.6 is demonstrated between the maximum ion temperature and the ion temperature at the front surface.

Design of the prototype of a beam transport line for handling and selection of low energy laser-driven beams

A first prototype of transport beam-line for laser-driven ion beams to be used for the handling of particles accelerated by high-power laser interacting with solid targets has been realized at INFN. The goal is the production of a controlled and stable beam in terms of energy and angular spread. The beam-line consists of two elements: an Energy Selection System (ESS), already realized and characterized with both conventional and laser-accelerated beams, and a Permanent Magnet Quadrupole system (PMQ) designed, in collaboration with SIGMAPHI (Fr), to improve the ESS performances. In this work a description of the ESS system and some results of its characterization with conventional beams are reported, in order to provide a complete explanation of the acceptance calculation. Then, the matching with the PMQ system is presented and, finally, the results of preliminary simulations with a realistic laser-driven energy spectrum are discussed demonstrating the possibility to provide a good quality beam downstream the systems.

Spreading and evolution behavior of coherent vortices of multi-Gaussian Schell-model vortex beams propagating through non-Kolmogorov turbulence

The explicit closed-form expressions for the beam width and angular spread of multi-Gaussian Schell-model vortex (MGSMV) beams propagating through atmospheric turbulence are derived in this paper. The spreading and evolution behavior of coherent vortices of MGSMV beams in non-Kolmogorov turbulence are investigated quantitatively by some typical numerical examples, where the evolution behavior of coherent vortices is stressed in particular. It is illustrated that MGSMV beams are more resistant to atmospheric turbulence than multi-Gaussian Schell-model (MGSM) non-vortex beams. By increasing the beam index of MGSMV beams, the deleterious turbulence effects can be reduced gradually. As MGSMV beams propagate in non-Kolmogorov turbulence, the position and number of coherent vortices are changeable. The impact of the beam index and fluctuations of atmospheric turbulence on the conservation distance of the topological charge is also explored in depth.

Insights into directional scattering: from coupled dipoles to asymmetric dimer nanoantennas.

Strong and directionally specific forward scattering from optical nanoantennas is of utmost importance for various applications in the broader context of photovoltaics and integrated light sources. Here, we outline a simple yet powerful design principle to perceive a nanoantenna that provides directional scattering into a higher index substrate based on the interference of multiple electric dipoles. A structural implementation of the electric dipole distribution is possible using plasmonic nanoparticles with a fairly simple geometry, i.e. two coupled rectangular nanoparticles, forming a dimer, on top of a substrate. The key to achieve directionality is to choose a sufficiently large size for the nanoparticles. This promotes the excitation of vertical electric dipole moments due to the bi-anisotropy of the nanoantenna. In turn, asymmetric scattering is obtained by ensuring the appropriate phase relation between the vertical electric dipole moments. The scattering strength and angular spread for an optimized nanoantenna can be shown to be broadband and robust against changes in the incidence angle. The scattering directionality is maintained even for an array configuration of the dimer. It only requires the preferred scattering direction of the isolated nanoantenna not to be prohibited by interference.

A Novel 3D Analytical Scattering Model for Air-to-Ground Fading Channels

A geometry-based three-dimensional (3D) novel stochastic channel model for air-to-ground (A2G) and ground-to-air (G2A) radio propagation environments is proposed. The vicinity of a ground station (GS) is modelled as surrounded by effective scattering points; whereas the elevated air station’s (AS) vicinity is modelled as a scattering-free region. Characterization of the Doppler spectrum, dispersion in the angular domain and second order fading statistics of the A2G/G2A radio communication channels is presented. Closed-form analytical expressions for joint and marginal probability density functions (PDFs) of Doppler shift, power and angle of arrival (AoA) are derived. Next, the paper presents a comprehensive analysis on the characteristics of angular spread on the basis of shape factors (SFs) for A2G/G2A radio propagation environments independently in both the azimuth and elevation planes. The analysis is further extended to second order statistics of the fading channel; where the behaviour of the level crossing rate (LCR), average fade duration (AFD), auto-covariance and coherence distance for the A2G/G2A radio propagation environment is studied. Finally, the impact of physical channel parameters, such as the mobility of AS, the height of AS, the height of GS and the delay of the longest propagation path, on the distribution characteristics of Doppler shift, angular spread and second order statistics is thoroughly studied.

Directional Fluorescence Emission Mediated by Chemically-Prepared Plasmonic Nanowire Junctions

The localized interaction between metallic nanostructures and surrounding fluorescent molecules can influence the emission characteristics of the molecule. With this hindsight, herein, by employing a Fourier optical fluorescence microscope, we experimentally show how fluorescence emission from molecules in the vicinity of a chemically prepared silver nanowire-dimer-junction can be directed in one or two channels. Measured forward-to-backward ratio of the fluorescence emission in a single channel was as high as 4.3 dB, and the observed polar and azimuthal angular spread was as narrow as 15° and 60°, respectively. Interestingly, the angle between the two emission channels mimicked the angle between the nanowires, thus, highlighting the prospect of geometrical control of the emitted light. These observations were further corroborated by three-dimensional finite-difference time-domain simulations. The presented results will have implications in momentum-space engineering of molecular fluorescence emission and can be extrapolated to single-emitter studies.