Angular Diversity Research Articles

On the Performance of Adaptive MIMO-OFDM Indoor Visible Light Communications

In this letter, we propose an adaptive indoor multiple input and multiple output (MIMO) orthogonal frequency division multiplexing (OFDM) visible light communication (VLC) system using a receiver module with angular diversity. In order to improve the capacity of indoor MIMO-OFDM VLC systems, tilted receivers are utilized to increase channel diversity, thus reducing channel correlation. With the help of singular value decomposition-based technique, which decomposes the MIMO VLC channels into independent parallel sub-channels, adaptive resource allocation, namely, bit and power loading, is used for these sub-channels to further improve the proposed system’s capacity. Based on a $4\times 4$ indoor MIMO-OFDM VLC system, we investigate bit error rate (BER) performance of the proposed adaptive system with different polar angles in two typical indoor scenarios. Numerical simulation results show that with 50-MHz modulation bandwidth, average BER can be improved from $4.97\times 10^{\mathrm {-3}}$ to $1.66\times 10^{\mathrm {-5}}$ and from $1.90\times 10^{\mathrm {-3}}$ to $1.59\times 10^{\mathrm {-6}}$ for the two scenarios, respectively.

Quantitative phase imaging of biological cells using spatially low and temporally high coherent light source.

In this Letter, we demonstrate quantitative phase imaging of biological samples, such as human red blood cells (RBCs) and onion cells using narrow temporal frequency and wide angular frequency spectrum light source. This type of light source was synthesized by the combined effect of spatial, angular, and temporal diversity of speckle reduction technique. The importance of using low spatial and high temporal coherence light source over the broad band and narrow band light source is that it does not require any dispersion compensation mechanism for biological samples. Further, it avoids the formation of speckle or spurious fringes which arises while using narrow band light source.

Performance Analysis of Indoor Diffuse VLC MIMO Channels Using Angular Diversity Detectors

We consider specular and diffuse reflection models for indoor visible light communications using a mobile receiver with angular diversity detectors in multiple input multiple output (MIMO) channels. We aim to improve the MIMO throughput compared to vertically oriented detectors by exploiting multipath reflections from different surfaces in the room. We then evaluate data throughput across multiple locations in the small room by using repetition coding, spatial multiplexing, and spatial modulation approaches. In spatial modulation, we also propose a novel approach called adaptive spatial modulation. This makes use of channel matrix rank information to decide which TX/RX setup to be used, and is developed to cope with rank deficient channels. In a scenario, where the receiver is moving, channel gains are weak in some locations due to the lack of line of sight (LOS) propagation between transmitters and receivers. This effect is mitigated by employing adaptive modulation and coding together with per antenna rate control. We then compare the throughput for LOS only channels against LOS with specular or diffuse reflection conditions, for both vertical and angular oriented receivers. The results show that exploiting specular and diffuse reflections provide significant improvements in link performance.

Fourier optics analysis of phase-mask-based path-length-multiplexed optical coherence tomography.

Optical coherence tomography (OCT) is an imaging technique that constructs a depth-resolved image by measuring the optical path-length difference between broadband light backscattered from a sample and a reference surface. For many OCT sample arm optical configurations, sample illumination and backscattered light detection share a common path. When a phase mask is placed in the sample path, features in the detected signal are observed, which suggests that an analysis of a generic common path OCT imaging system is warranted. In this study, we present a Fourier optics analysis using a Fresnel diffraction approximation of an OCT system with a path-length-multiplexing element (PME) inserted in the sample arm optics. The analysis may be generalized for most phase-mask-based OCT systems. A radial-angle-diverse PME is analyzed in detail, and the point spread function, coherent transfer function, sensitivity of backscattering angular diversity detection, and signal formation in terms of sample spatial frequency are simulated and discussed. The analysis reveals important imaging features and application limitations of OCT imaging systems with a phase mask in the sample path optics.

Ultra-short longitudinal spatial coherence length of laser light with the combined effect of spatial, angular, and temporal diversity

We demonstrate ultra-high axial-resolution topography and tomography of multilayered objects using pseudo thermal light source, i.e., laser. The longitudinal spatial coherence (LSC) length of light was significantly reduced by synthesizing a pseudo thermal source with the combined effect of spatial, angular, and temporal diversity. Thus, generating a low spatially coherent (i.e., broad angular frequency spectrum) light source having narrow temporal frequency spectrum. The LSC length was reduced less than 10 μm using a very low magnification lens. Experimental results of optical sectioning of multilayer objects with high axial-resolution of the order of 4 μm was achieved which is comparable to broadband light source. The present system does not require any dispersion compensation optical system for biological samples as a highly monochromatic light source is used.

Design and analysis of an angular‐segmented full‐mobility visible light communications receiver

ABSTRACTThis paper presents the concept, design and analysis for a visible light communications receiver utilising the angular and spatial diversity in order to achieve full mobility and protection from signal blocking in a typical home/office environment. The proposed receiver uses seven sub‐receivers, each dedicated to an independent viewing angle and direction. Employing selection combining, the receiver with the strongest signal is used for the communications channel. Full performance evaluation and numerical analysis of the proposed system have been performed in MATLAB and compared to that of a single receiver. The received power, impulse response, corresponding signal‐to‐noise ratios and root‐mean‐square delay spread are also investigated. Copyright © 2014 John Wiley & Sons, Ltd.

Sparsely sampled Fourier ptychography

Fourier ptychography (FP) is an imaging technique that applies angular diversity functions for high-resolution complex image recovery. The FP recovery routine switches between two working domains: the spectral and spatial domains. In this paper, we investigate the spectral-spatial data redundancy requirement of the FP recovery process. We report a sparsely sampled FP scheme by exploring the sampling interplay between these two domains. We demonstrate the use of the reported scheme for bypassing the high-dynamic-range combination step in the original FP recovery routine. As such, it is able to shorten the acquisition time of the FP platform by ~50%. As a special case of the sparsely sample FP, we also discuss a sub-sampled scheme and demonstrate its application in solving the pixel aliasing problem plagued in the original FP algorithm. We validate the reported schemes with both simulations and experiments. This paper provides insights for the development of the FP approach.

A Compressive Sensing Approach to Multistatic Radar Change Imaging

This paper describes a new approach for forming change images from multistatic radar data based on compressive sensing (CS). Broadly speaking, change images are naturally sparse in the raw image domain, which suggests a CS reconstruction method. Recent results show that the sparsity of the estimand dictates the number of samples required for faithful reconstruction, meaning a change image can be formed with far fewer measurements than used for conventional radar imaging. Our application has a small number of antennas arranged around the perimeter of a surveillance region, which provide large angular diversity but very poor angular sampling. Furthermore, due to application constraints, the scene is interrogated with limited frequency diversity. We aim to construct a high-resolution change image from the measurements, which are sub-Nyquist both spatially and in frequency. This paper first develops a radar imaging model in the context of CS, and then shows with collected data that a sparseness model improves image utility over conventional methods in our setting.

Research on speckle denoising by lensless Fourier transform holographic imaging with angular diversity

The signal-to-noise ratio and resolution of the reconstructed image is seriously influenced by speckle noise in digital holography, so it is essential to reduce the speckle noise and improve the image quality. Intensity correlation between two speckle patterns with the rectangle speckle spot is analyzed and deduced, and the minimal angular difference of illumination beams is given quantitatively under a special situation. A lensless Fourier transform holographic imaging system with angular diversity is designed based on the lens property, in which the direction of the wave can be changed by shifting the fiber instead of conventionally rotating the mirror, and the formor can change the direction of illumination with a fixed illumination size and location. Eighty one holograms with uncorrelated speckle patterns are recorded at different illumination angles by shifting the fiber. Then a digital reconstruction is achieved only by a fast Fourier transform, and the multiple reconstructed images are averaged. Experimental results show that the speckle contrast of the averaged reconstructed image can be reduced to 14.6% that from a single reconstructed image, and the signal-to-noise ratio is improved 6.9 times. This proposed digital holographic imaging method can suppress the speckle noise greatly, has a simple setup, and is easy to operate.

Speckle noise suppression in digital holography by angular diversity with phase-only spatial light modulator

A speckle noise suppression method in digital holography is proposed by the angular diversity with a phase-only spatial light modulator (SLM). The minimal angular difference of illumination beams is quantitatively analyzed to ensure the noncorrelation of any two speckle patterns, and then the phase-only SLM is employed to generate a series of tilted illumination beams. Comparing with the typical methods, the tilted illumination beams are controlled dynamically and accurately, which makes it possible to record a large number of holograms. Finally, using an image-plane digital holographic system, 117 holograms are recorded respectively, and the synthesized reconstructed images are obtained with the greatly suppressed speckle noise which is in good agreement with the theoretical results. The experimental results demonstrate the effectiveness, repeatability, and practicability of the proposed approach.

Scattering and reverberation from fish schools in the 500–1500 Hz band

We will report on the results from an experiment off Cape Hatteras, North Carolina, to look at scattering and reverberation from fish schools in the 500–1500 Hz band. The experiment, which was performed during the period May 12–29, 2012, was a joint acoustics, biology, and physical oceanography effort, with distinct, but coordinated, goals in each area. Acoustically, we wished to examine the scattering of sound from fish schools over a full range of azimuthal angles. To do this, we employed a source mounted on an autonomous vehicle and a moored, four element array receiver. The source traveled around the fish school and the receiver, giving the desired angular diversity. Video images, sidescan sonar, and direct sampling of the school allowed us to quantify the in-situ scattering field. Estimates for attenuation and scattering versus azimuthal angle will be presented. Directions for analysis and further research will be discussed.

Scattering and reverberation from fish schools in the 500–1500 Hertz band

We report here on the preliminary results from an experiment off Cape Hatteras, North Carolina, to look at acoustic scattering and reverberation from fish schools in the 500–1500 Hz band. The experiment, which was performed during the period May 12–29, 2012, was a joint acoustics, biology and physical oceanography effort, with distinct, but coordinated, goals in each area. Acoustically, our goal was to examine the scattering of sound from fish schools over a full range of azimuthal angles. To do this, we employed a source mounted on an autonomous vehicle and a moored, four element hydrophone array receiver. The source traveled around the fish school and the receiver, giving the desired angular diversity. Biologically, we were interested in mapping and imaging/classifying the fish (both individually and as schools) with the sidescan sonars on the vehicles, and contrasting/verifying this information with video images from high definition cameras attached to the vehicles. Oceanographically, the correlation between the ocean temperature field and the fish species encountered was of first order interest. Results from all three areas will be presented, including some interesting video images, and directions for analysis and further research will be discussed. [ Work sponsored by the Office of Naval Research.]

UTILIZATION OF THE WAVEFRONT SENSOR AND SHORT-EXPOSURE IMAGES FOR SIMULTANEOUS ESTIMATION OF QUASI-STATIC ABERRATION AND EXOPLANET INTENSITY

This paper provides a framework for the incorporation of the wavefront sensor measurements in the context of observing modes in which the science camera takes millisecond exposures. In this formulation, the wavefront sensor measurements provide a means to jointly estimate the static speckle and the planetary signal. The ability to estimate planetary intensities in as little as few seconds has the potential to greatly improve the efficiency of exoplanet search surveys. Unlike currently used methods, in which increasing the observation time beyond a certain threshold is useless, this method produces estimates whose error covariances decrease more quickly than inversely proportional to the observation time. This is due to the fact that the estimates of the quasi-static aberrations are informed by a new random (but approximately known) wavefront every millisecond. The method can be extended to include angular (due to diurnal field rotation) and spectral diversity. Numerical experiments are performed with wavefront data from the AEOS Adaptive Optics System sensing at 850 nm. These experiments assume a science camera wavelength $\lambda$\ of $1.1 \, \mu$, that the measured wavefronts are exact, and a Gaussian approximation of shot-noise. The effects of detector read-out noise and other issues are left to future investigations. A number of static aberrations are introduced, including one with a spatial frequency exactly corresponding the planet location, which was at a distance of $\approx 3\lambda/D$\ from the star. Using only 4 seconds of simulated observation time, a planetary intensity, of $\approx 1$ photon/ms, a stellar intensity of $\approx 10^5$\ photons/ms (contrast ratio $10^5$), the short-exposure estimation method recovers the amplitudes static aberrations with a 1% accuracy, and the planet brightness with a with 20% accuracy.

λ-diverse nearest neighbors browsing for multidimensional data

Traditional search methods try to obtain the most relevant information and rank it according to the degree of similarity to the queries. Diversity in query results is also preferred by a variety of applications since results very similar to each other cannot capture all aspects of the queried topic. In this paper, we focus on the \lambda-diverse k-nearest neighbor search problem on spatial and multidimensional data. Unlike the approach of diversifying query results in a postprocessing step, we naturally obtain diverse results with the proposed geometric and index-based methods. We first make an analogy with the concept of Natural Neighbors (NatN) and propose a natural neighbor-based method for 2D and 3D data and an incremental browsing algorithm based on Gabriel graphs for higher dimensional spaces. We then introduce a diverse browsing method based on the distance browsing feature of spatial index structures, such as R-trees. The algorithm maintains a Priority Queue with mindivdist of the objects depending on both relevancy and angular diversity and efficiently prunes nondiverse items and nodes. We experiment with a number of spatial and high-dimensional data sets, including Factual's (http://www.factual.com/) US points-of-interest data set of 13M entries. On the experimental setup, the diverse browsing method is shown to be more efficient (regarding disk accesses) than k-NN search on R-trees, and more effective (regarding Maximal Marginal Relevance (MMR)) than the diverse nearest neighbor search techniques found in the literature.

향상된 성능의 전술형 모노펄스 MIMO 레이더

본 논문에서는 지상의 추적 장비들이 무선망으로 연결되어 협력하는 전술형 모노펄스(Monopulse) MIMO 레이더의 성능 향상 기법을 연구하였다. 우선, 기존의 모노펄스 MIMO 레이더에 <TEX>${\alpha}{\beta}$</TEX> 필터를 적용하여 각도 다이버시티(Angular Diversity)를 충분히 얻지 못하는 환경에서의 위치추적 성능을 크게 향상시키는 방법을 제안하였다. 또한 필터링 기법과 연계하여 무선망을 이용하는 지상 추적 장비간 피드백(Feedback)을 줄일 수 있는 기법을 제안하였다. In this paper, we proposed an enhanced monopulse MIMO radar system for the tactical scenario where the ground receivers are connected wireless backhaul and closely spaced. By applying the <TEX>${\alpha}{\beta}$</TEX> filter to the conventional monopulse MIMO radar, we show that the localization performance can be improved significantly. We also propose an efficient localization algorithm for a system with lower rate feedback. Using numerical simulations, we demonstrate that the proposed scheme can improve the localization performance while reducing the feedback over conventional scheme.

激光阵列光源角度多样性抑制散斑方法

激光阵列光源角度多样性抑制散斑方法

Laser speckle reduction based on angular diversity induced by Piezoelectric Benders

Speckle reduction techniques have been investigated in display systems with coherent light sources. We propose a simple but efficient method to reduce speckle via two fast vibrating piezoelectric benders. The concept is modulating laser beams to have angle diversity and reducing speckle by temporal averaging in the integration time of the detector. Experiments demonstrate that the speckle contrast is suppressed down to 0.06. Both free space and imaging geometry are considered. In order to fairly evaluate the speckle reduction, the optical configuration as well as the camera settings, which largely affect the speckle contrast, is discussed.

Automatic realignment with electronic steering of free-space-optical transceivers in MANETs: A proof-of-concept prototype

Free-Space-Optical (FSO) communication has the potential to achieve very high wireless communication rates at tens of GHz. Although it has the advantage of high-speed optical modulation, FSO communication is prone to mobility and it requires establishment and maintenance of line-of-sight (LOS) between FSO transceivers since FSO transceivers are highly directional. We consider FSO structures with multiple transceivers placed on a spherical shape with angular diversity and tackle the problem of automatically detecting and maintaining LOS alignment among neighbor multi-transceiver FSO structures. We present a prototype implementation of such multi-transceiver electronically-steered communication structures. Our prototype uses a simple LOS detection and establishment protocol and assigns logical data streams to appropriate physical links. We show that by using multiple directional transceivers and an auto-alignment mechanism, it is possible to maintain optical wireless links in a mobile setting with minimal disruptions and overhead.

Three-Dimensional Positioning of Point Scatterers Based on Radargrammetry

This paper presents a new technique for the retrieval of 3-D point scatterer (PS) location and resolution cell configuration exploiting only the intensity of synthetic aperture radar (SAR) images. The implemented method is based on the principle of radargrammetry and makes use of Bayesian inference, wherein, directed graphs are utilized to represent dependencies of random variables and probability density functions are modeled by particle filter representations. Compared to published radargrammetric work, the newly developed algorithm optimally exploits stacks of acquisitions taken with at least three different incidence angles. Due to the large angular diversity, the method is insensitive to atmospheric propagation effects and motion of the scatterers in contrast to the coherent phase-based techniques such as persistent scatterer interferometry and SAR Tomography. Additionally, the method estimates absolute height, which is a big advantage compared to the relative estimates provided by the phase-based techniques, which moreover encounter phase unwrapping errors and temporal decorrelation. High-resolution spotlight TerraSAR-X data of Berlin central station is used as a processing example for this technique. The test case demonstrates the unambiguous absolute height estimation of PSs and resolving of complicated scattering situations (e.g., layover) in urban areas.

Laser speckle reduction by multimode optical fiber bundle with combined temporal, spatial, and angular diversity

We report significant speckle reduction in a laser illumination system using a vibrating multimode optical fiber bundle. The optical fiber bundle was illuminated by two independent lasers simultaneously. The beams from both lasers were first expanded and collimated and were further divided into multiple beams to illuminate the fiber optic bundle with normal and oblique incidence. Static diffusers were also placed at the input and output faces of the fiber bundle, thus introducing the spatial as well as angular diversity of illumination. Experiments were carried out both in free space and in imaging geometry configuration. Standard deviation, speckle contrast and signal-to-noise ratio of the images were computed, and the results were compared with those of white light illumination. Speckle contrast close to that of white light was obtained using a vibrating fiber bundle with combined temporal, spatial, and angular diversities of the illumination.