Over the years, a variety of array processing techniques have been applied to the problem of enhancing degraded speech to improve automatic speech recognition. In this context, linear beamforming has long been the approach of choice, for reasons including good performance, robustness and analytical simplicity. While various non-linear techniques - typically based to some extent on the study of auditory scene analysis - have also been of interest, they tend to lag behind their linear counterparts in terms of simplicity, scalability and exibility. Nonlinear techniques are also more difficult to analyze and lack the systematic descriptions available in the study of linear beamformers. This work focuses on a class of nonlinear processing, known as time-frequency (T-F) masking - a.k.a. spectro-temporal masking { whose variants comprise a significant portion of the existing techniques. T-F masking is based on accepting or rejecting individual time-frequency cells based on some estimate of local signal quality. Analyses are developed that attempt to mirror the beam patterns used to describe linear processing, leading to a view of T-F masking as "nonlinear beamforming". Two distinct formulations of these "nonlinear beam patterns" are developed, based on different metrics of the algorithms behavior; these formulations are modeled in a variety of scenarios to demonstrate the flexibility of the idea. While these patterns are not quite as simple or all-encompassing as traditional beam patterns in microphone-array processing, they do accurately represent the behavior of masking algorithms in analogous and intuitive ways. In addition to analyzing this class of nonlinear masking algorithm, we also attempt to improve its performance in a variety of ways. Improvements are proposed to the baseline two-channel version of masking, by addressing both the mask estimation and the signal reconstruction stages; the latter more successfully than the former. Furthermore, while these approaches have been shown to outperform linear beamforming in two-sensor arrays, extensions to larger arrays have been few and unsuccessful. We find that combining beamforming and masking is a viable method of bringing the benefits of masking to larger arrays. As a result, a hybrid beamforming-masking approach, called "post-masking", is developed that improves upon the performance of MMSE beamforming (and can be used with any beamforming technique), with the potential for even greater improvement in the future.

The thesis analyses and validates Beamforming methods using Convex Optimization.  CVX which is a Matlab supported tool for convex optimization has been used to develop this concept. An algorithm is designed by which an appropriate system has been identified by varying parameters such as number of antennas, passband width, and stopbands widths of a beamformer. We have observed the beamformer by minimizing the error for Least-square and Infinity norms. A graph obtained by the optimum values between least-square and infinity norms shows us a trade-off between these two norms. We have observed convex optimization for double passband of a beamformer which has proven the flexibility of convex optimization. On extension for this, we designed a filter in which stopband is arbitrary. A constraint is used by which the stopband would be varying depending upon the upper boundary (limiting) line which varies w.r.t y-axis (dB). The beamformer has been observed for feasibility by varying parameters such as number of antennas, arbitrary upper boundaries, stopbands and passband. This proves that there is flexibility for designing a beamformer as desired.

Adaptive heal-doming can significantly improve the image quality in biomedical ultrasound by reducing the clutter due to interfering signals arriving from undesired directions. Adaptive beamforming is computationally expensive, and the objective of this thesis is to expose and explore tradeoffs between computational complexity and quality of adaptive beamforming. We consider the conventional linearly constrained minimum variance (LCMV) adaptive beamformer, applied to B-mode ultrasound imaging, and study an alternative based on the well-known generalized sidelobe canceller (GSC) whose adaptation relies on unconstrained gradient-driven optimization. To our knowledge, this is the first time a GSC-based gradient-driven approach has been applied and evaluated in the context of ultrasound beamforming. As another alternative to the conventional LCMV method, we also propose and evaluate a simple idea of updating the beamformer's weight vector at a reduced rate. Both approaches have lead to significant computational savings, but they also sacrifice beamforming optimality. Our simulations show that despite suboptimal beamforming. the ultrasound image quality remains acceptable.

The idea of utilizing multiple antennas (MIMO) has emerged as one of the significant breakthroughs in modern wireless communications. MIMO techniques can improve the spectral efficiency of wireless systems and provide significant throughput gains. As such, MIMO will be increasingly deployed in future wireless systems. On the other hand, in order to meet the increasing demand for high data rate multimedia wireless services, future wireless systems are evolving towards universal frequency reuse, where neighboring cells may utilize the same radio spectrum. As such, the performance of future wireless systems will be mainly limited by inter-cell interference (ICI). It has been shown that the throughput gains promised by conventional MIMO techniques degrade severely in multi-cell systems. This definitely attributes to the existence of the ICI. A lot of related work has been performed on the ICI mitigation or cancellation strategies, in multi-cell MIMO systems. Most of them assume that the channel and even data information is available at the collaborating base stations (BSs). Different from the previous work, we are looking into certain low-complexity codebook-based multi-cell multi-user MIMO strategies. For most of our work, we derive the statistics of the selected user's signal-to-interference-and-noise-ratio (SINR), which enable us to calculate the achieved sum-rate accurately and e ciently. With the derived sum-rate expressions, we evaluate and compare the sum-rate performance for several proposed low-complexity ICI-mitigation systems with various system parameters for single-user per-cell scheduling case. Furthermore, in order to fully exploit spatial multiplexing gain, we are considering multi-user per-cell scheduling case. Based on the assumption that all CSI including intra-cell and inter-cell channels are available at each BS, we rstly look into the centralized optimization approach. Typically, since the sum-rate maximization problem is mostly non-convex, it is generally di cult to obtain the globally optimum solution. Through certain approximation and relaxations, we successfully investigate an iterative optimization algorithm which exploits the second-order cone programming (SOCP) approach. From the simulation results, we will observe that the iterative option can provide near-optimum sum capacity, although only locally optimized. Afterwards, inspired by the successful application of Per-User Unitary Rate Control (PU2RC) scheme, we manage to extend it into dual-cell environment, with limited coordination between two cells. / Graduate

Astronomy large-scale surveys require instrumentation to minimize the time required to complete observations of large sections of the sky. Optimizing receiver systems has been achieved through reducing the system temperature primarily by advances in low-noise amplifier technology to a point that the internally generated noise is now fast approaching the quantum limit. Instead, reflector-coupled focal plane arrays are now used to increase the field of view (FoV) by employing either multi-element horn feeds or phased array feeds. Widely spaced (2-3 wavelengths diameter) horn feeds inefficiently sample the available focal plane radiation, thus requiring multiple imaging passes. Alternatively, a more efficient method is to use a narrow element (0.5 wavelengths diameter) phased array feed with a beamformer to produce overlapping beams on the sky, fully Nyquist sampling the focal plane with a single pass. The FoV can be further increased with additional phased array feed (PAF) antenna-receiver modules adding to the contiguous fully sampled region. A 5 x 5 K-band (18 - 26 GHz) single polarization modular PAF incorporating an antenna array of planar axially symmetric elements is designed, simulated, manufactured and tested. Each narrow width tapered slot antenna element has an independent receiver chain consisting of a cryogenic packaged monolithic microwave integrated circuit (MMIC) GaAs amplifier and a packaged MMIC down converting mixer. Synthesized beams and beamformer characteristics are presented. The PAF imaging system performance is evaluated by survey speed and compared to the industry standard, the single pixel feed (SPF). Scientifically, K-band is attractive because it contains numerous molecular transitions, in particular the rotation-inversion lines of ammonia. These transitions are excited in dense gas, and can be used to directly measure kinetic temperatures and velocities of protostars throughout the Galaxy. Depending on the line detected, gas of different temperatures can be probed. It is concluded that even with a higher system temperature, a PAF with sufficient number of synthesized beams can outperform a SPF in imaging speed by more than an order of magnitude. / Graduate

Experimental aeroacoustic research was conducted on a wind turbine specific airfoil at low Reynolds numbers. The goal of this thesis was to study trailing edge noise generation from the airfoil and investigate correlations between the noise and the flow field. Before experiments were performed the current wind tunnel had to be modified in order to make it more suitable for aeroacoustic tests. Sound absorbing foam was added to the inside of the tunnel to lower the background noise levels and turbulence reduction screens were added which lowered the turbulence. An S822 airfoil was chosen because it is designed for low Reynolds flows attainable in the wind tunnel which are on the order of 104. Smoke wire flow visualization was used to gain insight into the airfoil wake development and oil film flow visualization was used to qualitatively assess the boundary layer development. Laser Doppler anemometry (LDA) was used to measure two components of velocity at high data rates in the airfoil wake. Wake profiles were measured in addition to single point measurements to determine the velocity spectrum. A microphone was mounted inside the test section in order to measure the trailing edge noise. Initial plans included measuring the trailing edge noise with a microphone array capable of quantifying and locating noise sources. Although an array was built and beamforming code was written it was only used in preliminary monopole source tests. Oil film results showed the behaviour of the boundary layer to be consistent with previous low Reynolds number experiments. LDA results revealed sharp peaks in the velocity spectra at 1100 Hz from U0 = 15–24 m/s, and 3100 and 3800 Hz, from U0 = 25–35 m/s, which were inconsistent with vortex shedding results of previous researchers. Also present were a series of broad peaks in the spectra that increase from 1200–1700 Hz in the U0 = 25–35 m/s range. The shedding frequency from the smoke wire flow visualization was calculated to be 1250 Hz at U0 = 26 m/s. These sharp peaks were also present in the acoustic spectrum. It was reasoned that these peaks are due to wind tunnel resonance which is a common occurrence in hard wall wind tunnels. In particular the tone at 1100 Hz is due to a standing wave with a wavelength equal to half the tunnel width. The shedding frequency from the smoke wire flow visualization was calculated to be 1100 Hz at U0 = 20 m/s. These tones exhibited a “ladder-like” relationship with freestream velocity, another aspect indicative of wind tunnel resonance. It was reasoned that the wind tunnel resonance was forcing the shedding frequency of the airfoil in the U0 = 15–24 m/s range, and in the U0 = 25–35 m/s range, the shedding frequency corresponded to the broad peaks in the LDA spectra.

We study the two-user multiple-input single-output (MISO) Gaussian interference channel where the transmitters have perfect channel state information and employ single-stream beamforming. The receivers are capable of performing successive interference cancellation, so when the interfering signal is strong enough, it can be decoded, treating the desired signal as noise, and subtracted from the received signal, before the desired signal is decoded. We propose efficient methods to compute the Pareto-optimal rate points and corresponding beamforming vector pairs, by maximizing the rate of one link given the rate of the other link. We do so by splitting the original problem into four subproblems corresponding to the combinations of the receivers' decoding strategies - either decode the interference or treat it as additive noise. We utilize recently proposed parameterizations of the optimal beamforming vectors to equivalently reformulate each subproblem as a quasi-concave problem, which we solve very efficiently either analytically or via scalar numerical optimization. The computational complexity of the proposed methods is several orders-of-magnitude less than the complexity of the state-of-the-art methods. We use the proposed methods to illustrate the effect of the strength and spatial correlation of the channels on the shape of the rate region.

<p>Beamforming of ultrasound signals in the presence of clutter, or partial aperture blockage by an acoustic obstacle can lead to reduced visibility of the structures of interest and diminished diagnostic value of the resulting image. We propose new beamforming methods to recover the quality of ultrasound images under such challenging conditions. Of special interest are the signals from large apertures, which are more susceptible to partial blockage, and from commercial matrix arrays that suffer from low sensitivity due to inherent design/hardware limitations. A coherence-based beamforming method designed for suppressing the in vivo clutter, namely Short-lag Spatial Coherence (SLSC) Imaging, is first implemented on a 1-D array to enhance visualization of liver vasculature in 17 human subjects. The SLSC images show statistically significant improvements in vessel contrast and contrast-to-noise ratio over the matched B-mode images. The concept of SLSC imaging is then extended to matrix arrays, and the first in vivo demonstration of volumetric SLSC imaging on a clinical ultrasound system is presented. The effective suppression of clutter via volumetric SLSC imaging indicates it could potentially compensate for the low sensitivity associated with most commercial matrix arrays. The rest of the dissertation assesses image degradation due to elements blocked by ribs in a transthoracic scan. A method to detect the blocked elements is demonstrated using simulated, ex vivo, and in vivo data from the fully-sampled 2-D apertures. The results show that turning off the blocked elements both reduces the near-field clutter and improves visibility of anechoic/hypoechoic targets. Most importantly, the ex vivo data from large synthetic apertures indicates that the adaptive weighing of the non-blocked elements can recover the loss of focus quality due to periodic rib structure, allowing large apertures to realize their full resolution potential in transthoracic ultrasound.</p> / Dissertation

Thesis (MEng) -- Stellenbosch University, 2014. / ENGLISH ABSTRACT: Beamforming is a technique used to combine signals from an array of antennas to effectively synthesize a single aperture and beam. In the Radio Astronomy community the technique is used to obtain a desirable beam pattern as well as to electronically point the beam of an array. Next generation radio telescopes such as the Square Kilometre Array (SKA) surpass current technology and will extensively make use of beamforming techniques. Various factors determine the output of a beamformer; however, given an array with a fixed configuration only the weights applied to the incoming signals affect the synthesized aperture and beam. Furthermore, the incoming data must be processed in real-time, at a rate equal to the input-output rate of the processor. Both the weighting function and the real-time implementation of beamforming, are the primary subjects of this thesis. In this thesis various deterministic weighting functions are investigated. The algorithms are implemented in a matlab program, serving as a simulation tool for investigating the techniques. The program is verified by comparing the expected theoretical outcomes to the simulated output. For the program the following functionalities are included: a steering technique, spectral weighting, Dolph-Chebychev, and the Least Square Error algorithm. Applications of these techniques is investigated and their prominence in the Radio Astronomy community is established. For the real-time beamformer implementation, the UniBoard platform configured with beamformer firmware, is investigated. This is important as the UniBoard is an excellent example of a beamformer implementation within the Radio Astronomy community. The architecture is used to emulate a linear array by implementing a python control script, where the output corresponded accurately with the expected theoretical values. The thesis also constitutes the design and implementation of a digital frequency domain beamformer on the ROACH board. This processing board is employed by the Karoo Array Telescope (KAT-7) in South Africa. This work is therefore important as it demonstrates a beamformer implementation on an architecture in use by the Radio Astronomy community. An antenna array is designed and built for the verification of the beamformer design. Results with a good degree of accuracy were obtained and where errors exist they are discussed. / AFRIKKANSE OPSOMMING: Bundelvorming is ’n tegniek waarmee die seine van ’n antenna samestelling gekombineer word om ’n enkele effektiewe stralingsvlak en stralingspatroon te sintiseer. In die Radio Astronomie gemeenskap word die tegniek gebruik om ’n gewenste stralingspatroon te sintiseer sowel as om die rigting van die patroon elektronies te beheer. Die Square Kilometre Array (SKA) is ’n toekomstige radioteleskoop en sal grootliks gebruik maak van bundelvorming tegnieke. Die uitset van bundelvormers word geaffekteer deur verskeie faktore, maar vir ’n gegewe samestelling is dit net die gewigsfunksies wat toegepas word op die inkomende seine wat die gesintiseerde patroon kan beheer. Verder moet die inkomende data verwerk word teen ’n tempo gelykstaande aan die inset-en-uitsetkoers van die verwerker. Die gewigsfunksie so wel as die implementasie van die bundelvormer is albei primêre onderwerpe van die tesis. ’n Verskeindenheid van deterministiese bundelvormingstegnieke sal ondersoek word in hierdie tesis. Die algoritmes is in ’n matlab program geïmplementeer vir simulasie doeleindes. Die program is geverifieër deur die uitset te vergelyk met die verwagte teoretiese waardes. Die program sluit die volgende funksies in: ’n rigting beheer algoritme, spektraalgewigte, Dolph-Chebychev, en die minste vierkantsfout algoritme. Hierdie tegnieke is van belang weens hul toepassing in die Radio Astronomie gemeenskap. Vir die implementasie van ’n bundelvormer is die UniBoard hardeware, geprogrameer in ’n bundelvormings modus, van gebruik gemaak. Hierdie aspek is belangrik omdat die Uni- Board ’n goeie voorbeeld van ’n geïmplementeerde bundelvormer in die Radio Astronomie gemeenskap is. Die UniBoard word gebruik om ’n lineêre samestelling te emuleer deur in python ’n beheer skrip te skryf, waar die uitset van die emuleerder akkuraat ooreenstem met die verwagte waardes. Die tesis behels ook die ontwerp en implementasie van ’n digitale frekwensiegebied bundelvormer op die ROACH platform. Hierdie verwerker word tans gebruik in die Karoo Array Telescope (KAT-7) in Suid-Afrika. Hierdie werk is dus belangrik omdat dit die implementasie van ’n bundelvormer op tegnologie wat huidiglik in die Radio Astronomie gemeenskap gebruik word demonstreer. Daarbenewens is ’n antenna samestelling ontwerp en gebou om die bundelvormer te verifieër. Die resultate is akkuraat tot ’n redelike mate. Waar daar ’n fout onstaan het word dit in die tesis bespreek.

La bande sub-millimétrique allant globalement de 300 GHz à 3 THz possède des propriétés similaires à la capacité de pénétration de photons non ionisants à travers des matériaux optiquement opaques. Pour l'imagerie THz, il est ainsi possible de détecter des objets cachés à l'intérieur de paquets, de vêtements ou de matelas... Avec l’évolution des technologies intégrées et l’augmentation des fréquences de coupure des transistors 〖(f〗_t/f_max), de nombreux circuits et systèmes ont été réalisés à des fréquences autour de 300 GHz, en particulier les systèmes de formation de faisceau. Ces systèmes permettent de générer un signal et de l’orienter électroniquement dans une direction définie de l’espace. Dans ce travail, une architecture originale d’un tel système est proposée. Elle repose sur la génération d’un signal dans la bande 270-300 GHz, tout en contrôlant sa phase à l’aide de déphaseurs implémentés au niveau de la voie LO dans la bande 45-50 GHz. La complexité du système impose une stratégie qui consiste à réaliser chaque bloc seul. Pour cela, l’émetteur dans la bande 270-300 GHz a été réalisé dans un premier temps. Il est composé d’un oscillateur verrouillé par injection sous-harmonique (45-50 GHz), d’un mélangeur passif et d'amplificateurs IF. Ensuite une architecture innovante de déphaseur a été réalisée, basée sur des lignes couplées à ondes lentes. Finalement, une chaîne de multiplication de fréquence a été réalisée afin de générer le signal d’injection à l’aide d’un signal basse fréquence (3-5 GHz). Les circuits ont été fabriqués en technologie BiCMOS 55 nm de STMicroelectronics. Les résultats de mesure correspondent sont en très bon accord avec les simulations, et les performances obtenues sont à l’état de l’art. Une fois les blocs élémentaires validés, des sous-systèmes ont été réalisés pour valider le bon fonctionnement d’une voie complète du réseau d'antennes. En termes de perspectives, ce travail ouvre la voie vers la conception et la réalisation d'un système complet d'orientation de faisceau contenant plusieurs voies/antennes. / The sub-millimeter wave band that covers the frequency range from 300 GHz to 3 THz has an interesting properties such the ability to penetrate materials. For THz imaging, it is possible to detect objects inside packages, clothes... With the evolution of integrated technologies and the increase of the cut-off frequencies of transistors 〖(f〗_t/f_max), many circuits and systems have been fabricated around 300 GHz, especially phased arrays for beamforming applications. These systems generate a signal and steer it electronically in a direction of the space. In this work, a novel architecture of phased array is proposed. It is based on the generation of a signal in the 270-300 GHz band, while controlling its phase by using phase shifters implemented in the LO path in the 45-50 GHz band. Each bloc should be measured in a stand-alone version, in order to get an idea about whole system performances. For this, the transmitter in the 270-300 GHz band has been realized first. It consists of a sub-harmonic injection locked oscillator, a passive mixer and IF amplifiers. Then, a novel architecture of phase shifter was proposed, it is based on slow waves coupled lines. Finally, a frequency multiplier chain was performed to generate the injection signal by using a lower frequency signal (3-5 GHz). The circuits are fabricated in a 55nm BiCMOS technology from STMicroelectronics. Measurements results are in a good agreement with simulations. Once the blocks are validated, sub systems are realized in order to validate one path of the array. The perspectives of this work include the design and realization of the complete phased array with multiple paths/antennas.