Browsing by Subject "Beamforming"

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    Beam steering in a half-frequency driven airborne CMUT transmitter array
    (IEEE Computer Society, 2019) Khan, Talha Masood; Taşdelen, Akif Sinan; Yılmaz, Mehmet; Atalar, Abdullah; Köymen, Hayrettin
    An airborne Capacitive Micromachined Ultrasonic Transducer (CMUT) transmit array was designed using electromechanical modelling for unbiased airborne operation. The array elements are designed for maximum swing at 10V p-p unbiased drive, whereas conventional practice is to bias CMUT close to the collapsed voltage to achieve higher swing. The devices were fabricated using a customized single photolithographic process with a combination of wet and dry etching. The wafer level fabrication enabled the usage of 2x2 and 3x3 arrays. Driving CMUTs in an unbiased mode at half frequency drives the ‘static pressure’ depressed silicon membrane at a larger swing without letting it collapse. The 2x2 array displays 3.375 kHz bandwidth when characterized in air. The phase and amplitude differences due to the dispersion of resonance frequencies of the elements are compensated for beamformed and beamsteered airborne operation.
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    Cooperative precoding and artificial noise design for security over interference channels
    (Institute of Electrical and Electronics Engineers Inc., 2015) Özçelikkale, A.; Duman, T. M.
    We focus on linear precoding strategies as a physical layer technique for providing security in Gaussian interference channels. We consider an artificial noise aided scheme where transmitters may broadcast noise in addition to data in order to confuse eavesdroppers. We formulate the problem of minimizing the total mean-square error at the legitimate receivers while keeping the error values at the eavesdroppers above target levels. This set-up leads to a non-convex problem formulation. Hence, we propose a coordinate block descent technique based on a tight semi-definite relaxation and design linear precoders as well as spatial distribution of the artificial noise. Our results illustrate that artificial noise can provide significant performance gains especially when the secrecy levels required at the eavesdroppers are demanding. © 1994-2012 IEEE.
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    Downlink beamforming under individual SINR and per antenna power constraints
    (IEEE, 2007-08) Yazarel, Y. K.; Aktaş, Defne
    In this paper we consider the problem of finding the optimum beamforming vectors for the downlink of a multiuser system, where there are individual signal to interference plus noise ratio (SINR) targets for each user. Majority of the previous work on this problem assumed a total power constraint on the base stations. However, since each transmit antenna is limited by the amount of power it can transmit due to the limited linear region of the power amplifliers, a more realistic constraint is to place a limit on the per antenna power. In a recent work, Yu and Lan proposed an iterative algorithm for computing the optimum beamforming vectors minimizing the power margin over all antennas under individual SINR and per antenna power constraints. However, from a system designer point of view, it may be more desirable to minimize the total transmit power rather than minimizing the power margin, especially when the system is not symmetric. Reformulating the transmitter optimization problem to minimize the total transmit power subject to individual SINR constraints on the users and per antenna power constraints on the base stations, the algorithm proposed by Yu and Lan is modified. Performance of the modified algorithm is compared with existing methods for various cellular array scenarios. ©2007 IEEE.
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    Faz-MIMO sistemlerinde geliş açısı kestirim performansı karşılaştırması
    (IEEE, 2021-07-19) Bahçeci, M. Umut; Güngör, Alper; Çetintepe, Çağrı; Tuncer, T. Engin
    Bu çalışmada, literatürde yer alan Fazlı Çoklu Giriş-Çoklu Çıkış (Fazlı-MIMO) dizi konsepti ile Sıkıştırılmış Algılama (SA) teknikleri ilk defa birlikte ele alınarak bir radar sisteminde çoklu hedefler için geliş açısı kestirim (GAK) problemi özgün olarak ele alınmıştır. Göndermeç tarafında ortalama güç kıstasları gözetilerek farklı altdizi sayıları için geleneksel huzme şekillendirme ve rasgele fazlı ağırlıklı tasarım yöntemleri incelenmi ş; almaç tarafında ise huzme şekillendirme, ayrık SA ve grup SA olmak üzere üç farklı geri çatım yöntemi ile çoklu hedefler için GAK başarımı araştırılmıştır. Anılan her senaryo için farklı Sinyal-Gürültü-Oranları (SGO) altında gerçekleştirilen parametrik Monte Carlo benzetimleri ile hedef başına hata sonuçları elde edilmiştir. Düşük SGO durumunda, Fazlı-MIMO ve SA teknikleri birlikte kullanılarak GAK başarımının artırılabileceği gösterilmiştir. Ayrıca grup seyreklik çözümü ile ayrık çözüme kıyasla kestirim hatasında 3 kata kadar bir azalma sağlanmıştır.
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    The fractional fourier transform and its applications to image representation and beamforming
    (ASME, 2003-09) Yetik, I. Ş; Kutay, M. A.; Özaktaş, Haldun. M.
    The ath order fractional Fourier transform operator is the ath power of the ordinary Fourier transform operator. We provide a brief introduction to the fractional Fourier transform, discuss some of its more important properties, and concentrate on its applications to image representation and compression, and beamforming. We show that improved performance can be obtained by employing the fractional Fourier transform instead of the ordinary Fourier transform in these applications.
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    Structuring research on conformal antennas a European collaboration
    (Institution of Engineering and Technology, 2007) Sipus, Z.; Persson, P.; Lanne, M.; Heckler, M.; Maci, S.; Masa-Campos, J. L.; Knott, P.; Ertürk, Vakur B.; Vandenbosch, G.
    The topic of this paper is the work carried out within work package 2.4-3 of the EU network "antenna centre of excellence" (ACE). This work package is concerned with structuring research on conformal antennas. In particular, the work is focused on the problems associated with full benchmarking of conformal antennas, on development of hybrid programs for analyzing different classes of conformal antennas, and on investigation of properties of algorithms for optimizing beam synthesis and beam-steering for conformal arrays.
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    Wide-angle reflection-mode spatial filtering and splitting with photonic crystal gratings and single-layer rod gratings
    (Optical Society of American (OSA), 2014) Serebryannikov, A. E.; Lalanne, P.; Petrov, A. Y.; Özbay, Ekmel
    New diffractive optical elements offering a frequency tolerant, very efficient, high-pass and bandpass spatial filtering over a broad range of incidence angles are demonstrated by numerical simulations. The device operates in reflection mode owing to the (nearly) perfect blazing. It relies on two-dimensional square-lattice photonic crystals composed of dielectric rods with simple corrugations at the interface. Similar performance can be obtained with gratings composed of a single rod layer placed in the near field of a metal mirror, indicating a route to geometries that can be easily fabricated with modern nanotechnologies. Also equal splitting between zero and first negative orders can be obtained for incidence-angle variations that are wider than 60°. © 2014 Optical Society of America