Browsing by Author "Viikari, Ville, Assist. Prof., Aalto University, Department of Radio Science and Engineering, Finland"

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    Multi-element Antennas for Mobile Communication Systems: Design, Evaluation and User Interactions
    (Aalto University, 2013) Azremi, Abdullah Al-Hadi; Holopainen, Jari, Dr., Aalto University, Department of Radio Science and Engineering, Finland; Radiotieteen ja -tekniikan laitos; Department of Radio Science and Engineering; Sähkötekniikan korkeakoulu; School of Electrical Engineering; Vainikainen, Pertti, Prof., Aalto University, Department of Radio Science and Engineering, Finland; Viikari, Ville, Assist. Prof., Aalto University, Department of Radio Science and Engineering, Finland
    Capacity improvement using Multiple-Input Multiple-Output (MIMO) technology necessitates multiple antennas at both transmitter and receiver ends. Antenna design challenges are related especially to mobile terminal, in which antennas need to be small, located relatively close to each other and interact with a user. Due to these limitations, mobile terminal antennas are potentially inefficient and do not provide a good MIMO performance. Traditional antenna design and characterization methods are not directly applicable to multiple antennas, and therefore research in these fields is needed. This thesis contributes two topics. First, characterization methods of multiple antennas in compact mobile terminals are studied. In the studied methods, the effects of propagation environment and user on the multiple antenna performance are taken into account. The time-varying propagation environment and user’s hand grip emulating the true scenario of the users in real-life environments are analyzed. In particular; different terminal orientations, hand grips and locations are investigated either with one or two hands holding the terminal. Second, the thesis has scientific contribution on introducing novel multi-antenna structures with elements ranging from two to eight, and frequencies ranging from 900 MHz UHF to 3500 MHz LTE bands. Specifically, this thesis advances the design, evaluation and user interaction of multiple antennas in mobile terminals operating at the 3500 MHz. The proposed structures are shown to provide good diversity and MIMO performances. Selection of suitable antenna type and their locations on the terminal chassis are shown to be important. Based on author’s experience, the proposed designs are particularly suitable for realizing a multi-element antenna structure that is tolerant to the user. Applicability of multiple antennas in compact mobile terminal for radio direction finding (RDF) is studied. Detailed analysis has also been carried out on the antenna topology on a mobile terminal and consideration of user interaction to ascertain the best use of antennas for such an application. It is found that ambiguity characterization method could be useful evaluation tool for designing a multi-antenna structure for RDF application.   
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    Studies on characterization of dielectric composite materials using radar and other microwave sensors
    (Aalto University, 2016) Olkkonen, Martta-Kaisa; Laitinen, Tommi, Dr.; Mikhnev, Valeri, D.Sc. (Tech.), National Academy of Sciences of Belarus Minsk, Belarus; Sähkötekniikan ja automaation laitos; Department of Electrical Engineering and Automation; Sähkötekniikan korkeakoulu; School of Electrical Engineering; Vainikainen, Pertti, Prof., Aalto University, Department of Radio Science and Engineering, Finland; Viikari, Ville, Assist. Prof., Aalto University, Department of Radio Science and Engineering, Finland; Eskelinen, Pekka, Prof., Aalto University, Department of Electrical Engineering and Automation, Finland
    This thesis describes several new advances in measurement of the properties of materials using microwave techniques. The developments were made through an interdisciplinary approach to explore the properties of composite materials such as biofuel, concrete, asphalt and the rock inclusions of asphalt. In this thesis, a composite material has been defined as inhomogeneous material with inclusions that have dimensions comparable to the measurement wavelength. Resonators were chosen to characterize materials in which the properties, such as moisture, are linked directly to the real or imaginary part of the measured signal, using either a linear or parabolic statistical fitting. The concepts were tested using online moisture measurement of biofuel employing a stripline cavity resonator at approximately 360 MHz, attached above and below the conveyor belt. In a similar manner, a resonant coaxial surface sensor operating at approximately 1.7 GHz was used to test the same concepts by determining the moisture content of concrete. Investigations into the use of radar techniques for material characterization employed a radar device (13–17 GHz), which was developed by colleagues to achieve a resolution (2–3 cm) sufficient for gaining an image of the structure of only the surface layer (< 5 cm). It is suggested that the use of polyoxymethylene (POM) or similar dielectric is preferable for radar calibration, as the reflectivity is of the same order as the material under test unlike in traditionally used metal sheets, which can cause saturation of the receiver. The permittivity of the asphalt aggregates was measured using both the resonator and waveguide methods as a precursor to measuring composites. Then, a scanner (7–17 GHz), which measured the transmission scattering parameters of a sample, was used as a laboratory reference method. This produced a two-dimensional color image of the permittivity and hence structure of the composite material with a resolution of 5 mm x 5 mm. Phase error due to multiple reflections was compensated by fitting a line to the fluctuating phase through the minima and maxima of the amplitude. Another signal processing routine was developed to filter out the multiple reflections so that the frequency-dependent permittivity was derived based on only the single-pass propagation through a concrete slab over 0.8–12 GHz. Further, a new algorithm was developed to perform direct interpretation of the GPR data. A color map was superimposed on top of a conventional grey-scale GPR image to distinguish materials based on their reflection-related phase.