26 research outputs found

    Physical bounds and radiation modes for MIMO antennas

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    Modern antenna design for communication systems revolves around two extremes: devices, where only a small region is dedicated to antenna design, and base stations, where design space is not shared with other components. Both imply different restrictions on what performance is realizable. In this paper properties of both ends of the spectrum in terms of MIMO performance is investigated. For electrically small antennas the size restriction dominates the performance parameters. The regions dedicated to antenna design induce currents on the rest of the device. Here a method for studying fundamental bound on spectral efficiency of such configurations is presented. This bound is also studied for NN-degree MIMO systems. For electrically large structures the number of degrees of freedom available per unit area is investigated for different shapes. Both of these are achieved by formulating a convex optimization problem for maximum spectral efficiency in the current density on the antenna. A computationally efficient solution for this problem is formulated and investigated in relation to constraining parameters, such as size and efficiency

    Investigation and comparison between radiation and phase center for canonical antennas

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    The phase center is defined as the point on an antenna from which the far field radiation seems to originate. Phase center calculations are often uncertain and vague, and very little concrete information is available on the subject. Recently, a replacement parameter called the radiation center was introduced in [5]. The radiation center is more rigorously defined than the phase center and possesses additional qualities, such as uniqueness, no need for user input, etc. This thesis evaluates the validity of the radiation center as a replacement for the phase center and also compares it with Muehldorf’s analytically calculated phase center [17]. The far-field is simulated in CST and evaluated in Matlab, using Ericsson Antenna ModelLibrary(eamlib)tocalculatetheradiationcenter. Phasecentercalculations are carried out in CST and the Muehldorf phase center is evaluated numerically in Matlab. The radiation center minimizes the phase well, achieving the same smoothness as the phase center. The radiation center varies according to the predicted behaviour of the phase center for most antennas. For the spiral antenna the radiation center does not adhere to the predicted behaviour of the phase center. For some antennas, specifically those that have wide or narrow beam widths in a certain direction, the radiation center seems to mainly be influenced by the phase function in the plane with wider radiation pattern. This strengthens the theoretical argument in [5] that the radiation center minimizes the phase according to far-field amplitude. Theradiationcenterproducesresultswithintheboundsoftheantenna structure for all antennas presented in this thesis. In contrast the phase center does not, specifically for the Yagi-Uda antenna and the Leaky Lens antenna [18]. The phase function in the main radiation lobe is regarded explicitly for some of the simulated antennas. The radiation center does not seem to minimize the phase for these antennas any better or worse than the phase center. These results suggest that the radiation center is a good candidate for origin of radiation for antennas

    Bandwidth-Constrained Capacity Bounds and Degrees of Freedom for MIMO Antennas

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    The optimal spectral efficiency and number of independent channels for MIMO antennas in isotropic multipath channels are investigated when bandwidth requirements are placed on the antenna. By posing the problem as a convex optimization problem restricted by the port Q-factor a semi-analytical expression is formed for its solution. The antennas are simulated by method of moments and the solution is formulated both for structures fed by discrete ports, as well as for design regions characterized by an equivalent current. It is shown that the solution is solely dependent on the eigenvalues of the so-called energy modes of the antenna. The magnitude of these eigenvalues is analyzed for a linear dipole array as well as a plate with embedded antenna regions. The energy modes are also compared to the characteristic modes to validate characteristic modes as a design strategy for MIMO antennas. The antenna performance is illustrated through spectral efficiency over the Q-factor, a quantity that is connected to the capacity. It is proposed that the number of energy modes below a given Q-factor can be used to estimate the degrees of freedom for that Q-factor.Comment: 13 pages, 17 figure

    Bandwidth-Constrained Capacity Bounds on MIMO Antennas

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    The optimal spectral efficiency of MIMO antennas in Rayleigh and ideal channels are investigated when bandwidth requirements are placed on the antenna. By posing the problem as a convex optimization problem restricted by the port Q-factor a semi-analytical expression is formed for its solution. The antennas are simulated by method of moments and the solution is formulated both for structures fed by discrete ports, as well as for design regions characterized by an equivalent current. It is shown that this solution is solely dependent on the so-called energy modes of the antenna.These modes are compared to the characteristic modes and how to effectively excite them is investigated for a linear dipole array as well as a plate with embedded, and raised, antenna regions. The performance is illustrated through spectral efficiency over the Q-factor, a quantity that is connected to the true capacity. It is demonstrated that the Q-factor and the spectral efficiency form a Pareto trade-off bound, and that a certain Q-factor is Pareto optimal

    Optimal Planar Electric Dipole Antenna

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    Considerable time is often spent optimizing antennas to meet specific design metrics. Rarely, however, are the resulting antenna designs compared to rigorous physical bounds on those metrics. Here we study the performance of optimized planar meander line antennas with respect to such bounds. Results show that these simple structures meet the lower bound on radiation Q-factor (maximizing single resonance fractional bandwidth), but are far from reaching the associated physical bounds on efficiency. The relative performance of other canonical antenna designs is compared in similar ways, and the quantitative results are connected to intuitions from small antenna design, physical bounds, and matching network design.Comment: 10 pages, 15 figures, 2 tables, 4 boxe

    Wireless Body Area Network for Heart Attack Detection

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    This article describes a body area network (BAN) for measuring an electrocardiogram (ECG) signal and transmitting it to a smartphone via Bluetooth for data analysis. The BAN uses a specially designed planar inverted F-antenna (PIFA) with a small form factor, realizable with low-fabricationcost techniques. Furthermore, due to the human body's electrical properties, the antenna was designed to enable surface-wave propagation around the body. The system utilizes the user's own smartphone for data processing, and the built-in communications can be used to raise an alarm if a heart attack is detected. This is managed by an application for Android smartphones that has been developed for this system. The good functionality of the system was confirmed in three real-life user case scenarios

    Fundamental bounds on transmission through periodically perforated metal screens with experimental validation

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    This paper presents a study of transmission through arrays of periodic sub-wavelength apertures. Fundamental limitations for this phenomenon are formulated as a sum rule, relating the transmission coefficient over a bandwidth to the static polarizability. The sum rule is rigorously derived for arbitrary periodic apertures in thin screens. By this sum rule we establish a physical bound on the transmission bandwidth which is verified numerically for a number of aperture array designs. We utilize the sum rule to design and optimize sub-wavelength frequency selective surfaces with a bandwidth close to the physically attainable. Finally, we verify the sum rule and simulations by measurements of an array of horseshoe-shaped slots milled in aluminum foil.Comment: 10 pages, 11 figures. Updated Introduction and Conclusion

    Herglotz functions and applications in electromagnetics

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    Herglotz functions inevitably appear in pure mathematics, mathematical physics, and engineering with a wide range of applications. In particular, they are the pertinent functions to model passive systems, and thus appear in modeling of electromagnetic phenomena in circuits, antennas, materials, and scattering. In this chapter, we review the basic theory of Herglotz functions and its applications to determine sum rules and physical bounds for passive systems.Peer reviewe

    Optimal Performance of Advanced Radiating Structures

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    Requirements on radiating structures are constantly increasing, demand for faster speed, smaller size, and higher reliability drives today's technical development. However, for electrically small structures, less than half-a-wavelength in size, performance is fundamentally limited by physical size. This thesis explores how to construct and calculate physical bounds for advanced antennas and complex environments that are in use in modern communication today. Previously, physical bounds have mainly been formulated for single feed, single resonance antennas in free space. However, in modern communication settings antennas are much more advanced. In all cellular networks after the third generation of mobile networks (3G) multiple input multiple output (MIMO) systems are being utilized, where antennas have multiple feeds. Formulating physical bounds for these antennas is not trivial due to classically limited performance metrics, such as the Q-factor, being difficult to define or calculate. It is not only the antennas themselves that are more advanced, antennas are also used in implants, medical devices, meta materials, and in plasmonics. Calculating physical bounds in these scenarios require new methods that reliably predict accurate results for all different types of materials.In this thesis a method for constructing physical bounds for general MIMO antennas is presented. By idealizing the channel and representing the antenna by the equivalent currents excited across it, a bound can be calculated with convex current optimization. It is shown that that bound is effectively reached by exciting different sets of modes depending on what constraints are put on the optimization. Different shapes and sub-regions are analyzed using the strength of these modes.A new method for calculating stored energy and Q-factor in the presence of complex media is presented and investigated. By viewing the antenna as a dynamic system the method of moments (MoM) impedance equation can be formulated as a state space model. The energy stored within in such a model is identified as the stored energy. This method can be generalized to dispersive and inhomogeneous media
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