Asher Yahalom

EHF for Satellite Communications: The New Broadband Frontier

The exploitation of extremely high-frequency (EHF) bands (30-300 GHz) for broadband transmission over satellite links is currently a hot research topic. In particular, the Q-V band (30-50 GHz) and W-band (75-110 GHz) seem to offer very promising perspectives. This paper aims at presenting an overview of the current status of research and technology in EHF satellite communications and taking a look at future perspectives in terms of applications and services. Challenges and open issues are adequately considered together with some viable solutions and future developments. The proposed analysis highlighted the need for a reliable propagation model based on experimental data acquired in orbit. Other critical aspects should be faced at the PHY-layer level in order to manage the tradeoff between power efficiency, spectral efficiency, and robustness against link distortions. As far as networking aspects are concerned, the large bandwidth availability should be converted into increased throughput by means of suitable radio resource management and transport protocols, able to support very high data rates in long-range aerospace scenarios.

The study of conical intersections between consecutive pairs of the five lowest 2A′ states of the C2H molecule

In this article we report findings regarding various conical intersections between consecutive pairs of the five lowest 2A′ states of the C2H molecule. We found that conical intersections exist between each two consecutive 2A′ states. We showed that except for small (high-energy) regions in configuration space, the two lowest adiabatic states (i.e., the 1 2A′ and the 2 2A′) form a quasi-isolated system with respect to the higher states. We also revealed the existence of degenerate parabolical intersections, those with a topological (Berry) phase zero, formed by merging two conical intersections belonging to the 3 2A′ and the 4 2A′ states, and suggested a Jahn-Teller-type model to analyze them. Finally, we examined the possibility that the “frozen” locations of the carbons can be considered as points of conical intersection. We found that the relevant two-state topological phase is not zero nor a multiple of π, but that surrounding both carbons yields a zero topological phase.

Study of ultrawide-band transmission in the extremely high frequency (EHF) band

The growing demand for broad-band wireless communication links and the lack of wide frequency bands within the conventional spectrum, causes us to seek bandwidth in the higher microwave and millimeter-wave spectrum at extremely high frequencies (EHF) above 30 GHz. One of the principal challenges in realizing modern wireless communication links in the EHF band are phenomena occuring during electromagnetic wave propagation through the atmosphere. A space-frequency approach for analyzing wireless communication channels operating in the EHF band is presented. Propagation of the electromagnetic radiation is studied in the frequency domain, enabling consideration of ultrawide-band modulated signals. The theory is employed for the analysis of a communication channel operating at EHF which utilizes pulse amplitude modulated signals. The atmospheric absorptive and dispersive effects on pulse propagation delay, pulse width and distortion are discussed. The theory and model are demonstrated in a study of ultrashort-pulse transmission at 60 GHz.

Helicity conservation via the Noether theorem

The conservation of helicity in ideal barotropic fluids is discussed from a group theoretical point of view. A new symmetry group is introduced, i.e., the alpha group of translations. It is proven via the Noether theorem that this group generates helicity conservation.

Propagation of ultra wide-band signals in lossy dispersive media

Development of a channel model for continuous frequencies enables the analysis of communications in an ultra wide band wireless network in indoor environment including a single transmitting and a single receiving antenna. In this work we will describe a model taking into account multiple reflections which are a consequence of the room in which both transmitter and receiver are localized including wall, ceiling and floor reflections. Moreover, our model enables the analysis of a communication channel between adjacent and distant rooms, in those cases we take into account the wide band signal propagation through separating walls. The model developed is in the frequency domain and thus allows analyzing dispersive effects in transmission and reflection of ultra short pulses in UWB communications from building materials which the room is made of in accordance with their complex dielectric coefficients. For this purpose a library of material characteristics of various materials (concrete, reinforced concrete, plaster, wood, blocks, glass, stone and more) in the standard frequency domain for wireless networks was assembled. One of the important phenomena for UWB communications which our research has revealed is the in-wall multiple reflections resulting in echoes of the narrow pulse transmitted. Our model takes into account antenna polarization and beam shape, the effect of those traits are clearly distinguishable. Space-frequency theory of the propagation of an ultra-wide band radiation in dielectric media is presented. The transfer function of a slab of material is derived in the frequency domain, considering polarization losses via a complex permittivity. It is shown that absorptive and dispersive effects play a role in the transmission and reflection coefficients of the electromagnetic incident field. The theory is applicable in the analysis of broadband communication links operating in wireless local or personal area networks. In an indoor scenario, the construction material of the walls attenuates the propagating waves in a dispersive manner, causing amplitude and phase distortions in the transmitted signal.

Simplified variational principles for barotropic magnetohydrodynamics

Variational principles for magnetohydrodynamics have been introduced by previous authors both in Lagrangian and Eulerian form. In this paper we introduce simpler Eulerian variational principles from which all the relevant equations of barotropic magnetohydrodynamics can be derived. The variational principle is given in terms of six independent functions for non-stationary barotropic flows with trivial topologies and three independent functions for stationary barotropic flows. This is less than the seven variables which appear in the standard equations of barotropic magnetohydrodynamics, which are the magnetic field B the velocity field v and the density ρ. For non-trivial topologies it is necessary to assume that some of the variables introduced in the non-stationary formalism are non-single-valued. That is, it is necessary to introduce a number of branch cuts in order to define single-valued branches of the field variables. In turn, these cuts along with the six field variables constitute an extended number of dynamic variables. The number of cuts necessary depends on the flow. The relations between barotropic magnetohydrodynamics topological constants and the functions of the present formalism will be elucidated. The equations obtained for non-stationary barotropic magnetohydrodynamics resemble the equations of Frenkel et al . ( Phys. Lett . A, vol. 88, 1982, p. 461). The connection between the Hamiltonian formalism introduced in Frenkel et al . (1982) and the present Lagrangian formalism (with Eulerian variables) will be discussed.

TIME-DEPENDENT AND TIME-INDEPENDENT APPROACHES TO STUDY EFFECTS OF DEGENERATE ELECTRONIC STATES

Two types of phases are discussed in this article: (1) The topological phase as introduced by Berry [Proc. R. Soc. London, Ser. A 392, 45(1984)] and Aharonov and Anandan [Phys. Rev. Lett. 58, 1593 (1987)] and (2) the Longuet–Higgins phase [Proc. R. Soc. London, Ser. A 344, 147 (1975)]. The two types of phases have a common origin, namely the multivaluedness of the electronic adiabatic basis, a phenomenon associated with the existence of a degeneracy in configuration space. It will be shown, by studying an electronic model Hamiltonian that arises from a two-state approximation to the Mathieu equation, that the two phases differ from each other substantially, coinciding only in the adiabatic limit upon completion of a cycle.

A Generalized Approach to Estimating Capacity Factor of Fixed Speed Wind Turbines

This letter presents a method for estimating the capacity factor of stall regulated wind turbines, based on the wind probability distribution function and manufacturer-provided power curve. It is further shown that the existing capacity factor formulations, e.g., for pitch regulated wind turbines, are a specific example of the derived method. Rather than using a particular model for approximating the power curve, polynomial fitting is employed in order to preserve the generality.

Propagation analysis of ultrashort pulses in resonant dielectric media

The space-frequency theory of the propagation of an ultrawideband radiation in dielectric media is presented. Characterization of the material via its susceptibility leads to a transfer function, which describes the response of the medium in the frequency domain. This description enables the consideration of broadband signals, taking into account inhomogeneous absorptive and dispersive effects of the medium. Analytical expressions are derived when a pulse-modulated signal is propagating in a general dielectric material. Conditions for apparent “superluminal” and pulse compression effects are identified. The theory is applied for a special case of transmission inside a resonant medium, revealing analytical approximations for the parameters of a Gaussian propagating pulse in terms of initial pulse width, carrier frequency, and medium parameters. Constraints of the derived analytical expressions are discussed, pointing out conditions of approximation validity. We demonstrate the approach by studying the propagation of ultrawideband signals, while transmitted in the vicinity of the 60 GHz absorption peak of the atmospheric medium at millimeter wavelengths.

A Numerical Model Based on Variational Principle for Airfoil and Wing Aerodynamics

Over the last few years, finite element algorithms for solution of the Euler flow equations have gained increased popularity. The objective of the current research is to develop a new method to solve the Euler flow equations numerically using a variational technique for airfoil and wing aerodynamics. A new formulation of Eularian variational principle satisfying the Kutta condition is suggested, and a numerical implementation is presented. The proposed method can obtain improved solution, especially for complicated geometries. A computer code named FLUIDEX was developed to analyze barotropic fluid dynamics. The solution of the flow problem is obtained by using numerical algorithm to find the extremum value of an “Action” i.e. by a variational principle. Predictions of the FLUIDEX numerical model were analyzed for particular cases of potential flow (compressible and incompressible). The results were successfully compared against exact analytical solutions for potential flow test problems. The proposed method obtains fast and stable solutions without the need to integrate the equations in time and space, and thus enables a considerable reduction of the time and cost of the solution. I. INTRODUCTION It is well known that a presentation of a physical problem in terms of a variational principle can lead to a better understanding of the problem. Moreover, a variational principle combined with a numerical technique can lead to improved solution, especially for complicated geometries. Early attempts have been made to formulate Eulerian fluid dynamics in terms of a variational principle (Herivel, 1955; Serrin, 1959; Lin, 1963). However, the variational principles developed by the above authors are very cumbersome containing quite a few “Lagrange multipliers” and “potentials”. The range of the total number of independent functions in the above formulations ranges from eleven to seven which exceeds by many the four functions appearing in the Eulerian and continuity equations of a baratropic flow. Therefore they did not have any practical use or applications. A compact four function variational principle was obtained by Seliger and Whitham (1968) and thus a new way of simulating fluid dynamics was made possible. In previous works we developed a new method to solve the Euler flow equations numerically using Seliger and Whitham Variational technique (Yahalom and Pinhasi 2002; Yahalom 2003). A numerical implementation of their formulation of the Eularian Variational principle was suggested, and results for analysis of flows around various geometries including wing profiles were presented.