Sharif Digital Repository

In this paper, the Becchi-Rouet-Stora-Tyutin (BRST) symmetry transformation is presented for the noncommutative U(N) gauge theory. The nilpotency of the charge associated with this symmetry is then proved. As a consequence of the spacelike noncommutativity parameter, the Hubert space of physical states is determined by the cohomology space of the BRST operator as in the commutative case. Further, the unitarity of the S-matrix elements projected onto the subspace of the physical states is deduced. © 2003 The American Physical Society  

In this paper, the Becchi-Rouet-Stora-Tyutin (BRST) symmetry transformation is presented for the noncommutative [Formula Presented] gauge theory. The nilpotency of the charge associated with this symmetry is then proved. As a consequence of the spacelike noncommutativity parameter, the Hilbert space of physical states is determined by the cohomology space of the BRST operator as in the commutative case. Further, the unitarity of the S-matrix elements projected onto the subspace of the physical states is deduced. © 2003 The American Physical Society  

This study presents a new ray tracing acceleration technique for site-specific propagation modelling in indoor environments. It overcomes one of the major problems regarding the computational efficiency of shooting-and-bouncing-ray (SBR) method, that is most of the rays emitted from the source do not reach the receiver whereas all of them must be traced. Our proposed method solves this problem in a two-step procedure based on the idea of line search theory. In the first step, called the bracketing phase, the solid angles around the transmitter that transport electromagnetic power to the receiver are determined. In the second step, called the sectioning or zoom phase, the accuracy is improved... 

Ray tracing has been widely accepted as a versatile and reliable method for indoor propagation modeling. Several published articles have addressed the computational efficiency and reliability as the most challenging problems of ray tracing and have presented various solutions. Such challenges are discussed in this paper and some possible solutions are presented  

The plan of the environment used in a ray tracing simulation is not usually in exact accordance with the actual environment plan. Errors on the order of several centimeters may be present in describing the lateral wall positions. Such errors are comparable to the wavelength in indoor wireless applications and hence, can severely alter the phases of multipath components and lead to inaccurate electromagnetic field at the receiver. In this paper, the impact of this type of geometrical errors on the ray tracing results is investigated. Lateral wall positions are considered as random variables to account for possible geometrical errors. The statistical moments of some channel parameters are... 

This study presents the modified wavefront decomposition method as a new ray-tracing acceleration technique for indoor propagation modelling. In this method, the exact propagation paths of the rays that reach the receiver after multiple reflections and/or refractions are calculated. Thus, the electromagnetic field at the receiver is obtained with a high accuracy. When only reflections are considered, our method simplifies to the multiple image theory or shooting and bouncing ray/image approach already available in the literature. However, the modified wavefront decomposition method calculates the exact propagation paths toward the receiver even if the rays encounter multiple refractions or a... 

In site-specific ultrawideband propagation modelling, electromagnetic field calculations need to be performed by ray tracing at numerous frequencies of the band in order to properly include the frequency variations of the propagation mechanisms and antenna patterns. This is computationally intensive especially in calculating the coverage map or finding the statistics of the channel parameters where field calculations need to be carried out at a lot of receiver locations. In this study, the cubic B-spline function is incorporated in ray tracing to increase the computational efficiency of site-specific ultrawideband propagation modelling. This function can accurately reconstruct a signal from... 

Site-specific ultrawideband propagation modeling involves dealing with frequency variations of the propagation mechanisms and antenna patterns in the ray tracing program. Typically, electromagnetic field calculations have to be performed in a lot of frequencies to accurately construct the frequency response of the channel at a receiving point. This imposes a heavy computational burden on ray tracing specially in calculating the coverage map or finding the statistics of channel parameters where the number of receiving locations is large. In this paper, the computational complexity of ray tracing in ultrawideband propagation modeling is reduced by using cubic B-spline interpolation. The... 

The complex dielectric constants of building materials are present in the reflection, transmission and diffraction coefficients of the ray tracing formulation. Any deviation of the dielectric constants included in ray tracing simulation from the actual material properties may be a source of inaccuracy of the simulation results. This paper explores the sensitivity of ray tracing results to the inaccuracy of the dielectric constants of building materials. To do this, the real and imaginary parts of the dielectric constants are considered as random variables to account for the present uncertainty. The propagation of uncertainty in the ray tracing simulation is then obtained by means of the... 

This paper presents an efficient way for overcoming the inaccurate path error encountered in ray tracing for site-specific indoor propagation modeling. The source is modeled using triangular ray tubes each of which consists of three rays which do not lie in the same plane. Each possible source ray inside an arbitrary ray tube is expanded in terms of the three rays defining that ray tube. The variations of the expansion coefficients during the intersections of the ray tubes with dielectric media are derived and used for correcting the paths of the rays toward the receiver. A verification example is presented which proves that the presented method has the ability to be efficiently used in... 

In ray-tracing-based propagation modelling, the electromagnetic field at the receiver is obtained by coherent summation of the fields of multipath components. It is therefore crucial to accurately calculate the phase of the electromagnetic field of each ray. In practice, when preparing the plan of the environment for ray tracing simulation, the lateral positions of the walls may not be included accurately in the database. This alters the phases of the fields as well as the delays of arrival of multipath components which may consequently lead to less accurate results. In this study, the sensitivity of ray tracing results to this type of geometrical inaccuracy is investigated through the... 

A new acceleration technique is presented for shooting and bouncing ray tracing in indoor environments. Tracing just a few rays, the algorithm finds the solid angles around the transmitter that transport electromagnetic power to the receiver. The accuracy is then improved by iteratively increasing the tessellation frequency of the source in the power-transporting solid angles. No rays will be sent through non-power-transporting solid angles, which results in significant reduction of the simulation time. An example of applying the method for studying indoor radio wave propagation is presented and the results are compared with a very high-resolution fully three-dimensional (3D) ray tracing... 

This paper applies the method of decomposition of wavefronts to ray tube tracing model. Energy ray tubes which are emitted from the source are modeled by three rays and the method of decomposition of wavefronts iteratively improves the accuracy of ray tracing. Comparison with applying the method of decomposition of wavefronts to conventional ray tracing is made afterwards. It is observed that by applying the method of decomposition of wavefronts to ray tube model, the accuracy of ray tracing can be improved much more efficiently and the simulation time decreases to almost a half. ©2009 IEEE  

This paper presents a new efficient and accurate method to overcome the double-counting problem of ray tracing in indoor environments. The conventional method of modeling the transmitter using an icosahedron inscribed in the unit sphere is used. The separation angle distribution of the source rays over the faces of the icosahedron is calculated prior to the start of the tracing procedure. These angles are used to cancel the double-counting problem in the post processing stage. To validate our proposed method, an electromagnetic problem that has analytical solution is presented and the ray tracing results are compared to this solution. The simulation results agree with analytical solution... 

A new acceleration technique for the ray tracing algorithm was presented in this paper. Instead of reducing the number of intersection tests, our proposed method finds the solid angles around the transmitter that transport electromagnetic power to the receiver. Then, high-resolution rays are sent out only in these solid angles and the procedure continues iteratively. Since a few rays are traced in each iteration, high resolution can be achieved within acceptable amount of time  

In this study, the Geant4 toolkit is utilized to present a new approach for simulation of the D-T interaction. The neutron production cross-section and the energy/angle distribution of neutrons produced as the D-T data libraries are incorporated in the Geant4, and their performance is evaluated. The spectra shape of the produced neutrons of the D-T interaction simulated by the Geant4 show good agreement compared to the results of the comparative codes (i.e. MCUNED, ENEA-JSI, and DDT). However, some differences are observed, especially, in angular range of 90-95 degrees. The reason for these differences as well as the results of the proposed approach in comparison with the results of the... 

The aim of this study was to determine the workspace of surgeon's body for designing more efficient surgical robots in the operation rooms. Five wearable inertial sensors were placed near the wrist and elbow joints and also on the thorax of surgeons to track the orientation of upper limb. Assuming that the lengths of five segments of an upper limb were known, measurements of the inertial sensors were used to determine the position of the wrist and elbow joints via an established kinematic model. subsequently, to assess the workspace of surgeon upper body, raw data were collected in the arthroscopy and laparoscopy operations. Experimental results demonstrated that the workspaces of surgeon's... 

When properties of the displacing fluid are considered, the rising profile of the penetrating fluid in a capillary tube deviates from its classical Lucas-Washburn profile. Also, shape of capillary tube can affect the rising profile in different aspects. In this article, effect of capillary tube's shape on the vertical capillary motion in presence of gravity is investigated by considering the properties of the displacing fluid. According to the fact that the differential equation of the capillary rising for a non-simple wall type is very difficult to solve analytically, a finite element simulation model is used for this study. After validation of the simulation model with an experiment that...