Zhengqing Yun

Propagation prediction models for wireless communication systems

A comprehensive review of the propagation prediction models for terrestrial wireless communication systems is presented in this paper. The classic empirical models are briefly described and the focus is placed on the application of ray-tracing techniques to the development of deterministic propagation models. Schemes to increase the computational efficiency and accuracy are discussed. Traditional statistical models are also briefly reviewed for completeness. New challenges to the propagation prediction are described and some new approaches for meeting these challenges are presented.

A ray-tracing method based on the triangular grid approach and application to propagation prediction in urban environments

This paper presents a fast ray-tracing procedure based on triangular division of the propagation environments. Unlike other accelerating methods that are mostly based on pre-establishment of visibility, this method does not require knowledge of the position information of the base station and/or receiving antennas and is thus more general. Although the triangulation is done in a two-dimensional (2-D) plane, this method is suitable for three-dimensional (3-D) simulations when a proper data structure for buildings is constructed. Validation results show good agreement between calculated and measured data from the European COST 231 project. The improvement in the computational efficiency is clearly demonstrated in examples.

Complex-wall effect on propagation characteristics and MIMO capacities for an indoor wireless communication environment

The effects of complex wall structures on the characteristics of fading and the capacity of multi-input multi-output (MIMO) wireless communication systems for some typical indoor propagation environments are investigated. Two cases of wall structures are examined in this paper. In the first case, the walls are considered to be homogenous solid slabs, while, in the second case, the walls are assumed to be of complex structures. A two-dimensional finite difference time domain method is employed to calculate the electric field distributions, and then, the local mean power, the Rician K factor, and the MIMO capacity are calculated and analyzed. It is found that the patterns of the local mean power distributions are different for the two wall-structure cases. As for the small-scale fading, it is shown that the Rician K factors for the two cases may be different by 5 dB. The resulting values of MIMO capacities are also quite different and are less than the ideal cases, where the elements of the transfer (H) matrix are assumed to be zero-mean Gaussians with unit variance. We also investigate the cases where complex walls are replaced by effective slab walls. It is found that complex walls cannot be appropriately characterized by simple effective slab walls as considerable difference exists between the two cases.

A ray-tracing approach for indoor/outdoor propagation through window structures

A ray-tracing approach for indoor/outdoor propagation through windows is proposed. Using both the finite-difference time-domain (FDTD) method and a ray-tracing algorithm, several cases of indoor/outdoor propagation through windows were investigated. It is shown that wave transmission through windows cannot generally be accounted for through a single transmission coefficient parameter. Instead, a full diffraction pattern needs to be accounted for and multiple-ray representation is therefore required. It is also shown that a single window model may be used to calculate transmission through set of windows in a typical building structure as a building block. Results from the implementation of a multiple-ray representation and FDTD simulations showed good agreement. Results were validated for both normal and oblique incident cases. The developed ray-tracing approach, therefore, facilitates the use of the developed window model in available ray-tracing algorithms often used for propagation characterization of urban environments. Simulation results were further validated by conducting measurements on scaled models at 30 GHz. The experimental results agreed well with the simulation data, thus validating the accuracy of the developed ray-tracing model for transmission through windows.

A new 3D FDTD multigrid technique with dielectric traverse capabilities

The finite-difference time-domain (FDTD) technique has become increasingly popular and is being used to model extremely complex and electrically large structures. These simulations are computationally demanding and often exceed available limits on computer resources. In this paper, we present an FDTD sub-gridding technique that allows for increased resolution in regions of interest without increasing the overall computational requirements beyond the available resources. Furthermore, the formulation presented here allows for traversing dielectric boundaries using any integer refinement factor and the maximum Courant number. By allowing the coarse-/fine-grid boundary to traverse dielectric boundaries, numerical simulations that were previously either extremely difficult or impossible to perform are now possible. The technique presented here uses a weighted current value from the coarse region at the boundary between the fine- and coarse-grid regions to update the fine-region tangential fields on that boundary. The weighting function depends on the material properties and the relative position of the fine-region electric field within the current contour at the boundary. The complete formulation of this new technique is described and some results of simulation cases are presented to validate the accuracy and stability of the newly developed FDTD code. Simulations include simple cases where the analytical solution exists and more complex cases, which were impossible to model using a uniform-grid FDTD code. In some simulation examples, computer memory savings as high as 70 times what would have been necessary with a uniform-grid code were achieved. It is shown that errors of less than 2% are achievable with ratios of coarse-to-fine grid sizes exceeding ten. The new technique is expected to be used in simulating many electrically large and complex structures in the biomedical microwave processing of materials and the wireless communications areas.

Hybrid smart antenna system using directional elements - performance analysis in flat Rayleigh fading

Smart antenna and associated technologies are expected to play a significant role in enabling broadband wireless communication systems. Smart antennas exploit space diversity to help provide high data rates, increased channel capacity, and improved quality of service at an affordable cost. In this paper we present a new procedure for implementing smart antenna algorithms. It is a hybrid approach that integrates the features of the switched beam method and the adaptive beam forming approach. Specifically it is shown that by using high gain antenna elements and combining the switched beam process with the adaptive beam forming procedure on a limited number of elements (as low as two in an eight-element array), a performance close to that of a more complex eight-element adaptive array may be achieved. The proposed hybrid method, therefore, is fast, is computationally efficient, and provides a cost effective approach for exploiting space diversity. Even with the inclusion of interference signals, the proposed hybrid approach out-performed the switched beam method, and provided performance similar to that of an adaptive array with less number of elements (three in an eight-element array). Implementation of an adaptive array also includes estimations; hence, reducing the number of elements in an array may lead to improved accuracy, in addition to fast convergence and reduced complexity.

Ray Tracing for Radio Propagation Modeling: Principles and Applications

This paper reviews the basic concepts of rays, ray tracing algorithms, and radio propagation modeling using ray tracing methods. We focus on the fundamental concepts and the development of practical ray tracing algorithms. The most recent progress and a future perspective of ray tracing are also discussed. We envision propagation modeling in the near future as an intelligent, accurate, and real-time system in which ray tracing plays an important role. This review is especially useful for experts who are developing new ray tracing algorithms to enhance modeling accuracy and improve computational speed.

Polarization and human body effects on the microwave absorption in a human head exposed to radiation from handheld devices

A multigrid finite-difference time-domain code was used to calculate specific absorption rate (SAR) distribution in a human head exposed to microwave radiation from handheld antennas. The effect of the human body was taken into account and different antennas and polarization conditions were considered, The distance between the antenna and human head were varied to examine the effect of the human body on the SAR distribution, From the numerical results, it is shown that the human body plays a significant role on the SAR value and its distribution in the head [as high as 53% monopole, 41% planar inverted F antenna (PIFA)]. It is also shown that the effect of the body is more dominant at lower frequencies (monopole 900 MHz versus 1.9 GHz). For the monopole case, effect of body is particularly important at larger separation distances from the head, e.g., at d=4 cm versus d=0.5 cm. Effect of body is particularly important for the vertical orientation cases for both the monopole and PIFA.

An Integrated Method of Ray Tracing and Genetic Algorithm for Optimizing Coverage in Indoor Wireless Networks

This letter presents a method for determining the required number and locations of transmit antennas to optimize wireless propagation coverage in a given indoor environment. The ray-tracing method is employed to calculate the field distribution due to one or more transmitting antennas and the genetic algorithm (GA) is used to determine the required number and the locations of these antennas to achieve optimized wireless coverage in a given area. In the ray-tracing method, detailed parameters such as thickness, dielectric constant, and the conductivity of individual walls are assigned for realistic field calculations. Obtained simulation results illustrate the feasibility of using the integrated ray-tracing/GA method in determining the optimized coverage of a wireless network.

Design and development of multiband coaxial continuous transverse stub (CTS) antenna arrays

Continuous transverse stub (CTS) technology has been adapted to use with coaxial lines to produce effective microwave antenna structures that radiate omnidirectionally, with high efficiency, low reflection, and useful radiation patterns. In this paper, we describe the design, construction, and testing of a new type of antenna array, that is, a six-element multiband (4.2 and 19.4 GHz) CTS antenna array. The design of the CTS array was optimized through simulation using finite-difference time-domain and then built and tested using both S-parameters and radiation pattern measurements. Simulation results agreed very well with measured data. These simple and low cost coaxial CTS structures could be adapted for base station applications in wireless communication, for satellite communication systems, and Identification Friend-or-Foe systems for the military.