Search

Nonspecular scattering from the facades of large buildings is dominated by windows and decorative masonry. The placement of these features is nearly periodic. As a result, scattering from these buildings takes the form of diffraction orders, rather than diffuse scattering from a randomly rough surface. Computing the scattering for many realizations of building facades allows us to average the scattering properties to achieve a single smoothed scattering coefficient that can be used to represent all of the buildings. Index Terms--Building reflection, coherent scattering, diffraction, noncoherent scatteirng.

Progressive and approximate techniques are proposed here for ray-tracing systems used to predict radio propagation. In a progressive prediction system, intermediate prediction results are fed back to users continuously. As more raypaths are processed, the accuracy of prediction results improves progressively. We consider how to construct a progressive system that satisfies the requirements of continuous observability and controllability as well as faithfulness and fairness. Adding a workload estimator to such a progressive prediction system allows termination of the computation when a desired accuracy (mean and standard deviation of the error) is achieved without knowing the final result that would be obtained if the prediction system runs to completion. The sample generator is at the core of the progressive prediction system and serves to cluster and prioritize raypaths according to their expected contributions to prediction results. Two types of progressive approaches, source-group-raypath-permute and raypath-interleave, are proposed. The workload estimator determines the number of raypaths to be processed to achieve the specified requirement on prediction accuracy. Two approximate models are described that adjust the workload dynamically during the prediction process. Our experiments show that the proposed progressive and approximate methods provide flexible mechanisms to trade prediction accuracy for prediction time in a relatively fine granularity. Index Terms--Faithfulness and fairness of prediction results, observability and controllability, prediction accuracy and prediction error, progressive and approximate prediction model, sample generator and workload estimator.

Propagation around comers in tunnels and urban street canyons is modeled using a modal approach. While the modal representation for a straight tunnel is well-known, coupling at L, T, and cross tunnels has not been studied. For the propagation into branches off the main tunnel, mode coupling at the discontinuities is obtained by hybrid ray-mode conversion. By accounting for the mode diffraction at each corner, we compute the coupling loss into cross junction, the arms of T-junctions, and L-bends. Because the cross tunnel has four edges, they generate more corner diffracted modal fields than other structures, thus have the strongest coupling. For low base station and mobile antennas, high rise urban street canyons can be modeled two-dimensional dielectric waveguides so that we can apply the modal results of tunnel to the mode propagation in urban street canyons. Index Terms--Corner propagation, coupling loss, modal analysis, mode coupling, mode diffraction, ray-mode conversion, tunnel propagation, urban street propagation.

For a low base-station (BS) antenna located on one street, signals propagate into crossing and parallel streets by reflection and diffraction at corners of buildings. Therefore, in order to accurately predict the received signals, it is necessary to properly model the diffraction coefficient at the building edge and to accurately represent the shape and the electrical properties of the building near the corner. This paper compares ray-tracing predictions to measurements of received power and root mean square (rms) delay spread and shows the need for a diffraction coefficient having larger values than suggested by the commonly used heuristic diffraction coefficient. A new heuristic diffraction coefficient is proposed that has higher diffracted field strength in the deep shadow region and in the region between the two shadow boundaries. The proposed diffraction coefficient shows better agreement with measurements of both received power and delay spread compared to the commonly used heuristic diffraction coefficient. The influence of building shape near the corner and its electrical properties on the ray-tracing predictions are also presented. The shape is shown to have an important role in accurately predicting both received power and delay spread. Index Terms--Heuristic diffraction coefficient, ray prediction, microcellular environment.

Implementation digital fixed wireless systems, such as digital FM, high definition TV (HDTV) and wireless local loop (WLL), require a high level of accuracy of propagation prediction in the VHF and UHF bands. This paper presents a theoretical model that predicts the average path loss from an elevated base station to subscriber antennas at rooftop level in residential areas for frequencies in the approximate band from 30 MHz up to 3 GHz. In addition, the model gives the standard deviation of the path loss at the average rooftop level and the height gain for subscriber antennas above or below the average rooftop level. The approach used here generalizes that of Walfisch and Bertoni, who developed a theoretical model of UHF propagation in urban environments. The rows of buildings in residential areas are modeled as a series of dielectric screens having random heights and uniform spacing. Propagation takes place over and through the dielectric screens, and ground reflections are included. The fields at the plane of each screen are obtained in terms of repeated numerical evaluation of the physical optics (PO) integral. Results obtained for the variation of the range dependence on frequency and row spacing are consistent with those previously found for high frequencies by assuming the screens to be totally absorbing. Using the results at different heights of receiving antenna allows computation of the height gain. The predictions are in agreement with the International Telecommunication Union Radio Communication (ITRU-R) recommendations in the VHF and UHF bands. Index Terms--Antenna height gain, multiple diffraction, path-loss model, radio propagation.

The design of future wireless communication systems employing microcells will require a flexible tool for determining the dependence of the coverage area on base-station antenna height and position. In this paper, the accuracy of a ray-based algorithm versus slope intercept models for propagation prediction in residential environments is investigated by comparison with measurements. The measurements were taken in two areas of Trenton, NJ, where building heights ranged from one to three stories. For base-station antennas at or above rooftop levels, the algorithm is shown to give good agreement with the measurements. For these high antennas, the average and standard deviations of the difference between the site-specific prediction and measurement are less than 1 and 5 dB, respectively. For base-station antennas below the rooftops, additional ray paths are required in order to achieve similar prediction accuracy.

Path-loss prediction algorithms for advanced wireless communication system planning have long considered the effect of electromagnetic propagation over buildings between the base station and subscriber. This phenomena is particularly important in residential areas, where the houses are typically a few stories high. For the most part, the buildings were assumed to lie on level terrain, although shadowing effects by terrain in the absence of buildings has been included. Previous works have offered a number of methods to quantitatively determine these effects from path profiles. This study examines propagation over buildings when the buildings are located on terrain features (hills). The buildings, which are represented by a series of absorbing half screens, are assumed to lie in rows that are equally spaced along parallel streets, with the streets running perpendicular to the terrain slope. Numerical results are obtained using successive repetition of the Kirchhoff-Huygens approximation. A phenomenological model based on ray optics for diffraction over a smooth surface is proposed as a way to interpret the numerical results. The dependence of model coefficients on terrain parameters are obtained from the numerical results. Index Terms - Propagation, terrain factors, urban areas.

A vertical-plane-launch (VPL) technique for approximating a full three dimensional (3-D) site-specific ray trace to predict propagation effects in cities for frequencies in the 300-MHz-3-GHz band is described and its predictions are compared with measurements for Rosslyn, VA. The VPL technique employs the standard shoot and bounce method in the horizontal plane while using a deterministic approach to find the vertical displacement of the unfolded ray paths. This approximation is valid since buildings walls are almost always vertical. The VPL method shows significant improvement compared with the slant-plane/vertical-plane (SP/VP) method for rooftop antennas. For a base station located at street level, the VPL method gives better predictions than the two-dimensional (2-D) method in locations where propagation over buildings is significant. Index Terms - Geometrical optics, propagation, urban areas.

A theoretical model is proposed to compute the path loss in a vegetated residential environment, with particular application to mobile radio systems. As in the past, the row of houses or blocks of buildings are viewed as diffracting cylinders lying on the earth and the canopy of the trees are located adjacent to and above the houses/buildings. In this approach, a row of houses or buildings is represented by an absorbing screen and the adjacent canopy of trees by a partially absorbing phase screen. The phase-screen properties are found by finding the mean field in the canopy of the tree. Physical optics (PO) is then used to evaluate the diffracting field at the receiver level by using a multiple Kirchhoff-Huygens integration for each absorbing/phase half-screen combination. Index Terms - Propagation, urban areas, vegetation.

The growing use of unlicensed wireless systems has spurred interest in the 2.4-GHz ISM band. In order to facilitate the design of such systems, measurements of the pulse response characteristics have been made inside commercial buildings. From the measured pulse response, the statistical properties of the amplitude variation for individual pulses was determined, in addition to path loss, mean excess delay, root mean square (rms) delay spread, and the coherence bandwidth of the indoor channel.

In defining path loss for narrowband systems, it is common to introduce the concept of the sector or local average in order to remove the rapid (Rayleigh or Rician) fading. As originally developed for cellular mobile radio (CMR) systems, the sector average is obtained from the spatial average of the received signal as the mobile traverses a path of 20 or so wavelengths. This approach has also been applied indoors with the assumption that a unique average could be obtained by moving either end of the radio link. Unlike the CMR path for indoor propagation, however, both ends of the radio link are in a rich multipath environment. It is shown both theoretically and experimentally that moving both ends of the link is required to achieve a unique average. When averaging is accomplished by moving only one end, the result can experience variations of as much as [+ or -]2 dB when the other end is moved by a fraction of a wavelength.

New microcellular systems have been proposed to operate over short radio paths by using low-base station antennas, and transmitting at low power. In order to study radio propagation in the microcellular environments for future personal communications services (PCS), comprehensive radio propagation measurements were conducted by Telesis Technologies Laboratory (TTL) in the San Francisco Bay area using three transmitting antenna heights of 3.2, 8.7, and 13.4 m and two frequencies in the 900 and 1900 MHz bands. This paper reports the path loss measurements made in urban and suburban areas where the receiving mobile was driven along preselected line-of-sight (LOS), zig-zag, and staircase routes to gather information about direct propagation along streets, as well as diffraction over the roofs in suburban areas, and diffraction around the corners in urban areas. The results obtained for the three antenna heights are studied, and show the height gain that can be expected for microcells in different environments.

Personal Communication Services (PCS) will make use of low antennas and low radiated power to achieve base station coverage over microcells having radius of about 500 m. For such small cells the configuration of buildings surrounding the base station will have a major influence on the propagation in different directions, and hence on cell shape. This paper presents a theoretical approach to determining cell shape for residential and commercial regions, where the buildings are of a relatively uniform height, and are closely spaced in rows along the streets. Because it is based in theory, the model treats base station antenna height and location, and frequency as input parameter, in addition to area specific descriptors of the buildings, such as average height, location with respect to the streets, and street geometry. As an example, contour plots of signal level have been computed for an area having two story town houses located on a rectangular street grid, and for base station antennas located mid-block, at a street intersection, and in the backyards of the buildings.

Buildings such as warehouses, supermarkets and retail department stores typically have walls constructed from concrete (cinder) blocks. The web and void design of the individual blocks and their arrangement within a wall creates a periodic structure, which exhibits frequency dependent transmission and reflections characteristics in the UHF bands proposed for future Personal Communication Systems (PCS). For higher frequencies, higher order Floquet modes excited at the periodic structure can propagate away from the wall, suggesting that significant power can be carried away from the wall in non-specular directions. Indoor propagation prediction models must consider the nonspecular paths eidted by walls with a periodic nature in order to account for all of the scattered power. In this work, plane wave reflection and transmission characteristics for typical concrete block walls are examined theoretically and experimentally to determine the frequency dependence of the specularly and nonspecularly transmitted and reflected fields.

To acquire a knowledge of radio propagation characteristics in the microcellular environments for personal communications services (PCS), a comprehensive measurement program was conducted by Telesis Technologies Laboratory (TTL) in the San Francisco Bay area using three base station antenna heights of 3.2 m, 8.7 m, and 13.4 m and two frequencies at 900 MHz and 1900 MHz. Five test settings were chosen in urban, suburban, and rural areas in order to study propagation in a variety of environments. This paper reports the LOS measurements in different environments, all of which show variations of signal strength with distance that have distinct near and far regions separated by a break point. It was also found that the location of the break point for different frequencies and antenna heights can be calculated based on first Fresnel zone clearance. The regression analysis reveals a slope that is less than two before the break point, while it is greater than two after the break point. This break distance can be used to define the size of microcell and to design for fast hand-off. Beyond the first Fresnel zone break distance the base station antenna height gain was observed to approximately follow the square power law of antenna height.

This paper presents theoretical results for the dependence on base station antenna height of the average received signal for mobiles at street level. The results apply to residential and commercial sections of cities and to all ranges of antenna height from well above to below that of the surrounding buildings. Assuming all buildings to be of equal heights, the range dependence of the average signal is found by evaluating multiple forward diffraction past rows of buildings. The solution for this diffraction problem for sources near to or below the rooftops gives the dependence of the range index on antenna height and base station height gain, which are in agreement with measurements. These results will be of importance for proposed systems for personal communication services (PCS), which envision the use of base station antennas at the height of lamp posts, as well as cellular mobile radio (CMR).

Because of the potential implementation of indoor wireless local area networks (LAN's) and personal communication networks (PCN) it is important to understand propagation of signals in the UHF band inside buildings. This paper explores features of office buildings of modern construction that influences propagation between transmitter and receiver located on the same floor. One feature is the clear space between ceiling and furnishings or floor that results in excess attenuation of the signal. A second feature is reflection and transmission at interior and exterior walls. Diffraction at corners and propagation along the exterior wall are also shown to be a significant means for radiation to reach the receivers. The influence of the first two features are combined into a computer program that evaluates the sector average signal, which is then compared with measurements.

A number of three-dimensional (3-D) direction-of-arrival measurements in a small campus environment at 1845 MHz are described. The impulse responses and the power have been measured using an 80-MHz wide-band receiver applying correlation techniques. These measurements are compared with theoretical predictions made with a site-specific ray-based propagation predictor tool to determine the coherently reflected part of the signal. Furthermore, radiosity is applied as a model to determine the power scattered by the surfaces. Index Terms--Cellular radio, dispersive channels, multipath channels.

Microcells for wireless communications can be realized with low base station antennas operating at low power. The low base station antennas expected for microcells make the propagation characteristics dependent on the direction relative to the street grid. Due to this anisotropic propagation, the shape of microcells is no longer circular, as is typically assumed so for macrocellular system planning. Therefore the infrastructure of microcells should be implemented with a different approach from that for macrocells. This paper aims at finding the effect of microcellular wave propagation on the development of cellular design for channelized systems in residential/commercial environments by examining key aspects of cell layout. Using a measurement-based anisotropic propagation model, cell shape and frequency reuse patterns are investigated for the downlink, and a methodology for frequency planning is presented. Index Terms--Cell plannings, cell shape, propagation modelings, time-division multiple-access (TDMA) microcells, traffic capacity.

This paper reports on studies of propagation in an urban forested park area to investigate the statistical nature of the time fading for frequencies ranging from 0.9 GHz to 1.8 GHz, as well as to examine the range dependence and the base station height gain. Measurements of the received signal over 30 s intervals for a stationary mobile were used to construct the distribution functions for the fading. In most cases, the distribution could be approximated by a Rician distribution, whose K-factor was found to depend on transmitter height. Index Terms--Communication systems, mobile cellular, vegetation, vegetation effects.

Limitations on the use of the uniform theory of diffraction (UTD) slope-diffraction method for propagation past knife edges are investigated. The settled field that is diffracted past rows of buildings, as computed by numerical integration, has the characteristics of amplitude and phase variation with a scale length that is large compared to wavelength and has small amplitude near the edges. Using this field, it is shown that the error in the UTD slope-diffraction method for diffraction past a final screen is connected with the Fresnel width, as compared to the scale length of the settled field. Index Terms--Diffraction, geometrical theory of diffraction, mobile communication, radio communication.

Author(s): Henry L. Bertoni (corresponding author) Because we love to talk, and because modern microchips have brought down the cost, cellular (mobile) phones have become a ubiquitous feature of Western [...]