Radio Propagation for Modern Wireless Systems

Description

To build wireless systems that deliver maximum performance and reliability, engineers need a detailed understanding of radio propagation. Drawing on over 15 years of experience, leading wireless communications researcher Henry Bertoni presents the most complete discussion of techniques for predicting radio propagation ever published. From its insightful introduction on spectrum reuse to its state-of-the-art real-world models for buildings, terrain, and foliage, Radio Propagation for Modern Wireless Systems delivers invaluable information for every wireless system designer. Coverage provides:

• A door to the understanding of radio wave propagation for the wireless channel.
• In-depth study of the effects on path loss of buildings, terrain, and foliage.
• A unified view of key propagation effects in narrowband and wideband systems, including spatial variation, angle of arrival, and delay spread.
• Readable account of diffraction at building corners, with worked out examples.
• Never-before-published coverage of mobile-to-mobile path loss in cities.
• Effective new ray-based models for site-specific predictions and simulation of channel statistics.
• Simulations of fast fading and shadow loss.

From start to finish, Radio Propagation for Modern Wireless Systems presents sophisticated models–and compares their results with actual field measurements. With thorough coverage and extensive examples from both narrowband and wideband systems, it can help any wireless designer deliver more powerful, cost-effective services.

Sample Content

Table of Contents

1. The Cellular Concept and the Need for Propagation Prediction.

Concept of spatial reuse. Linear cells as an example of FDMA spectrum reuse. Hexagonal cells for area coverage. a--Symmetric reuse patterns. b--Interference for symmetric reuse patterns. Sectored cells. Spatial reuse for CDMA. Summary. Problems. References.

Narrowband signal measurements. a--Signal variation over small areas: fast fading. b--Variations of the small-area average: shadow fading. c--Separating shadow fading from range dependence. Slope-intercept models for macrocell range dependence. Range dependence for microcells: influence of street geometry. a--LOS paths. b--Zigzag and staircase paths in Sunset and Mission districts. c--Non-LOS paths in the high-rise core of San Francisco. Multipath model for fast fading and other narrowband effects. a--Frequency fading. b--Time-dependent fading. c--Doppler spread. d--Depolarization. Narrowband indoor signal propagation. a--Fast fading for indoor links. b--Distance dependence of small-area average. Channel response for pulsed excitation. a--Power delay profile. b--Fading characteristics of individual pulses. c--Measures of time-delay spread. d--Coherence bandwidth. Multipath observed at elevated base station antennas. Summary. Problems. References.

Plane waves in an unbounded region. a--Phasor notation. b--Propagation oblique to the coordinate axes. c--Fast fading due to several plane waves. d--Correlation function and Doppler spread. e--Fading at elevated base stations. Reflection of plane waves at planar boundaries --62 3.2a--Snell's law. b--Reflection and transmission coefficients for TE polarization. c--Reflection and transmission coefficients for TM polarization. d--Height gain for antennas above ground. e--Reflection of circularly polarized waves. Plane wave incidence on dielectric layers. a--Reflection at a brick wall. b--Reflection at walls with loss. c--Transmission through walls of uniform construction. d--Transmission through in-situ walls and floors. Summary. Problems. References.

Radiation of spherical waves. Receiving antennas, reciprocity, and path gain or loss. a--Path gain or loss. b--Effective area of a receiving antenna. c--Received power in the presence of a multipath. Two-ray model for propagation above a flat earth. a--Breakpoint distance. b--Two-slope regression fit. LOS Propagation in an urban canyon. Cylindrical waves. Summary. Problems. References.

Local nature of propagation. a--Evaluation of the field distortion. b--Interpretation of the local region in terms of Fresnel zones. Plane wave diffraction by an absorbing half-screen. a--Field in the illuminated region y > 0. b--Field in the shadow region y < 0. c--Geometrical theory of diffraction. d--Evaluating the Fresnel integral for y near the shadow boundary. e--Uniform theory of diffraction. Diffraction for other edges and for oblique incidence. a--Absorbing screen. b--Conducting screen. c--Right-angle wedge. d--Plane waves propagating oblique to the edge. Diffraction of spherical waves. a--Diffraction for rays incident at nearly right angles to the edge. b--Diffraction for rays that are oblique to the edge. c--Path gain for wireless applications. Diffraction by multiple edges. a--Two parallel edges. b--Two perpendicular edges. Summary. Problems. References.

Modeling propagation over rows of low buildings. a--Components of the path gain. b--Modeling PG2 by diffraction of the rooftop fields. Approaches to computing the reduction PG1 of the rooftop fields. a--Physical optics approach to computing field reduction. b--Solutions for uniform row spacing and building height. Plane wave incidence for macrocell predictions. a--Solution in terms of Borsma's functions. b--Using the settled field to find the path loss. Cylindrical wave incidence for microcell predictions. a--Solution in terms of Borsma's functions. b--Path loss for low base station antennas. c--Path loss for mobile-to-mobile propagation. d--Propagation oblique to rows of buildings. Numerical evaluation of fields for variable building height and row spacing. a--Windowing to terminate the integration. b--Discretization of the integration. c--Height dependence of the settled field. d--Influence of roof shape. Summary. Problems. References.

Shadow fading statistics. a--Variation of the rooftop fields. b--Combined variations for street-level signal. Modeling terrain effects. a--Paths with LOS to the rooftops near the subscriber. b--Paths with diffraction over bare wedge-shaped hills. c--Paths with diffraction over bare cylindrical hills. d--Diffraction of cylindrical waves over hills with buildings. e--Path loss formulas for building-covered hills. Modeling the effects of trees. a--Propagation to subscribers in forested areas. b--Path loss to subscribers in forest clearings. c--Rows of trees in residential areas. Summary. Problems. References.

Outdoor predictions using a two-dimensional building database. a--Image and pincushion methods. b--Ray contributions to total power. c--Comparison of predictions with measurements. Two-dimensional predictions for a Manhattan street grid. a--Path loss in turning one corner. b--Predictions made using two-dimensional ray methods. Outdoor predictions using a three-dimensional building database. a--Three-dimensional pincushion method. b--Vertical plane launch method. c--Slant plane-vertical plane method. d--Monte Carlo simulation of higher-order channel statistics. Indoor site-specific predictions. a--Transmission through floors. b--Effect of furniture and ceiling structure on propagation over a floor. Summary. Problems. References.

Preface

Preface

The commercial success of cellular mobile radio since its initial implementation in the early 1980s has led to an intense interest among wireless engineers in understanding and predicting radio propagation characteristics within cities, and even within buildings. In this book we discuss radio propagation with two goals in mind. The first is to provide practicing engineers having limited knowledge of propagation with an overview of the observed characteristics of the radio channel and an understanding of the process and factors that influence these characteristics. The second goal is to serve as text for a master's-level course for students intending to work in the wireless industry. Books on modern wireless applications typically survey the issues involved, devoting only one or two chapters to radio channel characteristics, or focus on how the characteristics influence system performance. Now that the wireless field has grown in scope and size, it is appropriate that books such as this one examine in greater depth the various underlying topics that govern the design and operation of wireless systems. The material for this book has grown out of tutorials given by the author to engineering professionals and a course on wireless propagation given by the author at Polytechnic University as part of a program in wireless networks. It also draws upon the 15 years of experience the author and his students have had in understanding and predicting propagation effects.

Cellular telephones gave the public an active role in the use of the radio spectrum as opposed to the previous role of passive listener. This social revolution in the use of the radio spectrum ultimately changed governmental views of its regulation. Driven by the requirement to allow many users to operate in the same band, cellular telephones also created a technical revolution through the concept of spectral reuse. Systems that do not employ spectral reuse avoid interference by operating in different frequency bands and are limited in performance primarily by noise. In these systems, lack of knowledge of the propagation conditions can be compensated for by increasing the transmitted power, up to regulatory limits. In contrast, the concept of spectral reuse acknowledges that in commercially successful systems, interference from other users will be the primary factor limiting performance. In designing these systems, it is necessary to balance the desired signal for each user against interference from signals intended for other users. Finding the balance requires knowledge of the radio channel characteristics. Chapter 1 is intended to introduce the student reader to the concept of spectrum reuse and in the process to give examples of how the propagation characteristics influence the balance between desired signal and interference, and thereby influence system design. As in all chapters, examples are discussed to illustrate the concepts, and problems are included at the end of the chapter to give the students experience in applying the concepts.

In modern systems, the radio links are about 20 kilometers or less, the antennas that create the links lie near to or among the buildings or even inside the buildings, and the wavelength is small compared to the building dimensions. As a result, the channel characteristics are strongly influenced by the buildings as well as by vegetation and terrain. In this environment, signals propagate from one antenna to the other over multiple paths that involve the processes of reflection and transmission at walls and by the ground and the process of diffraction at building edges and terrain obstacles. The multipath nature of the propagation makes itself felt in a variety of ways that have challenged the inventiveness of communication engineers. Although initially a strong limitation on channel capacity, engineers have begun to find ways to harness the multipath signals so as to achieve capacities that approach the theoretical limit. However, each new concept for dealing with multipath calls for an even deeper understanding of the statistical characteristics of the radio channel. In Chapter 2 we describe many of the propagation effects that have been observed in various types of measurements, ranging from path loss for narrowband signals, to angle of arrival and delay spread for wideband transmission. As in other chapters, an extensive list of references is cited to aid the professional seeking a detailed understanding of particular topics. For the student reader, this chapter serves as an introduction to the types of measurements that are made, the methods used to process the data, and some of the statistical approaches used to represent the results. Understanding the measurements, their processing, and their representation also serves to guide the theoretical modeling described in subsequent chapters.

The level of presentation assumes that the reader has had an undergraduate course in electromagnetics with exposure to wave concepts. The presentation does not attempt to derive the propagation characteristics from Maxwell's equations rigorously; rather, the goal is to avoid vector calculus. The reader's background is relied on for acceptance of some wave properties; other properties are motivated through heuristic arguments and from basic ideas, such as conservation of power. For example, in Chapter 3 we start with the fundamental properties of plane waves and call on the reader's background in transmission lines when discussing reflection and transmission at the ground and walls. Wherever possible in this and following chapters, the theoretical results are compared to measurements. Thus plane waves are used to model observed interference effects, which are referred to as fast fading, and to model Doppler spreading. Plane wave properties and conservation of power are used in Chapter 4 to justify the properties of spherical waves radiated by antennas and to motivate the ray description of reflection at material surfaces. By accounting for these reflections, propagation on line-of-sight paths in urban canyons is modeled. Circuit concepts are used to obtain the reciprocity of propagation between antennas, and to derive expressions for path gain or loss.

Diffraction at building edges is an important process in wireless communications. It allows signals to reach subscribers who would otherwise be shadowed by the buildings. Because the reader is not expected to be familiar with this process, Chapter 5 explores diffraction in some detail. For simplicity, the scalar form of the Huygens-Kirchhoff integral is use as a starting point. We first use it to give physical meaning to the Fresnel ellipsoid about a ray, which is widely employed in propagation studies to scale physical dimensions. The geometrical and uniform forms of the fields diffracted by an absorbing half screen are derived. In these expressions we identify a universal component that applies to diffraction by any straight building edge or corner and a diffraction coefficient whose specific form is dependent on the nature of the edge. Diffraction coefficients for several types of edges and corners are given without derivation. Using heuristic ray arguments, the results obtained for plane waves are generalized to spherical waves radiated by antennas and to multiple edges. These results are cast in terms of path gain or loss, which is convenient for wireless applications.

Chapter 6 formulates the problem of average path loss in residential environments in terms of multiple diffraction past rows of buildings. Relying on the Huygens-Kirchhoff formulation, the diffraction problem is solved for various ranges of base station and subscriber antenna height. These results show how the frequency, average building height, and row separation influence the range dependence and height gain of the signal. This approach to diffraction is used in Chapter 7 to investigate the effects of randomness in building construction on shadow fading. Chapter 7 also makes use of diffraction to examine the effects of terrain and vegetation on the average path loss.

Propagation predictions that make use of a geometrical description of individual buildings are discussed in Chapter 8. Various ray-based models that incorporated the processes of reflection and diffraction at buildings have been developed to make such site-specific predictions. Their accuracy has been evaluated primarily by comparing predictions against measurements of the small area average received signal. However, the ray models have started to be used to predict higher-order channel statistics, such as time delay and angle spread, through Monte Carlo simulations. This approach can generate values for the statistical descriptors of the radio channel that are employed in advanced communication systems and show how these values depend on the distribution of building size and shape in different cities.

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