Propagation of Radiowaves (3rd Edition)
Propagation of Radiowaves introduces the basic concepts and mechanisms of radiowave propagation engineering in both the troposphere and ionosphere, an understanding of which is fundamental to the effective use of the radio spectrum for radiocommunication. Reflecting the wide experience of the exceptional group of authors, the contents provide a firm background to established theory and introduce the most appropriate models, methods and procedures which are of use to spectrum planners, system designers and operators in assessing the estimated performance of radio systems. The field of radio communications continues to change rapidly and the third edition of this outstanding and successful book has been fully updated to reflect the latest developments. The relevant Recommendations of ITU-R Study Group 3 are discussed and put into context. Propagation of Radiowaves, 3rd Edition is essential reading for professionals involved in the planning, design and operation of radio systems, as well as academics and postgraduate students in the field.
Inspec keywords: radiowave propagation
Other keywords: IEE; residential course; radiowave propagation; IET
Subjects: Radiowave propagation
- Book DOI: 10.1049/PBEW056E
- Chapter DOI: 10.1049/PBEW056E
- ISBN: 9781849195782
- e-ISBN: 9781849195799
- Page count: 467
- Format: PDF
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Front Matter
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1 Introduction
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Propagation is the fundamental property of electromagnetic waves upon which all radio usage depends. It is vital, in the crowded environment where there must be frequency reuse across the world, to have the tools for making performance assessments prior to the investment in equipment, so as to assure that a suitable interference controlled service may be obtained with the required quality. The studies of propagation and the modelling give these tools. It is the best we can do at present when faced with natural phenomena over which we have no control, and about which there has been inadequate study and measurement.
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2 Radio waves
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This chapter provides a physical description of radio waves, and defines basic radio parameters. It takes the opportunity to introduce some associated mathematical methods.
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3 Electromagnetic wave propagation
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For engineering applications, it is often sufficient to describe electromagnetic phenomena by means of a classical field theory, namely Maxwell's equations. These equations relate the electric field E and the magnetic field H to the current density and the charge density.
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4 Fading and statistics
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Signals and external noise are both subject to variations in time and with location. These changes in intensity arise from the nature of a random process, from multipath propagation, from changes in refractivity along the path, from movements of the system terminals or the reflecting medium, from changes in transmission loss, etc. A knowledge of the statistical characteristics of a received signal is likely to be required in the assessment of the performance of a radio system and for spectrum planning.
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5 Radio noise
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A signal reaching a receiver has to compete with noise which will be generated in the receiver itself and with external noise which also reaches the receiver through the antenna. In the absence of interference from other signals, the signal-to-noise ratio is a key parameter of the link budget calculated to determine the performance. The random movement of electrons in a resistor results in a fluctuating voltage across the resistor. At baseband this Gaussian, thermal or Johnson noise has a mean of zero and a Gaussian distribution of the instantaneous voltage. However, the mean noise power may be important, and at all radio frequencies the available mean noise power is given by p = kt0b (5.1) where k is the Boltzmann's constant, 1.38 x 10-23 (Joules per Kelvin), t0 is the temperature of the resistor, often taken as an ambient temperature of 290 K and b is the receiver noise equivalent bandwidth (Hz) (approximately the -3 dB bandwidth of the receiver).
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6 Clear-air characteristics of the troposphere
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This chapter considers the effects of refractive index variations on the propagation of radiowaves in the troposphere, and in particular those mechanisms that lead to propagation beyond the normal line-of-sight. Clear air implies that the effects of condensed water (clouds, rain, etc.) are ignored, although gaseous absorption is included. The influence of terrain diffraction is covered in Chapter 9, but terrain reflections are discussed here insofar as they contribute to the clear-air space wave. The frequencies of interest are above about 100 MHz; below this frequency refractive index variations are not strong enough to cause significant effects, and the ground wave and ionospheric mechanisms dominate at transhorizon ranges. The emphasis is on the meteorological mechanisms that give rise to anomalous propagation, and the basic models that have been developed to predict the effects of refractive index variations on radiowave propagation. Statistical procedures for the prediction of radio link reliability is the subject of Chapter 14.
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7 Reflection and scattering from rough surfaces
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Radio waves are reflected by the ground and from objects such as buildings. This can have various effects on radio systems. Reflections from buildings can permit a radio service to exist where the signal would otherwise be excessively attenuated by shadowing. Conversely, reflections can cause interference where shadowing alone would provide adequate attenuation of an unwanted signal. Reflections are also a major cause of multipath propagation. This can sometimes be exploited, particularly in a mobile radio system using code division multiple access (CDMA). For point-to-point links, on the other hand, ground reflections are generally viewed as an impairment, and every effort is made to minimise their effect.
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8 Introduction to multipath propagation
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Multipath propagation produces different versions of the transmitted signal which combine in the receiver. Multiple paths can be produced by reflection or scattering from the ground or objects such as buildings, inhomogeneity of atmospheric refractivity or multiple ray paths through the ionosphere. Each version of the signal will be affected by the path it has followed, including the effects of polarisation and direction-of-arrival at the receiving antenna. Unless one version dominates all the others, the received signal is a highly modified version of the transmitted signal. This chapter describes the general characteristics of multipath propagation and the type of analysis which is typically performed.
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9 Diffraction
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This chapter introduces the principles of diffraction, and describes a number of models used in propagation calculations for spectrum management and planning radio systems.
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10 Propagation in rain and clouds
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In this chapter, we consider the effect of atmospheric water particles on the radio wave. Water particles are found in air when they precipitate out from water vapour and are therefore found mainly in the troposphere, at higher levels they exist in a solid form as ice and at lower levels both ice and liquid is found. The scientific term for these particles is hydrometeors; this classification includes many forms with the most familiar being cloud, fog, rain, snow, hail and graupel. There are other forms, including, for example, needle-shaped ice and supercooled droplets of liquid water as well as intermediate forms, particularly melting particles in clouds. Each form has its own characteristic impact on radiowave propagation. The effect of hydrometeors on the passage of an electromagnetic wave depends very strongly on the size of the particle relative to the wavelength as well as the type and number of particles present. Hydrometeors can cause attenuation, scattering and depolarisation. In many cases, there will be several classes of hydrometeors occurring simultaneously in a region of precipitation and a link may pass through several different regions of precipitation.
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11 The ionosphere
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Ionospheric physics has many different aspects and this chapter is aligned to provide a foundation of knowledge for the radio systems user. The aim has been to give the reader an impression of the interacting atmospheric and space science that is needed to gain a full understanding of the composition and forces making up this complicated region of our environment. It is also important to gain an understanding of the measurement techniques. With these aspects in mind, it becomes clear that obtaining enough information to undertake reliable ionospheric now-casting and forecasting for radio system planning is a significant task that will continue to challenge us into the future.
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12 Ionospheric propagation
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For ionospheric signals the SNR is determined by a number of factors. For HF signals, a critical consideration is whether the signal is actually reflected from the ionosphere. All trans-ionospheric signals also experience some excess attenuation over free space, but because this is frequency dependent, the effects at higher frequencies are generally negligible. Multipath arises from various sources. A transmitted HF signal can be reflected from more than one of the several layers in the ionosphere. The transmission of a single pulse of energy is consequently received as a number of pulses which may be distinct or which may overlap. This situation is further complicated because the signals can also bounce off the ionosphere more than once, having been reflected from the ground in between. The earth's magnetic field also splits signals into two orthogonal polarisations which travel at a different speed and follow a slightly different path.
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13 Surface waves, and sky waves below 2 MHz
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The principal modes of radiowave propagation at frequencies below 2 MHz are the surface wave and sky wave. In this chapter these two modes are introduced and described. Rather than concentrating on the details of elaborate path loss prediction theories, they are merely introduced and the discussion then concentrates on their application by the planning engineer. The sky-wave propagation prediction methods described in this chapter are only applicable at frequencies below 2 MHz. However, the surface-wave models are based on more general theories and can also be applied in the HF band. Antenna and external-noise aspects of system planning are discussed. Throughout, readers are directed towards relevant data sources, prediction procedures and computer programs so that they might apply the planning methods described.
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14 Terrestrial line-of-sight links
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This chapter describes propagation issues relating to outdoor terrestrial line-of-sight (LOS) links, which are a widely used example of, in ITU terminology, a 'fixed service'. The significance of 'fixed' in this context is that both ends of the radio path terminate in equipment used under controlled conditions, unlike broadcasting or mobile services, where the user's equipment may be taken where there is no usable signal.
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15 Propagation for mobile and area coverage systems
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This chapter is concerned with propagation issues relating to systems in which one of the terminals of a link may be situated anywhere within a wide area; the most common such systems being cellular mobile radio, broadcasting and private mobile radio (PMR).
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16 Short-range and indoor propagation
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There is an increasing expectation that radio systems will work seamlessly as the user moves in and out of buildings. Furthermore, an increasing number of industrial and consumer devices incorporate a radio interface of some kind for portability or convenience of installation. Examples of such systems include wireless LANs, the Bluetooth and ZigBee standards, industrial telemetry, wireless microphones, smart metering networks, baby monitors and cellular picoand femto-cells. In addition to such short-range technologies, a greater or lesser degree of indoor coverage will be provided by cellular macroand microcells and by broadcast networks. A wide range of frequency allocations are in use for such services, often on a licence-exempt basis, ranging from low VHF to around 6 GHz. The lower limit tends to be set by bandwidth limitations, ambient noise and considerations of antenna efficiency, while the upper limit is dictated by acceptable path loss.
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17 Fixed wireless access and radio LANs
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This chapter is written within the framework of the provision of broadband data services to all locations using whatever technology is appropriate. We will concentrate on provision of high bandwidth data via radio systems to fixed terminals. Fixed terminals for this purpose may be defined as terminals where an antenna may be permanently installed on a mast, building or other structure. This type of service is distinguishable from fixed link service in several ways that are important from the propagation side. These networks are generally arranged in a point-to-multipoint or a mesh network topology. Radio paths are relatively short and terminals may not be optimally located for reliable propagation. The term fixed wireless access (FWA) will be used to describe these networks and applications. The main requirements placed on FWA systems when compared to mobile systems will be a relatively high data capacity, low latency, adequate coverage and high temporal reliability.
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18 Earth-space propagation
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Predictions of relevant propagation effects are required for the design, development, deployment and operation of all earth-space telecommunication systems. Three primary system issues arise in the design of earth-space telecommunication links: link availability, related to time periods when propagation impairments occurring on the path exceed a threshold that results in a system outage; degraded performance, when the link remains available but the received signal quality is insufficient to meet specified performance criteria; and unwanted interference, generated by other systems sharing the same bands, or possibly cross-polarisation interference (intersystem or intrasystem) between orthogonally polarised channels that results from signal depolarisation on the path, creating channel crosstalk if the system employs dual-polarized channels for frequency reuse.
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19 Terahertz propagation
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Radio waves are part of the total spectrum of electromagnetic waves and are arbitrarily stated to have an upper frequency limit of 3 THz (3,000 GHz). Nevertheless, as far as technical studies within the ITU are concerned, radiocommunication extends to higher frequencies without limit. Because of the ever increasing demand for more data transmission with higher data rates, attention is turning to ever higher frequencies. The challenge is to see if the higher frequencies are useful for communications. It is worthwhile noting that frequencies roughly beyond the visible range have additional features that should be taken into account.
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20 Computer modelling
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In principle, a propagation model for any radiowave problem is provided by a solution of Maxwell's equations. This type of model is deterministic in the sense that if the propagation medium is characterised exactly, the solution would also be an exact description of the propagation conditions for the given path at the given time. In practice, of course, approximations and simplifications are required and no practical solution of Maxwell's equation will be applicable to all propagation problems. Different propagation mechanisms require different approximations, and this has led to the development of various numerically intensive methods in different parts of the radio spectrum.
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21 Numerically intensive propagation prediction methods
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This chapter aims to introduce a number of deterministic radiowave propagation prediction methods and discuss briefly the computational issues arising in their practical implementation. Ideally, we wish to solve Maxwell's equations exactly by specifying a boundary value problem to a sufficient degree of accuracy, subject to some initial conditions. The boundary value problem in this case is the geometrical and electrical description of the radio environment down to sub-wavelength accuracy, whereas the initial conditions are the current distribution on the transmitting antenna.
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Back Matter
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