The field of radio communications continues to change rapidly, and the second edition of this outstanding book, based on a popular IEE Vacation School, has been fully updated to reflect the latest developments. The introduction of new services and the proliferation of mobile communications have produced a growing need for wider bandwidths and the consequent need for frequency reuse. This book introduces the basic concepts and mechanisms of radiowave propogation engineering in both the troposphere and ionosphere, and includes greater emphasis on the needs of digital technologies and new kinds of radio systems. The content reflects the wide experience of an exceptional group of authors and the emphasis throughout is on modelling and prediction methods and the relevant ITU Radiocommunication Sector recommendations. Propagation of Radiowaves, 2nd Edition is essential reading for professionals involved in planning, designing and operating radio systems, as well as postgraduate students in the field.
Inspec keywords: electromagnetic wave reflection; electromagnetic wave scattering; radiowave propagation; troposphere; ionosphere; radiocommunication
Other keywords: VHF broadcasting; ionospheric prediction; sky waves; ionospheric propagation; short-range propagation; electromagnetic wave propagation; radiowave propagation; spectrum use; clear air characteristics; outdoor mobile propagation; diffraction; Earth-space propagation; radiocommunications system planning; reflection; mobile spread spectrum networks; surface waves; scattering; UHF broadcasting; propagation data requirements; rain; troposphere; clouds
Subjects: Electromagnetic wave propagation and interactions in the lower atmosphere; Radiowave propagation; Radio links and equipment; Ionospheric electromagnetic wave propagation
This book, and the lecture course on which it is based, is intended to deal with the practical engineering aspects of radiowave propagation, emphasising the propagation concerns and models and the associated prediction procedures which are appropriate for the system applications of current interest. For many years, a Radiocommunication Study Group of the International Telecommunication Union (ITU) has been studying propagation on a world wide basis and producing the Recommendations which give descriptions and prediction techniques for the propagation of radiowaves. These Recommendations are regularly reviewed and revised by international experts in ITU-R Study Group 3 and probably represent the latest and best tools which the engineer may use. The course deals with a number of these Recommendations.
This chapter introduces a number of topics which should be useful for the succeeding chapters. Antenna gain, radiated power and transmission loss are commonly used terms when describing systems, but the precision given by the internationally agreed definitions of these terms is necessary if ambiguity is to be avoided. System performance is governed not only by the transmission loss, under some stated conditions, but also by the variability of the signal in time or space, which can then be described in statistical terms, and by the level of background signals either broadband noise or interfering transmissions. The statistical probability distributions in common use are introduced, and the benefits of diversity reception are outlined. The types of radio noise are described together with the ways in which noise power from a number of sources may be combined for use in performance prediction.
Although for many purposes a radiowave can be treated simply as a power flow, there are other situations where it is necessary to take into account the fact that it consists of vector fields. Multiple radio paths (or rays) between a transmitter and a receiver can occur for various reasons, such as the combination of ground and sky waves, multimode sky-wave propagation, atmospheric refraction and layer reflection, and reflections from the ground and objects such as buildings. It is called multipath propagation, and can give rise to several effects on the received signal. A simple mathematical approximation which is widely used in radiowave propagation concerns situations where a right-angled triangle is long and thin, i.e. the hypotenuse is only slightly longer than one of the other sides.
It is neither economically realistic nor spectrally efficient to engineer a radio communications system to the extent that it can be guaranteed to work for 100 percent of the time, and realistic allowances have to be made for equipment failures, propagation problems, and interference. When planning a system it is therefore necessary to have design targets for the maximum percentages of time for which the circuit may be interrupted, or its error performance degraded. The amount of time for which a degraded level of service can be accepted is dependent on the particular application. High-grade telecommunications systems are very tightly specified in this respect, typically needing to be fully operational for all but few hundredths of a per cent of the time. Even local access radio systems generally need to work to 99.99 percent of the time, or better. Broadcasting is more tolerant, with perhaps 0.1 percent of time being allowed for interruptions. However, mobile networks, which live in an interference-dominated environment in which performance is statistically less certain, are normally designed for median conditions and are not so concerned about the smaller time percentages.
This chapter introduces the principles of electromagnetic theory which are essential to an understanding of radiowave propagation. Where possible, a practical engineering approach has been adopted. However, the material has been structured to provide those with a more mathematical background with a satisfactory account of the subject, together with references to further reading material. Initially, propagation between two antennas in free space is considered by means of power flux concepts. This enables important ideas such as free-space path loss, antenna gain and effective aperture to be introduced. Maxwell's equations are presented. and plane-wave solutions are derived as a means of introducing polarisation, wave impedance and electromagnetic power flow. Finally, radiation from a current distribution is examined and illustrated by deriving the fields of a dipole antenna.
Radiowaves are reflected by the ground and from objects such as buildings. This can have various effects on radio systems. Reflections from buildings, etc., 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.
This chapter considers the effects of refractive index variations on the propagation of radiowaves in the troposphere, and in particular those mechanisms which 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 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.
Diffraction can be treated as a highly theoretical subject, although the under lying process can be visualised quite readily. It is usually interpreted in terms of the wave nature of electromagnetic radiation, although there is an argument that quantum uncertainty provides a better physical description. Whatever the case, this chapter uses the concept of radiowaves to describe the cause and effects of diffraction.
Short-range radio systems are used for many purposes for telemetry, remote control and games, as well as for communications. Communications uses include cordless telephony, radio local area networks (RLANs), radio fixed access and microcellular systems. For some of these applications, very low powers are used, with poor and poorly located antennas where the user only expects the range to be of the order of a few metres. For example, for remote car door locks, the user will expect to point the transmitter towards the car and probably has some understanding of the need to provide a near line-of-sight path. For such appli cations there seems to be very little requirement to attempt to provide good propagation models. For RLANs and other indoor applications there will be a need for some kind of generic modelling of the effects of the room size and shape, obstructions in the room, the construction materials and the penetration through walls and floors. For high-speed data transmission, it may also be necessary to model the multipath time spreads. But for outdoor microcellular systems and similar applications it will be necessary to model propagation at distances ranging out to a kilometre or so, where the longer-range area coverage prediction methods may take over. 1 km is the dividing distance used by the ITU-R in its Recommendations between short-range and longer-range prediction methods. However, the above considerations mainly apply to VHF and UHF systems, where although the distances are short the propagation is in the far-field regime and the usual propagation techniques are applicable. Another interpretation of short range would be to consider paths within the near field regime around the transmitting antenna. This is mainly of significance at lower frequencies.
This chapter introduces 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 subwavelength accuracy, whereas the initial conditions are the current distribution on the transmitting antenna.
This chapter describes key modelling concepts for mobile communication systems operated in outdoor environments.
Water appears in the atmosphere in a variety of forms, usually referred to by the term hydrometeor, which includes particles as diverse as cloud, rain drops, snowflakes, ice crystals, hail and graupel. Of these, rain, hail, graupel and snow are generally recognised as precipitation. The effects that hydrometeors have on communications systems are dependent both on the system frequency and the type of particle present. At any given instant, of course, more than one type of particle will affect a given link. For example, an earth-space link may often encounter rain over the lower part of the path, and snow at greater heights. We will consider first the various forms of hydrometeor, together with their relevance to radiowave propagation. A brief theoretical framework will then be presented as a background to the model development process and, finally, specific effects of hydrometeors on systems will be considered.
Broadcasting is defined in the ITU Radio Regulations as 'a radiocommunication service in which the transmissions are intended for direct reception by the general public'. European broadcasting at VHF and UHF takes place in the frequency ranges shown in Figure 13.1. In the UK, 88-108 MHz is used for FM radio, 218-230 MHz is used for digital radio and 470-860 MHz is used for analogue and digital television.
Spread spectrum has been adopted as the core radio access method in many of the third-generation (3G) wireless standards, where high capacity networks supporting multimedia-like services are desired. This selection was based on numerous research initiatives conducted both privately within numerous organisations, as well as publicly under initiatives such as the UK DTI LINK programme and the European RACE1 and ACTS2 programmes, addressing the selection of the primary air interface technique for the universal mobile telecommunication system (UMTS). Relevant examples include the RACEII ATDMA and CODIT projects, as well as the UK DTI/SERC LINK CDMA programme. More recently, a body called the Third Generation Partnership Project (3PP) was established with the aim of harmonising the numerous proposals for 3G systems on a global basis. This chapter describes the fundamental aspect of high capacity CDMA cellular network.
This chapter discusses basic concepts regarding ionospheric electromagnetic wave propagation.
The ionosphere, which extends from ~60 km to ~1000 km, significantly affects the propagation of high-frequency (HF) to ultrahigh-frequency (UHF) signals which pass through it. The effects are varied but include refraction, retardation and scintillation. Ground-ground HF communications systems, ground-space communications systems, single frequency GPS (global positioning system), HF over-the-horizon radars, satellite altimeters and space-based radars are examples of radio systems constrained by this medium. Topics of interest include ionospheric morphology; ray tracing; MUF, multipath and other HF issues; fading and Doppler effects; and modem requirements on SNR, Doppler and multipath.
The ionosphere (the ionised region of the atmosphere between about 60 km and 1000 km) affects all radio signals below ~5 GHz which pass through or travel via it. Some of the associated radio systems can only operate because of the ionosphere, but it can also degrade radio system operation. Sometimes the effects of the ionosphere are highly significant, sometimes they can all but be ignored. For high-frequency (HF) sky-wave propagation, the ionosphere is, of course, a prerequisite. If the ionosphere were stable in time and constant in space, it would be relatively easy to determine the effect of the ionisation on the radio propagation and hence on the radio system. Unfortunately, stability is not the norm, particularly in the high-latitude regions, and, as a consequence ionospheric, prediction methods and models are required to support system design, service planning and frequency management. These methods and models characterise the medium and estimate the system performance. Some operate in real (or close to real) time and they may even directly advise the radio operator on a course of action which would improve system performance.
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 concentrate 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.
Recommendation ITU-R R530 (1999) presents propagation data for planning terrestrial line-of-sight radio systems, typically operating in frequency bands between 1 GHz and 45 GHz, and gives step-by-step methods for predicting the performance of these systems for percentage times down to 0.001 per cent. The recommendation deals only with effects related to the wanted signal. The interference aspects of terrestrial line-of-sight link design are covered in Recommendation ITU-R P.452 (1999). Although the prediction methods presented in Recommendation ITU-R P.530 are based on the physics of radio propagation, measured data was used as the main foundation and the majority of the formulae are semi-empirical statistical relationships. The Recommendation divides broadly into methods for determining the antenna heights (with and without antenna height diversity) and methods for calculating the statistics of signal fading, enhancement and depolarisation. In the case of clear air effects the methods are subdivided into a quick (initial planning) method and a more detailed method, which requires terrain data for the area of the link. Other topics covered are: the correlation of simultaneous fading on multihop links, the frequency and polarisation scaling of rain attenuation statistics, the statistics of rain fade duration and the prediction of the error performance and availability of digital systems.
Nowadays, many new technologies and additional users are seeking access to an increasingly busy radio spectrum. An evolving propagation and interference modelling capability is clearly needed to support the necessary spectrum management changes and to resolve the spectrum sharing and system deployment issues that arise within a multioperator, multiservice situation. In the middle of the 1980s it became clear that the methods then available to model interference propagation at microwave frequencies would not be adequate for the tasks which lay ahead. Considerable effort was therefore invested in devising more flexible and reliable means of predicting microwave interference. These notes provide an introduction to one such development, a new set of propagation models developed by a major European collaborative project (COST 210), and subsequently adopted in 1992 by ITU-R Study Group 3 as Recommendation ITU-R P.452. Since then the ITU-R Study Group has progressively extended the capabilities and accuracy of the original COST 210 method to cope with new interference scenarios and to provide full global applicability.
Recommendation ITU-R R618 presents propagation data and step-by-step methods for predicting the performance of earth-space radio systems operating in frequency bands between about 1 GHz and 55 GHz and applicable to average annual percentage times in the range 0.001 per cent to 50 per cent. This recommendation deals only with effects relating to the wanted signal. Interference aspects are covered in Recommendations ITU-R P.452, P.619 and P.1412. There is insufficient space within this chapter to present all the details of the prediction methods given in Recommendation ITU-R P.618 and, therefore, a broad description will be given together with references to related ITU documents.