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Chapter 1 Introduction 1.1 BACKGROUND The history of mobile radio goes back almost to the origins of radio communication itself. The very early work of Hertz in the 1880s showed that electromagnetic wave propagation was possible in free space and hence demonstrated the practicality of radio communications. In 1892, less than ®ve years later, a paper written by the British scientist Sir William Crookes [1] predicted telegraphic communication over long distances using tuned receiving and transmitting apparatus. Although the ®rst radio message appears to have been transmitted by Oliver Lodge in 1894 [2], it was the entrepreneur Marconi [3] who initially demonstrated the potential of radio as a powerful means of long-distance communication. In 1895, using two elevated antennas, he established a radio link over a distance of a few miles, and technological progress thereafter was such that only two years later he succeeded in communicating from The Needles, Isle of Wight, to a tugboat over a distance of some 18 miles (29 km). Although it seems highly unlikely that Marconi thought of this experiment in terms of mobile radio, mobile radio it certainly was. Nowadays the term `mobile radio' is deemed to embrace almost any situation where the transmitter or receiver is capable of being moved, whether it actually moves or not. It therefore encompasses satellite mobile, aeromobile and maritime mobile, as well as cordless telephones, radio paging, traditional private mobile radio (PMR) and cellular systems. Any book which attempted to cover all these areas would have to be very bulky and the present volume will therefore be concerned principally with the latter categories of use, which are covered by the generic term `land mobile radio'. This, however, is not a book that deals with the systems and techniques that are used in land mobile communications; it is restricted primarily to a discussion of the radio channel ± the transmission medium ± a vital and central feature which places fundamental limitations on the performance of radio systems. The majority of the book is concerned with the way in which the radio channel aects the signal that propagates through it, but there are other chapters treating related topics. These have been included to make the book more comprehensive without straying too far from the main theme. It is not pro®table at this point to discuss details; they can be left until later. Suce it to say that in the vast majority of cases, because of complexity and variability, a The Mobile Radio Propagation Channel. Second Edition. J. D. Parsons Copyright & 2000 John Wiley & Sons Ltd Print ISBN 0-471-98857-X Online ISBN 0-470-84152-4 deterministic approach to establishing the parameters of the propagation channel is not feasible. Almost invariably it is necessary to resort to measurements and to the powerful tools of statistical communication theory. One point worth clarifying at this stage, however, is that signal transmission over a mobile radio path is reciprocal in the sense that the locations of the transmitter and receiver can be interchanged without changing the received signal characteristics. The discussion can therefore proceed on the basis of transmission in either direction without loss of generality. However, a word of caution is needed. The levels of ambient noise and interference at the two ends of the link may not be the same, so reciprocity with respect to the signal characteristics does not imply reciprocity with respect to the signal-to-noise or signal-to-interference ratios. Some years ago the primary concern of a book such as this would undoubtedly have been the propagation aspects related to traditional mobile radio services which are based on the concept of an elevated base station on a good site, communicating with a number of mobiles in the surrounding area. Such systems, known as PMR systems, developed rapidly following World War II, especially once the transistor made it possible to design and build compact, lightweight equipment that could easily be installed in a vehicle and powered directly from the vehicle battery. These are often termed dispatch systems because of their popularity with police forces, taxi companies and service organisations who operate ¯eets of vehicles. The frequency bands used for dispatch systems lie in the range 70±470 MHz and have been chosen because the propagation characteristics are suitable, the antennas have a convenient size and adequate radio frequency (RF) power can be generated easily and eciently. The operational strategy is to divide the available spectrum into convenient channels with each user, or user group, having access to one or more of these channels in order to transmit a message, usually speech, by amplitude modulation or frequency modulation. The technique of providing a service to a number of users in this way is known as frequency division multiple access (FDMA), and because each channel carries only one message the term single channel per carrier (SCPC) is also used. In the early post-war days, channels were spaced by 100 kHz, but advances in technology, coupled with an ever increasing demand for licences, has led to several reductions to the point where currently in the UK, channels in the VHF band (30± 300 MHz) are 12.5 kHz apart, whereas 25 kHz separation is still used for some channels in the UHF band (300±3000 MHz). For these PMR systems, indeed for any mobile radio system with a similar operating scenario, the major propagation-related factors that have to be taken into consideration are the eect of irregular terrain and the in¯uence on the signal of trees, buildings and other natural and man-made obstacles. In recent years, however, expanded services have become available, for example radio pagers, which are now in common use. Hand-portable, rather than vehicle-borne equipment is also being used by security guards, police ocers and by subscribers to cellular radio-telephone systems. Hand-portable equipment can easily be taken into buildings, so a book concerned with propagation must also consider the properties of the signal inside buildings and in the surrounding areas. For cordless telephones and the like, there is also a need to study propagation totally within buildings. Neither can we restrict attention to frequencies below 470 MHz; ®rst- and second-generation analogue and 2 The Mobile Radio Propagation Channel digital cellular radio telephone systems, e.g. AMPS, TACS, GSM and DCS1800, use frequencies up to 1900 MHz, and third-generation wideband systems will probably use even higher frequencies to solve the problems of spectrum congestion and required bandwidth. What then are the matters of primary concern? For transmissions of the traditional type, in which the signals are restricted to fairly narrow radio channels, two major factors have to be quanti®ed: . Median signal strength . Signal variability The ability to predict the minimum power a transmitter must radiate to provide an acceptable quality of coverage over a predetermined service area and the ability to estimate the likely eect of such transmissions on services in adjacent areas, are both critical for improving frequency reuse techniques, for implementing band-sharing schemes between dierent services and for the success of radio-telephone systems. This is not easy and there is a vital and continuing need for a better understanding of the in¯uence of the dierent urban and terrain factors on the mobile radio signal. As far as signal variability is concerned, it is often convenient to separate the eects into those which occur over a short distance and those which are apparent only over much longer distances. Short-distance eects include the rapid fading caused by multipath propagation in urban areas; longer-distance eects include the much slower variations in average signal strength as the receiver moves from one area to another. For digital systems it is neither ecient nor desirable to use FDMA/SCPC as a multiple-access technique, and spectrum utilisation is substantially improved by allowing each user access to a wider-bandwidth radio channel, but only for a small percentage of the time. This time division multiple access (TDMA) strategy is used in the GSM and DCS1800 systems. Third-generation systems will be based around wideband code division multiple access (CDMA) and these spread-spectrum systems will oer even greater capacity and security together with access to multimedia communications. First developed for military purposes, CDMA has virtually no noise or crosstalk and is well suited to high-quality multimedia services. The characterisation of wideband channels will be discussed in Chapter 6; for now it will suce to note that if digital (pulse) signals propagate in a multipath environment then interference can occur between a given pulse and a delayed version of an earlier pulse (an echo) that has travelled via a longer path. This is known as intersymbol interference (ISI) and can cause errors. The extent of the problem can be quanti®ed by propagation studies which measure parameters such as the average delay and the spread of delays. Finally, in this introductory section, it is worth making two further points. Firstly, the geographical service area of many mobile radio systems is too large to be economically covered using a single base station, and various methods exist to provide `area coverage' using a number of transmitters. We will return to this topic in Section 1.3.2. Secondly, in order to maximise the use of the available spectrum, channels that are allocated to one user in a certain geographical area are reallocated to a dierent user in another area some distance away. The most common example Introduction 3 of this is cellular radio, which relies on frequency reuse to achieve high spectrum eciency. However, whenever frequencies are reallocated, there is always the possibility that interference will be caused and it should therefore be understood that adequate reception conditions require not only an acceptable signal-to-noise ratio but also, simultaneously, an acceptable signal-to-interference ratio. This subject will be treated in Chapter 9. Throughout the book the term `base station' will be used when referring to the ®xed terminal and the term `mobile' to describe the moving terminal, whether it be hand-portable or installed in a vehicle. 1.2 FREQUENCY BANDS Having set the scene, we can now discuss some of the topics in a little more detail. It is very important to understand how RF energy propagates and in preparation for a brief general discussion let us de®ne more clearly what is meant by the term `radio wave' and how waves of dierent frequencies are classi®ed. The part of the electromagnetic spectrum that includes radio frequencies extends from about 30 kHz to 300 GHz, although radio wave propagation is actually possible down to a few kilohertz. By international agreement the radio frequency spectrum is divided into bands, and each band is given a designation as in Table 1.1. Electromagnetic energy in the form of radio waves propagates outwards from a transmitting antenna and there are several ways in which these waves travel, largely depending on the transmission frequency. Waves propagating via the layers of the ionosphere are known as ionospheric waves or sky waves; those that propagate over other paths in the lower atmosphere (the troposphere) are termed tropospheric waves, and those that propagate very close to the Earth's surface are known as ground waves. Ground waves can be conveniently divided into space waves and surface waves, and space waves can be further subdivided into direct waves which propagate via the direct path between transmitting and receiving antennas and ground-re¯ected waves that reach the receiving antenna after re¯ection from the ground. Figure 1.1 gives a simple picture. The surface waves are guided along the Earth's surface and because the Earth is not a perfect conductor, energy is extracted from the wave, as it propagates, to supply losses in the ground itself. The attenuation of this wave (sometimes known as the Norton surface wave) is therefore directly aected by the ground constants 4 The Mobile Radio Propagation Channel Table 1.1 Designation of frequency bands Frequency band Frequency range Extremely low frequency (ELF) 53kHz Very low frequency (VLF) 3±30 kHz Low frequency (LF) 30±300 kHz Medium frequency (MF) 300 kHz±3 MHz High frequency (HF) 3±30 MHz Very high frequency (VHF) 30±300 MHz Ultra high frequency (UHF) 300 MHz±3 GHz Super high frequency (SHF) 3±30 GHz Extra high frequency (EHF) 30±300 GHz (conductivity and dielectric constant) along the transmission path. The importance of each of these waves in any particular case depends upon the length of the propagation path and the frequency of transmission. We can now discuss each frequency band in turn. 1.2.1 VLF In the VLF range the wavelength is very long, typically 10 5 m, and antennas are therefore very large. They have to be very close to the Earth and are often buried in the ground. The radio waves are re¯ected from the ionosphere and a form of Earth± ionosphere waveguide exists that guides the wave as it propagates. Because of diurnal variations in the height of the ionospheric D-layer, the eective height of the terrestrial waveguide also varies around the surface of the Earth. The uses of VLF include long-distance worldwide telegraphy and navigation systems. Frequencies in the VLF range are also useful for communication with submerged submarines, as higher frequencies are very rapidly attenuated by conducting sea water. Digital transmissions are always used but the available bandwidth in this frequency range is very small and the data rate is therefore extremely low. 1.2.2 LF and MF At frequencies in the range between a few kilohertz and a few megahertz (the LF and MF bands) ground wave propagation is the dominant mode and the radiation characteristics are strongly in¯uenced by the presence of the Earth. At LF, the surface wave component of the ground wave is successfully utilised for long-distance communication and navigation. Physically, antennas are still quite large and high- power transmitters are used. The increased bandwidth available in the MF band allows it to be used for commercial AM broadcasting, and although the attenuation Introduction 5 Figure 1.1 Modes of radio wave propagation. of the surface wave is higher than in the LF band, broadcasting over distances of several hundred kilometres is still possible, particularly during the daytime. At night, sky wave propagation via the D-layer is possible in the MF band and this leads to the possibility of interference between signals arriving at a given point, one via a ground wave path and the other via a sky wave path. Interference can be constructive or destructive depending upon the phases of the incoming waves; temporal variations in the height of the D-layer, apparent over tens of seconds, cause the signal to be alternatively strong and weak. This phenomenon, termed fading, can also be produced by several other mechanisms and always occurs when energy can propagate via more than one path. It is an important eect in mobile radio. 1.2.3 HF Ground wave propagation also exists in the HF band, but here the ionospheric or sky wave is often the dominant feature. For reasons which will become apparent later, the HF band is not used for civilian land mobile radio and it is therefore inappropriate to go into details of the propagation phenomena. Suce it to say that the layers of ionised gases within the ionosphere (the so-called D, E and F layers) exist at heights up to several hundred kilometres above the Earth's surface, and single and multiple hops via the various ionospheric layers permit almost worldwide communications. The height of the dierent layers varies with the time of day, the season of the year and the geographical location [4]; this causes severe problems which have attracted the attention of researchers over many years and are still of great interest. 1.2.4 VHF and UHF Frequencies in the VHF and UHF bands are usually too high for ionospheric propagation to occur, and communication takes place via the direct and ground- re¯ected components of the space wave. In these bands, antennas are relatively small in physical size and can be mounted on masts several wavelengths above the ground. Under these conditions the space wave is predominant. Although the space wave is often a negligible factor in communication at lower frequencies, it is the dominant feature of ground wave communication at VHF and UHF. The bandwidth available is such that high-quality FM radio and television channels can be accommodated, but propagation is normally restricted to points within the radio horizon and coverage is therefore essentially local. The analysis of space wave propagation at VHF and UHF needs to take into account the problems of re¯ections both from the ground and from natural and man-made obstacles. Diraction over hilltops and buildings, and refraction in the lower atmosphere are also important. 1.2.5 SHF Frequencies in the SHF band are commonly termed microwaves, and this term may also be used to describe that part of the UHF band above about 1.5 GHz. Propagation paths must have line-of-sight between the transmitting and receiving antennas, otherwise losses are extremely high. At these frequencies, however, it is possible to design compact high-gain antennas, normally of the re¯ector type, which 6 The Mobile Radio Propagation Channel concentrate the radiation in the required direction. Microwave frequencies are used for satellite communication (since they penetrate the ionosphere with little or no eect), point-to-point terrestrial links, radars and short-range communication systems. 1.2.6 EHF The term `millimetre wave' is often used to describe frequencies in the EHF band between 30 and 300 GHz. In comparison with lower frequencies, enormous bandwidths are available in this part of the spectrum. Line-of-sight propagation is now predominant and although interference from ground-re¯ected waves is possible, it is often insigni®cant, because the roughness of the ground is now much greater in comparison with the wavelength involved. It is only when the ground is very smooth, or a water surface is present, that the ground-re¯ected waves play a signi®cant role. This topic will be treated in Chapter 2. In the millimetre waveband the most important eects that have to be taken into account are scattering by precipitation (rain and snow) and, at certain frequencies, absorption by fog, water vapour and other atmospheric gases. A detailed treatment of millimetre wave propagation is well beyond the scope of this book and, in any case, is not directly relevant to current mobile radio systems. However, Figure 1.2 shows the attenuation by oxygen and uncondensed water vapour [5] as a function of frequency. At some frequencies there are strong absorption lines, e.g. the water vapour absorption at 22 GHz and the oxygen absorption at 60 GHz. However, between these lines there are windows where the attenuation is much less. Specialised applications such as very short range secure communication systems and satellite-to-satellite links are where millimetre waves Introduction 7 Figure 1.2 Attenuation by oxygen and water vapour at sea level, T 208C; water content 7.5 g/m 3 . ®nd application, although in the 1980s there was some interest in the absorption bands as they appeared to have some potential for future microcellular systems. At present there is no volume market in this frequency range, so component and system costs are very high. 1.3 MOBILE RADIO FREQUENCIES There are several factors that have to be taken into account in deciding what frequency band should be used for a particular type of radio communication service. For the speci®c application of interest, two-way mobile radio operations, communication is required over ranges that do not normally exceed a few tens of kilometres, often much less. Clearly, unnecessary interference would be caused to other users if the signals propagated too far. It is also evident that if mobiles are to communicate freely with their base, or with each other, throughout a given area (which may or may not be the total service area of the system) the transmitters involved must be able to provide an adequate signal strength over the entire area concerned. Operating frequencies must be chosen in a region of the RF spectrum where it is possible to design ecient antennas of a size suitable for mounting on base station masts, on vehicles and on hand-portable equipment. Since the mobiles can move around freely within the area covered by the radio system, their exact location is unknown and the antennas must therefore radiate energy uniformly in all directions in the horizontal (azimuth) plane; technically this is known as omnidirectional radiation.* It is also vital that the frequencies chosen are such that the transmitters used at base stations and mobiles can generate the necessary RF power while remaining fairly small in physical size. For two-way mobile radio, particularly in urban areas, it is seldom that the mobile antenna has a direct line-of-sight path to the base station. Radio waves will penetrate into buildings to a limited extent and, because of diraction, appear to bend slightly over minor undulations or folds in the ground. Fortunately, due to multiple scattering and re¯ection, the waves also propagate into built-up areas, and although the signal strength is substantially reduced by all these eects, sensitive receivers are able to detect the signals even in heavily built-up areas and within buildings. The choice of frequency is therefore limited by the need to minimise the losses due to buildings while continuing to satisfy the other constraints mentioned above. For these reasons, traditional two-way mobile radio originally developed almost exclusively around the VHF and latterly UHF bands. In a city, for example, there are many mobile radio users such as emergency services and taxi companies. In the case of a police force, the central control room receives reports of incidents in the city area, often by emergency telephone calls. The control room radio operator puts out a call to a police ocer believed to be in the appropriate area; who may be on foot with a personal radio or in a vehicle equipped with mobile radio. On receipt of the call, the ocer acknowledges it, investigates the incident and reports back by radio. Because of the FDMA/SCPC method of operation, police forces have radio channels allocated for their exclusive use and there is no mutual interference between them 8 The Mobile Radio Propagation Channel *Omnidirectional is not to be confused with isotropic which means `in all directions'. and other users on dierent channels in the same frequency band. However, all police ocers who carry a receiver tuned to the appropriate frequency will hear the calls as they are broadcast. The range over which signals propagate is also a fundamental consideration since in order to use the available spectrum eciently, it is necessary to reallocate radio channels to other users operating some distance away. If, in the above example, the message from the control room had been radiated on HF, then it is possible that the signals could have been detected at distances of several hundred kilometres, which is unnecessary, undesirable and would cause interference to other users. The VHF and UHF bands therefore represent an optimum choice for mobile radio because of their relatively short-range propagation characteristics and because radio equipment designed for these bands is reasonably compact and inexpensive. Vertical polarisation is always used for mobile communications; at frequencies in the VHF band it is preferable to horizontal polarisation because it produces a higher ®eld strength near the ground [6]. Furthermore, mobile and hand-portable antennas for vertical polarisation are more robust and more convenient to use. In an overall plan for frequency reuse, no worthwhile improvement can be achieved by employing both polarisations (as in television broadcasting) because scattering in urban areas tends to cause a cross-polar component to appear. Although this may have some advantages, for example it is often inconvenient to hold the antenna of a hand- portable radio-telephone in a truly vertical position, it is apparent that no general bene®t would result from the transmission of horizontally polarised signals. There are many other services, however, which also operate in the VHF and UHF bands, for example, television, domestic radio, Citizens' Band radio, marine radio, aeromobile radio (including instrument landing systems) and military radio. Several of these services have a `safety of life' element and it is vital that their use is tightly regulated to ensure maximum eciency and freedom from interference. The exact frequencies within the VHF and UHF bands that are allocated for various radio systems are agreed at meetings of the International Telecommunications Union (ITU) and are legally binding on the member states. Every twenty years the ITU organises a world administrative radio conference (WARC) at which regulations are revised and updated and changes in allocations and usage are agreed. In each country, use of the radio frequency spectrum is controlled by a regulatory authority; in the UK this is the Department of Trade and Industry (DTI) and in the USA it is the Federal Communications Commission (FCC). The regulatory authority is responsible for allocating speci®c portions of the available spectrum for particular purposes and for licensing the use of individual channels or groups of channels by legitimate users. Because of the attractive propagation characteristics of VHF and UHF, it is possible to allocate the same channel to dierent users in areas separated by distances of 50±100 km with a substantial degree of con®dence that, except under anomalous propagation conditions, they will not interfere with each other. 1.3.1 Radio links For obvious reasons, VHF or UHF radio transmitters intended to provide coverage over a fairly large area are located at strategic points (usually high, uncluttered sites) within the intended area. However, the control room may be at some completely Introduction 9 dierent location, so a method has to be found to get the intended message information (which may be voice or data) to the transmitter sites. This can be achieved by using telephone lines or by a further radio link. The technical speci®cations for telephone lines and the policy for their use often rule out this possibility, and the necessary quality and reliability of service can only be achieved by using a radio link between the control room and each of the VHF/UHF transmitter sites. The kind of radio link used for this purpose has requirements quite dierent from those of the two-way VHF/UHF systems used to communicate with mobiles. In this case we are only communicating between one ®xed point (the control room) and another ®xed point (the site concerned), and for this reason such links are commonly termed point-to-point links. Omnidirectional radio coverage is not required, in fact it is undesirable, so it is possible to use directional antennas which concentrate the radio frequency energy in the required direction only. In addition, there is a substantial degree of freedom to locate the link transmitters and receivers at favourable locations where a line-of-sight path exists and the radio path does not need to rely on the propagation mechanisms, discussed earlier, which make the VHF and UHF bands so attractive for communications to and from mobiles. These features have been exploited extensively in link planning, particularly with regard to allocation of frequencies. Because of congestion in the frequency bands best suited to communications with mobiles, link activity has been moved into higher frequency bands and modern links operate typically at frequencies above 2 GHz. This presents no problems since compact high-gain directional antennas are readily available at these frequencies. Two frequencies are necessary for `go' and `return' paths, since if a link serves more than one base transceiver then one may be transmitting while others are receiving; this means that full-duplex operation is needed, i.e. messages can pass both ways along the link simultaneously. When several channels are operated from the same transmitter site, a choice has to be made between using several link frequencies, one for each transmitter, or using a multiplexed link in which the messages for the dierent transmitters at the remote site are assembled into an FDM baseband signal which is then modulated onto the radio bearer. The multiplex approach can be more ecient than the SCPC alternative in requiring only one transceiver at each end of the link, and this technique is widely implemented. Naturally, the bandwidth occupied by a multiplexed link transmission is proportionally greater than an SCPC signal, but a 10-channel multiplexed link connection occupies no more spectrum than 10 separate links spaced out in frequency. Certain conditions have to be satis®ed for radio links to operate satisfactorily. Firstly it is vital that the direct path between the two antennas (the line-of-sight path) is clear of obstructions. However, this in itself is not enough; it is highly desirable that there are no obstructions close to the line-of-sight path since they could cause re¯ections and spoil reception. Figure 1.3 shows a simple link path of the kind we are considering; the dotted line de®nes a region known as the ®rst Fresnel zone. The theory in Chapter 2 enables us to calculate the dimensions of this zone, and shows that for satisfactory radio link operation it should be almost free of obstructions. 1.3.2 Area coverage A traditional mobile radio system comprises several transceivers which communicate with a single, ®xed base station. In most cases the base station is centrally located 10 The Mobile Radio Propagation Channel [...]... infrastructure [7,8] Although this is acceptable for a high-quality nationwide radio-telephone network, it is not attractive for a more localised PMR dispatch system For traditional mobile radio services, if the area is too large to be economically covered by one base station or if geographical conditions produce di culties, a more 12 The Mobile Radio Propagation Channel suitable solution is to transmit from several... far away from a transmitter it is necessary to go before its frequency can be reused without risk of mutual interference in either direction This will be discussed in Chapter 9 but the distance is in fact quite large, at least ®ve times the radius of the coverage area, depending on how comprehensively the service area is provided with strong receivable signals If a single high mast were situated in the...Introduction Figure 1.3 11 Simple point-to-point radio link within the area to be served and is connected to the control room via a telephone line or radio link A straightforward approach to the problem of providing coverage over very large areas would therefore be to erect a very high tower somewhere near the centre of the... requirement means that the time delay involved in sending the message from control to the various transmitters in the system must be the same In other words, all the radio links in the system must be delay-equalised The situation seems to be even more critical in digital systems using the TETRA standard, which are now reaching the implementation stage Di culties are likely to arise as a result of timing... data, building shape and height information, and vegetation data For determining building penetration losses, the characteristics of building materials may well be important It is important to know the resolution and accuracy of such databases, as well as the relationship between database accuracy and prediction accuracy Although a clear relationship is intuitively present, it is not immediately apparent... mobile radio systems in general, and channel characterisation in particular, propagation models are required to deal with a number of situations as outlined in Section 1.1 These models are necessary for accurate coverage planning, the characterisation of multipath eects and for interference calculations Moreover, they are required for a wide variety of environments from rural areas to in-building 14... environments from rural areas to in-building 14 The Mobile Radio Propagation Channel situations, and for special cases such as in tunnels and along railways The overall scenario encompasses the full range of macrocells, microcells and picocells; some have the base station antenna well above the local clutter and others do not In second-generation cellular radio systems, the network planning process (Chapter... have a large number of low-power transmitters radiating from short masts, each covering a small territory but permitting reuse of the frequencies assigned to them many times in a de®ned geographical area This is the basis of the `cellular radio' approach to area coverage and is extremely eective However, implementing this technique requires ®rstly a large number of available channels, and secondly a... electricity Fortnightly Review, 173±81 Austin B.A (1994) Oliver Lodge ± the forgotten man of radio? Radioscientist, 5(1), 12±16 Marconi Co Ltd (1981) Gugliemo Marconi Betts J.A (1967) High Frequency Communications English Universities Press, London Collin R.E (1985) Antennas and Radiowave Propagation McGraw-Hill, New York Knight P (1969) Field strength near the ground at VHF and UHF: theoretical dependence... Research Report 1969/3 7 Appleby M.S and Garrett J (1985) Cellnet cellular radio network Br Telecommun Engng, 4, 62±9 8 Department of Trade and Industry (1985) A Guide to the Total Access Communication System DTI, London 9 Dernikas D (1999) Performance evaluation of the TETRA radio interface employing diversity reception in adverse conditions PhD thesis, University of Bradford . 2000 John Wiley & Sons Ltd Print ISBN 0-4 7 1-9 8857-X Online ISBN 0-4 7 0-8 415 2-4 deterministic approach to establishing the parameters of the propagation channel is not feasible. Almost invariably. size. For two-way mobile radio, particularly in urban areas, it is seldom that the mobile antenna has a direct line-of-sight path to the base station. Radio waves will penetrate into buildings to. UHF bands, for example, television, domestic radio, Citizens' Band radio, marine radio, aeromobile radio (including instrument landing systems) and military radio. Several of these services have a
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