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`.(If:.579:1"..inSlgtallwul55.1.51»...
`
`Exhibit 1022
`U.S. Patent No. 6,108,388
`
`

`

`
`
`
`
`"-7:"mp13‘$1-'fi‘nlu-r-rig—fitl‘fm'wmmmzaJenna—u...
`
`
`
`
`
`Published in the USA and
`Canada by Halsted Press,
`a Division of John Wiley & Sons, Inc.
`New York
`
`© Copyright J. D. Parsons, 1992
`
`
`
`many“'.'..'.'.'
`3:31ngI.‘4+m-_.__
`
`Library of Congress Cataloging-in-Publication Data
`Parsons, J. D. (John David)
`The mobile radio propagation channel / J.D. Parsons.
`p.
`cm.
`includes bibliographical references and index.
`ISBN 0-470-21824-X
`2. Radio--Transmitters and
`1. Mobile radio stations.
`transmission.
`3. Radio wave propagation.
`I. Title.
`TK6570.M6P38
`1991
`621.3845--dc20
`
`91-31593CIP
`
`Printed in Great Britain by BPCC Wheatons Ltd, Exeter
`
`
`
`

`

`The figbile Radio
`Progagatior; {Zhagrgel
`
`]. D. i’arsons, DSdEng), FEng, FlEE
`Professor of Electrical Engineering, University of Liverpool
`
`Halsted Press: a division of
`jOHN WILEY & SONS
`New York=Toronto
`
`
`
`

`

`Contents
`
`1
`
`INTRODUCTION
`
`1.1 Background
`1.2 Modes of propagation and the uses of different frequency bands
`1.2.1 VLF
`1.2.2 LF and MF
`1.2.3 HF
`1.2.4 VHF and UHF
`1.2.5 SHF
`1.2.6 EHF
`1.3 Choice of operating frequency for mobile radio systems
`1.3.1 Radio links
`
`1.3.2 Area coverage
`References
`
`2
`2.1
`2.2
`2.3
`
`FUNDAMENTALS OF VHF AND UHF PROPAGATION
`Introduction
`Propagation in free space
`Propagation over a reflecting surface
`2.3.1 The reflection coefficient of the earth
`2.3.2 Propagation over a curved reflecting surface
`2.3.3 Propagation over a plane reflecting surface
`2.4 Ground roughness
`2.5 The effect of the atmosphere
`2.5.1 Atmospheric ducting and non-standard refraction
`References
`
`3
`PROPAGATION OVER IRREGULAR TERRAIN
`3.1
`Introduction
`3.2 Huygen’s Principle
`3.3 Diffraction over terrain obstacles
`3.3.1 Fresnel-zone ellipsoids
`3.3.2 Diffraction losses
`3.4 Multiple knife-edge diffraction
`3.4.1
`Bullington’s equivalent knife-edge
`3.4.2
`The Epstein-Peterson method
`3.4.3
`The Japanese method
`3.4.4
`The Deygout method
`Comparison
`
`3.4.5_
`
`
`
`MNOOONNQONONUI-PF—‘F‘
`
`r—i—H—n
`
`51
`
`l6
`16
`17
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`22
`24
`26
`28
`31
`34
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`37
`39
`40
`48
`48
`48
`49
`50
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`

`

`viii
`
`3.5
`
`CONTENTS
`
`Path loss prediction models
`3.5.1 The Egli model
`3.5.2 The JRC method
`
`3.5.3 The Blomquist — Ladell model
`3.5.4 The Longley — Rice models
`3.5.5 CCIR methods
`3.5.6 The BBC method
`3.5.7 Other methods
`Discussion
`
`References
`
`PROPAGATION IN BUILT-UP AREAS
`Introduction
`
`Built-up areas: a classification problem
`4.2.1 A classification approach
`4.2.2 Classification methods: a brief review
`Propagation prediction techniques
`4.3.1 Young’s measurements
`4.3.2 Allsebrook’s method
`4.3.3 The Okumura method
`4.3.4 The Ibrahim and Parsons method
`4.3.5 Lee’s model
`4.3.6 Other methods
`
`4.4
`
`Microcellular systems
`4.4.1 Microwave measurements
`4.4.2 UHF measurements
`
`References
`
`CHARACTERISATION OF MULTIPATH PHENOMENA
`Introduction
`
`The nature of multipath propagation
`Short-term fading
`‘
`5.3.1
`The scattering model
`Angle of arrival and signal spectra
`The received signal envelope
`The received signal phase
`Envelope autocorrelation and baseband power spectrum
`Level crossing rate and average fade duration
`The PDF of phase difference
`Random FM
`
`Rician fading
`Spatial correlation of field components
`5.12.1 Cross-coreelation
`
`The signal received at the base station
`5.13.1 Vertically separated antennas
`5.13.2 Horizontally separated antennas
`The magnetic field components
`Signal variability
`5.15.1 Statistics of the fast fading
`5.15.2 Statistics of the local mean
`
`5.10
`5.11
`5.12
`
`5.13
`
`53
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`61
`64
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`69
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`131
`134
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`140
`142
`143
`145
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`149
`151
`
`
`
`

`

`CONTENTS
`
`5.15.3 Large area statistics
`References
`
`WIDEBAND CHANNEL CHARACTERISATION
`Introduction
`
`Frequency-selective fading
`6.2.1 Channel characterisation
`Characterisation of deterministic channels
`6.3.1 Time-domain function
`
`6.3.2 Frequency-domain function
`6.3.3 Time-variant transfer function
`6.3.4 Delay/Doppler-spread function
`6.3.5 Relationships between system functions in different domains
`Characterisation of randomly time-variant linear channels
`6.4.1 Channel correlation functions
`
`6.4.2 Relationships between channel correlation functions
`Classification of practical channels
`6.5.1 The wide-sense stationary channel
`6.5.2 The uncorrelated scattering channel
`6.5.3 The wide—sense stationary uncorrelated scattering channel
`Channel characterisation using the scattering function
`6.6.1 The point-scatterer description
`6.6.2 Statistical point-scatterer model
`6.6.3 The scattering function
`Mobile radio channel characterisation
`6.7.1
`Small-scale channel characterisation
`
`6.4
`
`6.5
`
`6.6
`
`6.7
`
`Large—scale channel characterisation
`6.7.2
`References
`
`FURTHER MOBILE RADIO CHANNELS
`Introduction
`
`Building penetration losses
`Propagation inside buildings
`7.3.1 Propagation characteristics
`7.3.2 Wideband measurements
`
`Radio propagation in tunnels
`Propagation in rural areas
`7.5.1
`Introduction
`
`75.2 Signal statistics
`7.5.3 Comparison of various distributions to model the small-scale
`signal variations
`References
`
`SOUNDING, SAMPLING AND SIMULATION
`Channel sounding
`Narrowband channel-sounding techniques
`Signal sampling
`Sampled distributions
`8.4.1
`Sampling to obtain the local mean value
`8.4.2 Sampling 3 Rayleigh-distributed variable
`
`ix
`
`153
`159
`
`161
`161
`162
`164
`165
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`166
`167
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`206
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`212
`212
`212
`214
`215
`216
`217
`
`
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`

`

`
`
`CONTENTS
`
`8.5
`
`8.6
`8.7
`8.8
`
`8.9
`
`8.10
`
`Mean signal strength
`8.5.1 Confidence interval
`Normalisation revisited
`
`Wideband channel sounding
`Wideband sounding techniques
`8.8.1
`Periodic pulse sounding method
`8.8.2 Pulse compression techniques
`8.8.3 Convolution matched-filter technique
`8.8.4 The swept time—delay cross-correlation method
`System requirements
`8.9.1 Dynamic range
`8.9.2 Multipath resolution
`8.9.3 Scaling factor for the STDCC
`8.9.4 Doppler-shift resolution
`8.9.5 Accuracy of frequency standards
`8.9.6 Phase noise in signal sources
`8.9.7
`Improved sounder design
`Experimental data processing
`8.10.1 Frequency-domain characterisation
`8.10.2 Large-scale characterisation
`8.10.3 Summary
`Radio channel simulation
`Hardware simulation of narrowband channels
`Wideband channels
`8.13.1 Software simulation
`8.13.2 Hardware simulation
`
`References
`
`MAN-MADE NOISE AND INTERFERENCE
`Introduction
`
`The Nature of impulsive noise
`Characterisation of pulses
`9.3.1 The spectrum amplitude of a rectangular pulse
`9.3.2
`Impulse generators
`Characterisation of impulsive noise
`9.4.1 Measurement parameters
`Measuring equipment
`9.5.1 Bandwidth considerations
`
`9.5.2 Dynamic range considerations
`9.5.3 Receiver sensitivity and noise figure considerations
`9.5.4
`Impulse response considerations
`Practical measuring systems
`Measurement of noise amplitude distribution
`Statistical characterisation of noise
`
`Impulsive noise measurements
`9.9.1 Measurements using NAD
`9.9.2 Measurements using APD
`Discussion
`
`Prediction of performance degradation
`
`9.6
`9.7
`9.8
`9.9
`
`9.10
`9.11
`
`217
`218
`220
`221
`223
`223
`224
`225
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`229
`230
`230
`230
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`231
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`
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`269
`273
`273
`275
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`277
`289
`291
`
`

`

`’i
`
`lntrsénetion
`
`1.1 BACKGROUND
`
`to the origins of
`The history of mobile radio goes back almost
`radiocommunication itself. The very early work of Hertz in the 1880’s showed
`that electromagnetic wave propagation was possible in free space and hence
`demonstrated the practicality of radiocommunications. In 1892, less than 5
`years later, a paper written by the British scientist Sir William Crookes [I]
`predicted telegraphic communication over
`long distances using tuned
`receiving and transmitting apparatus and this early work was soon turned
`into the first practical communication system by the entrepreneur Marconi
`[2]. He established a radio link over a distance of a few miles in 1895 using
`two elevated antennas and progress 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. 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,
`aero-mobile 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, of necessity, be very
`bulky and the present volume will therefore be concerned only 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 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 affects the signal that propagates through it, but there are further
`chapters, one concerned with topics such as channel sounding, channel
`simulation and signal sampling and the other devoted to characterisation of
`the noise environment.
`they can be left until
`It is not profitable, at this point, to discuss details
`later. Suflice it to say at the beginning that in the vast majority of cases,
`because of complexity and variability,
`a deterministic approach to
`establishing the parameters of the propagation channel is not feasible. Almost
`1
`
`
`
`

`

`2
`
`THE MOBILE RADIO PROPAGATION CHANNEL
`
`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 ambient noise levels at the two ends of the link may not be the same
`and so reciprocity with respect to the signal characteristics does not imply
`reciprocity with respect to the signal—to-noise ratio.
`If this book had been written some years ago its primary concern would
`undoubtedly have been the propagation aspects related to traditional mobile
`radio services which are based on the concept of an elevated base station,
`located on a good site, communicating with a number of mobiles in the
`surrounding area. Such systems, known as private mobile radio (PMR)
`systems, developed rapidly following World War 11, particularly so when
`after the invention of the transistor it became possible to design and build
`compact, lightweight equipment that could easily be installed in a vehicle
`and powered directly from the vehicle battery. They are often termed
`‘Dispatch Systems” because of their popularity with police forces,
`taxi
`companies and service organisations who operate fleets of vehicles. The
`frequency bands used for such systems lie in the range 70~47O MHz and have
`been chosen because the propagation characteristics are suitable, antennas
`are of a convenient size and adequate radio frequency (RF) power can be
`generated easily and efficiently. The operational strategy used 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 means of either amplitude 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 lead to several reductions to the point where
`currently in the UK, channels in the VHF band (30—300 MHz) are 12.5 kHz
`apart, whilst 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 effect of irregular terrain and the influence
`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, policemen
`and, more recently by subscribers to cellular radio telephone systems. Such
`equipment can easily be taken into buildings, so a book concerned with
`propagation must consider additionally, the properties of the signal inside
`buildings and in the surrounding areas. For cordless telephones and the like,
`
`

`

`INTRODUCTION
`
`3
`
`there is also a need to study propagation totally within buildings. Neither
`can we restrict attention to frequencies below 470MHz; current telepoint
`and cellular radio services operate at frequencies up to 900 MHZ, and the
`first generation of
`‘personal’ communication systems
`is
`likely to be
`developed using frequencies around 1.8GHz. In the future even higher
`frequencies will probably be used to implement microcellular schemes and
`in order to ease spectrum congestion.
`What
`then are the matters of primary concern? In connection with
`transmissions of the traditional type in which the signals are restricted to
`fairly narrow radio channels, important factors which have to be quantified
`include:
`
`(1) The median signal strength, and
`(2) The signal variability.
`
`large to be economically covered using a single base station and various
`
`The ability to predict the minimum power it is necessary to radiate from a
`given transmitter to provide an acceptable quality of coverage over a
`pre-determined service area and to estimate the likely effect of such
`transmissions on services in adjacent areas is critical in improving frequency
`re-use techniques, in implementing band-sharing schemes between different
`services and for the success of radio telephone systems. This is not easy and
`there is a vital need for a better understanding of the influence of the different
`urban and terrain factors on the mobile radio signal.
`As far as signal variability is concerned, it is often convenient to separate
`the effects, on a basis of scale, into those which occur over a short distance
`and those which are apparent only over much longer distances. The former
`category includes the rapid fading caused by multipath propagation in urban
`areas whilst the much slower variations in average signal strength as the
`receiver moves from one area to another lie in the latter category.
`For the future, digital techniques are of increasing interest and in the new
`Pan-European Cellular Radio Scheme [3] which will become operational in
`the early 1990s, digitised speech will be used. At carrier frequencies above
`lGHz, and with digital
`techniques,
`it may no longer be convenient or
`desirable to use the FDMA/SCPC systems previously described and spectrum
`efficiency may be substantially improved by allowing each user access to a
`very wide radio channel
`(perhaps several MHz) but only for a small
`percentage of the time. This strategy is known as time division multiple
`access (TDMA). The characterisation of wideband channels will be discussed
`in Chapter 6; for the present it will be sufficient to point out that if digital
`(pulse) signals propagate in an environment where more than one path exists
`between the transmitter and the receiver, 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 (181)
`and can cause errors. The extent of the problem can be quantified 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
`
`

`

`4
`
`THE MOBILE RADIO PROPAGATION CHANNEL
`
`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 re-allocated to a different user in another
`area some distance away. The most common example of this is cellular radio,
`which relies on frequency re—use to achieve high spectrum efficiency. 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 fixed terminal and the term
`‘mobile’ to describe the moving terminal, whether it be hand-portable or
`installed in a vehicle.
`
`1.2 MODES 0F PROPAGATION AND THE USES OF
`DIFFERENT FREQUENCY BANDS
`
`Having set the scene for the book, we can now move on to discuss some of
`the topics in a little more detail. It is very important to understand the
`manner in which radio frequency energy propagates and in preparation for
`a brief general discussion of that topic let us define more clearly what is
`meant by the term radio wave and how waves of different frequencies are
`classified.
`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 at frequencies down to a few kHz. By international
`agreement the radio frequency spectrum is divided into ‘bands’, each band
`being given a designatory name 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, much depending on the transmission frequency. Waves propagated
`via the layers of the ionosphere are known as ionospheric waves or sky-waves.
`
`Table 1.1 DESIGNATION OF FREQUENCY BANDS
`————————_‘__________
`Frequency band
`Frequency range
`
`< 3 kHz
`Extremely low frequency (ELF)
`3—30 kHz
`Very low frequency (VLF)
`30—300 kHz
`Low frequency (LF)
`300 kHz—3 MHz
`Medium frequency (MP)
`3—30 MHZ
`High frequency (HF)
`30—300MHZ
`Very high frequency (VHF)
`300 MHz—3 GHZ
`Ultra high frequency (UHF)
`3-30 GHZ
`Super high frequency (SHF)
`30—300 GHz -
`Extra high frequency (EHF)
`————————________________
`
`
`
`
`E—m—Smn
`
`

`

`INTRODUCTION
`
`5
`
`Radiowaves
`
`
`
`Ionospheric or
`sky-waves
`
`Ground anes
`
`Tropospheric waves
`
`Space waves
`
`Surface waves
`
`Direct waves
`
`Ground-reflected
`waves
`
`Fig. 1.1 Modes afradiowave propagation
`
`Those that propagate over other paths in the lower atmosphere (the
`troposphere) are termed tropospheric waves whilst those that propagate very
`close to the Earth’s surface are known by the generic term ground-waves.
`Ground waves can be conveniently divided into space waves and surface
`waves and space waves can be further sub-divided into direct waves which
`propagate via the direct path between transmitting and receiving antennas
`and ground-reflected waves that reach the receiving antenna after reflection
`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 affected by the ground
`constants (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 105 m, and antennas
`are therefore very large. They are of necessity very close to the earth and
`are often actually buried in the ground. The radio waves are reflected 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
`D-layer the effective height of the terrestrial waveguide also varies around
`
`

`

`6
`
`THE MOBILE RADIO PROPAGATION CHANNEL
`
`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 kHz and a few MHz (the LF and
`MF bands) groundwave propagation is the dominant mode and the radiation
`characteristics are strongly influenced 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 of the surface wave is higher than
`in the LF band, broadcasting over distances of several hundred miles is still
`possible particularly during the daytime. At night, skywave propagation Via
`the ionospheric 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 groundwave and the other via a skywave path.
`Interference can be
`constructive or destructive depending upon the phases of the incoming waves
`and temporal variations in the height of the D-layer, which are apparent
`over periods measured in 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 effect in Mobile Radio as will be
`seen later.
`
`1.2.3 HF
`
`Groundwave propagation also exists in the HF band, but here the ionospheric
`or skywave 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.
`Suffice 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 different 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.
`
`___..___._.n_....-MV
`
`i
`
`

`

`INTRODUCTION
`
`7
`
`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-reflected components of the spacewave. 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 spacewave is
`predominant. Although the spacewave is often a negligible factor
`in
`communication at lower frequencies, it is the dominant feature of groundwave
`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 spacewave propagation
`at VHF and UHF needs to take into acount the problems of reflections both
`from the ground and from natural and man-made obstacles. Diffraction over
`hill tops and buildings, and refraction in the lower atmosphere are also
`important.
`
`directly relevant to current mobile radio systems. However, Fig. 1.2 shows
`
`The term millimetre wave (mm-wave) is often used to describe frequencies
`in the EHF band between 30 and 300 GHz. By 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-reflected waves is possible, it is often insignificant, 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-reflected waves play a significant role.
`This topic will be treated in Chapter 2. In the min—wave band the most
`important effects 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 ofmm-wave
`propagation is well beyond the scope of this book and, in any case, is not
`
`1.2.5 SHF
`
`Frequencies in the SHF band are commonly termed microwaves and this
`term is sometimes used, in addition, to describe that part of the UHF band
`above about 1.5GHz. Propagation paths must have line.of.sight between
`the transmitting and receiving antennas, otherwise losses are extremely high.
`At these frequencies it is possible to design compact high-gain antennas
`normally of the reflector-type which concentrate the radiation in the required
`direction. Microwave frequencies are used for satellite communication (since
`they penetrate the ionosphere with little or no effect), point-to-point terrestrial
`links, radars and short-range communication systems.
`
`1.2.6 EHF
`
`L______ _
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`

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`8
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`THE MOBILE RADIO PROPAGATION CHANNEL
`
`dB/km
`
`Attenuation,
`
`3
`
`6
`
`15
`
`60
`3O
`Frequency, GHz
`
`150
`
`300
`
`Fig. 1.2 Attenuation by oxygen and water vapour at sea level. T = 20°C. Water content: 7.5 g/m3
`
`the attenuation by oxygen and uncondensed water vapour [5] as a function
`of frequency. It can be seen that at some frequencies there are strong
`absorption lines, for example 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 mm—waves find application but the absorption bands have
`some potential for microcellular mobile communications in the future. At
`present there is no volume market in this frequencies range so component
`and system costs are very high.
`
`13 CHOICE OF OPERATENG FREQUENCY FOR MOBILE
`RADIO SYSTEMS
`
`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 communications
`service. For the specific application of interest,
`two-way mobile radio
`operations, communication is required over ranges that do not normally
`exceed a éew tens of km — indeed it would cause unnecessary interference 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
`
`

`

`INTRODUCTION
`
`9
`
`over the entire area concerned. Operating frequencies must be chosen in a
`region of the RF spectrum where it is possible to design efficient 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 ‘omni-directional’
`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 whilst 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
`diffraction, appear to bend slightly over minor undulations or folds in the
`ground. Fortunately, due to multiple scattering and reflection, the waves
`also propagate into built up areas, and although the signal strength is
`substantially reduced by all these various effects, 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 whilst continuing to satisfy the other constraints mentioned
`above.
`
`For these reasons, traditional two-way mobile radio has 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, taxi companies
`and the like. The central police 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 officer, 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 officer acknowledges it,
`investigates the incident and reports back by radio. Because of the
`FDMA/SCPC method of operation, police forces have radio channels
`exclusively allocated for their use and there is no mutual interference between
`them and other users such as taxis who have been allocated different channels
`in the same frequency band. All police oflicers on duty, however, 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 efficiently,
`it is necessary to
`re-allocate radio channels to other users operating some distance away. If
`in the example given above the message had been radiated on HF, then it
`is possible that the signals could have been detected at distances of several
`hundred km, which is unnecessary, undesirable and in any case, 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
`
`* Omni-directional is not to be confused with isotropic, which means “in all directions’.
`
`.W3Z'Q’A’”
`
`,...,.,._,...-p...-.__4,..._mflm
`
`-—«-—~-..-...-u'.
`
`

`

`l
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`10
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`THE MOBILE RADIO PROPAGATION CHANNEL
`
`used for mobile communications. At frequencies in the VHF band it is better
`than horizontal polarisation because it produces a higher field 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 re-use, no worthwhile improvement can be achieved by
`employing both polarisations (as used 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 benefit 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, aero-mobile radio (including instrument lan