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
aects the signal that propagates through it, but there are other chapters treating
related topics. These have been included to make the book more comprehensive

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 eect 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 ocers 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,

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 dierent 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
eciency. 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 dierent 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

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 eective 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

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
eect), 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 eects 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

the signal strength is substantially reduced by all these eects, 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 ocer 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 ocer 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 dierent channels in the same frequency band. However, all
police ocers 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 eciently, 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

within the intended area. However, the control room may be at some completely
Introduction 9
dierent 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 dierent 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

spectrum for independent and unrelated services is the dominant issue here, since
there are far too many user groups and far too little spectrum available to allocate a
unique segment of it to each group. The same frequencies therefore have to be reused
many times in dierent parts of the country.
The question is therefore, how 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 middle of, say, the London area with sucient transmitter power to
cover all of Greater London, then that frequency would not be reusable anywhere in
the south of England, nor in a large part of Wales.
What are the alternatives? An obvious one is to 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
eective. However, implementing this technique requires ®rstly a large number of
available channels, and secondly a complicated and costly 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
Introduction 11
Figure 1.3 Simple point-to-point radio link.
suitable solution is to transmit from several locations at once. In this case the
transmitters are all operated at nominally the same frequency so that whatever the
location of the mobile within the overall coverage area, it is within range of at least
one of the base stations and its receiver does not have to be retuned. This method of
operation is well established and is known as quasi-synchronous or simulcast
operation. It exploits the fact that although a transmission frequency cannot be used

principal consideration was to ensure that the transmitter network provided an
adequate signal at a high percentage of locations. But in considering the receive
problem, it is clear that a mobile wishing to access control, transmits on a vacant
channel and a signal is received at each of the various base station sites. Usually, one
base receiver will receive a stronger signal than the others because the mobile is
nearest to the site in question.
The radio system needs to decide which site is nearest to the mobile and to
establish communications via that site. This means the system must compare the
radio signals from the mobile at all the base station receivers and then choose the
strongest. This is known as receiver voting. In the absence of other factors,
comparison of received signal strengths around a ring connection might be ecient
12 The Mobile Radio Propagation Channel
in terms of link deployment, but this type of connection involves accumulated delay
in reaching a decision on which is the `best' receiver and this delay is unacceptable in
emergency service applications. Star connections are therefore preferable.
1.4 POSTSCRIPT
In the context of 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 eects and for interference calculations. Moreover,
they are required for a wide variety of environments from rural areas to in-building
Introduction 13
Figure 1.4 Con®guration of link networks: (a) star connection, (b) ring connection.
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 11)
includes not only coverage planning but also frequency assignment strategies and
aspects of base station parameters. Third-generation (UMTS) systems will
incorporate a hierarchical cell structure and for this, coverage planning, frequency

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.
14 The Mobile Radio Propagation Channel