Xem mẫu
- Networks and Telecommunications: Design and Operation, Second Edition.
Martin P. Clark
Copyright © 1991, 1997 John Wiley & Sons Ltd
ISBNs: 0-471-97346-7 (Hardback); 0-470-84158-3 (Electronic)
1 ransmzsszon
Systems
The basic line transmission theory that we have considered so far, together with theprinciples of
analogue and digital signal transmission over pairs of electrical wires, is quite suitable for short
range conveyance of a small number of circuits. However, it is not always practical or economic
to use many multiple numbers of physical ‘pairs’ between exchanges, so we have recourse to
frequency division and time division multiplexing reduce the numberof physical pairs of wires
to
needed to convey a large number of long haul circuits between common end-points. There are, of
course,differenttransmissionmedia to be considered,some of themmuchbettersuited to
particular applications than plain electric wire. They include coaxial cable, radio, satellites and
optical fibre. In this chapter we discuss the main features of each of these transmission types, to
ease the decision about which one is best suited to any particular application.
8.1 AUDIO CIRCUITS
The two-wire and four-wire transmission systems described in the first chapters of this
book, comprising, respectively, one or two physical pairs of copper (or aluminium)
wires together with amplifiers and equalizers as appropriate, are correctly called audio
circuits. (In actual use the wires themselves are often wound or twisted around each
other inside the cable, hence the terms twisted pairs and twisted pair cables). The title
audio or baseband circuit is given because the frequencies of the electrical signals carried
by thecircuit are adirectmatchwiththose of theaudio signal theyrepresent.
In other words, theelectrical signal frequencies are in the audio range; the signal has not
been processed in any way, neither frequency-shifted nor multiplexed. As a reminder of a
typical audio circuit configuration, Figure 8.1 shows a four-wire repeatered circuit as
already discussed in Chapter 3 on long haul communication.
Audio circuits may be used to carry any of a wide range of bandwidths, although in
general the greater the bandwidth to be carried, the shorter the maximum range of the
circuit, because the electrical properties o f the line (its impedance, reactance and
capacitance) tend to interfere. It is known that high frequency signals are carried only
141
- 142 TRANSMISSION SYSTEMS
Repeater
I 1
Figure 8.1 A four-wirerepeatered'audiocircuit'
by the outer surface of wire conductors (the skin e f e c t ) , and this makes high frequency
signals proneto interference from electrical and magnetic fields emanating from
adjacent wires.
Unamplified audio circuits arecommonly used toprovide relatively short range
telephone junction circuits of 4 kHz bandwidth between exchanges over a range up to
about 20 km, and amplification can be used to extend this range. A typical use of an
audio circuit is to connect an analoguelocal (or end ofice or central ofice) exchange to
the nearest trunk (or toll) exchange. Beyond a distanceof about 15 km, even if amplifiers
are used, economics tend to favour other transmission means. As an example, an un-
amplified pair of wires using 0.63 mm diameter conductors has an insertion loss (i.e. a
line loss which must be added to other circuit losses) of 3 dB at a range of 7 km. The
transmission range for speech is increased to 33 km by the use of an amplifier, but this
range is usually precluded by the line quality needs of the signalling system.
Asalsoexplained in Chapter 3, therate of signaldegradation(i.e. attenuation
and distortion) in tlvisted pair transmission media can be reduced by loading the cable
(see Chapter 3) and more simply, by increasing the diameter (gauge) of the wires them-
selves. Bigger conductors cause less signal attenuation because they have less resistance.
However, the relative cheapness of alternative transmission media makes it uneconomic
to employ very heavy gauge (larger diameter)twisted pairs over more than about km. 20
of
At this distance the conductors an unamplified circuit need to be around 1 mm thick,
and becausethe amount of conductingmaterialrequired(copperoraluminium) is
proportional to the square the diameter the cost escalates rapidly as the cable gauge
of is
increased. On a more cheerfulview, the converse also applies, so that at shorter
distances
narrowercablesareeconomicallyfavourable.Inone way andanother anetwork
operator has extensive information available to help him choose the right conductor
of
(copper, aluminium, etc.), and the appropriate gauge wire, thereby cutting costs and
still giving the transmission performance required.
Because they are susceptible to external electro-magnetic interference, audio circuits
are not normally suitable for use on very wide-bandwidth systems, including FDM
(frequency division multiplex) systems, although early systems used screened copper
wires. Audio circuits are most commonly foundin small scale networks, and within the
local area of a telephone exchange. A number of twisted pairs are normally packed
together into large cables. A cable may vary in size from about 3 mm diameter to about
- STANDARD
TWISTED
PAIR TYPES
CABLE INDOOR
FOR USE 143
80mm, carrying anything between one and several thousand individual pairs. Thus,
running out from a 10 000 line local exchange, a small number (say 10-20) of main
backbone cables of several hundred or a thousand pairs may run out along the street
conduits to streetside cabinets where a simply cross-connect frame isused to extend
the pairs out in a star fashion on smaller cables (say 25 or 50 pairs). Finally, at the
distribution point individual pairs of wires may be crimped within a cable joint (at the
top of a telegraph pole) and run out from here to individual customer’s premises.
8.2 STANDARD TWISTED PAIRCABLE TYPES FOR INDOOR USE
For indoor use in conjunction with structured cabling systems (Chapter 45), there are a
number of standardized unshielded twisted pair ( U T P ) and shielded twisted pair ( S T P )
cabling types. Table 8.1 provides a brief overview of them.
8.3 TRANSVERSE SCREEN AND COAXIALCABLE TRANSMISSION
A considerable problem with twistedpairs is signal distortion caused by the skin efSect.
The interference may be manifested as noise or ‘hum’ on the line, or in extreme cases a
stronger voice or other signal may be induced from adjacent wire pairs, causing an
effect known as crosstalk. The likelihood of such interference is increased when higher
bandwidth signals (including FDM) are carried. Two options are available to reduce
the interference. The first is to screen the cable with a light metal foil, usually wound
into the cable sheath. This is called screened or shielded cable. In addition, transverse
screencablealsoincludes screensbetweenwires withinthecable,isolating transmit
from receive pairs. A second type of cable not prone to electromagnetic interference of
this kind is coaxial cable. This is constructed so that most of the signal is carried by an
electromagnetic field near the centre of the cable. Coaxial cable was the workhorse of
Table 8.1 Standardized UTP and STP (horizontal) cabling types
~~
UTP or STP cable type Intended application
Category 1 (Cat. 1) andCategory 2 (Cat. 2) voice and low speed data
Category 3 (Cat. 3) specification allows use up to 16 MHz, typically
allowing use for combined voice and ethernet or
4 Mbit/s token ring LANs
Category 4 (Cat. 4) specification allows use up to 20 MHz, allowing
for 16Mbit/s token ring LAN use as well as
voice
Category 5 (Cat. 5 ) specification allows use up to 100 MHz, allowing
use for voice or data up to lOOMbit/s
- 144 TRANSMISSION
frequency divisionmultiplex (FDM), high bandwidth analogue early
and digital
transmission systems. Now optical fibre is supplanting it for new digital lines.
Instead of a twisted pair of wires, a coaxial cable consists of two concentric con-
ductors, made usually of aluminium or copper. The central or pole, is separated from
wire,
the cylindrical outer conductor by a cylindrical spacing layer of insulation, as shown in
Figure 8.2.
The outer conductor is a metal foil or mesh, wound spirally around the insulation
layer, and outside this is a layer of sheathing toprovide external insulation and physical
protection for the cable.
A single coaxial cable is equivalent to a single pair of twisted wires. To achieve the
equivalent of four-wire transmission, two coaxial cables must be used, one to carry the
transmit signal and one to carry the receive signal.
Coaxial cables function in much the same way as twisted pair circuits, but there are
some important differences in performance. Unlike twisted pair audio circuits, coaxial
cable circuits are said to be unbalanced. By this we mean that the two conductors in a
coaxial cable do not act equally in conveying the signals. In a balanced circuit such
as a twisted pair, where the signal is carried by the electrical currents in the two wires
passing in opposite directions, the currents generate equal but opposite electromagnetic
fields around them which tend to cancel each other out. By contrast, the signal in a
coaxial cable is carried largely by the electromagnetic field surrounding the inner and
outerconductors, which is largely induced by the central
conductor. outer
The
conductor, sometimes called the shield, is usually operated at earth or ground voltage,
and prevents the electromagnetic field from radiating outside the cable sheath. The
signal is thereby protected from interference to some extent.
Unfortunately the overall rate of signal attenuation is greater on unbalanced circuits
like coaxial cables than on balanced ones such as twisted pairs, and this needs to be
counteracted by the use of amplifiers. On the other hand, unlike twisted pair circuits
which are liable to lose balance as the result of damp or damage, coaxial cables are
more stableand suffer less from variable
the attenuation extraneous
and signal
reflections whichsometimes affect twisted pairs.Coaxialcables are more reliable in
service, are easier to install, and they require less maintenance. Further, because of their
insensitivity to electromagnetic interference they perform better than twisted pairs when
the cable route passes near metal objects or other electrical cables.
\ sheath Protective
Outer
conductor
(mesh or spiral band)
Insulation
/ (cylindrical s p o c e r )
Central conductor
or pole
Figure 8.2 Coaxial cable
- DIVISION FREQUENCY (FDM) 145
8.4 FREQUENCY DIVISION MULTIPLEXING (FDM)
FDM, orfrequency division multiplexing is a means of concentrating multiple analogue
circuits over a common physical four-wire circuit. It was used predominantly in the ana-
logue era of telecommunication before glass fibres and digital techniques were available.
Sophisticated equipment at the two ends of a long distance four-wire transmission line
are used to break up the available signal bandwidth into many different and independent
speech of other analogue signal circuits. In effect, the multiplexing equipment multiplies
many times the number of circuits which may be carried, thus saving the expense of
multiple long distance transmission lines.
FDM was used predominantly in conjunction with analogue trunk radio systems and
long distance coaxial transmission lines. FDM equipment remains in widespread use,
buttheadvent of cheaper and higherqualitydigitaltransmissionalternatives (in
particular, TDM or time division multiplexing) is leading to its obsolescence.
FDM allows many analogue transmission circuits (or channels) to share the same
physical pair of wires or othertransmission medium. It requires sophisticated and expen-
sive transmission line terminating equipment and a high quality line, but it has the
potential for overall cost saving because the number of wire pairs between the endpoints
can be reduced. The longer the length of the line, the greater the overall saving.
Typical transmission systems used in conjuntion with FDM are coaxial cables and
analogue trunk radio systems. With analogue satellite systems, a slight variant of the
technique is used, FDMA or frequency divisionmultipleaccess. This uses a similar
technique, but in addition it allows multiple earth stations to communicate using a
single satellite.
The lowest constituent bandwidth that makes up an FDM system designed for the
carriageofmultipletelephonechannelshasachannelbandwidthof4kHz.This
comprises the 3.1 kHz needed for the speech itself, together with some spare bandwidth
on either side which buffers the channel from interferenceby its neighbouring channels.
The speech normally resides in the bandwidth between 300Hz and 3400Hz.
In order to prepare the baseband signal which forms the basic building block of an
F D M system, each of the individual input speech circuits are filtered to remove all the
signals outside of the 300-3400Hz band. The resulting signal is represented by the
baseband signal (schematically a triangle) shown in Figure 8.3.
amplitude
A
carrier
frequency signal
)/e
300
original
spectrum (baseband)
3400 4600 7700
I
l
I
8300
WO'sideband'
reproductions of
original signal
I1400
* signal
frequency
sideband
sideband
upper
baseband
lower
Figure 8.3 The principle of FDM, frequency shifting by carrier modulation
- 146 TRANSMISSION SYSTEMS
circuit 1
.” channel
translating group
circuit 12 ..... equipment one four-wire
circuit
circuit 13
... channel
translatmg
circuit 24 supergroup
GTE
circuit 25 (of 60 tributary channels)
... group one four-wire
translattng translating
circuit 36 circuit
equipment (typically coax)
circuit 37 I CTE I
channel
... translating
circuit 48 eauiment
circuit 49
channel
... translating
circuit 60 equipment
Figure 8.4 Circuit multiplexing using FDM
The next step is that the signal is frequency shifted by modulating the baseband signal
with one of a number of standard carrier signals. The frequency of the carrier signal
used will be equal to the value of the frequency shift required. In our diagram,a carrier
signal frequency of 8000 Hz is being used. This creates two new sideband images of the
signal, the lowersideband and the upper sideband. All the original information from
the baseband signal is held by each of the sideband images, so it is normal during the
transmission to save electrical energy andbandwidth by transmittingonlya single
sideband having first suppressed the carrier (suppressed carrier mode; note, however,
that suppressed carrier mode, while common for line systems is not common in radio
systems as it is often inconvenient to make a carrier signal generator available at the
receiver). Demodulation of the signal into its tributary channels is a similar process to
that of modulation.
F D M is usually conducted in a heirarchical manner as shown in Figure 8.4. The
heirarchy allows progressively more circuits to be multiplexed onto the same line. Of
course, for each further stage of multiplexinga further line quality and bandwidth
threshold must be exceeded for the system to operate reliably. The various different
stages of the F D M hierarchy are achieved by using different translating equipments (the
modulating and demodulating device), as we discuss next.
The carrier frequencies used to in a CTE to createa basic group are 64 kHz, 68 kHz,
7 2 kHz, . . . , 108 kHz, and each of the lower sidebands is used. The resulting group
frequency pattern is as shown in Figure 8.5.
This is the basic building block when further multiplexing into supergroup, but when
sending to line it is normal to frequency shift the group into the lowest frequency range
- FREQUENCY DIVISION MULTILEXING (FDM) 147
channelnumber 12 11 10 9 8 7 6 5 4 3 2 1
Basic group
structure
Frequency 6068768492100 108
6472808896 104 kHz
Figure 8.5 Basic groupstructure
channel
number 1 2 3 4 5 6 7 8 9 10 11 12
FDM group
structure
by the use of a group modulating frequency of 120 kHz. This brings the signal into the
frequency range 12-60 kHz (Figure 8.6).
The further multiplexing steps are listed briefly in Table 8.2.
FDM systems formed the backbone of public telephone networks until the mid-1970s
when digital transmission and pulse code modulation took over as the cheaper and
higher quality alternative. FDM was commonly used on the long distance and inter-
national links between trunk (toll) telephone exchanges. The transmission lines were
typically either coaxial cable or analogue radio. Although the technique is now falling
into disuse for optical fibre and copper line systems, it is likely to remain important
in conjunction radio
with systems,where associated
the technique FDMA (fre-
quency division multiple access) is a very effective wayof sharing radio bandwidth
between multiple communicating endpoints. We return to the subject of radio later in
the chapter.
Table 8.2 FDM hierarchy
Translating
equipment Tributary channel
Signal capacity name output name
CTE channel translating 12 voice channels
Group
equipment
GTE translating
group 5 groups (60 voice
channels)
Supergroup
equipment
STE supergroup translating Hypergroup or Mastergroup
equipment
- 148 TRANSMISSION SYSTEMS
8.5 HDSL(HIGH BIT-RATEDIGITAL SUBSCRIBER LINE) AND
ADSL(ASYMMETRIC DIGITAL SUBSCRIBER LINE)
Although modern optical fibre cable (discussed later in this chapter) has meant that
copper pair and coaxial cabling has to some extent fallen out of fashion, the huge
existing base of such cabling has caused recent development effort to be concentrated
on seeking new methods of using it for high speed digital connectionof customers. Two
noteworthy methods have emerged. Theseare H D S L (high bitrate digital subscriber line)
and A D S L (asymmetric digital subscriber line).
H D S L uses two or three copper pairs to provide a full duplex 2Mbit/s access line,
enabling high bitrate business applications to operate over existing copper lineplant.
A D S L , meanwhile, provides a high bitrate of 4 Mbit/s or 6 Mbit/s in the downstream
channel(e.g.forbroadcastingavideosignaltoresidentialcustomers)with,say,a
64 kbit/s upstream channel for control of the received signal. Likely uses of A D S L are
for video-on-demand (VOD), remote shopping (teleshopping), distancelearning (tele-
education) and other interactive multimedia services. We return to HDSL and ADSL in
Chapter 17.
8.6 OPTICAL FIBRES
Mosttransmission typescanbe used tosupporteitheranalogueor digitalsignal
transmission: twisted pair circuits, coaxial cables and radio systems can all be used as
the basis of either analogue or digital line transmission systems. However, the fact that
digitaltransmissionrequiresonlytwodistinctlinestates (on/oR mark/space)has
opened up a new wider range of digital propagation methods; and the most important
new digital transmission medium is optical fibre.
The importance of optical fibres and their extensive use stems from their extremely
high bit-rate capacity and their low cost. Optical fibres are a hair’s breadthin diameter
and are made from a cheap raw material: They are easy to install because they are
glass.
small, and because they allow the repeaters to be relatively far apart (well over 100 km
spacing is possible) they are easy to maintain.
An optical fibre conveys the bits of a digital bit pattern as either an ‘on’ or an ‘off’
state of light. The light (of wavelength 1.3 or 1.5 pm) is generated at the transmitting
end of the fibre, either by a laser, or by a cheaper device called a light emitting diode
(LED for short). A diodeis used for detection. The light stays within the fibre other (in
words is guided by the fibre) due to the reflective and refractive properties of the outer
skin of the fibre,. which is fabricated in a ‘tunnel fashion’.
Fibre-optic technology is already into its third generation. In the first generation,
fibres had two cylindrical layers of glass, called the core and the cladding. In addition a
cover or sheath of a different material (say plastic) provided protection. The core and
the cladding were both glass, but of different refractive index. A step in the refractive
index existed at the boundary of core and cladding, causing reflection of the light rays
at this interface, so guiding the rays along the fibre. Unfortunately however, because of
the relatively large diameter of the core, a number of different light ray paths could be
produced, reflecting
all off thecladding at different
angles, an effect known as
- OPTICAL FIBRES 149
Figure 8.7 An optical fibre cable, showing clearly overall construction.At the centre, a steel
the
twisted wire provides strength, enabling cable to be pulled through ducts during installation.
the
Around it are ten plastic tubes, each one containing and protecting a hair’s breadth fibre, each
capable of carrying many megabits of information per second. (Courtesy of British Telecom)
dispersion. The different overall ray paths would be of different lengths and therefore
take different amountsof time to reach the receiver. The received signal therefore not
is
as sharp; it is dispersed. It is most problematic when the user intends
very high bit rate
operation. The different wave paths (or modes) explain the namestep index multimode
fibre which has been given to these fibres. Step index multimode fibre is illustrated in
Figure 8.8(a).
A refinement of step index multimode fibre is achieved by using a fibre with a more
gradual grading of the refractive index from core to cladding.A graded index multimode
fibre is shown in Figure 8.8(b). This has aslightly improved high bit rate performance
- 150 TRANSMISSION SYSTEMS
- OPTICAL FIBRES 151
over step-index multimode fibres. Finally, monomode,fibres (illustrated in Figure 8..8(c)
are the third generation optical fibre development, have the best performance. In
of and
monomode fibre, more advanced fibre production techniques have produced a very
narrow core area in fibre, surrounded by a cladding area; there a step change the
the is in
refractive index of the glass at the boundary of the core and the cladding. The narrow
core of amonomodefihre allows only oneof the ray paths (ormodes) to exist. As a result
there is very little light pulse dispersion in monomode fibre, and much higher bit rates
can be carried.
The qualities of different types of optical are laid out in ITU-T recommendations
fibre
G.652, G.653 and G.654. Recommendation G.652 specifies a monomode fibre optimized
for operation at 10 nm. Recommendations G.653 and G.654
13 specify monomode cables
optimized for operation at 1550 nm. All the types of cable may used for both 1310 nm
be
and 1550 nm, but may not operateat maximum efficiency for both wavelengths.
For in-building and campus cabling of office, industrial or university sites it is now
commonplace to use optical fibre at least in the building risers (the conduits between
floors or buildings). The cable generally used contains multimode fibre, usually suitable
for transmission between relatively cheap, LED transmitting devices over distances up
to 3 km or 15 km. Although the multimodejbre itself is nowadays more expensive than
monomode fibre, this is usually compensated forby the much cheaper enddevice costs.
In contrast, monomodejbre is the preferred technology of network operators, and
this is the cable mainly deployed for metropolitan, national and undersea usage. The
highvolumesof production have brought the cost below that of multimode fibre,
despitethemorecomplexmanufacture.Themain benefit is the very highbitrates
achievable and the lengthy inter-repeater distances. The disadvantage is the need for
expensive laser devices as transmitters to produce the high quality single mode (single
wavelength) light required.
An important factor in the design and installation of optical fibre networks is the
careful jointing or interconnection of fibre sections. Thereflections caused at the joint,
and the losses caused by slight misalignment of the two fibre cores can lead to major
losses of signal strength. The latest generation of cabling splicing devices and cable
connectors have improved real network performance dramatically in recent years. The
ST-connector (a bayonet-type connector) now common for optical fibre patch frames
and equipment connections, for example, has been designed to ensure correct alignment
of the fibres. The SC-connector specially developed for ATM (asynchronous transfer
mode, see Chapter 26) also shares the ability for exact alignment.
Recent development in optical fibre technology has concentrated on achieving longer
operational life of installed cable, without the need to replace ‘active’ components. One
of the problems with the original systems was the useof optical signal regenerators
specific to a given light wavelength and transmission bitrate. This means that the early
opticalfibrescannot easily be upgraded in bitratewithoutmajorre-investment in
regenerators and other active components.For this reason, considerable effort has been
put into opticalamplifiers and regenerators which work without reconverting the signal
to electrical form and then re-creating a new light signal for onward transmission. Such
amplifiers can be built to give greater range of optical wavelength and bitrate. Bitrate,
wavelength or multiplexing upgrades can thus to some extent belimited to the end
devices, giving scope for new developments like tcavelengtlz division multiplexing
( W D M ) and passive optical networks, which we discuss next.
- 152 TRANSMISSION
1300
coupler
transmitter
1500 nm 1500 nm
Figure 8.9 The principle of wave division multiplex (WDM)
8.6.1 WavelengthDivisionMultiplexing(WDM)
Wave division multiplexing ( W D M ) is the use of a single fibre to carry multiple optical
signals. Thus, for example, two signals at 1300 nm and 1500nm light wavelength could
carry independent 155 Mbit/s digital signals over the same fibre. In this way, transmit
and receive signals could be consolidatedto a single fibre,rather than requiring separate
fibres (Figure 8.9). Alternatively, the capacity of two existing fibres can be doubled.
8.6.2 BroadbandPassiveOpticalNetwork(BPON)
Broadband passive optical networks (BPONs) are intended for deployment in customer
connection networks between network operators local exchange sites (central ofices)
and customer premises. Here the challenge is to achieve maximum efficiency in the use
of local lineplant, creating the effect of a star-topology (direct connection of each
twisted cable
coaxial
ORS = optical repeater station
OSH = optical splitter head
Figure 8.10 Anopticalfibreaccessnetwork
- customer to the exchange) without needing thousands of fibres and also giving the
potential of signal broadcasting (for cable television for example) to each customer over
a common cabling infrastructure. Figure 8.10 illustrates a typical optical fibre access
network. Optical repeater stations are coupling units for boosting signal strength or for
connecting various fibre rings, meanwhile splitters are passive components which use an
optical cavity of the appropriate length to divide the incomingsignalbetweentwo
fibres. The optical nodes of Figure 8.10 are the receiving devices.
8.7 RADIO
In 1887 Heinrich Hertz produced the first man-made radio waves. Radio waves, like
electricity and light, are forms of electromagnetic radiation; the energy is conveyed by
waves of magnetic and electrical fields. In a wire, these waves are induced and guided
by an electrical current passing along an electrical conductor, but that is not the only
way of propagating electromagnetic ( E M ) waves. By usinga very strongelectrical
signal as a transmitting source,an electromagnetic wave can be made to spread far and
wide through thin air. That is the principle of radio. The radio waves are produced by
transmitters, which consist of a radio wave source connected to some form of antenna,
examples of which are
0 low frequency antenna (aerial) or radio mast
0 H F (high frequency), VHF (very HF), or UHF (ultra-HF) antenna or mast
0 microwavedish antenna
0 troposhericscatterantenna
0 satellite antenna
Radio is a particularly effective means of communication between remote locations and
over difficult country, where cable laying and maintenance is not possible or is pro-
hibitively expensive.It is also effective as a meansof broadcasting the same information
to multiple receivers and cost-effective for low volume use-sharing resources between
many users.
One way of communicating information by radio waves is by encoding (or more
correctly modulating) a frequency
high carrier signal
prior to transmission. The
technique of modulationinradio is very similar to that used in FDM, and single
sideband (SSB) operation, as we discussed earlier in the chapteris common, though the
carrier is rarely suppressed. A distinctive feature of a radio carrier signal is its high
frequency relative to the frequency bandwidth of the information signal. The frequency
of the carrier needs to be high for it to propagate as radio waves.
The modulation of the radio carrier signal may follow either analogue or a digital
an
regime. Analogue modulation is carried out in a similar way to FDM. Digital modula-
tion can be by ‘on/off’ carrier signal modulation (i.e. by switching the carrier on and
off), or by other methods such as frequency sh$t keying ( F S K ) , phase shift keying
( P S K ) or quadrature amplitude modulation ( Q A M ) , details of which are given in the
next chapter.
- 154 TRANSMISSION SYSTEMS
Oscillator RF
Amplifier (carrier
signal
generator)
t
v* 2/
QD
[ )-
(typical amplrfier
Filter
Power
li
Radio
waves
1 GHz)
Modulator
-A+-
I '
Audlo Audio
Filter
amplifier stgnal
input
(typical
20-20 k H 2 )
Figure 8.11 A simplified radio transmitter
Aftermodulationofthecarrierfrequency,thecombinedsignal is amplified and
applied to a radio antenna. Amplification boosts the signal strength sufficiently for the
antenna to convert the electrical current energy into a radio wave.
Figure 8.1 1 illustrates a simple radio transmitter: an audio signal is being filtered (to
cut out extraneous signalsoutside the desired bandwidth) and amplified. Next it is
modulated onto a radio-frequency ( R F ) carrier signal which is produced by a high
quality oscillator; then the modulated signal is filtered again to prevent possible inter-
ference with other radio waves of adjacent frequency. Finally the signal is amplified by
a high power amplifier and sent to the antenna where it is converted into radio waves.
Figure 8.12 illustrates a corresponding radio receiver, the components of which are
similar to those of the transmitter shown in Figure 8.1 The radiowaves are received by
1.
the antenna and are reconverted into electrical signals. A filter removes extraneous and
Oscillator Antenna
Demodulator
Filter Equalizers
Amplifier
Output
sianal
Figure 8.12 A simplified radio receiver
- RADIO WAVE PROPAGATION 155
interfering signals before demodulation. As an alternative to a filter, a tunedcircuit
could have been used with the antenna. A tuned circuit allows the antenna to select
which radio wave frequencies will be transmitted or received.
Then comes demodulation.Inthe receiver shown in Figure8.12 (typical ofa
microwave receiver) demodulation is carried out by subtracting a signal equivalent
to the original carrier frequency, leaving only the original audio or information signal.
A cruder and cheaper method of demodulation, not requiring an oscillator, employs a
rectif)ing circuit. This serves to have the same subtracting effect on the carrrier signal.
After demodulation, the information signal is processed to recreate the original audio
signal as closely as possible. The signal is nextamplifiedusing an amplifier with
automatic gain control ( A G C ) to ensure that the output signal volume is constant, even
if the received radio wave signal has been subject to intermittent fading. Finally, the
output is adjusted remove
to various signal distortions called group delay and
frequency distortion by devices called equalizers. We will consider the cause and cure of
these distortions in Chapter 33.
The simplified transmitter and receiver shown in Figures 8.1 l and 8.12 could be used
together for simplex or one-way radio transmission. Such components in use are thus
equivalent to one pair of a four-wire transmission line discussed in Chapter 3. For full
duplex or two-way transmission, equivalent to a four-wire system, the equipment must
be duplicated, with two radio transmitters and two receivers, one of each at each end of
the radio link. Two slightly different carrier frequencies are normally used for the two
directions of transmission, to prevent the two channels interfering with one another.
Alternatively, by duplicating senders and receivers at each end of the link, but using
only one radio channel, half-dup1e.x operation could be used (communication in both
directions, but in only one direction at a time).
8.8 RADIO WAVE PROPAGATION
When radio waves are transmitted from a point, they spread and propagate spherical
as
wavefronts. The wavefronts travel in a direction perpendicular to the wavefront, as
shown in Figure 8.13.
Figure 8.13 Radiowavepropagation
- 156 TRANSMISSION SYSTEMS
Refractlon Dlfferent
density
.\ ky-wave layers-In
tonosphere
Reflectlon
Troposphere
Dlffractlon
Slght-llnepath
Transmitter Recelve
Figure 8.14 Different modes of radio wave propagation
Radiowaves and lightwaves are both forms of electromagnetic radiation, and they
display similar properties. Just as a beam light maybe reflected, refracted (i.e. slightly
of
bent), and diffracted (slightly swayed around obstacles), so may a radio wave, and
Figure 8.14 gives examples of various different wave paths between transmitter and
receiver, resulting from these different wave phenomena.
Four particular modes of radio wave propagation are shown in Figure 8.14.A radio
transmission system is normally designed to take advantage of one of these modes. The
four modes are
0 line-ofsight propagation
0 surfacewave (diffracted)propagation
e tropospheric scatter (reflected and refracted) propagation
0 skywave (refracted)propagation
A line-of-sight ( L O S ) radio system (microwave radio above about 2 GHz) relies on the
fact that waves normally travel in a straight line. The range of a line-of-site system is
limited by the effectof theearth’scurvature,asFigure8.14shows.Line-of-sight
systems therefore can reach beyond the horizon only when they have tall masts. Radio
systems can also be used beyond the horizon by utilizing one of the other three radio
propagation effects also shown in Figure 8.14.
Surface waves have a good range, dependending on their frequency. They propagate
by diffraction using the ground as a waveguide. Low frequency radio signals are the best
suited to surfacewave propagation, because the amount of bending (the effect properly
called dzflraction) is related to the radio wavelength. The longer the wavelength, the
greaterthe effect of diffraction.Thereforethelowerthefrequency,thegreaterthe
bending. A second means of over-the-horizon radio transmission isby tropospheric
scatter. This is a form of radio wave reflection. It occurs in a layer of the earth’s
atmosphere called the froposhere and works best on ultra high frequency (UHF) radio
waves. The final last example of over-the-horizon propagation given in Figure 8.14 is
- RADIO ANTENNAS 157
Refroctlon
ongle 9 >angle +
Figure 8.15 Refractioncausingskywaves
known as skywave propagation. This comes about through refraction (deflection) of
radiowaves by the earth’s atmosphere, and it occurs because the different layersof the
earth’s upper atmosphere (the ionosphere) have different densities, with the result that
radiowaves propagate more quickly in some of the layers than in others, giving rise to
wave deflection between the layers, as Figure 8.15 shows in more detail. However, as
Figure 8.15 also demonstrates, not all waves are refracted back to the ground; some
escape the atmosphere entirely, particularly if their initial direction of propagation
relative to the vertical is too low (angle 4).
8.9 RADIO ANTENNAS
Eachindividualradiosystem is requiredtoperformslightlydifferenttasks.The
distinguishing qualities of a radio system are
0 its
range
0 its transmit and receive signal power
0 its directionality (i.e. whether the transmitted signal is concentrated in a particular
direction or not)
0 its mode of use (e.g. point-to-point or point-to-multipoint)
These qualities are determined by the mode of propagation for which the system is
designed (e.g. surface wave, tropospheric scatter, line-of-sight, skywave, etc.), by the
frequencyoftheradiocarrierfrequency, by thepower of thesystemandmost
particularly by thesuitability of thetransmittingand receiving antennas.Antenna
design has a significant effect on radio systems’ range, directionality and signal power.
- 158 TRANSMISSION SYSTEMS
The simplest type of radio antenna is called a dipole. A dipole antenna consists of a
straight metal conductor. Examples can be found fitted on many household domestic
radios, to receive V H F (or F M ) radio signals. Another, larger, example of a dipole
aerial is a radiomast.Radiomastsmay be upto severalhundred feet in height,
consisting of one enormous dipole.
The qualities of dipole antennas makethem suitable for broadcast or omnidirectional
transmission and receipt of radio signals. A dipole antenna sends out a signal of equal
strength in all directions, and a receiving antenna can receive signals equally well from
most directions. Dipole aerials are therefore much used for broudcusf transmission from
one transmitting station to many receiving stations. Dipole antennas are also used for
point-to-pointtransmission when the location of the receiver or transmitter is
unknown, as in the case of mobile radio or ships at sea.
Thedisadvantage of omnidirectional receiving antennas is their susceptibility to
radio interference undesired
from sources.
Omnidirectional transmitting antennas
also have a disadvantage: they expend a lot of power by transmitting their radio wave
in all conceivabledirections,but only asmallnumberofpoints within theoverall
coverage area pick up a very smallfraction of the signal power;theremainder is
wasted. For this reason, some antennas are especially designed to work directionally.
One example of adirectional antenna is the dish-shapedtype used formicrowave,
satellite, and tropospheric scatter systems; see Figure 8.16(a). Dish-shaped antennas
work by focusing the transmitted radio waves in a particular direction. Another type
called an array antenna is illustrated in Figure 8.16(b). It is similar to the type used for
domestic receivers.
By using a directional rather than an omnidirectional antenna, the range of the radio
system and the signal power transmitted in or received from a given direction can be
deliberately controlled.
Directional antennas areideal for use on point-to-point radio links; the overall power
needed for transmission is reduced, and the received signal suffers less from interference.
The directionality of an antenna is best illustrated by its lobe diagram, and two
examples are given in Figure 8.17.
The lobes of an antenna lobe diagram show the transmitting and receiving efficiency
of the antenna in the various directional orientations. The length of a lobe on a lobe
diagram is proportional to the relative signal strength of the radiowave transmitted by
theantenna in thatparticulardirection. Hence,
the circle in omnidirectional
the
antenna lobe diagramrepresents an equal strengthof signal transmitted in all directions
(isotropic radiation). Compare with this the directional aerialwhich has one very strong
lobe in one particular direction, and a number of much smaller ones. This sort of lobe
pattern is typical of many directional aerial types.
Figure 8.16 (a) Microwave radio repeater station. A 68-foot tower and associated microwave
radio repeater station, at thebrow of a hill. The antennadishes on opposite sides of the tower are
slightly offset in direction to prevent the possibilityof signal overshoot. (Courtesy o BTArchives)
f
(h) Array antenna.Sixteen metre sterba array antenna. The elements the antenna are the
of barely
visible wires, strung from the main pylon, and arranged in a regular and rectangular formation.
The pattern of repeated elements makes the antenna highly directional ~ cutting out all signals
except that from the desired direction. (Courtesy o j BT Archives)
- RADIO ANTENNAS 159
- 160 TRANSMISSION SYSTEMS
Omnidirectional Highly directional
lobe pattern lobe pattern
c) Figure 8.17 Aerial lob diagrams
Any antenna can be used either to transmit or to receive, and it has the same lobe
pattern when used foreitherpurpose.Thelobepattern shows an antenna’sdirec-
tionality. On the transmission side we have seen that the lobe pattern shows the relative
signal strengths transmitted in various directions. In ‘receive mode’, the lobe pattern
shows the antenna’s relative effectiveness in picking up radio waves from sources in the
various directions. As we have said, a directional receive antenna also helps to reduce
thelikelihoodofinterference fromotherradio sources;thelobe pattern shown in
Figure 8.18 shows a directional antenna being used to eliminate interference from a
second radio source.
The receiving antenna R in Figure 8.18 is oriented so that its principle lobeis pointing
at the transmitting source T1. Meanwhile,sourceT2aligns with a null in thelobe
pattern. Thereceiving antenna will thus be much more effective in reproducing the signal
from T1 than in reproducing thesignal from T2, so that the signal from T2 is suppressed
relative to that from TI.
An antenna is required at each end of any radio link, but the two antennas need not
be similar. For example, the transmitting antenna may be a high power, omnidirec-
tional, broadcast radio mast, whereas receiving antennas may be highly directional, and
perhaps relatively small and cheap, as would be the case with television broadcasting;
whereas with ships’ radio it is difficult to keep directional antennas correctly oriented,
and so an omnidirectional antenna will be a cheaper solution.
In the next few sections, some practical radio systems are considered in more detail.
The characteristics of the antennas, including their directionality, are described and an
explanation is given of how the various antennas induce the desired modeof radiowave
propagation (e.g. surface wave, skywave, etc.).
Interfering
-X l2 transmitter
d
c
Receiving 4
antenna R A - /
4-H
-c
/ H
- - -X T1
Lobe pattern
Figure 8.18 Excludinginterferingradiotransmitters
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