Mạng và viễn thông P8

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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....

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  1. 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
  2. 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
  3. 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
  4. 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
  5. 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
  6. 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
  7. 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
  8. 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
  9. 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
  11. 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.
  12. 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
  13. 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.
  14. 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
  15. 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
  16. 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
  17. 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.
  18. 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)
  20. 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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