Chapter Fifteen COMMUNICATION SYSTEMS 15.1 Inrropuction Communication is the act of transmission of information. Every living creature in the world experiences the need to impart or receive information almost continuously with others in the surrounding world. For communication to be successful, it is essential that the sender and the receiver understand a common language. Man has constantly made endeavors to improve the quality of communication with other human beings. Languages and methods used in communication have kept evolving from prehistoric to modern times, to meet the growing demands in terms of speed and complexity of information. It would be worthwhile tolook at the major milestones in events that promoted developments in communications, as presented in Table 15.1. Modern communication has its roots in the 19" and 20" century in the work of scientists like J.C. Bose, F.B. Morse, G. Marconi and Alexander Graham Bell. The pace of development seems to have increased dramatically after the first half of the 20"" century. We can hope to see many more accomplishments in the coming decades. The aim of this chapter is to introduce the concepts of communication, namely the mode of communication, the need for modulation, production and detection of amplitude modulation. 15.2 Evements or a Communication SysTEM ‘Communication pervades all stages of life of al living creatures. Irrespective ofits nature, every communication system has three essential elements- 2018-19,"a Physics Tante 15.1 Some MAJOR MILESTONES IN THE HISTORY OF COMMUNICATION Around ‘The reporting ofthe delivery of _It is believed that minister Birbal 1565A.D. achild by qucen using drum _ experimented with the arrangement to beats from a distant place to decide the number of drummers posted King Akbar. between the place where the queen stayed and the place where the king stayed. 1835 Invention of telegraph by It resulted in tremendous growth of Samuel F.B. Morse and Sir __ messages through post offices and Charles Wheatstone reduced physical travel of messengers considerably. 1876 Telephone invented by Perhaps the most widely used means of Alexander Graham Bell and communication in the history of Antonio Meucci ‘mankind, 1895 Jagadis Chandra Bose and It meant a giant leap — from an era of Guglielmo Marconi communication using wires to demonstrated wireless communicating without using wires. telegraphy. (wireless) 1936 Television broadcast(John _ First television broadcast by BBC Logi Baird) 1955 First radio FAX transmitted The idea of FAX transmission was across continent,(Alexander _ patented by Alexander Bain in 1843. Bain) 1968 ARPANET- the first Internet. ARPANET was a project undertaken by came into existence(J.C.R. _ the U.S. defence department. It allowed Licklider) file transfer from one computer to another connected to the network. 1975 Fiber optics developed at Bell Fiber optical systems are superior and Laboratories more economical compared to traditional communication systems. 1989-91 Tim Berners-Lee invented the WWW maybe regarded as the mammoth World Wide Web. encyclopedia of knowledge accessible to everyone round the clock throughout the year. 514 2018-19Communication Systems transmitter, medium/channel and receiver. The block diagram shown in Fig, 15.1 depicts the general form of a communication system. ‘Communication System formation |Message Transmitted [Received ieee SES A ransaer| Channel Receiver User of Information| Signal FIGURE 15.1 Block diagram of a generalised communication systein. Ina communication system, the transmitter is located at one place. the receiver is located at some other place (far or near) separate from the transmitter and the channel is the physical medium that connects them. Depending upon the type of communication system, a channel may be in the form of wires or cables connecting the transmitter and the receiver or it may be wireless. The purpose of the transmitter is to convert the message signal produced by the source of information into a form suitable for transmission through the channel, Ifthe output of the information source isa non-electrical signal like a voice signal, a transducer converts it to electrical form before giving it as an input to the transmitter. When a transmitted signal propagates along the channel it may get distorted due to channel imperfection. Moreover, noise adds to the transmitted signal and the receiver receives a corrupted version of the transmitted signal. ‘The receiver has the task of operating on the received signal. It reconstructs a recognisable form of the original message signal for delivering it to the user of information. ‘There are two basic modes of communication: point-to-point and broadcast. In point-to-point communication mode, communication takes place over a link between a single transmitter and a receiver, Telephony is an ‘example of such a mode of communteation. In contrast, in the broadcast mode, there are a large number of receivers corresponding to a single transmitter. Radio and television are examples of broadcast mode of communication. 15.3 Basic TerminoLocy Usep in ELECTRONIC Communication SysTEMS By now, we have become familiar with some terms like information source, transmitter, receiver, channel, noise, ctc. It would be easy to understand the principles underlying any communication, if we get ourselves acquainted with the following basic terminology. 2018-19 i i I I Source [Signal 7 (Siena ‘Signal I Messaue i i i 515" Physics (0 Transducer: Any device that converts one form of energy into another can be termed asa transducer. In electronic communication systems, we usually come across devices that have either their inputs or outputs in the electrical form. An electrical transducer may be defined as a device that converts some physical variable (pressure, displacement. force. temperature. etc.) into corresponding variations in the electrical signal at its output. (@) Signal: Information converted in electrical form and suitable for transmission is called a signal. Signals can be either analog or digital. Analog signals are continuous variations of voltage or current. They are essentially single-valued functions of time. Sine wave is a fundamental ‘analog signal. All other analog signals can be fully ‘understood in terms of their sine wave components. Sound and picture signals in TV are analog in nature. Digital signals are those which can take only discrete stepwise values, Binary system that Is extensively used in digital electronics employs Just two levels ofa signal. ‘0’ corresponds to alow level and ‘I’ corresponds to a high level of voltage/ current. There are several coding schemes useful for digital communication. They employ suitable combinations of number systems such as the binary coded decimal (BCD)*. American Standard Code for Information Interchange (ASCID)** is a universally popular digital code to represent numbers, letters and certain characters. (Nowadays, optical signals are also in use.) (di) Noise: Noise reters to the unwanted signals that tend to disturb the transmission and processing of message signals in a communication system. ‘The source generating the noise may be located inside or outside the system. () Transmitter: A transmitter processes the incoming message signal so as to make it suitable for transmission through a channel and subsequent reception. () Receiver: A receiver extracts the desired message signals from the received signals at the channel output. (i) Attenuation: The loss of strength of a signal while propagating through a medium is known as attenuation. TTR BGD. a digit & usually represented by four binary (0 or 1} bits. For example the numbers 0, 1, 2, 3,4 in the decimal system are written 2 0000, 0001, 0010, 0011 and 0100. 1000 would represent eight © Tis a character encoding in terms of numbers based on English alphabet since the computer can only understand numbers. 516 2018-19wit) eo) ) a) aii) Communication Systems Amplification: It is the process of increasing the amplitude (and consequently the strength) of signal using an electronic efreuit called the amplifier (reference Chapter 14). Amplification is necessary to compensate for the attenuation of the signal in communication systems, The energy needed for additional signal strength is obtained from a DC power source. Amplification is, done at a place between the source and the destination wherever signal strength becomes weaker than the required strength. Range: Itis the largest distance between a source and a destination up towhich the signal is received with sutflictent strength. Bandwidth: Bandwidth refers to the frequency range over which an equipment operates or the portion of the spectrum occupied by the signal. Modulation: The original low frequency message /information signal cannot be transmitted to long distances because of reasons given in Section 15.7. Therefore, at the transmitter, information contained in the low frequency message signal 1s, superimposed on a high frequency wave, which acts as a carrier of the information. This process is known as modulation. As will be explained later, there are several types of modulation, abbreviated as AM, FM and PM, Demodutation: The process of retrieval of information from the carrier wave at the receiver Is termed demodulation. This is the reverse process of modulation. Repeater: A repeater is a combination of a receiver and a transmitter. A repeater, picks up the signal from the transmitter, amplifies and retransmits it to the receiver sometimes with a change in carrier frequency. Repeaters are used to extend the range of a communication system as shown in Fig. 15.2. A communication satellite is essentially a repeater station in space. aE * FIGURE 15.2 Usé of répeaier station to increase the range of communication, 15.4 Banpwrs or SIGNALs In a communication system, the message signal can be voice, music, picture or computer data. Each of these signals has different ranges of frequencies. The type of communication system needed for a given signal depends on the band of frequencies which is considered essential for tte communication process. For speech signals, frequency range 300 Hz to 3100 Hzis considered adequate. Therefore speech signal requires a bandwidth of 2800 Fiz (3100 Hz ~ 300 Hy for commercial telephonic communication. To transmit music, 2018-19 517"a Physics 15. -15: an approximate bandwidth of 20 kHz 1s required because of the high frequencies produced by the musical instruments. The audible range of frequencies extends from 20 Hz to 20 kHz, Video signals for transmission of pictures require about 4.2 MHz of bandwidth. A TV signal contains both voice and picture and is usually allocated 6 MHz of bandwidth for transmission. In the preceeding paragraph, we have considered only analog signals. Digital signals are in the form of rectangular waves as shown in Fig. 15.3, One can show that this rectangular wave can be decomposed into a superposition of sinusoidal waves of frequencies Vj, 2v,, BV, 4¥p ..- RY, where nis an integer extending to infinity and v,= 1/T,, The fundamental (v,}, fundamental (v,) + second harmonic (2v,), and fundamental (¥,) + second harmonic (2v,) + third harmonic (8v,), are shown in the same figure to illustrate this fact. Itis clear that to reproduce the i netneirven | rectangular wave shape ersemaaia | exactly we need to superimpose all the harmonies ,, 2v,, 3Y,, Gehumonceas”] AV)... Which implies an infinite bandwidth. However, for practical purposes, the contribution from higher harmonics can be neglected, thus limiting FIGURE 15.3 Approximation of a rectangular wave int terms of athe bandwidth. Asa result, 518 fundamental sine wave and its harmonics. received waves are a distorted version of the transmitted one. If the bandwidth is large enough to accommodate a few harmonies, the information is not lost and the rectangular signal is more or less recovered. This is so because the higher the harmonic, less ts its contribution to the wave form. 15.5. Banpwiptx or Transmission Mepium Similar to message signals, different types of transmission media offer different bandwidths. The commonly used transmission media are wire, free space and fiber optic cable. Coaxial cable is a widely used wire medium, which oifers a bandwielth of approximately 750 MHz. Such cables are normally operated below 18 GHz. Communication through free space using radio waves takes place over a very wide range of frequencies: from a few hundreds of kHz to a few GHz. This range of frequencies is further subdivided and allocated for various services as indicated in Table 15.2. Optical communication using fibers is performed in the frequency range of 1 THz to 1000 THz (microwaves to ultraviolet). An optical fiber can offer a transmission bandwidth in excess of 100 GHz. Spectrum allocations are arrived at by an international agreement, ‘The International Telecommunication Union (ITU) administers the present system of frequency allocations. 2018-19Communication Systems ‘Taste 15.2 Some IMPORTANT WIRELESS COMMUNICATION FREQUEN‘ aod Standard AM broadcast. 540-1600 kHz FM broadcast 88-108 MHz ‘Television 54-72 MHz, VHF (very high frequenctes) 76-88 MHz, ™v 174-216 MHz UHF (ultra high frequencies) 420-690 MHz, 1 Cellular Mobile Radio 896-901 MHz, Mobile to base station 840-935 MHz Base station to mobile Satellite Communteation _5.925-6.425 GHz Uplink. 3.7-4.2 GHz Downlink 15.6 Propacation or ELECTROMAGNETIC Waves In communication using radio waves, an antenna at the transmitter radiates the Electromagnetic waves (em waves), which travel through the space and reach the receiving antenna at the other end. As the em wave travels away from the transmitter, the strength of the wave keeps on decreasing. Several factors influence the propagation of em waves and the path they follow. At this point, it fs also important to understand the composition of the earth's atmosphere as it plays a vital role in the propagation of em waves. A brief discussion on some useful layers of the atmosphere is given in Table 15.3. 15.6.1 Ground wave ‘To radiate signals with high efficiency, the antennas should have a size comparable to the wavelength { of the signal (at least ~ 4/4). At longer wavelengths (i.c., at lower frequencies), the antennas have large physical size and they are located on or very near to the ground. In standard AM broadcast, ground based vertical towers are generally used as transmitting antennas. For such antennas, ground has a strong influence on the propagation of the signal. The mode of propagation is called surface wave propagation and the wave glides over the surface of the earth. A wave induces current in the ground over which it passes and it is attenuated asa result of absorption of energy by the earth. The attenuation of surface waves increases very rapidly with increase in frequency. The maximum range of coverage depends on the transmitted power and frequency (less than a few MHZ). 519 2018-19"a Physics PU BE Oe eR Shar eRe ey ater santero ark ‘Troposphere D (part of stratosphere) E (partof Stratosphere) F, (Part of ‘Mesosphere) (Thermosphere) 520 MAMTsBOZ0— 70 Gaze Day and VHF (up to several GHz) night 65-75 km Day only Reflects LF, absorbs MF and HF to some degree 100 km Day only Helps surface waves, reflects HF 170-190 km Daytime, Partially absorbs HF merges with waves yet allowing them F, atnight toreachF, 300 km at night, Day and Efficiently reflects HF 250-400 km. night. waves, particularly at during daytime night 15.6.2 Sky waves In the frequency range from a few MHz up to 30 to 40 MHz, long distance communication can be achieved by ionospheric reflection of radio waves back towards the earth. This mode of propagation is called sky wave propagation and is used by short wave broadcast services. The ionosphere {s so called because of the presence of a large number of tons or charged particles, It extends from a height of ~ 65 Km to about 400 Km above the earth's surface. Ionisation occurs due to the absorption of the ultraviolet and other high-energy radiation coming from the sun by air molecules. ‘The ionosphere is further subdivided into several layers, the details of which are given in Table 15.3. The degree of ionisation varies with the height. The density of atmosphere decreases with height. At great heights the solar radiation is intense but there are few molecules to be ionised. Close to the earth, even though the molecular concentration is very high, the radiation intensity is low so that the ionisation is again low. However, at some intermediate heights, there occurs a peak of ionisation density. ‘The ionospheric layer acts as a reflector for a certain range of frequencies (Sto 30 MHz), Electromagnetic waves of frequencies higher than 30 MHz penetrate the ionosphere and eseape. These phenomena are shown in the Fig. 15.4. The phenomenon of bending of em waves so that they are diverted towards the earth is similar to (otal internal reflection in opties*. * Compare this with the phenomenon of mirage, 2018-19Communication Systems Tonosphertc Layers FIGURE 15.4 Sky wave propagation. The layer nomenclature is given in Table 15.3. 15.6.3 Space wave Another mode of radio wave propagation is by space waves. A space wave travels in a straight line from transmitting antenna to the receiving antenna. Space waves are used for line-of-sight (LOS) communication as well as satellite communication. At frequencies above 40 MHz, communication is essentially limited to line-of-sight paths. At these frequencies, the antennas are relatively smaller and can be placed at heights of many wavelensths above the ground. Because of line-of-sight nature of propagation, direct waves get blocked at some point by the curvature of the earth as illustrated in Fig. 15.5. If the signal is to be received beyond the horizon then the receiving antenna must be high enough to intercept the line-of-sight waves. If the transmitting antenna is at a height h,, then you can show that the distance to the horizon d, is given as dy = /2Rh, . where Ris the radius of the earth (approximately 6400 km). dis also called the radio horizon of the transmitting antenna. With reference to Fig. 15.5 the maximum line-of-sight distance d,, between the two antennas having heights h,and h,above the earth is given by dy = J2Rh, + /2Rhy 5.1) where h,is the height of receiving antenna. 2018-19,"a Physics ‘Television broadcast, microwave links and satellite communication are some examples of communication systems that use space wave mode of propagation. Figure 15.6 summarises the various modes of wave propagation discussed so far. Communication satellite Space wave Ionosphere — FIGURE 15:6 Various propagation modes for em waves. Example 15.1 A transmitting antenna at the top of a tower has a height 32 m and the height of the receiving antenna is 50 m. What is the maximum distance between them for satisfactory communication in LOS mode? Given radius of earth 6.4 x 10° m. Solution yp, = 2X64 x10" x32 + /2x64%10" x50 m = 64x10? xJ10 + 8x 10° xJ10 m =144x10? x0 m = 45.5km 15.7 Moputarion anp 1s Necessity As already mentioned, the purpose of a communication system is to transmit information or message signals. Message signals are also called baseband signals, which essentially designate the band of frequencies representing the original signal, as delivered by the source of information, No signal, in general, is a single frequency sinusoid, but it spreads over a range of frequencies called the signal bandwidth. Suppose we wish to transmit an electronic signal in the audio frequency (AF) range (baseband signal frequency less than 20 kHz) over a long distance directly, a Let us find what factors prevent us from doing so and how we overcome these factors. 2018-19Communication Systems 15.7.1 Size of the antenna or aerial For transmitting a signal, we need an antenna or an aerial. This antenna should have a size comparable to the wavelength of the signal (at least 2/4 in dimension) so that the antenna properly senses the time variation of the signal. For an electromagnetic wave of frequency 20 kHz, the wavelength Ais 15 km. Obviously, such a long antenna is not possible to construct and operate. Hence direct transmission of such baseband signals is not practical. We can obtain transmission with reasonable antenna Iengths if transmission frequency is high (for example, if vis 1 MHz, then 21s 300 m). Therefore, there 1s a need of translating the information contained (1 our original low frequency baseband signal into high or radio frequenctes before transmission. 15.7.2 Effective power radiated by an antenna A theoretical study of radiation from a linear antenna (length 0) shows that the power radiated is proportional to (I/2)2. This implies that for the same antenna length, the power radiated increases with decreasing 2, i.e., increasing frequency. Hence, the effective power radiated by a long wavelength bascband signal would be small. For a good transmission, we need high powers and hence this also points out to the need of using high frequency transmission. 15.7.3 Mixing up of signals from different transmitters Another important argument against transmitting baseband signals directly is more practical in nature. Suppose many people are talking at the same time or many transmitters are transmitting baseband information signals simultaneously. All these signals will get mixed upand there is no simple way to distinguish between them. This points out towards a possible solution by using communication at high Amplitude frequencies and allotting a band of frequencies to each message signal for its transmission. The above arguments suggest that there is a need for translating the original low frequency baseband message or injormation signal into high Puise frequency wave before transmission such that the duration, ‘=Time per translated signal continues to possess the pulse] Jpus information contained in the original signal. In sise fal doing so, we take the help of a high frequency signal, \I known as the carrier wave, and a process known i Pulse amplitude as modulation which attaches information to it. The carrier wave may be continuous (sinusoidal) or in, the form of pulses as shown in Fig. 15. Asinusoidal carrier wave can be represented as eft) =A, sin (gt + 52) where o{t)is the signal strength (voltage or current), A, is the amplitude, (= 2xv,)is the angular frequency and gis the initial phase of the carrier wave. During the process of modulation, any of the three parameters, viz A.. @, and @ of the carrier wave can be controlled by the message or 2018-19 o FIGURE 15.7 (a) Sinusoidal. and {b) pulse shaped signals. 523,524, En "a Physics information signal. This results in three types of modulation: (i) Amplitude modulation (AM), (il) Frequency modulation (i) Phase modulation (PM), as shown in Fig. 15.8. (eM) “A i mag ~ A 2 ole i ix 2 2 } crea (=< w Ci] i a 1 i MM et) for FM O| ann J l\ VV : efor PM A i if Tura RT ih Wl ] | Mi Mm | modulation: and (@) phase modulation to amplitude modulation only. 15.8 Ampuitupe Moputation FIGURE 15.8 Modulation of a carrier wave: (a) a sinusoidal carrier wave: (©) a modulating signal: (c) amplitude modulation: (q) frequency and @ to) @ a a Similarly, the significant characteristics ofa pulse are: pulse amplitude, pulse duration or pulse Width, and pulse position (denoting the time of rise or fallof the pulse amplitude) as shown in Fig. 15.7(b). Hence, different types of pulse modulation are: (a) pulse amplitude modulation (PAM), (b) pulse duration modulation (PDM) or pulse width modulation (PWM), and (¢) pulse positfon modulation (PPM). In this chapter, we shall confine In amplitude modulation the amplitude of the carrier is varied in accordance with the information signal. Here we explain amplitude modulation process using a sinusoidal signal as the modulating signal. Let eft)=A. sin oy trepresent carrier wave and mit) =A,, sin @,trepresent the message or the modulating signal where w,, = 22f,, is the angular frequency of the message signal. The modulated signal c,,(t) can be written as en lt = A, An sina,t) sin o,f \ =A, coo sinayyt) sinegt can also be seen from Fig. 15.8(c). From Eq. (15.3), we can write, ent) = A, sina,t+ uA, sino,t sinat 2018-19 (15.3) Note that the modulated signal now contains the message signal. This (5.4)Communication Systems Here u=A,,/A, isthe modulation index: in practice, wiskept <1 to avoid distortion. Using the trignomatric relation sind sinB= '% (cos(A—B)—cos (A+B), we can write c,, (0 of Eq. (15.4) as eqlt= Ax sin ost + “26 cos, - ty) tA costa, +t 55) side frequencies. The modulated signal now consists of the carrier wave of frequency @, phis two sinusoidal waves each with a frequency slightly different from, known as side bands. The frequency spectrum of the amplitude modulated signal is shown in Fig. 15.9. Here @-o,, and @+ ©, are respectively called the lower side and upper A Amplitude ya, 2 @-e) @ +o) @inrdians FIGURE 15.9 A plot of amplitude versus o for an amplitude modulated ‘signal. As long as the broadeast frequencies (carrier waves) are sufficiently spaced out so that sidebands do not overlap, different stations can operate without interfering with each other. Example 15.2 A message signal of frequency 10 kFz and peak voltage of 10 volts is used (0 modulate a carrier of frequency 1 MHz and peak voltage of 20 volts. Determine (a) modulation index, (b) the side bands. produced. Solution (a) Modulation index =10/20 = 0.5 (b) The side bands are at (1000+10 kHz)=1010 kHz and (1000 =10 Ktiz) = 990 ti. 15.9 Propuction or AmptirupE MopuLatep WavE Amplitude modulation can be produced by a variety of methods. A conceptually simple method Is shown in the block diagram of Fig. 15.10. BANDPASS Aa m0, SQUARE 0) “FILTER AW TAW Device centieD vccnas pC | Sie (Modulating BxtieCxtte Signal) ¢(0) A, sin at (carne) FIGURE 15.10 Block diagram of a simple modulator for obtaining an AM signal. ee 2018-19526 "a Physics Here the modulating signal A,, sin qt is added to the camer signal A, sin gf to produce the signal x (J, This signal x (0 = A,, sing,t +A, sinqt is passed through a square law device which is a non-linear device witich produces an output y(t)= Bx()+Cx2(t) 05.6) where Band Care constants. Thus, y (0 BA, sin «t+ BA, sin @t +C At sin* @,t+ AZ sin? 0 +2A,,A.sing,tsin ot 05.7) BA, sin @,{ + BA, sin at CAR CAE CAS ogo, CAF cosdant 2°22 2 + CA,A,c08 (ay ~ @,) t- CA,,A, 008 (+ 0) t 05.8) where the trigonometric relations sin®A = (1 ~cos2A)/2 and the relation for sinA sin B mentioned earlier are used. In Bq, (15.8), there is a de term /2 (A342). and sinusoids of frequencies @,, 20). @, 20. @-@, and o)+ @,. As shown in Fig. 15.10 this signal is passed through a band pass filter’ which rejects de and the sinusoids of frequencies @, 20, and 2 @ and retains the frequencies 0, Q- g, and ©+ @,."The output of the band pass filter therefore is of the same form as Eq, (15.5) and is therefore an AM wave. It is to be mentioned that the modulated signal cannot be transmitted as such. The modulator is to be followed by a power amplifier which provides the necessary power and then the modulated signal is fed to an antenna of appropriate size for radiation as shown in Fig. 15.11. mii ‘AMPLITUDE: MODULATOR AMPLIFIER Message eignal * Carrier “TRANSMITTING ANTENNA, FIGURE 15.11 Block diagram of a transmitter 15.10 Detection or Ampuitupe Moputarep Wave ‘The transmitted message gets attenuated in propagating through the channel. The receiving antenna is therefore to be followed by an amplifier and a detector. In addition, to facilitate further processing, the carrier frequency is usually changed to a lower frequency by what is called an intermediate frequency (IF) stage preceding the detection. The detected signal may not be strong enough to be made use of and hence is required © Aband pass filter rejects low and high frequencies and allows a band of frequencies to pass through. 2018-19