Communication System

Communication System transmitter, medium/channel and receiver. The block diagram shown in Fig. . depicts the general form of a communication system.

FIGURE . Block diagram of a generalised communication system. In a 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. If the output of the information source is a 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 communication. 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. Communication System (vii) Amplification: It is the process of increasing the amplitude (and consequently the strength) of a signal using an electronic circuit called the amplifier (reference Chapter ).

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. (viii) Range: It is the largest distance between a source and a destination up to which the signal is received with sufficient strength.

(ix) Bandwidth: Bandwidth refers to the frequency range over which an equipment operates or the portion of the spectrum occupied by the signal. (x) Modulation: The original low frequency message/information signal cannot be transmitted to long distances because of reasons given in Section . . Therefore, at the transmitter, information contained in the low frequency message signal is 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. (xi) Demodulation: The process of retrieval of information from the carrier wave at the receiver is termed demodulation. This is the reverse process of modulation.

(xii) 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. .

. A communication satellite is essentially a repeater station in space. FIGURE . Use of repeater station to increase the range of communication.

Communication System . . 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 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 wavelengths 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.

. . 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. FIGURE .

Sky wave propagation. The layer nomenclature is given in Table . . FIGURE .

Line of sight communication by space waves. If the transmitting antenna is at a height hT, then you can show that the distance to the horizon dT is given as T T d Rh , where R is the radius of the earth (approximately km). dT is also called the radio horizon of the transmitting antenna. With reference to Fig.

. the maximum line-of-sight distance dM between the two antennas having heights hT and hR above the earth is given by M T R d Rh Rh ( . ) where hR is the height of receiving antenna. Television broadcast, microwave links and satellite communication are some examples of communication systems that use space wave mode of propagation.

Figure . summarises the various modes of wave propagation discussed so far. FIGURE . Various propagation modes for em waves.

EXAMPLE . Example . A transmitting antenna at the top of a tower has a height m and the height of the receiving antenna is m. What is the maximum distance between them for satisfactory communication in LOS mode?

Given radius of earth . × m. Solution m dm m m . km Communication System .

. 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 #/ in dimension) so that the antenna properly senses the time variation of the signal. For an electromagnetic wave of frequency kHz, the wavelength # is 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 lengths if transmission frequency is high (for example, if + is MHz, then # is m). Therefore, there is a need of translating the information contained in our original low frequency baseband signal into high or radio frequencies before transmission.

. . Effective power radiated by an antenna A theoretical study of radiation from a linear antenna (length l) shows that the power radiated is proportional to (l/#) . This implies that for the same antenna length, the power radiated increases with decreasing #, i.e., increasing frequency.Hence, the effective power radiated by a long wavelength baseband 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. . . 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 up and there is no simple way to distinguish between them. This points out towards a possible solution by using communication at high 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 information signal into high frequency wave before transmission such that the translated signal continues to possess the information contained in the original signal.

In doing so, we take the help of a high frequency signal, known as the carrier wave, and a process known as modulation which attaches information to it. The carrier wave may be continuous (sinusoidal) or in the form of pulses as shown in Fig. . .

A sinusoidal carrier wave can be represented as c(t) = Ac sin (,ct + -) ( . ) where c(t) is the signal strength (voltage or current), Ac is the amplitude, ,c ( = +c) is the angular frequency and - is the initial phase of the carrier wave. During the process of modulation, any of the three parameters, viz Ac, ,c and -, of the carrier wave can be controlled by the message or FIGURE . (a) Sinusoidal, and (b) pulse shaped signals.

information signal. This results in three types of modulation: (i) Amplitude modulation (AM), (ii) Frequency modulation (FM) and (iii) Phase modulation (PM), as shown in Fig. . .

FIGURE . Modulation of a carrier wave: (a) a sinusoidal carrier wave; (b) a modulating signal; (c) amplitude modulation; (d) frequency modulation; and (e) phase modulation. Similarly, the significant characteristics of a pulse are: pulse amplitude, pulse duration or pulse Width, and pulse position (denoting the time of rise or fall of the pulse amplitude) as shown in Fig. .

(b). Hence, different types of pulse modulation are: (a) pulse amplitude modulation (PAM), (b) pulse duration modulation (PDM) or pulse width modulation (PWM), and (c) pulse position modulation (PPM). In this chapter, we shall confine to amplitude modulation on ly. Communication System Here )= Am/Ac is the modulation index; in practice,) is kept to avoid distortion.

Using the trignomatric relation sinA sinB = ½ (cos(A – B) – cos (A + B), we can write cm (t) of Eq. ( . ) as ( ) sin cos( ) cos( ) c c c c c c A A c t A t t t ( . ) Here ,c –,m and ,c +,m are respectively called the lower side and upper side frequencies.

The modulated signal now consists of the carrier wave of frequency ,c plus 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. . .

FIGURE . A plot of amplitude versus ,for an amplitude modulated signal. EXAMPLE . As long as the broadcast frequencies (carrier waves) are sufficiently spaced out so that sidebands do not overlap, different stations can operate without interfering with each other.

Example . A message signal of frequency kHz and peak voltage of volts is used to modulate a carrier of frequency MHz and peak voltage of volts. Determine (a) modulation index, (b) the side bands produced. Solution (a) Modulation index = / = .

(b) The side bands are at ( + kHz)= kHz and ( – kHz) = kHz. Communication System Detection is the process of recovering the modulating signal from the modulated carrier wave. We just saw that the modulated carrier wave contains the frequencies ,c and ,c . ,m.

In order to obtain the original message signal m(t) of angular frequency ,m, a simple method is shown in the form of a block diagram in Fig. . . FIGURE .

Block diagram of a receiver. to be amplified. A block diagram of a typical receiver is shown in Fig. .

FIGURE . Block diagram of a detector for AM signal. The quantity on y-axis can be current or voltage. The modulated signal of the form given in (a) of fig.

. is passed through a rectifier to produce the output shown in (b). This envelope of signal (b) is the message signal. In order to retrieve m(t), the signal is passed through an envelope detector (which may consist of a simple RC circuit).

In the present chapter we have discussed some basic concepts of communication and communication systems. We have also discussed one specific type of analog modulation namely Amplitude Modulation (AM). Other forms of modulation and digital communication systems play an important role in modern communication. These and other exciting developments are taking place everyday.

So far we have restricted our discussion to some basic communication systems. Before we conclude this chapter, it is worth taking a glance at some of the communication systems (see the box) that in recent times have brought major changes in the way we exchange information even in our day-to-day life: Communication System SUMMARY . Electronic communication refers to the faithful transfer of information or message (available in the form of electrical voltage and current) from one point to another point. .

Transmitter, transmission channel and receiver are three basic units of a communication system. . Two important forms of communication system are: Analog and Digital. The information to be transmitted is generally in continuous waveform for the former while for the latter it has only discrete or quantised levels.

. Every message signal occupies a range of frequencies. The bandwidth of a message signal refers to the band of frequencies, which are necessary for satisfactory transmission of the information contained in the signal. Similarly, any practical communication system permits transmission of a range of frequencies only, which is referred to as the bandwidth of the system.

. Low frequencies cannot be transmitted to long distances. Therefore, they are superimposed on a high frequency carrier signal by a process known as modulation. .

In modulation, some characteristic of the carrier signal like amplitude, frequency or phase varies in accordance with the modulating or message signal. Correspondingly, they are called Amplitude Modulated (AM), Frequency Modulated (FM) or Phase Modulated (PM) waves. . Pulse modulation could be classified as: Pulse Amplitude Modulation (PAM), Pulse Duration Modulation (PDM) or Pulse Width Modulation (PWM) and Pulse Position Modulation (PPM).

. For transmission over long distances, signals are radiated into space using devices called antennas. The radiated signals propagate as electromagnetic waves and the mode of propagation is influenced by the presence of the earth and its atmosphere. Near the surface of the earth, electromagnetic waves propagate as surface waves.

Surface wave propagation is useful up to a few MHz frequencies. . Long distance communication between two points on the earth is achieved through reflection of electromagnetic waves by ionosphere. Such waves are called sky waves.

Sky wave propagation takes place up to frequency of about MHz. Above this frequency, electromagnetic waves essentially propagate as space waves. Space waves are used for line-of-sight communication and satellite communication. .

If an antenna radiates electromagnetic waves from a height hT, then the range dT is given by T Rh where R is the radius of the earth. . Amplitude modulated signal contains frequencies ( ), c ,c and ( ). c .

Amplitude modulated waves can be produced by application of the message signal and the carrier wave to a non-linear device, followed by a band pass filter. . AM detection, which is the process of recovering the modulating signal from an AM waveform, is carried out using a rectifier and an envelope detector. POINTS TO PONDER .

In the process of transmission of message/ information signal, noise gets added to the signal anywhere between the information source and the receiving end. Can you think of some sources of noise? . In the process of modulation, new frequencies called sidebands are generated on either side (higher and lower than the carrier frequency) of the carrier by an amount equal to the highest modulating frequency.

Is it possible to retrieve the message by transmitting (a) only the side bands, (b) only one side band? . In amplitude modulation, modulation index ) & is used. What will happen if ) ?

Communication System FIGURE . The carrier wave is given by ( ) 2sin( ) c t t volts. (i) Sketch the amplitude modulated waveform (ii) What is the modulation index? .

For an amplitude modulated wave, the maximum amplitude is found to be 10V while the minimum amplitude is found to be 2V. Determine the modulation index, ). What would be the value of ) if the minimum amplitude is zero volt? .

Due to economic reasons, only the upper sideband of an AM wave is transmitted, but at the receiving station, there is a facility for generating the carrier. Show that if a device is available which can multiply two signals, then it is possible to recover the modulating signal at the receiver station. Absorption spectra AC current AC Generator AC voltage applied to a capacitor applied to a resistor applied to an inductor applied to a series LCR circuit Accelerators in India Accommodation of eye Activity of radioactive substances Additivity of charges Alpha decay Alpha particle scattering Ammeter Ampere Amperes circuital law Amplification Amplitude modulation Analog signal AND gate Andre, Ampere Angle of deviation of incidence of reflection of refraction Angular magnification Apparent depth Area element vector Astigmatism Atomic mass unit number spectra Attenuation Aurora Boriolis Band gap Bandwidth of signal Bandwidth of transmission medium Bar magnet as solenoid Barrier potential Base Becquerel Beta decay Binding energy per nucleon Biot-Savart law Bohr magneton Bohr radius Bohr’s model of atom Bohr’s postulates Brewster’s angle Brewster’s law C A. Volta Capacitance Capacitive reactance Capacitive circuit Capacitor parallel plate in parallel in series Cartesian sign convention Cassegrain telescope Cells in parallel in series Chain reaction Channel Charging by induction Charles August de Coulomb Chromatic aberration Ciliary muscles Coercivity Coherent source Collector Colour code of resistors Combination of lenses