Introduction to communication system chapter 1.pptx

Introduction to communication system chapter 1.pptx

1. Introduction to communication system Chapter 1 1

2. Introduction Communication is state of exchanging information between entities. Some of the communication systems and communicating media in which exchange of information take place includes: Telephone Radio Television and internet are some of it 1.1 E lements of an electrical communication system Electrical communication systems are designed to send messages or information from a source that generates the messages to one or more destinations . Figure 1.1 Functional block diagram of a communication system Output signal Information source and input transducer Transmitter Output transducer Receiver Channel 2

3. Source : generates the information. A source of information is basically a signal (single valued function of time) which carries the information. 2. Transmitter : converts electrical signal into a form that is suitable for transmission through the physical channel or transmission media Translate the information signal to be transmitted into the appropriate frequency range that much the frequency allocation assigned to the transmitter. C hannel : is the physical medium that is used to send the signal form transmitter to the receiver. Depending on the mode of transmission communication channels are classified into two categories Channel based on guided propagation (telephone channels, coaxial cable, optical fiber) Channels based on free propagation (wireless broadcast channels, satellite channels, mobile radio channels) 3

4. 3. Receiver: recovering the message signal contained in the received signal. There are 2 means of communication Broadcasting : which involves the use of a single powerful transmitter and numerous receivers that are relatively inexpensive to build. In this class of communication systems, information-bearing signals flow only in one direction, from the transmitter to each of the receivers out there in the field. Point-to-point communications : in which the communication process takes place over a link between a single transmitter and a single receiver . In this second class of communication systems, there is usually a bidirectional flow of information-bearing signals, which, in effect, requires the use of a transmitter and receiver (i.e., transceiver)at each end of the link. 4

5. 1.2 Elements of Analog communication system Analog communication is the type of communication in which the message or information signal to be transmitted is analog in nature. This means that in analog communication the modulating signal i.e baseband signal is an an analog signal. An analog message is a physical quantity that varies with time, usually in a smooth and continuous fashion. examples of analog sources of information are Speech, image, and video in a form of TV and Radio . In analog communication, the analog message signal modulates some high carrier frequency inside the transmission to produce modulated signal. This modulated signal is then transmitted with the help of a transmitting antenna to travel through the transmission channel. At the receiver this modulated signal is received and processed to recover the original message signal. 5

6. 6 Modulator Demodulator Transmission Channel Input transducer Transmitter Receiver Output transducer Carrier EM waves (modulated signal) Baseband signal (electrical signal) Baseband signal (electrical signal) Basic analog communications system

7. 1.3 Frequency D omain A nalysis O f D eterministic S ignals and Systems In particular signals are used to transmit the information over a communication channel. Such signals are usually called information-bearing signals . In the transmission of an information-bearing signal over a communication channel, the shape of the signal is changed, or distorted, by the channel. In other words, the output of the communication channel, which is called the received signal, is not an exact replica of the channel input due to the channel distortion . 1.3.1 FOURIER SERIES The analysis of signals and linear systems in the frequency domain is based on representation of signals in terms of the frequency variable and this is done through employing: Fourier series Fourier transforms. 7

8. 1. Fourier series Let the signal x(t) be a periodic signal with period T 0. If the following conditions are satisfied. x(t ) is absolutely integrable over its period; i.e . 2. The number of maxima and minima of x(t) in each period is finite, 3. The number of discontinuities of x(t) in each period is finite, then x(t) can be expanded in terms of the complex exponential signals = x n = 1∕ dt The coefficients x n are called the fourier series coefficients of the signal is equal to = 0 and = ∕ 2 Where f =1∕ is the fundamental frequency of the signal   8

9. T he fourier series expansion can be expressed in terms of the angular frequency ω = 2 π f , Therefore = 2 π ∕ ω x n = ω 0∕ 2 π And x(t) =   9

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12. Here T 0 = 2 and it is convenient to choose α = − 1/2 12

13. 2. Fourier transform If the signal x(t) satisfies certain conditions, namely x(t) is absolutely integrable on the real line; i.e., 2 . The number of maxima and minima of x(t) in any finite interval on the real line is finite, 3. The number of discontinuities of x(t) in any finite interval on the real line is finite, then, the Fourier transform (or Fourier integral) of x(t) , defined by 13

14. Example 1 14

15. Example 2 15

16. 1.4 Signal transmission in Baseband Communication systems can be classified into two groups depending on the range of frequencies they used to transmit information. These communication systems are classified into BASEBAND or PASSBAND system. Baseband is to designate the band of frequency representing the original signal as determined by a source of information. This baseband signal can be a combination of two or more message signals. If baseband signal is transmitted directly, then it is known as baseband transmission. The baseband signal may be analog as well as digital Analog baseband signals varies continuously with time and has continuous amplitude. The digital signal is discrete in both time and amplitude. Baseband transmission is preferred at low frequencies and for short distance. However, inter symbol interference (ISI) is the major problem associated with this transmission. 16

17. The source of information, for example, could be a computer that produces a stream of binary data made up of the symbols 0 and 1. The task of a digital communication system is to transport the data stream from the source to its destination over a channel in a reliable manner. To accomplish this task, we need to use a modulation technique that involves varying the amplitude, phase or frequency of transmitted pulses in accordance with the raw data in some discrete manner . 17

18. 1.4.1 SIGNAL DISTORTION IN TRANSMISSION Distortion is any change in the shape of a signal's waveform. T ransmission systems always produce some amount of signal distortion. For the purpose of studying distortion effects on various signals , we'll define two major types of distortion : Linear distortion Nonlinear distortion 18

19. Linear Distortion Linear distortion includes any amplitude or delay distortion associated with a linear transmission system . 1.1 Amplitude distortion: is easily described in the frequency domain ; it means simply that the output frequency components are not in correct proportion . Since this is caused by H(f) not being constant with frequency. A mplitude distortion is sometimes called frequency distortion . The most common forms of amplitude distortion are excess attenuation or enhancement of extreme high or low frequencies in the signal spectrum . If the low-frequency or high-frequency component is attenuated by one-half, the resulting outputs are as shown in Fig. 3.2-4. As expected, loss of the high-frequency term reduces the "sharpness " of the waveform. 19

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21. 1.2 Phase or Delay distortion T he phase shift is not linear , the various frequency components suffer different amounts of time delay, and the resulting distortion is termed phase or delay distortion . For an arbitrary phase shift , the time delay is a function of frequency and can be found by writing arg with all angles expressed in radians. Thus 21 which is independent of frequency only if arg H(f) is linear with frequency.

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24. 24 In order to recover the original signals Xl and X2, the group delay must be constant. Therefore, from Eq. (4) this implies that tg can be found directly from the derivative of arg H(f) = Ѳ (f) as Note that this condition on arg H(f) is less restrictive than in the general case presented earlier . If then the general conditions of distortionless transmission are met and td = tg

25. Equalization Linear distortion-both amplitude and delay is theoretically curable through the use of equalization networks . 25 Figure 3.2-6 shows an equalizer Heq (f ) in cascade with adistorting transmission channel Hc (f). Since the overall transfer function is H(f ) = Hc (f) Heq (f) the final output will be distortionless if Hc (J) Heq (f) = where K and td are more or less arbitrary constants. Therefore, we require that

26. T he tapped-delay-line equalizer, or transversal filter, has emerged as a convenient and flexible device . To illustrate the principle, Fig. 3.2-8 shows a delay line with total time delay 2 having taps at each end and the middle. The tap outputs are passed through adjustable gains, C -l , Co and C l summed to form the final output. Thus 26

27. 27 Example Radio systems suffer from multipath distortion caused by two (or more) propagation paths between transmitter and receiver. Reflections due to mismatched impedance on a cable system produce the same effect. suppose the channel output is

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29. 2. Nonlinear Distortion and Companding sine wave components of the signal are bent (sharply curved or an angle ) that the distortion is called nonlinear distortion. T he instantaneous values of input and output are related by a curve or function y(t ) = T[x(t)], commonly called the transfer characteristic. 29

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31. Companding The joint use of compressing and expanding is called companding . High amplitude analog signals are compressed prior to transmission and then expanded in the receiver. A compressor has greater amplification at low signal levels than at high signal levels , similar to Fig. 3.2-9, and thereby compresses the range of the input signal. If the compressed signal falls within the linear range of the channel, the signal at the channel output is proportional to Tcomp [x(t)] which is distorted by the compressor but not the channel. Ideally, then, the expander has a characteristic that perfectly complements the compressor so the expanded output is proportional to Texp { Tcomp [x(t)]} = x(t ), as desired . 31

32. Besides reducing nonlinear distortion, companding tends to compensate for the signal-level difference between loud and soft talkers. The key advantage of companding compared to the simpler technique of linearly attenuating the signal at the input (to keep it in the linear range of the channel) and linearly amplifying it at the output. 32