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Adama Science & Technology University
School of Engineering and Information Technologies
Electrical Engineering Department
Introduction to Communication systems (EEng-3210)
Chapter 1: Introduction
The purpose of a communication system is to transmit information signals from a source, located at one point in space, to a user destination, located at another point. Mostly, the message produced by the source is not electrical in nature. Thus an input transducer is used to convert the message generated by the source in to time varying electrical signal called the message signal. By using another transducer at the receiver, the original message will be reproduced at the user destination. The message signal can be analog form or digital form.
1.1. Major Parts of Communication Systems Basically communication system consists of three major parts: Transmitter, Communication Channel and Receiver.
Fig.1.1: Basic parts of communication system
Transmitter: - The main purpose of the transmitter is to modify the message signal into a form suitable for transmission over the channel. It involves modulation and amplification. Modulation is a process of mixing the message signal with a very high frequency carrier, which is suitable for propagation.
Communication Channel: - The communication channel may be a transmission line (as in Telephony and Telegraphy), an optical fiber (as in optical communication), or merely free space in which the signal is radiated as an EM wave (as in radio and Television).
Receiver: - The main purpose of the receiver is to reproduce the original message signal from the degraded version of the transmitted signal after propagation through the channel. This is accomplished by using a process of demodulation and amplification. Demodulation is a reverse of modulation. It is a process of extracting the original message from the received signal.
1.2. Classification of Communication Systems There are three ways in which communication systems are classified: analog or digital systems, one-way (simplex) or two-way (half & full duplex) systems, and base band or modulated systems.
According to the message signal communicated, communication systems can be classified in to two types: Analog and Digital. Analog communication system is the one in which message signal is transmitted and received in analog form. Digital communication systems are systems in which message signal is transmitted and received in digital form. Analog systems were the
Transmitter Channel Receiver
first to be developed, however in recent years digital systems have become more popular due to its superior performance.
There are also two basic types of communication systems. The simplest is one-way communication, normally referred to as simplex. In simplex, information travels in one direction only. For example radio and TV broadcasting are simplex. The bulk of communication system, however, is two way which is referred as full duplex. Another form of two-way communications is where only one party transmits at a time. This is known as half duplex.
In a communication system, the information signal may be transmitted by itself over the medium or may be used to modulate a carrier for transmission over a long distance. The former is a base band communication while the later is a bandpass (Modulated signal) communication.
1.3. Typical Communication Systems
Analog Communication System The block diagram of a typical analog communication system is shown in fig 1.2. The analog signal to be transmitted can be a voice waveform, television signal, or any other information- bearing signal. Typically, this message signal must first be filtered to eliminate undesired components and amplified to a suitable level, depending on the source.
Fig. 1.2: Typical Block diagram of Analog communication Systems
The message signal often modulated onto a carrier, which can be a sinusoidal signal, pulse train, or a light wave. In the modulation process, the signal affects some parameter of the carrier in a predetermined way. The modulated signal is then amplified and radiated from an antenna. Various things may happen to the signal in transmission through the channel usually attenuated an attacked by noise.
The corrupted signal received from the channel are amplified to a suitable level and filter to eliminate noise and interfering signals that are all outside the frequency range of the desired signal. The amplified signal is then demodulated to recover the original message. After it is filter the original signal will be reproduced.
Digital Communication System Nowadays digital communication systems are found better in performance than analog communications systems, and therefore communication systems are becoming fully digital. Digital systems demand efficient and reliable digital data transmission and storage systems. This demand has been accelerated by the emergence of large scale, high speed data networks for the exchange, processing and storage of digital information in military, government and
Analog Message IN
Modulation
Carrier Osc.
Power Amp
Channel Analog Message Out
Filter Demodulation LNA
1.4. Frequency Spectrum The frequency spectrum is divided into segments for the purpose of classifying the various portions and how they are used. These are given in table below.
Name Frequency Wavelength Applications Extremely Low frequency (ELF)
30-300Hz 107 -10^6 m Ac line frequency, low end of human hearing Voice frequency (VF) 300-3000Hz 106 -10^5 m Normal range of human speech. Very Low Frequency (VLF)
3-30KHz 105 -10^4 m Higher end of human hearing, sounds from musical instruments Low frequency (VF) 30-300KHz 104 -10^3 m Used as sub carrier, and also in marine navigation. Medium frequency (MF)
300-3000KHz 103 -10^2 m MW AM radio broadcasting
High frequency (HF) 3-30MHz 102 -10^1 m SW AM radio, Two way communications Very High frequency (VHF)
30-300MHz 101 -1m FM radio, VHF TV channels, marine and aeronautical Comn, Mobile Ultra High frequency (UHF)
300-3000MHz 1-10-^1 m UHF TV channels, Cellular Mobile,
Super High frequency (SHF)
3-30GHz 10 -^1 -10-^2 m Microwave frequency used in satellite, radar and long distance comn. Extremely High frequency (EHF)
30-300GHz 10 -^2 -10-^3 m Limited activities so far
Infrared - (^) 0.7 – 10 μm In astronomy to detect stars, for guidance in weapons, TV remote control. The Visible Spectrum - (^) 0.4 –0.8 μm Optical communication
1.5. Fundamental Limitations of Communication Systems
The goal of a communication system engineer is to design systems that provide high quality service for the maximum number of user with the smallest cost and least usage of limited resources. The resources to be conserved include hardware for generating, transmitting, and receiving information signal, the channel bandwidth, and the transmitter power. In other words, engineers attempt to design communication systems that transmit information at a high rate, with high performance, using the minimum amount of transmitted power and bandwidth. Given these requirements, what is the fundamental limitation of a digital communication system? The most significant limitations on the performance of communication system are noise, distortion and bandwidth. Usually, the transmitter and the receiver are carefully designed so as to minimize the effects of noise and distortion on the quality of reception. Bandwidth of a communication system is the range of frequencies that it can pass through. The information capacity of a communication system is a measure of how much information can be carried through the system in a given period of time. It is a function of system bandwidth.
Is it possible to invent a system with no bit error at the output even when we have noise introduced in to the channel? The answer is stated by Shannon-Hartely capacity theorem, according to which digital communication systems that attain as close to zero error probability as described are theoretically possible, provided that the rate of information transmitted is less
than the capacity of the channel C. In other words, Shannon showed that if the rate of information R (in b/s) is less than C , the probability of error would approach zero. The channel capacity C (in b/s) could be calculated by using the equation given below which is
referred to as Shannon equation. Here, B is bandwidth in Hz, N
S
is signal-to-noise power ratio
and as already mentioned C is the channel capacity. Signal-to-noise power ratio indicates the measure of noise relative to information signal.
. (^)
N
S
C B log 2 1 (1.1)
In analog systems the optimum system might be defined as the one that achieves the largest signal to noise ratio at the receiver output subject to design constraints such as channel bandwidth and transmitted power. Is it possible to design a system with infinite signal to noise ratio at the output when noise is introduced by the channel? The answer is of course no.
1.6. Signal Distortion in Transmission A signal transmission system is the electrical channel between an information source and destination. These systems range in complexity from a simple pair of wires to a sophisticated laser-optics link. But all transmission systems have two physical attributes of particular concern in communication: internal power dissipation that reduces the size of the output signal, and energy storage that alters the shape of the output (distortion).
Distortionless Transmission Distortionless transmission means that the output signal has the same "shape" as the input. More precisely, given an input signal x(t), we say that the output is undistorted if it differs from the input by a multiplying constant and a finite time delay. Analytically, we have distortionless transmission if
Where K and td are constants. The properties of a distortionless system are easily found by examining the output spectrum
The stringent demands of distortionless transmission can only be satisfied approximately in practice, so transmission systems always produce some amount of signal distortion. For the purpose of studying distortion effects on various signals, we'll define three major types of distortion:
A common area of confusion is constant time delay versus constant phase shift. The former is desirable and is required for distortionless transmission. The latter, in general, causes distortion. Suppose a system has the constant phase shift not equal to 0° or + m180°. Then each signal
frequency component will be delayed byθ^2 π cycles of its own frequency; this is the meaning of
constant phase shift. But the time delays will be different, the frequency components will be scrambled in time, and distortion will result.
That constant phase shift does give distortion is simply illustrated by returning to the test signal of Fig. 1.4 and shifting each component by one-fourth cycle θ=-90°. Whereas the input was roughly a square wave, the output will look like the triangular wave in Fig. 1.6.
Fig. 1.6: Test signal with constant phase shift θ=-90°.
Nonlinear Distortion A system having nonlinear elements cannot be described by a transfer function. Instead, the instantaneous values of input and output are related by a curve or function y(t) = T[x(t)], commonly called the transfer characteristic. Fig. 1.7 shows a representative transfer characteristic; the flattening out of the output for large input excursions is the familiar saturation- and-cutoff effect of transistor amplifiers. We'll consider only memoryless devices, for which the transfer characteristic is a complete description.
Under small-signal input conditions, it may be possible to linearize the transfer characteristic in a piecewise fashion, as shown by the thin lines in the figure. The more general approach is a polynomial approximation to the curve, of the form
and the higher powers of x(t) in this equation give rise to the nonlinear distortion. Even though we have no transfer function, the output spectrum can be found, at least in a formal way, by transforming the above equation_._ Specifically, invoking the convolution theorem,
Fig. 1.7: Transfer characteristic of a nonlinear device
Equalization Linear distortion-both amplitude and delay-is theoretically curable through the use of equalization networks. Fig. 1.8 shows an equalizer Heq(f) in cascade with a distorting transmission channel HC( f ). Since the overall transfer function is H( f)= HC(f)Heq(f), the final output will be distortionless if H (^) C (f)Heq(f)=Ke-jwtd, where K and td are more or less arbitrary constants. Therefore, we require that
H (f )
Ke H (f)= C
-jwt eq
d , where X( f)≠0.
Channel Equalizer
Fig. 1.8 Channel with equalization for linear distortion
Rare is the case when an equalizer can be designed to satisfy the above equation exactly which makes equalization be a theoretical cure. But excellent approximations often are possible so that linear distortion can be reduced to a tolerable level.
Companding Although nonlinear distortion has no perfect cure, it can be minimized by careful design. The basic idea is to make sure that the signal does not exceed the linear operating range of the channel's transfer characteristic. Ironically, one strategy along this line utilizes two nonlinear signal processors, a compressor at the input and an expander at the output, as shown in Fig. 1.9.
Fig.1.9: Companding System
A compressor has greater amplification at low signal levels than at high signal levels, and thereby compresses the range of the input signal. Ideally, then, the expander has a characteristic that perfectly complements the compressor so the expanded output is proportional to the input, as desired. The joint use of compressing and expanding is called companding.
x(t) HC(f) Heq (f) y(t)
x(t) Compressor Channel Expander y(t)
