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Chapter 1-1

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88 views8 pages

Chapter 1-1

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Firaa'ol Gizaachoo
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1. Introduction to Electronic Communication The goal of communication is to transfer information from one place to another. This is done by sending the information as electromagnetic energy through vacuum, air, wire, or strands of glass and plastic fiber. Modern communication systems use a wide range and variety of electronic equipment to meet the needs of users: hand held radios, satellite dish antennas, commercial radio and television stations, radars ete. All communication systems have at least two end points. At one end there is source of signals (voice or computer data), circuitry for converting the signal source into a signal that is compatible with the rest of the system, (wire, air, or light carrying fiber). After the transmitted signal passes through this link, it is received by the far end, converted into a signal format as needed, and finally passed to the user. 1.1. The Importance of Communications The importance of communication can be seen in different facets in our daily lives. lm The telephones in our hands m The radios and televisions in our living room m™ The computer terminals with access to the internet in our offices & homes and ete Communication provides senses for ships on the high seas, aircraft in flight, and rockets and satellites in space. Communication keeps weather forecaster informed of conditions measured by multitude of sensors. 1.2. Elements of Communication System Communication involves implicitly the transmission of information from one point to another through succession of processes: @ Generation of message signal: voice, music, computer data, picture © Description of that message signal with certain measure of precision, by set of symbols: electrical, aural, visual @ Encoding of the symbols in a form that is suitable for transmission over physical medium @ Transmission of the encoded symbols to the desired destination @ Decoding and reproduction of the original symbols © Recreation of the original message signal, with definable degradation in quality due to the imperfections in the system There are three basic elements in every communication system: transmitter, channel, and receiver, as shown in figure 1. The transmitter is located at some point in space, the receiver is located at some other point, and the channel is the physical medium that connects them. 1, Transmitter: it converts the message signal from the source of information into a form suitable for transmission over the channel 2. Channel: it distorts the signal due to some imperfections. Noise and interfering signals also added to the transmitted signal 3. Receiver: it reconstructs the received corrupted signal due to channel imperfections, and noise and interference into a recognizable form of the original message signal for the user. Communication System Source of . L ‘User of Figure 1: Elements of Communication system The transducer which is not shown in figure 1 is used first. As a rule, the message produced by the source is not electrical in nature. Accordingly, an input transducer is used to convert the message generated by the source into a time-varying electrical signal called the message signal. By using another transducer at the receiver, the original message is recreated at the user destination. Basically there are two modes of communication “+ Broadcasting: it involves the use of powerful signal transmitter and numerous receivers that ore relatively inexpensive to build, Information bearing signals flow in only in one direction. Eg. radio and television communication + Point-to-Point communication: communication process takes place over a link between a single transmitter and receiver. In this case, there is usually «a bidirectioncl flow of information-bearing signals, which requires the use of @ transmitter and receiver at each end of the link. Eg Telephone communication Communication resources: The two primary communication resources in communication systems are: transmitted power and channel bandwidth. A general system design objective is to use these two resources as efficiently as possible. One resource can be considered as more important than the other. So we can have power limited and band limited communication channels. E.g, telephone circuit is « typicel band limited channel and space communication link or satellite channel is typically power limited channel. Sources of information: In telecommunications we have dominantly four important sources of information: Speech, Music, Pictures, Computer data. Modulation Process: The purpose of a communication system is to deliver a message signal from an information source in recognizable form to a user destination, with the source and user being physically separated from each other. To do this, the transmitter modifies the message signal into a form suitable for transmission over the channel. This modification is achieved by mecns of c process known as modulation. I+ involves varying some parameters of a corrier wave (amplitude, frequency or phase) in accordance with the message signal. The receiver re-creates the original message signal from a degraded version of the transmitted signal after propagation through the channel. This re-creation is accomplished by using « process known as demedulation. It isa reverse of the modulation process used in the transmitter. NEED for modulatior D> Rediscey —the a 4 ork + — : Height: 4 artenna 2 a Function “NT Winimum height ortemre ¢ gon by Ni. Whose, = -S , C= SxWF, vwelecty gp lg fe “Tha walling Pag vorty, exs- > fe isuna, , de _ aute® | height: 4. antenna = t= GE =Ssaes = Soo meledy iD f.amna, bash antinra = = = - Suk $ oe “Wet Lex doe = Femehor. Prom abruc. Two exorvply FY Cheat, Wha 08 —the Weary iting Punyuinty if Sniseoied., height “the. - artenna ¢ ducovoped DB Arsotdy rising ¢) Segrak + ~ FUL audio @reoy) Signoly Gange Prom 9042 +e KHZ. Couteg the. wating Signal and then ab y AiYfecue te Sepotuche hye Gagnoly ok the Weceine end. dn the. Communicertion |= % Low quency Supralt hae peti asiabon ona thay gt pty Sermon Spa cont dittantoy . be -Krargmiied dietily cus ling ® Medulelion intoeogee the Fragquency Qhite Siete “rag “tay Cosy be. “Pranteilad. oh Img fborca ‘ - ok Madulaion almus the fe Be os raion oy Sere oi al out the Sarre. Communication Channel. FS wombe, 4-1 Sasa cpraieg Strationotly Numb) hedio Stokiors ay = raautbon Serublarumdly, i > Allow Jn—the_bandusitth:— ah qe A Signal. «may be mode Smoller Lrmprows, ualihy q rotaption :— ete Guedaatey he, move tn otent, Reducdion dh rnaje. tmpowy pissin The modulation process can be classified as 1, Continuous-Wave (CW) Modulation: a sinusoidal wave is used as the carrier. When the amplitude of the carrier is varied in accordance with the message signal, we call it amplitude modulation (AM), and when the angle of the carrier is varied, we call it angle modulation. The angle modulation may be further subdivided into frequency modulation (FM) and phase modulation (PM),in which the instantaneous frequency and phase of the carrier, respectively ,are varied in accordance with the message signal 2. Pulse-Modulation: The carrier consists of a periodic sequence of rectangular pulses. Pulse-modulation can itself be of an analog or digital type. In analog pulse modulation, the amplitude, duration, or position of a pulse is varied in accordance with sample values of the message signal. In such a case, we speak of pulse-amplitude modulation (PAM) pulse-duration modulation (PDM), and pulse-position modulation (PPM). The standard digital form of pulse modulation is known as pulse-code modulation (PCM) that has no CW counterpart. 1.3. Communication Channels Depending on the mode of transmission, communication channels can be distinguished as guided propagation and free propagation. Guided communication includes: telephone channels, coaxial cables, and optical fibers whereas free propagation includes wireless broadcast channels, mobile radio channels, and satellite channels. 1.4. Analog and Digital Communication Asa student of communication you may be aware of these two terms analog and digital communication. 1. Analog: The electrical message signal which is continuously varying with time. For example Speech, video and variation in temperature with time. 2. Digital: The electrical message signal which has finite number of discrete levels. Text and Data are example of digital communication. ‘Advantages Disadvantages “Analog Communication = Smaller bandwidth ‘System * Synchronization problem is relatively easier Expensive analog components No privacy Cannot merge data from different sources No error correction capability Digital Communication + Inexpensive digital System circuits + Privacy preserved/data encryption/ © Canmerge different data/voice, video and data/ é&transmit over a common digital transmission system * Error correction by coding Larger bandwidth Synchronizetion problem is relatively difficult Energy and Power Signals In electrical systems, a signal may represent a voltage or a current. Consider a voltage v(t) developed across a resistor R, producing a current i(t). The instantaneous power dissipated in this resistor is defined by _ or PEER Or, equivalently, P= Ri? (1.2) (2:3) In both cases, the instantaneous power P is proportional to the squared amplitude of the signal. Furthermore, for a resistor R equal tot-ohm, we see that Eqs(1.2)and (1.3) take on the same mathematical form. Accordingly, in signal analysis it is customary to work with o 1-ohm resistor, so that, regardless of whether a given signal 9(+) represents voltage or a current. For this reason |g(t)|* is generally referred to as the normalized power of g(t). when |however, no confusion results from so doing we shall often follow the generally accepted practice of dropping the word “normalized” and refer instead simply "power". We may express the instantaneous power associated with the signal as P=(|g0F° (4) Based on this convention, we define the total energy of a signal g(t) os E= Jim f Ig(e)|? dt Tae) =| wore as) And its average power as aes 7 2 Palins | eco a as) We say that the signal g(t) is cn energy signal if and only if the total energy of the sigral satisfies the condition O

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