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Principles of communication engineering


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Principles of communication engineering

  1. 1. DEX2201EX, Principle of Communication Engg. Unit 1, prepared by: Er Lochan Raj NeupaneManmohan Memorial Polytechnic Page | 1Principles of communication engineeringUnit 1 INTRODUCTION1.1 history of communicationCommunication is the process of establishing connection or link between two points for informationexchange.The history of telecommunication - the transmission of signals over a distance for the purposeof communication - began thousands of years ago with the use of smoke signals and drums in Africa,America and parts of Asia. In the 1790s the first fixed semaphore (system of conveying information by meansof visual signals) systems emerged in Europe however it was not until the 1830s that electricaltelecommunication systems started to appear.Basic electrical signals: 1838: Electrical telegraph 1858: First trans-Atlantic telegraph cable 1876: Telephone 1880: Telephony via lightbeam photophonesAdvanced electrical and electronic signals: 1893: Wireless telegraphy 1896: Radio 1914: First North American transcontinental telephone calling 1927: Television 1927: First commercial radio-telephone service, U.K.–U.S. 1930: First experimental videophones 1934: First commercial radio-telephone service, U.S.–Japan 1936: Worlds first public videophone network 1946: Limited capacity Mobile Telephone Service for automobiles 1956: Transatlantic telephone cable 1962: Commercial telecommunications satellite 1964: Fiber optical telecommunications 1965: First North American public videophone network 1969: Computer networking 1973: First modern-era mobile (cellular) phone 1979: INMARSAT ship-to-shore satellite communications 1981: First mobile (cellular) phone network 1982: SMTP email 1983: Internet 1998: Mobile satellite hand-held phones 2003: Skype Internet telephony
  2. 2. DEX2201EX, Principle of Communication Engg. Unit 1, prepared by: Er Lochan Raj NeupaneManmohan Memorial Polytechnic Page | 21.2 Modulation, need for modulation and demodulation in communication systemModulation is the process of varying one or more properties of a periodic waveform, called the carriersignal, with a modulating signal which typically contains information to be transmitted. In other wordsModulation may be defined as the process by which some characteristics of a signal called carrier is variedin accordance with the instantaneous value of another signal called modulating signal. Characteristics that canbe varied are amplitude or frequency or phase.The information containing signal are referred as modulating signals. The signal resulting from the processof modulation is called modulated signal.Need for modulation or Benefits of ModulationModulation is needed in a communication system to achieve the following basic needs1)MultiplexingSimultaneous transmission of multiple signal over the common channel is possible only with modulation process.2) Practicability of antennasFor efficient radiation and reception of signal, antenna should have length comparable to quarter-wavelengthof the frequency used.For example if 5 kHz signal were to be transmitted without modulation then length of antenna would bel= λ/4 = c/4f =(3x108)/ (4x5x103) =15kmObviously, it will be impracticable to construct and install such antenna. However length of antenna can bereduced using modulating technique. If audio frequency is translated to a radio frequency carrier of frequency5MHz, length of antenna would bel= λ/4 = c/4f =(3x108)/ (4x5x106) =15 metersThe antenna height may be achieved practically.
  3. 3. DEX2201EX, Principle of Communication Engg. Unit 1, prepared by: Er Lochan Raj NeupaneManmohan Memorial Polytechnic Page | 33) Remove interference: Due to transmission over same frequency range, signals interfere with each other.Hence, in order to keep the various signals separate, it is necessary to translate or shift them to differentportion of electromagnetic spectrum using modulation.4) Reduce noise: Noise is major limitation in communication. Effect of noise can be minimized using severalmodulation techniques.Demodulation is the reverse process of modulation, which is used to get back the original message signal.Modulation is performed at the transmitting end whereas demodulation is performed at the receiving end.1.3 Analog Communication SystemAnalog communication is that type of communication in which the message or information signal to betransmitted is analog in nature.In analog communication, the analog message signal modulates some high carrier frequency inside thetransmitter to produce modulated signal. This modulated signal is then transmitted with the help of atransmitting antenna to travel through the transmission channel. At the receiver, this modulated signal isreceived and processed to recover the original message signal. Figure below shows block diagram of analogcommunication system.Presently all the AM, FM radio transmission and T.V. transmission are examples of analog communication.1.4 Classification of telecom system and their basic characteristicsTELEPHONE: A telephone, or phone, is a telecommunications device that permits two or more users toconduct a conversation when they are not in the same locality of each other to be heard directly. A telephoneconverts sound, typically and most efficiently the human voice, into electronic signals suitable fortransmission via cables or other transmission media over long distances, and replays such signalssimultaneously in audible form to its user. First patented in 1876 by Alexander Graham Bell and furtherdeveloped by many others, the telephone was the first device in history that enabled people to talk directlywith each other across large distances.The essential elements of a telephone are a microphone (transmitter) to speak into andan earphone (receiver) which reproduces the voice of the distant person. In addition, most telephonescontain a ringer which produces a sound to announce an incoming telephone call, and a dial used to enter atelephone number when initiating a call to another telephone. Until approximately the 1970s mosttelephones used a rotary dial, which was outdated by the modern Touch-Tone push-button dial, firstintroduced by AT&T in 1963. The receiver and transmitter are usually built into a handset which is held up tothe ear and mouth during conversation. The dial may be located either on the handset, or on a base unit towhich the handset is connected by a cord containing wires.
  4. 4. DEX2201EX, Principle of Communication Engg. Unit 1, prepared by: Er Lochan Raj NeupaneManmohan Memorial Polytechnic Page | 4A landline telephone is connected by a pair of wires to the telephone network, while a mobile phone,such as a cellular phone, is portable and communicates with the telephone network by radio transmissions.A cordless telephone has a portable handset which communicates by radio transmission with the handsetbase station which is connected by wire to the telephone network.The transmitter converts the sound waves to electrical signals which are sent through the telephonenetwork to the receiving phone. The receiving telephone converts the signals into audible sound in thereceiver, or sometimes a loudspeaker. Telephones are a duplex communications medium, meaning theyallow the people on both ends to talk simultaneously. The telephone network, consisting of a worldwide netof telephone lines, fiberoptic cables, microwave transmission, cellular networks, communications satellites,and undersea telephone cables connected by switching centers, allows any telephone in the world tocommunicate with any other. Each telephone line has an identifying number called its telephone number. Toinitiate a telephone call the user enters the other telephones number into a numeric keypad on the phone.Graphic symbols used to designate telephone service or phone-related information in print, signage, andother media include.Although originally designed for simple voice communications, most modern telephones have manyadditional capabilities. They may be able to record spoken messages, send and receive text messages, takeand display photographs or video, play music, and surf the Internet. A current trend is phones that integrateall mobile communication and computing needs; these are called smartphones.TELEX: The telex network is a switched network of teleprinters similar to a telephone network, for thepurposes of sending text-based messages. This network provided the first common medium for internationalrecord communications using standard signaling techniques and operating criteria as specified bythe International Telecommunication Union (ITU).Telex messages are routed by addressing them to a telex address, e.g., "14910 ERIC S", where 14910 isthe subscriber number, ERIC is an abbreviation for the subscribers name and S is the country code. Solutionsalso exist for the automatic routing of messages to different telex terminals within a subscriber organization,by using different terminal identities, e.g., "+T148".A major advantage of telex is that the receipt of the message by the recipient could be confirmed with ahigh degree of certainty by the "answerback". At the beginning of the message, the sender would transmit aWRU (Who aRe yoU) code, and The WRU code would also be sent at the end of the message, so a correctresponse would confirm that the connection had remained unbroken during the message transmission. Thisgave telex a major advantage over less verifiable forms of communications such as telephone and fax.FASCIMILE: Fax (short for facsimile), sometimes called telecopying or telefax, is the telephonictransmission of scanned printed material (both text and images), normally to a telephone number connectedto a printer or other output device. The original document is scanned with a fax machine (or a telecopier),which processes the contents (text or images) as a single fixed graphic image, converting it into a bitmap,and then transmitting it through the telephone system. The receiving fax machine reconverts the codedimage, printing a paper copy. For many decades before digital technology became widespread the scanneddata was transmitted as analog.Although businesses usually maintain some kind of fax capability, the technology has faced increasingcompetition from Internet-based alternatives. Fax machines still retain some advantages, particularly in thetransmission of sensitive material which, if sent over the Internet unencrypted, may be vulnerable tointerception, without the need for telephone tapping. In some countries, because electronic signatures on
  5. 5. DEX2201EX, Principle of Communication Engg. Unit 1, prepared by: Er Lochan Raj NeupaneManmohan Memorial Polytechnic Page | 5contracts are not recognized by law while faxed contracts with copies of signatures are, fax machines enjoycontinuing support in business.The once ubiquitous (found everywhere) fax machine has also begun to disappear from the small office/ home office environment too. Personal computers have also long been able to handle incoming andoutgoing faxes using analogue modems or ISDN, eliminating the need for a stand-alone fax machine. Thesesolutions are often ideally suited for users who only very occasionally need to use fax services.A videophone is a telephone with a video display, capable of simultaneous video and audio forcommunication between people in real-time. Videophone service provided the first form of video telephony,later to be followed by videoconferencing, webcams, and finally high-definition tele presence.At the beginning of its commercial deployment from the 1950s through the 1990s, video telephony alsoincluded image phones which would exchange still images between units every few seconds overconventional type telephone lines, essentially the same as slow scan TV systems. The development ofadvanced technology video codecs and high bandwidth Internet telecommunication services allowedvideophones to provide high quality color service between users almost any place in the world that theInternet is available, often at low or nominal costs.In the present day videophones have become widely available at reasonable cost, although not widelyused in everyday communications for a variety of reasons. However, they are particularly useful tothe deaf and speech-impaired who can use them with sign language, and are becoming increasingly popularfor educational instruction, telemedicine and to those with mobility issues.A pager is a wireless telecommunications device that receives and displays numeric or text messages, orreceives and announces voice messages. One-way pagers can only receive messages, while responsepagers and two-way pagers can also acknowledge, reply to, and originate messages using an internaltransmitter. Pagers operate as part of a paging system which includes one or more fixed transmitters (or inthe case of response pagers and two-way pagers, one or more base stations), as well as a number of pagerscarried by mobile users. These systems can range from a restaurant system with a single low-powertransmitter, to a nationwide system with thousands of high-power base stations.One of the first practical paging services was launched in 1950 for physicians in the New York City area.Physicians paid US$12 per month for the service and carried a 200 gram (6 oz) pager that would receivephone messages within 40 km (25 mi) of a single transmitter tower. In 1960, John Francis Mitchell combinedelements of Motorolas Walkie Talkie and automobile radio technologies to create the first transistorizedpager, and from this point, paging technology continued to advance, and pager adoption continued toexpand, until the early 1990s. However, by the mid-1990s, as cellular technologies became cheaper and morewidely available, advanced services began to displace paging as a commercial product. Today, pagers existlargely as niche products, finding preferential use in applications such as hospitals, publicsafety, and retail locations where their simplicity, high reliability, and low cost represent significantadvantages.A mobile phone (also known as a cellular phone, cell phone, and a hand phone) is a device that can makeand receive telephone calls over radio link while moving around a wide geographic area. It does so byconnecting to a cellular network provided by a mobile phone operator, allowing access to the publictelephone network. By contrast, a cordless telephone is used only within the short range of a single, privatebase station.
  6. 6. DEX2201EX, Principle of Communication Engg. Unit 1, prepared by: Er Lochan Raj NeupaneManmohan Memorial Polytechnic Page | 6In addition to telephony, modern mobile phones also support a wide variety of other services such as textmessaging, MMS, email, Internet access, short-range wireless communications (infrared, Bluetooth),business applications, gaming and photography. Mobile phones that offer these and more general computingcapabilities are referred to as smartphones.The first hand-held mobile phone was demonstrated by John F. Mitchell and Dr. MartinCooper of Motorola in 1973, using a handset weighing around 2.2 pounds (1 kg). In 1983, the DynaTAC8000x was the first to be commercially available. From 1990 to 2011, worldwide mobile phone subscriptionsgrew from 12.4 million to over 6 billion, penetrating about 87% of the global population and reachingthe bottom of the economic pyramidA satellite telephone, satellite phone, or sat phone is a type of mobile phone that connects toorbiting satellites instead of terrestrial cell sites. They provide similar functionality to terrestrial mobiletelephones; voice, short messaging service and low-bandwidth internet access are supported through mostsystems. Depending on the architecture of a particular system, coverage may include the entire Earth, oronly specific regions.The mobile equipment, also known as a terminal, varies widely. Early satellite phone handsets had a sizeand weight comparable to that of a late-1980s or early-1990s mobile phone, but usually with a largeretractable antenna. More recent satellite phones are similar in size to a regular mobile phone whilesome prototype satellite phones have no distinguishable difference from an ordinary smartphone. Satphones are popular on expeditions into remote areas where terrestrial cellular service is unavailable.1.5 Electromagnetic Spectrum and its various rangesThe electromagnetic spectrum is the range of all possible frequencies of electromagnetic radiation. Radiospectrum refers to the part of the electromagnetic spectrum corresponding to radio frequencies – that is,frequencies lower than around 300 GHz (or, equivalently, wavelengths longer than about 1 mm).
  7. 7. DEX2201EX, Principle of Communication Engg. Unit 1, prepared by: Er Lochan Raj NeupaneManmohan Memorial Polytechnic Page | 7Different parts of the radio spectrum are used for different radio transmission technologies andapplications. Radio spectrum is typically government regulated in developed countries and, in some cases, issold or licensed to operators of private radio transmission systems (for example, cellular telephone operatorsor broadcast television stations). Ranges of allocated frequencies are often referred to by their provisioneduse (for example, cellular spectrum or television spectrum)1.6 Communication channelsIn telecommunications, a communication channel, or channel, refers either to a physical transmissionmedium such as a wire, or to a logical connection over a multiplexed medium such as a radio channel. Achannel is used to convey an information signal, for example a digital bit stream, from one orseveral senders (or transmitters) to one or several receivers. A channel has a certain capacity for transmittinginformation, often measured by its bandwidth in Hz or its data rate in bits per second.Communicating data from one location to another requires some form of pathway or medium. Thesepathways, called communication channels, use two types of media: cable (twisted-pair wire, cable, and fiber-optic cable) and broadcast (microwave, satellite, radio, and infrared). Cable or wire line media uses physicalwires of cables to transmit data and information. Twisted-pair wire and coaxial cables are made of copper,and fiber-optic cable is made of glass.Communication channel may be divided asi) Wired channel: it always has some pair of cable for ex. Co-axial cable, optical fibre, twistedpair cable etc. communication through wire channel is called line communicationName of Band Symbols Frequency Range Wavelength Range ITU band numberExtremely low ELF 3 to 30 Hz 10000 km to 100000 km 1Super low SLF 30 to 300 Hz 1000 km to 10000 km 2Ultra low ULF 300 to 3000 Hz 100 to 1000 km 3Very low VLF 3 to 30 kHz 10 to 100 km 4Low LF 30 to 300 kHz 1 to 10 km 5Medium MF 300 to 3000 kHz 100 to 1000 m 6High HF 3 to 30 MHz 10 to 100 m 7Very high VHF 30 to 300 MHz 1 to 10 m 8Ultra high UHF 300 to 3000 KHz 10 to 100 cm 9Super high SHF 3 to 30 GHz 1 to 10 cm 10Extremely high EHF 30 to 300 GHz 1 to 10 mm 11Tremendously high THF 300 to 3000 GHz 0.1 to 1 mm 12
  8. 8. DEX2201EX, Principle of Communication Engg. Unit 1, prepared by: Er Lochan Raj NeupaneManmohan Memorial Polytechnic Page | 8ii) Wireless channel: in wireless communication message signal is transmitted through openspace by electromagnetic wave also called radio wave. These waves are radiated fromtransmitter in open space through a device called antenna.1.7 Noise in communicationNoise is random, undesirable electrical energy that enters the communications system via thecommunicating medium and interferes with the transmitted message. However, some noise is also produced inthe receiver.Classification of Noise Noise may be put into following two categories.1 external 2. Internal noise1) External noises, i.e. noise whose sources are external. External noise cannot be reduced except bychanging the location of the receiver or the entire system. Internal noise on the other hand can be easilyevaluated mathematically and can be reduced to a great extent by proper design. As already said, because ofthe fact that internal noise can be reduced to a great extent, study of noise characteristics is a very importantpart of the communication engineering.External noise may be classified into the following three types:i) Atmospheric noisesAtmospheric noise or static is caused by lighting discharges in thunderstorms and other natural electricaldisturbances occurring in the atmosphere. These electrical impulses are random in nature. Hence the energy isspread over the complete frequency spectrum used for radio communication.Atmospheric noise accordingly consists of spurious radio signals with components spread over a widefrequency range. These spurious radio waves constituting the noise get propagated over the earth in the samefashion as the desired radio waves of the same frequency. Accordingly at a given receiving point, the receivingantenna picks up not only the signal but also the static from all the thunderstorms, local or remote. The fieldstrength of atmospheric noise varies approximately inversely with the frequency. Thus large atmospheric noiseis generated in low and medium frequency (broadcast) bands while very little noise is generated in the VHF andUHF bands. Further VHF and UHF components of noise are limited to the line-of-sight (less than about 80 Km)propagation. For these two-reasons, the atmospheric noise becomes less severe at Frequencies exceeding about30MHz.ii) Extraterrestrial noisesThere are numerous types of extraterrestrial noise or space noises depending on their sources. However,these may be put into following two subgroups.Solar NoiseThis is the electrical noise emanating from the sun. Under quite conditions, there is a steady radiation ofnoise from the sun. This results because sun is a large body at a very high temperature (exceeding 6000°C onthe surface), and radiates electrical energy in the form of noise over a very wide frequency spectrum includingthe spectrum used for radio communication. The intensity produced by the sun varies with time. In fact, the sunhas a repeating 11-Year noise cycle. During the peak of the cycle, the sun produces some amount of noise that
  9. 9. DEX2201EX, Principle of Communication Engg. Unit 1, prepared by: Er Lochan Raj NeupaneManmohan Memorial Polytechnic Page | 9causes tremendous radio signal interference, making many frequencies unusable for communications. Duringother years. The noise is at a minimum level.Cosmic noiseDistant stars are also suns and have high temperatures. These stars, therefore, radiate noise in the sameway as our sun. The noise received from these distant stars is thermal noise (or black body noise) and isdistributing almost uniformly over the entire sky. We also receive noise from the center of our own galaxy (TheMilky Way) from other distant galaxies and from other virtual point sources such as quasars and pulsars.iii) Man-made noises or industrial noises.By man-made noise or industrial- noise is meant the electrical noise produced by such sources asautomobiles and aircraft ignition, electrical motors and switch gears, leakage from high voltage lines,fluorescent lights, and numerous other heavy electrical machines. Such noises are produced by the arc dischargetaking place during operation of these machines. Such man-made noise is most intensive in industrial anddensely populated areas. Man-made noise in such areas far exceeds all other sources of noise in the frequencyrange extending from about 1 MHz to 600 MHz2) Internal noise in communication, i.e. noises which get, generated within the receiver or communicationsystem. Internal noise may be put into the following categories.i) Thermal noise or white noise or Johnson noiseConductors contain a large number of free" electrons and "ions" strongly bound by molecular forces. Theions vibrate randomly about their normal (average) positions, however, this vibration being a function of thetemperature. Continuous collisions between the electrons and the vibrating ions take place. Thus there is acontinuous transfer of energy between the ions and electrons. This is the source of resistance in a conductor.The movement of free electrons constitutes a current which is purely random in nature and over a long timeaverages zero. There is a random motion of the electrons which give rise to noise voltage called thermal noise.Thus noise generated in any resistance due to random motion of electrons is called thermal noise or whiteor Johnson noise.Thus noise power generated by resistor isPn ∝ TBPn = KTBWhere Pn = Maximum noise power output of a resistor.K = Boltzmann’s constant = 1.38 x10-23 joules I Kelvin.T = Absolute temperature.B = Bandwidth over which noise is measured.ii) Shot noise.The most common type of noise is referred to as shot noise which is produced by the random arrival ofelectrons or holes at the output element, at the plate in a tube, or at the collector or drain in a transistor. Shotnoise is also produced by the random movement of electrons or holes across a PN junction. Even through currentflow is established by external bias voltages, there will still be some random movement of electrons or holes dueto discontinuities in the device. An example of such a discontinuity is the contact between the copper lead and
  10. 10. DEX2201EX, Principle of Communication Engg. Unit 1, prepared by: Er Lochan Raj NeupaneManmohan Memorial Polytechnic Page | 10the semiconductor materials. The interface between the two creates a discontinuity that causes randommovement of the current carriers. .iii) Flicker noise.Flicker noise or modulation noise is the one appearing in transistors operating at low audio frequencies.Flicker noise is proportional to the emitter current and junction temperature. However, this noise is inverselyproportional to the frequency. Hence it may be neglected at frequencies above about 500 Hz and it, Therefore,possess no serious problem.1.8 Fundamental limitations of communication systemNoise limitation:Bandwidth limitation:Equipment limitation:Noise limitation: we know that noise is unwanted form of energy which tends to interfere withtransmission and reception of desire signal in communication system. They cannot be eliminated completelyhowever the effect of noise can be minimized with the help of several technique in fact the noise limits ourability to identify the intended or desire message correctly and it limits the information transmission. In largesignal variation with the effect of noise is small and can be ignored. However in long distance communicationsignal may be as small as noise. Thus in such a case the presence of noise severally limits the capability ofcommunication system.Bandwidth limitation: it is another major constraint in a communication system. The frequency range orband of frequency needed for a particular given transmission is known as bandwidth. This band of frequencyis always allocated by some international agency. But for given transmission this allocated bandwidth maynot be sufficient to convey the entire information. Thus greater is the transmission bandwidth of the systemmore is the information transmitted.Equipment limitation: the equipment limitation is another major constraint in communication system.The noise and bandwidth limitation dictate theoretically what can or cannot be achieved in terms ofperformance in communication system. This theoretical limit may not be realized in practical system due toequipment limitation. Example: they might require band pass filter with quality factor with 100 at centerfrequency of 1 kHz, such a filter cannot be realized in practice.