Principles of communication engineering

DEX2201EX, Principle of Communication Engg. Unit 1, prepared by: Er Lochan Raj Neupane

Manmohan Memorial Polytechnic Page | 1

Principles of communication engineering

Unit 1 INTRODUCTION

1.1 history of communication

Communication is the process of establishing connection or link between two points for information

exchange.

The history of telecommunication - the transmission of signals over a distance for the purpose

of 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 means

of visual signals) systems emerged in Europe however it was not until the 1830s that electrical

telecommunication systems started to appear.

Basic electrical signals:

1838: Electrical telegraph 1858: First trans-Atlantic telegraph cable 1876: Telephone 1880: Telephony via lightbeam photophones

Advanced 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: World's 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



1.2 Modulation, need for modulation and demodulation in communication system

Modulation is the process of varying one or more properties of a periodic waveform, called the carrier

signal, with a modulating signal which typically contains information to be transmitted. In other words

Modulation may be defined as the process by which some characteristics of a signal called carrier is varied

in accordance with the instantaneous value of another signal called modulating signal. Characteristics that can

be varied are amplitude or frequency or phase.

The information containing signal are referred as modulating signals. The signal resulting from the process

of modulation is called modulated signal.

Need for modulation or Benefits of Modulation

Modulation is needed in a communication system to achieve the following basic needs

1) Multiplexing Simultaneous transmission of multiple signal over the common channel is possible only with modulation process.

2) Practicability of antennas For efficient radiation and reception of signal, antenna should have length comparable to quarter-wavelength of the frequency used.

For example if 5 kHz signal were to be transmitted without modulation then length of antenna would be l= λ/4 = c/4f =(3x108)/ (4x5x103) =15km Obviously, it will be impracticable to construct and install such antenna. However length of antenna can be reduced using modulating technique. If audio frequency is translated to a radio frequency carrier of frequency 5MHz, length of antenna would be

l= λ/4 = c/4f =(3x108)/ (4x5x106) =15 meters The antenna height may be achieved practically.



3) 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 different

portion of electromagnetic spectrum using modulation.

4) Reduce noise: Noise is major limitation in communication. Effect of noise can be minimized using several

modulation 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 System

Analog communication is that type of communication in which the message or information signal to be transmitted is analog in nature. In analog communication, the analog message signal modulates some high carrier frequency inside the transmitter 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. Figure below shows block diagram of analog communication 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 characteristics

TELEPHONE: A telephone, or phone, is a telecommunications device that permits two or more users to conduct a conversation when they are not in the same locality of each other to be heard directly. A telephone converts sound, typically and most efficiently the human voice, into electronic signals suitable for transmission via cables or other transmission media over long distances, and replays such signals simultaneously in audible form to its user. First patented in 1876 by Alexander Graham Bell and further developed by many others, the telephone was the first device in history that enabled people to talk directly with each other across large distances.

The essential elements of a telephone are a microphone (transmitter) to speak into and an earphone (receiver) which reproduces the voice of the distant person. In addition, most telephones contain a ringer which produces a sound to announce an incoming telephone call, and a dial used to enter a telephone number when initiating a call to another telephone. Until approximately the 1970s most telephones used a rotary dial, which was outdated by the modern Touch-Tone push-button dial, first introduced by AT&T in 1963. The receiver and transmitter are usually built into a handset which is held up to the ear and mouth during conversation. The dial may be located either on the handset, or on a base unit to which the handset is connected by a cord containing wires.



A 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 handset base station which is connected by wire to the telephone network.

The transmitter converts the sound waves to electrical signals which are sent through the telephone network to the receiving phone. The receiving telephone converts the signals into audible sound in the receiver, or sometimes a loudspeaker. Telephones are a duplex communications medium, meaning they allow the people on both ends to talk simultaneously. The telephone network, consisting of a worldwide net of telephone lines, fiberoptic cables, microwave transmission, cellular networks, communications satellites, and undersea telephone cables connected by switching centers, allows any telephone in the world to communicate with any other. Each telephone line has an identifying number called its telephone number. To initiate a telephone call the user enters the other telephone's number into a numeric keypad on the phone. Graphic symbols used to designate telephone service or phone-related information in print, signage, and other media include.

Although originally designed for simple voice communications, most modern telephones have many additional capabilities. They may be able to record spoken messages, send and receive text messages, take and display photographs or video, play music, and surf the Internet. A current trend is phones that integrate all 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 the purposes of sending text-based messages. This network provided the first common medium for international record communications using standard signaling techniques and operating criteria as specified by the International Telecommunication Union (ITU).

Telex messages are routed by addressing them to a telex address, e.g., "14910 ERIC S", where 14910 is the subscriber number, ERIC is an abbreviation for the subscriber's name and S is the country code. Solutions also 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 a high degree of certainty by the "answerback". At the beginning of the message, the sender would transmit a WRU (Who aRe yoU) code, and The WRU code would also be sent at the end of the message, so a correct response would confirm that the connection had remained unbroken during the message transmission. This gave 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 telephonic transmission of scanned printed material (both text and images), normally to a telephone number connected to 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 coded image, printing a paper copy. For many decades before digital technology became widespread the scanned data was transmitted as analog.

Although businesses usually maintain some kind of fax capability, the technology has faced increasing competition from Internet-based alternatives. Fax machines still retain some advantages, particularly in the transmission of sensitive material which, if sent over the Internet unencrypted, may be vulnerable to interception, without the need for telephone tapping. In some countries, because electronic signatures on



contracts are not recognized by law while faxed contracts with copies of signatures are, fax machines enjoy continuing 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 and outgoing faxes using analogue modems or ISDN, eliminating the need for a stand-alone fax machine. These solutions 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 for communication 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 also included 'image phones' which would exchange still images between units every few seconds over conventional type telephone lines, essentially the same as slow scan TV systems. The development of advanced technology video codecs and high bandwidth Internet telecommunication services allowed videophones to provide high quality color service between users almost any place in the world that the Internet is available, often at low or nominal costs.

In the present day videophones have become widely available at reasonable cost, although not widely used in everyday communications for a variety of reasons. However, they are particularly useful to the deaf and speech-impaired who can use them with sign language, and are becoming increasingly popular for educational instruction, telemedicine and to those with mobility issues.

A pager is a wireless telecommunications device that receives and displays numeric or text messages, or receives and announces voice messages. One-way pagers can only receive messages, while response pagers and two-way pagers can also acknowledge, reply to, and originate messages using an internal transmitter. Pagers operate as part of a paging system which includes one or more fixed transmitters (or in the case of response pagers and two-way pagers, one or more base stations), as well as a number of pagers carried by mobile users. These systems can range from a restaurant system with a single low-power transmitter, 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 receive phone messages within 40 km (25 mi) of a single transmitter tower. In 1960, John Francis Mitchell combined elements of Motorola's Walkie Talkie and automobile radio technologies to create the first transistorized pager, and from this point, paging technology continued to advance, and pager adoption continued to expand, until the early 1990s. However, by the mid-1990s, as cellular technologies became cheaper and more widely available, advanced services began to displace paging as a commercial product. Today, pagers exist largely as niche products, finding preferential use in applications such as hospitals, public safety, and retail locations where their simplicity, high reliability, and low cost represent significant advantages.

A mobile phone (also known as a cellular phone, cell phone, and a hand phone) is a device that can make and receive telephone calls over radio link while moving around a wide geographic area. It does so by connecting to a cellular network provided by a mobile phone operator, allowing access to the public telephone network. By contrast, a cordless telephone is used only within the short range of a single, private base station.



In addition to telephony, modern mobile phones also support a wide variety of other services such as text messaging, MMS, email, Internet access, short-range wireless communications (infrared, Bluetooth), business applications, gaming and photography. Mobile phones that offer these and more general computing capabilities are referred to as smartphones.

The first hand-held mobile phone was demonstrated by John F. Mitchell and Dr. Martin Cooper of Motorola in 1973, using a handset weighing around 2.2 pounds (1 kg). In 1983, the DynaTAC 8000x was the first to be commercially available. From 1990 to 2011, worldwide mobile phone subscriptions grew from 12.4 million to over 6 billion, penetrating about 87% of the global population and reaching the bottom of the economic pyramid

A satellite telephone, satellite phone, or sat phone is a type of mobile phone that connects to orbiting satellites instead of terrestrial cell sites. They provide similar functionality to terrestrial mobile telephones; voice, short messaging service and low-bandwidth internet access are supported through most systems. Depending on the architecture of a particular system, coverage may include the entire Earth, or only specific regions.

The mobile equipment, also known as a terminal, varies widely. Early satellite phone handsets had a size and weight comparable to that of a late-1980s or early-1990s mobile phone, but usually with a large retractable antenna. More recent satellite phones are similar in size to a regular mobile phone while some prototype satellite phones have no distinguishable difference from an ordinary smartphone. Sat phones are popular on expeditions into remote areas where terrestrial cellular service is unavailable.

1.5 Electromagnetic Spectrum and its various ranges

The electromagnetic spectrum is the range of all possible frequencies of electromagnetic radiation. Radio spectrum 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).



Different parts of the radio spectrum are used for different radio transmission technologies and applications. Radio spectrum is typically government regulated in developed countries and, in some cases, is sold or licensed to operators of private radio transmission systems (for example, cellular telephone operators or broadcast television stations). Ranges of allocated frequencies are often referred to by their provisioned use (for example, cellular spectrum or television spectrum)

1.6 Communication channels

In telecommunications, a communication channel, or channel, refers either to a physical transmission

medium such as a wire, or to a logical connection over a multiplexed medium such as a radio channel. A

channel is used to convey an information signal, for example a digital bit stream, from one or

several senders (or transmitters) to one or several receivers. A channel has a certain capacity for transmitting

information, 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. These

pathways, 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 physical

wires 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 as

i) Wired channel: it always has some pair of cable for ex. Co-axial cable, optical fibre, twisted

pair cable etc. communication through wire channel is called line communication

Name of Band Symbols Frequency Range Wavelength Range ITU band number

Extremely low ELF 3 to 30 Hz 10000 km to 100000 km 1

Super low SLF 30 to 300 Hz 1000 km to 10000 km 2

Ultra low ULF 300 to 3000 Hz 100 to 1000 km 3

Very low VLF 3 to 30 kHz 10 to 100 km 4

Low LF 30 to 300 kHz 1 to 10 km 5

Medium MF 300 to 3000 kHz 100 to 1000 m 6

High HF 3 to 30 MHz 10 to 100 m 7

Very high VHF 30 to 300 MHz 1 to 10 m 8

Ultra high UHF 300 to 3000 KHz 10 to 100 cm 9

Super high SHF 3 to 30 GHz 1 to 10 cm 10

Extremely high EHF 30 to 300 GHz 1 to 10 mm 11

Tremendously high THF 300 to 3000 GHz 0.1 to 1 mm 12



ii) Wireless channel: in wireless communication message signal is transmitted through open

space by electromagnetic wave also called radio wave. These waves are radiated from

transmitter in open space through a device called antenna.

1.7 Noise in communication

Noise is random, undesirable electrical energy that enters the communications system via the communicating medium and interferes with the transmitted message. However, some noise is also produced in the receiver.

Classification of Noise Noise may be put into following two categories.1 external 2. Internal noise

1) External noises, i.e. noise whose sources are external. External noise cannot be reduced except by changing the location of the receiver or the entire system. Internal noise on the other hand can be easily evaluated mathematically and can be reduced to a great extent by proper design. As already said, because of the fact that internal noise can be reduced to a great extent, study of noise characteristics is a very important part of the communication engineering.

External noise may be classified into the following three types:

i) Atmospheric noises

Atmospheric noise or static is caused by lighting discharges in thunderstorms and other natural electrical disturbances occurring in the atmosphere. These electrical impulses are random in nature. Hence the energy is spread over the complete frequency spectrum used for radio communication.

Atmospheric noise accordingly consists of spurious radio signals with components spread over a wide frequency range. These spurious radio waves constituting the noise get propagated over the earth in the same fashion as the desired radio waves of the same frequency. Accordingly at a given receiving point, the receiving antenna picks up not only the signal but also the static from all the thunderstorms, local or remote. The field strength of atmospheric noise varies approximately inversely with the frequency. Thus large atmospheric noise is generated in low and medium frequency (broadcast) bands while very little noise is generated in the VHF and UHF 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 about 30MHz.

ii) Extraterrestrial noises

There are numerous types of extraterrestrial noise or space noises depending on their sources. However, these may be put into following two subgroups.

Solar Noise

This is the electrical noise emanating from the sun. Under quite conditions, there is a steady radiation of noise from the sun. This results because sun is a large body at a very high temperature (exceeding 6000°C on the surface), and radiates electrical energy in the form of noise over a very wide frequency spectrum including the spectrum used for radio communication. The intensity produced by the sun varies with time. In fact, the sun has a repeating 11-Year noise cycle. During the peak of the cycle, the sun produces some amount of noise that



causes tremendous radio signal interference, making many frequencies unusable for communications. During other years. The noise is at a minimum level.

Cosmic noise

Distant stars are also suns and have high temperatures. These stars, therefore, radiate noise in the same way as our sun. The noise received from these distant stars is thermal noise (or black body noise) and is distributing almost uniformly over the entire sky. We also receive noise from the center of our own galaxy (The Milky 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 as automobiles 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 discharge taking place during operation of these machines. Such man-made noise is most intensive in industrial and densely populated areas. Man-made noise in such areas far exceeds all other sources of noise in the frequency range extending from about 1 MHz to 600 MHz

2) Internal noise in communication, i.e. noises which get, generated within the receiver or communication system. Internal noise may be put into the following categories.

i) Thermal noise or white noise or Johnson noise

Conductors contain a large number of 'free" electrons and "ions" strongly bound by molecular forces. The ions vibrate randomly about their normal (average) positions, however, this vibration being a function of the temperature. Continuous collisions between the electrons and the vibrating ions take place. Thus there is a continuous 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 time averages 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 white or Johnson noise.

Thus noise power generated by resistor is

Pn ∝ TB

Pn = KTB

Where 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 of

'electrons or holes at the output element, at the plate in a tube, or at the collector or drain in a transistor. Shot noise is also produced by the random movement of electrons or holes across a PN junction. Even through current flow is established by external bias voltages, there will still be some random movement of electrons or holes due to discontinuities in the device. An example of such a discontinuity is the contact between the copper lead and



the semiconductor materials. The interface between the two creates a discontinuity that causes random movement 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 inversely proportional 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 system Noise limitation: Bandwidth limitation: Equipment limitation: Noise limitation: we know that noise is unwanted form of energy which tends to interfere with

transmission and reception of desire signal in communication system. They cannot be eliminated completely however the effect of noise can be minimized with the help of several technique in fact the noise limits our ability to identify the intended or desire message correctly and it limits the information transmission. In large signal variation with the effect of noise is small and can be ignored. However in long distance communication signal may be as small as noise. Thus in such a case the presence of noise severally limits the capability of communication system.

Bandwidth limitation: it is another major constraint in a communication system. The frequency range or

band of frequency needed for a particular given transmission is known as bandwidth. This band of frequency is always allocated by some international agency. But for given transmission this allocated bandwidth may not be sufficient to convey the entire information. Thus greater is the transmission bandwidth of the system more 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 of performance in communication system. This theoretical limit may not be realized in practical system due to equipment limitation. Example: they might require band pass filter with quality factor with 100 at center frequency of 1 kHz, such a filter cannot be realized in practice.

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