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WEEK-I

ELECTROMAGNETIC WAVES COMMUNICATION SYSTEMS

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  • Electromagnetic waves
  • How are Electromagnetic waves formed
  • Generation of Electromagnetic Waves
  • Electromagnetic Spectrum
  • Applications of Electromagnetic Waves
  • Uses of Electromagnetic Waves
  • Communication systems
  • Basic Elements of communication system
  • Assignment

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ELECTROMAGNETIC WAVES

  • Electromagnetic radiations are composed of electromagnetic waves that are produced when an electric field comes in contact with the magnetic field.
  • It can also be said that electromagnetic waves are the composition of oscillating electric and magnetic fields.

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GENERATION OF ELECTROMAGNETIC WAVES

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  • Electromagnetic waves are a form of radiation that travel though the universe. They are formed when an electric field couples with a magnetic field.
  • Both electricity and magnetism can be static (respectively, what holds a balloon to the wall or a refrigerator magnet to metal), but when they change or move together, they make waves.
  • Magnetic and electric fields of an electromagnetic wave are perpendicular to each other and to the direction of the wave.

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APPLICATIONS OF ELECTROMAGNETIC WAVES

  • Electromagnetic radiations can transmit energy in vacuum or using no medium at all
  • Electromagnetic waves play an important role in communication technology
  • Electromagnetic waves are used in RADARS
  • UV rays are used to detect forged bank notes. Real bank notes don’t turn fluorescent under the UV light
  • Infrared radiation is used for night vision and is used in security camera

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USES OF ELECTROMAGNETIC WAVES

Different parts of the EM spectrum have different uses:

  • Radio waves - radio and television
  • Microwaves - satellite communications and cooking food
  • Infrared - Electrical heaters, cooking food and infrared cameras
  • Visible light - Fibre optic communications
  • Ultraviolet - Energy efficient lamps, sun tanning
  • X-rays - Medical imaging and treatments
  • Gamma rays - Medical imaging and treatments

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ELECTROMAGNETIC SPECTRUM

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The electromagnetic spectrum covers electromagnetic waves with frequencies ranging from below one hertz to above 1025 hertz, corresponding to wavelengths from thousands of kilometers down to a fraction of the size of an atomic nucleus.

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ELECTROMAGNETIC SPECTRUM

Computer Science & Engineering – 20CS33P

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Wavelength (m)

Frequency (Hz)

Energy (J)

Radio

> 1 x 10-1

< 3 x 109

< 2 x 10-24

Microwave

1 x 10-3 - 1 x 10-1

3 x 109 - 3 x 1011

2 x 10-24- 2 x 10-22

Infrared

7 x 10-7 - 1 x 10-3

3 x 1011 - 4 x 1014

2 x 10-22 - 3 x 10-19

Optical

4 x 10-7 - 7 x 10-7

4 x 1014 - 7.5 x 1014

3 x 10-19 - 5 x 10-19

UV

1 x 10-8 - 4 x 10-7

7.5 x 1014 - 3 x 1016

5 x 10-19 - 2 x 10-17

X-ray

1 x 10-11 - 1 x 10-8

3 x 1016 - 3 x 1019

2 x 10-17 - 2 x 10-14

Gamma-ray

< 1 x 10-11

> 3 x 1019

> 2 x 10-14

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COMMUNICATION SYSTEMS

Basic Elements of communication system

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The block diagram of a communication system will have five blocks, including the information source, transmitter, channel, receiver and destination blocks.

  1. Source
  2. The objective of any communication system is to convey information from one point to the other.
  3. The information comes from the source, which originates it. Information is a very generic word signifying at the abstract level anything intended for communication, which may include some thoughts, news, feeling, visual scene, and so on.
  4. The source converts this information into physical quantity. The physical manifestation of the information is termed as message signal.

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2.Transmitter

  • The objective of the transmitter block is to collect the incoming message signal and modify it in a suitable fashion, such that, it can be transmitted via the chosen channel to the receiving point.
  • Channel is a physical medium which connects the transmitter block with the receiver block. The functionality of the transmitter block is mainly decided by the type or nature of the channel chosen for communication.

3.Channel

  • Channel is the physical medium which connects the transmitter with that of the receiver.
  • The physical medium includes copper wire, coaxial cable, fibre optic cable, wave guide and free space or atmosphere.
  • The choice of a particular channel depends on the feasibility and also the purpose of the communication system.

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4.Receiver

  • The receiver block receives the incoming modified version of the message signal from the channel and processes it to recreate the original (non-electrical) form of the message signal.
  • There are a great variety of receivers in communication systems, depending on the processing required to recreate the original message signal and also final presentation of the message to the destination.

5.Destination

  • The destination is the final block in the communication system which receives the message signal and processes it to comprehend the information present in it.
  • Usually, humans will be the destination block.

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Commonly used terms in Electronic Communication Systems

  • Information: Message or information is the entity that is to be transmitted. It is often within the sort of audio, video, temperature, picture, pressure, etc.
  • Signal: The single-valued function of time that carries the information. For transmission, the information is converted into an electrical form.
  • Transducer: A device or an arrangement that converts one form of energy to the other. An electrical transducer converts physical variables like pressure, force, temperature into corresponding electrical signal variations. Example: Microphone, Photodetector.

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  • Amplifier: The electronic circuit or device that increases the amplitude or the strength of the transmitted signal is named an amplifier. When the signal strength becomes but the specified value, amplification is often done anywhere in between transmitter and receiver. The amplification is provided by a  DC power source.
  • Modulator: As the original message signal can't be transmitted over an outsized distance due to their low frequency and amplitude, they're superimposed with high frequency and amplitude waves called carrier waves. This phenomenon of superimposing a message signal with a carrier wave is called modulation. And the resultant wave is a modulated wave which is to be transmitted.

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Different Types Of Modulation.

i. Amplitude Modulation (AM)

ii. Frequency Modulation (FM)

iii. Phase Modulation (PM)

  • Transmitter: It is the arrangement that processes the message signal into an appropriate form for transmission and subsequently reception.
  • Antenna: An Antenna is a structure or a device that radiates and receives electromagnetic waves. So, they are used in both transmitters and receivers.

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  • Channel: It refers to a physical medium such as wire, cables, space through which the signal is passed from transmitter to the receiver.

  • Noise: Noise is one of the channel imperfections or impairment in the received signal at the destination. External sources include interference, interference generated by natural sources like solar, lightning, or radiation, from automobile generated radiation. The external noise is often minimized and eliminated by the appropriate design of the channel, shielding of cables. Digital transmission external noise is often minimized.

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  • Attenuation: Attenuation is a problem caused by the medium. When the signal is propagating for an extended distance through a medium, counting on the length of the medium the initial power decreases. The loss in initial power is directly proportional to the length of the medium. Using amplifiers, the signal power is strengthened or amplified so on reducing attenuation. Digital signals are comparatively less susceptible to attenuation than analog signals.
  • Distortion: It is also another type of channel problem. The signal may have frequency and bandwidth different from the transmitted signal when the signal is distorted. The variation in the signal frequency can be linear or nonlinear.

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  • Receiver: The message or information from the transmitted signal at the output end of the channel is extracted by an arrangement that is called a receiver and as the original message signal is a receiver it is reproduced in a suitable form.
  • Demodulator: It is the inverse phenomenon of modulation. The process of separation of message signals from the carrier takes place within the demodulator. The information is retrieved from the modulated wave.
  • Repeaters: Repeaters are placed at different locations in between the transmitter and receiver. A repeater receives the transmitted signal, amplifies it, and sends it to the next repeater without distorting the original signal.

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Communication Systems

  • Communicate means sharing of information.
  • This sharing can be local or remote.
  • The term telecommunication means communication at a distance.(telephony, telegraphy, television)
  • The word data refers to information presented in whatever form is agreed upon by the parties creating and using the data.
  • Data communications are the exchange of data between two devices via some form of transmission medium such as a wire cable.
  • Communicating devices must be part of a communication system made up of a combination of hardware and software.

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Elements of Communication Systems

  • A Data Communication system has 5 components.

Fig: Data Communication System

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Data Communication System characteristics

  • Delivery: The system must deliver data to the correct destination. Data must be received by the intended device or user and only by that device or user.
  • Accuracy: The system must deliver the data accurately. Data that have been altered in transmission and left uncorrected are unusable.
  • Timeliness: The system must deliver data in a timely manner. Data delivered late are useless. In the case of video and audio, timely delivery.
  • Jitter: The variation in the packet arrival time.

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Data Representation

  • text

text symbols are represented with a sequence of bits 0 or 1. Each sequence is called a code, and the process is called coding.

  • numbers

numbers are also represented with a sequence of 0 and 1. ASCII is not used for number representation.

  • Images

  • audio
  • video

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DATA FLOW

  •  Communication between two devices – Sender and Receiver over a medium, can be –
    • simplex
    • half-duplex
    • full-duplex

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  • Simplex mode - communication is unidirectional, as on a one-way street. Only one of the two devices on a link can transmit; the other can only receive.
  • Keyboards and traditional monitors are examples of simplex devices. The keyboard can only introduce input; the monitor can only accept output.
  • The simplex mode can use the entire capacity of the channel to send data in one direction.

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  •  Half-Duplex mode - each station can both transmit and receive, but not at the same time.
  • When one device is sending, the other can only receive, and vice versa.
  • The half-duplex mode is like a one-lane road with traffic allowed in both directions.
  • When cars are traveling in one direction, cars going the other way must wait. In a half-duplex transmission, the entire capacity of a channel is taken over by one of the two devices is transmitting at the time.
  • For example, Walkie-talkies and citizens band radios (CB) are both half-duplex systems.
  • Half-duplex mode is used where there is no need for communication in both directions at the same time; the entire capacity of the channel can be utilized for each direction.

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  • Full-Duplex mode or duplex mode - both stations can transmit and, receive simultaneously.
  • Here, signals going in one direction share the capacity of the link:with signals going in the other direction.
  • This sharing can occur in two ways: Either the link must contain two physically separate transmission paths, one for sending and the other for receiving; or the capacity of the channel is divided between signals traveling in both directions.
  • For example - telephone network. When two people are communicating by a telephone line, both can talk and listen at the same time.
  • This mode is used when communication in both directions is required all the time.
  • The capacity of the channel must be divided between the two directions.

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ANALOG AND DIGITAL SIGNALS

  • Data can be analog or digital.
  • The term analog data refers to information that is continuous
  • The Digital data refers to information that has discrete states.
  • Analog data take on continuous values.
  • Digital data take on discrete values.
  • Analog signals can have an infinite number of values in a range.
  • Digital signals can have only a limited �number of values.

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Data can be Analog or Digital.

  1. Analog data refers to information that is continuous; ex. sounds made by a human voice
  2. Digital data refers to information that has discrete states.
  3. Digital data take on discrete values.
  4. For example, data are stored in computer memory in the form of Os and 1s.
  5. Signals can be of two types:

1. Analog Signal: They have infinite values in a range.

2. Digital Signal: They have limited number of defined values

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Fig: Comparison of analog and digital signals

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PERIODIC ANALOG SIGNALS

  • Signals which repeat itself after a fixed time period are called Periodic Signals.
  • Signals which do not repeat itself after a fixed time period are called Non-Periodic Signals. Periodic analog signals can be classified as simple or composite.
  • A simple periodic analog signal, a sine wave, cannot be decomposed into simpler signals.
  • A composite periodic analog signal is composed of multiple sine waves.

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Sine Wave

  • Signals means the value of any quantity taken over a period of time.
  • Signals are time varying in nature.
  • A graph can be plotted between values at different time instants which can be called as graphical representation of signal.

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A sine wave is characterized by three parameters:

  1. Peak Amplitude
  2. Frequency
  3. Phase

Peak Amplitude

  • The amplitude of a signal is the absolute value of its intensity at time t.
  • The peak amplitude of a signal is the absolute value of the highest intensity.
  • The amplitude of a signal is proportional to the energy carried by the signal.

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Frequency

  • Frequency refers to the number of cycles completed by the wave in one second.
  • Period refers to the time taken by the wave to complete one second.

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Phase

  • Phase describes the position of the waveform with respect to time (specifically relative to time O).

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DIGITAL SIGNALS

  • In addition to being represented by an analog signal, information can also be represented by a digital signal.
  • For example, a 1 can be encoded as a positive voltage and a 0 as zero voltage.
  • A digital signal can have more than two levels. In this case, we can send more than 1 bit for each level.

Bit Rate

The bit rate is the number of bits sent in 1s, expressed in bits per second (bps)..

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Bit Length

The bit length is the distance one bit occupies on the transmission medium.

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Modulation

  • Modulation is the process of converting data into radio waves by adding information to an electronic or optical carrier signal.
  • A carrier signal is one with a steady waveform -- constant height, or amplitude, and frequency.
  • The process of modulation changes the characteristics of the wave to be transmitted by super imposing the message signal on the high-frequency signal.
  • In this process, video,voice and other data signals modify high-frequency signals- also known as the carrier wave.
  • This carrier wave can be DC or AC depending on the application used.

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  • Modulation techniques are classified into 2 types:
  • Analog Modulation
  • Digital Modulation

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Frequency Modulation: Frequency modulation occurs when a signal's amplitude and frequency have a constant state, but the carrier wave frequency changes or varies.

Phase Modulation: Phase modulation refers to a situation where the phase of a high-frequency carrier wave varies or changes due to a phase shift in the modulated signal, while the amplitude and frequency remain unchanged.

Amplitude Modulation: Amplitude modulation refers to a change in the amplitude of a carrier wave caused by a change in the modulation of the signal. The signal phase and frequency remain constant.

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Working of Modulation

  • During modulation, data is converted into electrical signals or optical carrier signals that are optimized for transmission. Carrier waves are of a constant frequency, similar to a sine wave.
  • For a wide range of applications, we need to adjust the signal to higher recurrence bands, which are referred to as Radio Frequency (RF) bands, in order to suit the spread characteristics of correspondence channels.

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  • Non-dispersive channels provide the simplest RF bands since they alter signal amplitude and period as the message is transmitted. Non-dispersive channels can be demodulated in two different ways.
  • The first path involves measuring phase bending and using a coordinated filter as in a baseband correspondence framework.
  • Sound demodulation refers to the process of identifying frequencies.
  • The second method, which we call non-coherent demodulation, does not use phase data at all in the demodulation process. 

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Demodulation

  • Demodulation is the process of extracting information from the transmitted signal.
  • Many factors influence how faithfully the extracted information replicates the original input information.
  • Electromagnetic interference can degrade signals and make the original signal impossible to extract.
  • Demodulators typically include multiple stages of amplification and filtering in order to eliminate interference.

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  • Amplitude Shift Keying ASK is a type of Amplitude Modulation

which represents the binary data in the form of variations in the

amplitude of a signal.

  • Any modulated signal has a high frequency carrier.
  • The binary signal when ASK modulated, gives a zero value for

 Low input while it gives the carrier output for High input.

  • The following figure represents ASK modulated waveform along with its input.

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  • Frequency Shift Keying  FSK is the digital modulation technique in which the frequency of the carrier signal varies according to the digital signal changes.
  • FSK is a scheme of frequency modulation.
  • The output of a FSK modulated wave is high in frequency for a

binary High input and is low in frequency for a binary Low input.

  • The binary 1s and 0s are called Mark and Space frequencies.

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  • Phase Shift Keying PSK is the digital modulation technique in which the phase of the carrier signal is changed by varying the sine and cosine inputs at a particular time.
  • PSK technique is widely used for wireless LANs, bio-metric, contactless operations, along with RFID and Bluetooth communications.

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Difference between Demodulation and Modulation

Parameters

Demodulation

Modulation

Description

Demodulation is the recovery of original information at the carrier’s far end.

The process of influencing data information on the carrier is known as modulation.

Implementation

Demodulation occurs on the receiving end of a communication system.

Modulation occurs on the transmission side of a communication system.

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Message Transmission

In demodulation, the carrier and message signals are separated to produce the original information signal.

The original message signal is mixed with a carrier wave whose parameters must be changed during modulation.

Requirement

To recover the original signal, demodulation is required.

Modulation necessitates the use of a modulator section to mix the two signals.

Conversion of Signals

The low-frequency signal is obtained from the high-frequency signal during demodulation.

Modulation transforms a low-frequency signal into a high-frequency signal.

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Purpose

Demodulation is used to re-establish the original message signal.

Modulation is primarily used to send data over longer distances.

Complexity

Demodulation is typically difficult.

Modulation is a relatively simple process.