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Communication system

Communication is the act of transmission of information. A setup that transfer information from one point to another is called communication system. There are three basic elements of a communication system.

1.      Transmitter : A device which transmit message signal through a channel to receiver .

2.      Receiver  : A device which recovers the desired message signals from the received signals at the channel output.

3.      Channel : it is a physical medium which connect transmitter and reciver.  Depending upon the type of communication system, a channel may be in the form of wires or cables connecting the transmitter and the receiver or it may be wireless.

There are two basic modes of communication:

1.      Point-to-point communication takes place over a link between a single transmitter and a receiver. Telephony is an example of such a mode of communication.

2.      broadcast. ; there are a large number of receivers corresponding to a single transmitter. Radio and television are examples of broadcast mode of communication.

Transducer: Any device that converts one form of energy into another can be termed as a transducer. An electrical transducer may be defined as a device that converts some physical variable (pressure, displacement, force, temperature, etc) into the electrical signal at its output.

Signal : when Information converted in electrical form and suitable for transmission is called a signal. These are genereted by transducer which convey information. The main charterstic of a signal are amplitude, frequancy and phase.

There are two types of Signals :

Analog signals :

In  this type of signals, voltage or  current varies continuously with time. They are single-valued functions of time. It is given by,      E=E0 sin (wt+o)

Sine wave is a fundamental analog signal. Sound and picture signals in TV are analog in nature.

Digital signals : In  this type of signals, there is a stepwise variation of voltage or  current with time. it is a discrete valued function of time.

The presence of signal is denoted by digit 1 and absence of signal denoted by 0.

Binary system

Noise: Noise refers to the unwanted signals that tend to disturb the transmission and processing of message signals in a communication system. The source generating the noise may be located inside or outside the system.

Attenuation: The loss of strength of a signal while propagating through a medium.

Antenna : range and hight.

Repeater : A repeater is a device which have combination of a receiver and a transmitter. A repeater, picks up the signal from the transmitter, amplifies and retransmits it to the receiver sometimes with a change in carrier frequency.

Repeaters are used to extend the range of a communication system as shown. Example : a communication satellite is essentially a repeater station in space.

Modulation:

It is a process of superimposing the low frequency message signal on a high frequency carrier wave. The resultant wave is called modulated wave.

Nessesity of modulation is due to :

1.      Size of antena : Size of antenna should be compareble with wavelength of a signal.

2.      Power radiated by antenna :

3.      Mixing up of signal from different transmitters:

There are basically two types of modulation :

a.      Amplitude modulation

b.      Frequancy modulation

AMPLITUDE MODULATION :

the amplitude of the carrier is varied in accordance with the information signal.

Explaination :  Let a carrier wave  

and a modulating signal is              

 Then modulated signal cm (t) can be written as

                                          

Put   which is the modulation index; in practice,is kept ≤1 to avoid distortion.

Using the trignomatric relation sinA sinB = ½ (cos(A – B) – cos (A + B),

we can write cm (t)

PRODUCTION OF AMPLITUDE MODULATED WAVE

DETECTION OF AMPLITUDE MODULATED WAVE

Detection is the process of recovering the modulating signal from the

modulated carrier wave. block diagram

 The transmitted message gets attenuated in propagating through the channel. The receiving antenna is therefore to be followed by an amplifier and a detector.

The modulated signal is passed

through a rectifier to produce . This envelope of

signal (b) is the message signal. In order to retrieve m(t ), the signal is

passed through an envelope detector (which may consist of a simple RC

circuit).

Demodulation: The process of retrieval of information from the carrier wave at the receiver is termed demodulation. This is the reverse process of modulation.

Bandwidth: it is the frequency range in which frequncy of a signal varies. In communication system, information may be speech (audio), voice, picture of digital data.

1.      Speech signal :

2.      TV signal :

3.      Digital data :

internal reflection

long distance communication can be achieved by ionospheric reflection of radio waves back towards the earth. This mode of propagation is called sky wave propagation and is used by short wave broadcast services.

It extends from a height of ~ 65 Km to about 400 Km above the

earth’s surface.

The ionospheric layer acts as a reflector for a certain range of frequencies

(3 to 30 MHz). Electromagnetic waves of frequencies higher than 30 MHz

penetrate the ionosphere and escape.

Space wave

A space

wave travels in a straight line from transmitting antenna to the receiving

antenna. Space waves are used for line-of-sight (LOS) communication as

well as satellite communication. At frequencies above 40 MHz,

communication is essentially limited to line-of-sight paths. At these

frequencies, the antennas are relatively smaller and can be placed at

heights of many wavelengths above the ground. Because of line-of-sight

nature of propagation, direct waves get blocked at some point by the

curvature of the earth as illustrated in Fig. 15.5. If the signal is to be

received beyond the horizon then the receiving antenna must be high

enough to intercept the line-of-sight waves.

The Internet

It is a system which permits communication of all types of information between any two or more computers connected through network.

Its applications include

1.       E mail – It permits exchange of text/graphic material using email software.

2.       File transfer – A FTP (File Transfer Programmes) allows transfer of files/software from one computer to another

3.       E-commerce – Customers can receive all the information about various products or services of companies through their websites. They can do on-line shopping from home/office.

4.       Chat – Real time conversation among people through typed messages is made easy through internet.

Facsimile (FAX)*

It scans the contents of a document to create electronic signals. These signals are then sent to the destination (another FAX machine) in an orderly manner using telephone lines. At the destination, the signals are reconverted into a replica (copy) of the original document.

Mobile telephony

The central concept of this system is to divide the service area into a suitable number of cells centred on an office called MTSO (Mobile Telephone Switching Office).

Each cell contains a low-power transmitter called a base station and caters to a large number

of mobile receivers (popularly called cell phones). Each cell could have a service area of a few

square kilometers or even less depending upon the number of customers. When a mobile

receiver crosses the coverage area of one base station, it is necessary for the mobile user to be

transferred to another base station. This procedure is called handover or handoff. This process

is carried out very rapidly, to the extent that the consumer does not even notice it. Mobile

telephones operate typically in the UHF range of frequencies (about 800-950 MHz).

GPS system :