communication system

COMMUNICATION

SYSTEM

Presented by Kailash Sharma

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Communication System

Communication is the act of transmission of information

➢ They both are communicating but there is limitation on

the distance between them.

➢ We are more interested in distance communication through

radio waves.

➢ In modern days we have mobile phones are best example of

distance communication.

➢ We all have tried one simple tool in our school day

Two persons taking to each other

(They both must understand

language of each other.)


There are two basic modes of communication

➢ Point to point communication (single transmitter and single receiver)

Mobiles phone

➢ Broadcast (Single transmitter but multiple receiver)

Radio and television are example


Elements of Communication System →


Channel:

➢ Physical medium that connects transmitter and receiver

➢ Depending upon type of communication channel may be in form of wire (cables) or

wireless.


Elements of communication system:

(i) Transducer→ Any device that converts one form of energy into another form of energy is

termed as transducer.

Input or output of transducer is in the electrical form.

Pressure variation

(microphone)

Transduce Electrical Signal


Signal:

➢ Information converted in electrical form and suitable for transmission is called signal.

➢ Signals can be either analog or digital.

Digital signal: Voltage or current have only two value 0 or 1

(those can take only discrete stepwise values)


Analog signal: Continuous variation of voltage or current with time.

Fundamental analog signal is sine wave

➢ Sound and picture signals in TV are analog.

Noise: Unwanted signals (घुस पठैिये)

➢ Source of noise may be inside or outside.

v or i

t


Transmitter: Transmitter converts the message signal produced by source of information

into a form suitable for transmission through channel (radio waves).

Receiver: Receiver re-construct or extract the desired message signal at the channel output.

Attenuation: The loss of strength of a signal while being transmitted through medium is

known as attenuation.


Amplification:

➢ The process of increasing amplitude of signal using an electronic circuit called the

amplifier.

➢ It is necessary to compensate the attenuation of the signal in communication system.

➢ The energy needed for additional signal strength is obtained from a DC source.

Range: Largest distance between source and destination upto which signal is received with

sufficient strength.


Bandwidth: Frequency range over which an equipment operates or portion of spectrum

occupied by signal.

Bandwidth of different signals:

In a communication system message signals can voice, music, picture or computer date.

Each of these difference ranges of frequency.

Speech signal → 300 Hz to 3100 Hz

Bandwidth = 2800 Hz (Telephonic communication)

Music signal→ 20 Hz → 20 kHz

Bandwidth = 20 kHz (High frequency of musical instrument)

Video signals→ Picture transmission (bandwidth 4.2 MHz)

Sound + Picture (6 MHz)


Bandwidth of transmission medium:

Coaxial cables → 750 MHz (bandwidth)

Normal operate below 18 GHz.

Optical fiber→ 100 GHz (bandwidth)

Operating frequency (1 THz to 1000 THz)

(microwave to UV)

Communication through free space → (Radio wave)

(few hundreds of kHz to few GHz)


Free space propagation:

Antenna at transmitter radiates EM waves which travel through the space and reach the

receiving antenna. (Many factor play vital role)

To radiate signal size of antenna should be at lest λ/4.

Ground wave:

(i) Wave glide over surface of Earth also known as surface wave.

(ii) Attenuation of surface wave increases with frequency.

(iii) Frequency less than few MHz for efficient max range transmission.


Sky wave:

Few MHz to 30 MHz

Ionosphere → 65 km to 400 km

↑

presence of large number of ions

(Ionization occurs due to UV ray absorption)


Space wave:

(i) Space wave travel in straight line form transmitting antenna to receiving antenna.

(ii) They are used for line of sight communication as well as satellite.

(iii) Frequency of operation above 40 MHz.

dT → radio horizon of transmitting antenna,

m T Rd 2Rh 2Rh= +


Television Broadcast, Microwave link, Satellite communication are example of

communication system that uses space wave mode of propagation.


MODULATION AND ITS NECESSITY

As already mentioned, the purpose of a communication system is to transmit information or

message signals. Message signals are also called baseband signals, which essentially

designate the band of frequencies representing the original signal, as delivered by the source

of information. No signal, in general, is a single frequency sinusoid, but it spreads over a

range of frequencies called the signal bandwidth. Suppose we wish to transmit an electronic

signal in the audio frequency (AF) range (baseband signal frequency less than 20 kHz) over

a long distance directly. Let us find what factors prevent us from doing so and how we

overcome these factors,


Size of the antenna or aerial

For transmitting a signal, we need an antenna or an aerial. This antenna should have a size

comparable to the wavelength of the signal (at least λ/4 in dimension) so that the antenna

properly senses the time variation of the signal. For an electromagnetic wave of frequency

20 kHz, the wavelength λ is 15 km. Obviously, such a long antenna is not possible to

construct and operate. Hence direct transmission of such baseband signals is not practical.

We can obtain transmission with reasonable antenna lengths if transmission frequency is

high (for example, if v is 1 MHz, then λ is 300 m). Therefore, there is a need of translating

the information contained in our original low frequency baseband signal into high or

radio frequencies before transmission.


Effective power radiated by an antenna

A theoretical study of radiation from a linear antenna (length l) shows that the power

radiated is proportional to (l/λ)2. This implies that for the same antenna length, the power

radiated increases with decreasing λ, i.e., increasing frequency. Hence, the effective power

radiated by a long wavelength baseband signal would be small. For a good transmission,

we need high powers and hence this also points out to the need of using high frequency

transmission.


Mixing up of signals from different transmitters

Another important argument against transmitting baseband signals directly is more

practical in nature. Suppose many people are talking at the same time or many transmitters

are transmitting baseband information signals simultaneously. All these signals will get

mixed up and there is no simple way to distinguish between them. This points out towards

a possible solution by using communication at high frequencies and allotting a band of

frequencies to each message signal for its transmission.

communication system

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