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PRINCIPLES OF COMMUNICATION

GRADE XII

OUR OWN HIGH SCHOOL, AL WARQA’A

DUBAI

•COMMUNICATION

- Transmission of information /message from one point to another

Communication enters our lives in many ways:

- Telephone – makes us talk to any person anywhere.- Radio and television – entertain and educate.- Communication signals as navigational aids – ships, aircrafts and satellites .- Weather forecasting – conditions measured by a multitude of sensors are communicated to forecasters.- Videophones, voicemail and satellite conferencing – enable seeing live images instantly and communicate directly with people located far away.- Digital data transmission and retrieval – has made realization of e-mail, FAX and internet possible.

We communicate through speech. In modern communication systems, the information is first converted into electrical signals and then sent electronically.

•COMMUNICATION SYSTEM

Two persons talking to each other constitute the simplest communication system. The person who speaks is the source, the person listening is the receiver and the intervening air is the communication link between them.

A communication system consists of three basic components:

- Transmitter (source)- Communication channel (link – medium)- Receiver

Nature/details of these components depend on:

1. Nature of the signal/ message to be communicated.2. Distance which separates the source and the receiver.

- Direct talking is possible over short distances – sound waves attenuate fast.- Long distance communication requires the signal/message to be converted into an electrical signal/a set of signals /electromagnetic waves.- Long distance communication requires a link between the source and the receiver

Message signal Transmitter

Communication

channel Receiver

Out put

signal

CUMMUNICATION SYSTEM

NOISENOISE

Transmitter:

Transmits the message/signal over the communication channel. Quite often the original signal is not suitable for transmission over the communication channel to the receiver. It requires to be modified to a form suitable for transmission.

Communication Channel:

Provides a link between the transmitter and the receiver. It can be a transmission line (telephone and telegraphy), an optical fibre (optical communication) or free space in which the signal is radiated in the form of electromagnetic waves.

Receiver:

Reconstructs the original message/signal after propagation through the communication channel.

•TRANSMITTER

- We communicate through a message or a signal.

- A transmitter transmits the message over the communication channel to the receiver.

- As a rule, the message produced by the source is not suitable for transmission over the communication channel. Accordingly, a suitable transducer converts it into a time varying electrical signal called the message signal.

A transmitter, in its simplest form, is a setup which boosts the power of message signal and feeds it into the communication channel

Microphone

Transmitter

Amplifier

Receiver

Antenna Antenna

Amplifier Loud speaker

Basic constituents of a transmitter are: 1. Message signal 2. Modulation 3. Antenna

Message signal:

A single valued function of time that conveys the information.

Analog Signals Discrete or digital

Analog Signal Is a continuous function of time, with the amplitude (instantaneous value of the signal) being continuous.

In its simplest form, the transmitter has following problems:

1. Size of the antenna or aerial For transmitting a signal we need an antenna. It should have a size comparable to the wavelength of the electromagnetic wave representing the signal ( at least /4) so that the time variation of the signal is properly sensed by the antenna. For an electromagnetic wave of frequency 20 kHz, the wavelength is 15 km. Obviously such a long antenna is not possible. Therefore, direct transmission of such a signal is not possible. If the frequency of the signal is 1MHz, the corresponding wavelength is 300m and transmission of such a signal is possible. Therefore, there is a need of translating the information contained in the original low frequency signal into high or radio-frequencies before transmission.2. Effective power radiated by an antenna The power radiated from a linear antenna For a good transmission we need high power hence there is need for high frequency transmission.

2/ l

3. Mixing up of signals from different transmitters

Direct transmission of baseband signal leads to interference from

multiple transmitters. Thus multiple user friendly communication is

not possible. A possible solution is provided by employing

communication at high frequencies and then allotting a band of

frequencies to each user.

The above arguments suggest that there is a need for translating the

original signal ( low frequency) into a high frequency wave before

transmission such that the translated signal continues to possess the

information contained in the original signal. The high frequency wave

carrying the information is called the carrier wave. The process of

transformation is called Modulation.

Modulation

Transformation of the signal into a form suitable for transmission

through a given communication channel

Modulator Amplifier

Transmitter Receiver

Tunable

Amplifier

Demodu-lator

Audio

AmplifierSignal

To

Speaker

Antenna Antenna

Simplest form of an analog signal is a sinusoidal signal having a single frequency

g(t) = A sin t

Signals generated by different sources have their own characteristics - amplitude, frequency or nature.

Nature – Simple single frequency or a complex superposition of several frequency components

The signals associated with music or speech are complex; can be considered as superposition of several sinusoidal signals of varying amplitudes and frequencies.

The range over which the frequencies in a signal vary is called the bandwidth (B) (the frequency range between the lowest and highest frequency components). Bandwidth for audio signals is 20 Hz to 20 kHz.

Discrete Signals

Discrete signals are discontinuous in time; they are defined only at discrete times.

In case of discrete signals the independent variable (time) takes only discrete values which are usually uniformly spaced. Consequently, discrete-time signals are described as sequences of samples whose amplitudes may take a continuum of values. When each sample of a discrete-time signal is quantized I.e. its amplitude is only allowed to take on a finite set of values (e.g. in a binary representation low and high signals are designated as 0 and 1) and then coded, the resulting signal is referred to as a digital signal.

• Modulation

We have seen:- Direct transmission of audio frequency signals is not possible – associated wavelengths are large. - Quite often, the message signals require a carrier and a communication channel for transmission from source to the receiver. - An efficient utilization of the communication channel requires a shift of the range of base band frequencies to another frequency range suitable for transmission. A shift of the range of frequencies in a signal is accomplished by employing a process called modulation. In this process the signal is

superimposed over the carrier wave. The frequency, fc, of the carrier wave must be much higher than the highest frequency component of the message signal.

The form of modulation depends on the specific characteristics of the carrier wave, nature of the message signal/data and the communication channel. The carrier wave may be (I) continuous or (ii) pulse. The signal may be analog or digital.

LESSON - 3

DIFFERENT TYPES OF MODULATION TECHNIQUES

Forms of Modulation

Amplitude Modulation

Analog Signals Angle/Frequency Modulation

Pulse Modulation

Amplitude shift keying (ASK)

Digital Signals Frequency shift keying (FSK)

Phase shift keying (PSK)

ANALOG SIGNALSAmplitude & Frequency Modulation

A sinusoidal wave conveys no information. To transmit information by the usual sinusoidal waveform, the characteristics of the wave must be varied in some manner. A sinusoidal carrier wave C(t) is defined by

C(t) = Ac cos (c t + o )

The modulation of the carrier wave can be accomplished in two ways: (1) The amplitude of the carrier wave is varied about a mean value,

linearly with the baseband signal m(t), the angular frequency c remaining constant. This mode of modulation is termed as amplitude modulation. (2) The phase angle of the carrier wave is varied according to the baseband signal, the amplitude of the carrier wave being kept constant. This mode of modulation is termed as angle modulation. There are two variations of angle modulation - phase modulation (PM) and frequency modulation (FM).

Amplitude Modulation:

- Employed for commercial broadcasting of voice signals. Carrier frequencies – 0.5 to 20 MHz.- Broadcast noisy – noise signals created by atmospheric static or man made electric discharges also get amplitude modulated.

Frequency Modulation:

- TV broadcast , VHF, UHF, SHF and EHF broadcasts.- Requires higher carrier wave frequencies.- Noise generated by atmospheric or man made electric discharges does no harm to intelligence. - Higher Signal/Noise ratio, quality of broadcast very good.

FM Radio – 88 to 108 MHz VHF TV – 47 to 230 MHz UHF TV – 470 to 960 MHz

HOMEWORK

• A carrier wave of peak voltage 12V is used to transmit a message signal. What should be the peak voltage of the modulating signal in order to have a modulation index of 75%?

• For an amplitude modulated wave , the maximum amplitude is found to be 10V while the minimum amplitude is found to be 2V. Determine the modulation index,µ.

LESSON - 4

BANDWIDTH & PROPAGATION OF

E.M WAVES

BANDWIDTH OF SIGNALS

• Bandwidth refers the frequency range over which the portion of the spectrum occupied by the signal.

• In a communication system, the message signal can be voice, music, picture or computer data.

• Each of these has different ranges of frequencies. The type of communication system needed for a given signal depends on the band of frequencies which is considered essential for the communication process.

• For speech signals, frequency range 300Hz to 3100Hz is considered adequate. Therefore speech signal requires a bandwidth of 2800Hz(3100Hz-300Hz) for commercial telephonic communication.

• For speech signals, frequency range 300Hz to 3100Hz

is considered adequate. Therefore speech signal requires a bandwidth of 2800Hz(3100Hz-300Hz) for commercial telephonic communication.

• To transmit music an approximate bandwidth of 20khz is required.

• Video signals for transmission of pictures require about 4.2MHz of bandwidth.

• A T V signal contains both voice and picture and is allocated 6 MHz of bandwidth for transmission.

* Digital signals are in the form of rectangular waves

as shown in fig.* This rectangular wave can be assumed to be the superposition of sinusoidal waves of frequencies nνo. This implies an infinite band width.* For practical purposes, the contribution from higher and lower frequencies can be neglected thus

limiting the bandwidth.

Bandwidth of Transmission medium

• Different types of media offer different bandwidths.

• Commonly used transmission media are wire, free space and optic cable.

• Coaxial cable is widely used wire medium which offers a bandwidth of 750MHz.

• An optical fibre can offer a transmission bandwidth in excess of 100GHz.

• Spectrum allocations are arrived at by an international agreement. The international Telecommunication Union(ITU) administers the present system of frequency allocations.

Communication through free space

Propagation of EM waves

• Ground wave propagation:• A radio wave that can travel directly from one point to

another following the surface of the earth is called a ground wave.

• Ground wave propagation is possible only when the transmitting and receiving antenna are close to the surface of the earth.

• When the em wave glides over the earth surface, its electric field component induces charges hence current on the earth’s surface.

• Ground wave gets attenuated due to diffraction effect.

• Ground wave propagation is possible at low frequencies, 500 kHz

– 1500 kHz. This region of the AM band is called medium wave band.

• Maximum range of ground wave propagation depends on two factors: a) the freq. of the transmitted wave and b) the power of the transmitter.

• ADVANTAGES: i) Given enough transmitting power, ground waves can be used to communicate between any two locations on the earth. (ii) Ground waves are relatively unaffected by changing atmospheric conditions.

• Disadvantages:

• i) Ground waves require a relatively high transmitting power.

ii) Ground losses vary with surface material and composition.

iii) Large antennas are required for transmission.

Space wave propagation

• If a radio wave transmitted from antenna, travelling in a straight line, directly reaches the receiving antenna, it is called a space wave.

• Its also called tropospheric or line of sight propagation.

• The range of space wave propagation is limited by the line of sight distance and the curvature of the earth.

Range of a transmitting antenna

• The Range of coverage of transmission from an antenna is

d = √2hR, where R= radius of the earth and h = height of the antenna.

• The range of space wave propagation between two towers on earth’s surface is dM = dT + dR.

SKY WAVE PROPAGTION

• A radio wave directed towards the sky and reflected by the atmosphere towards the desired location of the earth is called a sky wave.

• Radio waves freq. 2MHz – 20MHz can be reflected by the ionosphere.

• This region of the AM wave band is called short wave band.

• The radio waves of different frequencies sent from earth will be reflected back the appropriate layers of the ionosphere. The critical frequency for reflection is, therefore, given by 2/1

max )(9 Nfc

HOME WORK

1) The TV signals have a bandwidth of 4.7MHz. What is the number of channels that can be accommodated in a bandwidth of 4700GHz.

(106 channels)

1) A transmitting antenna at the top of a tower has a height 32m and that of the receiving antenna is 100m. What is the maximum distance between them for satisfactory communication in LOS mode? Radius of earth 6400km.

2) The TV tower at a particular station has a height of 160m.

a) What is its coverage range?

b) How much population is covered by the transmission. If the average

density around the tower is 1200 /km2.

c) By how much should the height be increased to double its coverage

range? Given radius of the earth = 6400km.

( 45km, 76.37lakhs, 480m)