1368.pdf

CASIRJ Volume 5 Issue 1 [Year - 2014] ISSN 2319 9202

International Research Journal of Commerce Arts and Science http:www.casirj.com Page 246

Types of Modulation Schemes used in Communication Systems

Swati Gupta

Abstract

Communication plays a vital role in our lives, it is impossible to think life without it. So, it is very

important for us to understand the technology behind it which has made our lives so simple and fast.

This paper is about the different types of modulation schemes like AM, FM, PM, PAM, PWM, PPM,

PCM etc used in the communication system and their importance.

1. What is communication?

Communication is the basic process of exchanging information or transferring information from one

place to another.

ex. When we are talking to someone, we exchange the information in form of voice signals; sending

information through computers; talking on phone (mobile communication), radio broadcasting (FM),

television broadcasting etc.

If we talk about the stone age, how the data was exchanged between two points. There was no

television, mobile phone, INTERNET etc. Then how communication was possible? The people in the

ancient time used to send the message by using a pigeon or someone had to carry the physical message

from one place to another. But the biggest disadvantage is speed of transmission of data was very very

slow.

If we see the situation now, you just need to press enter on your computer and the data is transmitted

from one place to another in fraction of seconds.

We are going to discuss the technology behind this. How this communication process has become so

simple and fast.



2. Elements of a communication system

information

signal

(input)

Input

transducer

Transmitter

Channel

information

signal

(output)

Output

transducer

Receiver



Basic Block Diagram of Communication System

a. Information

The data or the message which is to be transmitted from one place to another. Ex. When you make a

call, your voice signal need to be sent from one place to the another. In this case data will be the voice

signal. Ex. When you mail a file from your computer, the information to be transmitted will be the file.

b. Input Transducer

A transducer is a device which converts one form of energy to another. The data which is to be

transmitted may not be in the form suitable for transmission. It needs to be converted to a suitable

format. Hence we need a transducer. Ex. When we speak, the voice signal can not be transmitted

directly. It must be converted to electrical form. This is done with the help of transducer like

microphone which converts sound waves into electrical signals.

c. Transmitter

Main function of the transmitter is to amplify the signal, modulate it with a high frequency carrier and

transmit it. The voice signal which is now electrical in nature (with the help of transducer) has the

frequency range 20Hz-20KHz. This signal is not capable of traveling long distances (which is

explained later). In transmitter, the signal is modulated with a high frequency carrier which is called as

modulation.

d. Communication channel

The message travels from one point (transmitter) to another (receiver) through communication channel.

So, channel is the medium through which information is transmitted. Ex. Talking on phone from Delhi

to US, your voice signal need to travel through some channel. So there must be some physical

connection between two points.

Channel can be of three types:

i. Copper wires

Signals are converted to electrical form and they travel through these copper wires.

ii. Free space

Signals are converted to electromagnetic radiations and then transmitted through antennas into

the air. They travel the free space with the speed of light. The signals are received by the

antenna at the receiving end. Ex. Radio broadcasting, TV broadcasting.

iii. Optical fibers

The optical fiber acts as a waveguide and light travels through it. Message signal to be

transmitted is modulated with the light signal which is launched into the fiber with the help of

LED or Laser. Ex. Optical fibers are used for data transfer through internet.

ex. pair of wires that carry voice signals from microphone to a headset.



e. Receiver

Communication channel adds some noise (unwanted signal) to the transmitted signal. Function of the

receiver is to extract the modulated data first from the noise and then demodulate it to get back the

original data (in electrical form) from the high frequency carrier. This process is called as

Demodulation.

f. Output Transducer

Since, a transducer converts one form of energy to another, it converts the incoming electrical signal

back to the original form. Ex. A loudspeaker will convert electrical signals back to sound signal.

3. Modulation

Suppose a person wants to travel from India to US. He will travel either by plane or by ship. The

person can be seen as message signal who wants to travel from one place to another and plane or ship

can be seen as the carrier signal which will carry the person.

Hence two signals are involved: the message signal also called as modulating signal and the carrier

signal.

Message signal: which is to be sent from one point to another.

Carrier signal: it is a high frequency signal which carries the message signal along.

Since carrier signal carries the message signal, so some parameter of the carrier signal should be

changed according to the message signal. Hence, either amplitude, phase or frequency of the carrier

signal is varied according to the instantaneous value of the message signal. The resulting signal is

called as modulated signal which is then transmitted through the transmitter.

4. Need of modulation

The message signal cannot be transmitted directly through the communication channel. The message

signal which has low frequency is modulated with the high frequency carrier i.e. the message signal is

shifted to high frequency range.

Reasons for modulation:

a. Height of Antenna

The message signal has a low frequency. Ex. Voice signal has the frequency from 20Hz-20KHz. We

know that,

where c = speed of light

= frequency

= wavelength

c =

Since frequency of message signal is less, its wavelength will be large.



Antenna height is given as,

Height = / 4

Therefore, the antenna height will be large which is practically impossible.

After modulation with a high frequency carrier, the frequency will be large. Hence less wavelength and

therefore antenna height will be small.

b. Energy

Since,

where E = energy

h = Planck's constant

= frequency

E = h

Message signal has less frequency. Hence the energy will be less. Therefore, the signals will not be able

to travel long distances. They will die out because of less energy.

After modulation with a high frequency carrier, the frequency will be large. Hence energy will be more.

Therefore, with increase in the frequency, signals can travel longer distance.

c. Mixing of signals

Voice and music signals are in range of audio frequency i.e. 20Hz to 20KHz, if different message

signals are transmitted from the different transmitters,

ex. m1(t) -> 20Hz-20KHz

m2(t) -> 20Hz-20KHz

m3(t) -> 20Hz-20KHz

all the signals will interfere with each other because of the same frequency range. Hence receiver will

not be able to separate the message signals.

To avoid the interference, the information of message signal is converted to different high frequency

band so that they occupy different slots in frequency domain.

d. Multiplexing

Multiplexing means mixing of signals i.e. more than two signals can be transmitted over the same

communication channel simultaneously. Hence many signals use the same channel without any

interference with each other.

5. Classification of Electronic Communication System

Based on the nature of the message signal, the communication system can be classified into two

categories:



Communication

System

Analog

Communication

Digital

Communication

a. Analog Communication

In this technique, message to be transmitted is continuous i.e analog in nature. An analog signal is a

variable signal which is continuous in both time and amplitude. It is modulated by a high frequency

carrier signal. One of the parameters of the carrier signal like amplitude, phase, frequency is varied

according to the instantaneous value of the message signal. It can transmit data including voice, image,

video etc.

b. Digital Communication

In this, the message to be transmitted is in the form of digits 0 and 1 with constant amplitude, constant

frequency and constant phase. Both time and amplitude are discrete in nature.



Depending on these two, we have different types of modulation:

Modulation

Analog modulation Digital modulation

Continuous wave

modulation

Pulse modulation PCM

Amplitude

modulation

Angle

modulation

PAM PWM PPM

AM FM PM



6. Analog Modulation

6.1 Continuous wave modulation

The carrier signal, used for modulating the message signal, is a sine wave i.e. continuous in nature.

Hence it is called as continuous wave modulation.

6.1.1. Amplitude Modulation (AM)

We have a message signal with frequency fm and a carrier signal (sine wave) with frequency fc

fc >> fm

Role of the carrier signal is used to carry the message signal along.

In case of AM, the amplitude of the carrier signal is varied according to the instantaneous value of the

message signal. Hence the information is contained in its amplitude variation. The frequency of carrier

remains constant.

Message signal

carrier signal

AM wave



Advantages:

a. Simple circuitry. AM transmitters and receivers are simple to design.

b. AM signals can be transmitted over longer distances.

Disadvantages:

a. One of the biggest disadvantages of AM is noise. Noise always affects the amplitude. Since

amplitude of the carrier wave contains the information, if it is disturbed by the noise it will be difficult

to get the original message back from the modulated signal at the receiver.

Applications:

a. If we check our radio systems, we have AM radio channel. AM is short form of amplitude

modulation. AM radio stations are also called as medium wave stations in some countries. It was the

first technique to broadcast radio signals to the public. AM radio is in a band of 550KHz 1700KHz.

b. In TV broadcasting, AM is used for transmitting the video signals. Since video contains very low

frequency as well as very high frequency components, bandwidth requirement is very high. If we use

FM instead of AM, the circuitry will become very complex.

6.1.2. Frequency Modulation (FM)


fc >> fm

In case of FM, the frequency of the carrier signal is varied according to the instantaneous value of the

message signal. Hence the information is contained in its frequency variation. The amplitude of carrier

remains constant.

Message signal

Carrier signal

FM wave


Advantages:

a. FM is less prone to noise as compared to AM. Since frequency of the carrier contains the message

information and noise always affects the amplitude, hence it is less prone to noise.

b. It has better sound quality than AM.

c. Power requirement is less as compared to AM.

Disadvantages:

a. Transmitters and Receivers are complex to design as compared to FM

b. It cannot be transmitted over long distances. It can only be used for line-of-site communication.

Applications:

a. We often use the term FM while listening to the radio. This FM is frequency modulation. The music

which we are listening to comes to us using frequency modulation technique. FM radio is in a band of

88MHz 108MHz. All the channels in FM are within this range.

b. FM is used in satellite communication because of less power requirement.

6.1.3. Phase Modulation (PM)


fc >> fm

In case of PM, the phase of the carrier signal is varied according to the instantaneous value of the

message signal. Hence the information is contained in its phase variation. The amplitude and the

frequency of carrier remain constant.

Message signal

Carrier signal


PM wave

Advantages:

a. Phase modulation and demodulation is easy as compared to frequency modulation.

Applications:

a. Phase modulator is used to determine the velocity of moving target.

6.2. Pulse Modulation

The carrier used to modulate the message signal is a pulse train. Hence it is called as pulse modulation.

Pulse train

6.2.1 Advantages of Pulse modulation over continuous wave modulation

a. Since we transmit the information using a pulse train, we do not need to generate power

continuously. Power is transmitted in short bursts.

b. Since, time between two pulses is free, it can be utilized to send samples of other message signals.

This is called as Time Division Multiplexing.

6.2.2 Disadvantages of Pulse modulation over continuous wave modulation

a. It needs large bandwidth.

6.2.3 Pulse Amplitude Modulation (PAM)

PAM is the simplest form of analog pulse modulation.


In PAM, the amplitude of the pulse train is varied in accordance to the instantaneous value of the

message signal.

i. PAM using instantaneous sampling

The message signal is multiplied with the periodic train of pulses with unit amplitude and width dt->0,

and we get instantaneous sampling.

Disadvantages

a. An instantaneous sample has an infinitesimal energy. When such a sample is transmitted by the

transmitter, because of the very less energy, it will be lost in the background noise.

b. If we see the Fourier transform of an impulse, it has all the frequency components in it. Hence an

instantaneous pulse requires an infinite bandwidth to get transmitted. We can say, transmission

bandwidth is inversely proportional to the width of the pulse. Narrower is the width of the pulse, more

is the bandwidth required.

Hence an instantaneous sampling is hardly feasible and we need to wider the width of the periodic

pulse train.


ii. PAM using natural sampling

The message signal is multiplied with the periodic train of pulses with unit amplitude and width dt. The

top of the pulse follows the shape of the message signal.

Disadvantages:

a. The top of the pulse follows the shape of the message signal. When transmitted by the transmitter,

communication channel adds some noise to the signal and noise always affects the amplitude. At the

receiver, it will be difficult to detect the shape of the top of the pulse. Therefore, exact amplitude

detection cannot be done.

iii. PAM using flat top sampling


Advantages:

a. Since the top of the pulse is flat, noise can be easily removed from the signal.

Disadvantages:

a. Flat top sampling results in distortion of the signal. The high frequency components are lost. But this

can be corrected by using an equilizer.

iv. Advantages of PAM

a. Generation and detection of PAM are simple process. Circuitry is simple.

v. Disadvantages of PAM

a. Noise performance is bad. Noise affects the amplitude and the amplitude of the pulse train carries the

information about the message signal. It cannot be used for long distance transmission.

b. Power depends on the amplitude and the width. All the pulses in a PAM wave differs in amplitude,

therefore power will also differ. Hence transmitter must be able to handle the power required to

transmit pulse having maximum amplitude.

6.2.4 Pulse Width Modulation (PWM) or Pulse Duration Modulation (PDM) or Pulse Length

Modulation (PLM)

In PWM, the width of the pulse train is varied in accordance to the instantaneous value of the message

signal.

Hence, the width of the sample contains the information about the message signal.

As the amplitude of message signal increases, the pulse width increases. As the amplitude of message

signal decreases, the pulse width also decreases.


Advantages:

a. Noise is less as compared to PAM since information is contained in the varying width and not in the

amplitude.

b. If noise affects the amplitude of the pulse, it can be easily removed by using a Schmitt trigger.

Disadvantages:

a. Power depends on the amplitude and the width of the pulse. Since all the pulses in PWM wave

differs in width, therefore power will also differ. Transmitter must be able to handle the power required

to transmit the pulse with maximum width.

b. Time division multiplexing is difficult to achieve because of the varying pulse width. The pulses

from different sample may overlap with each other.

6.2.5 Pulse Position Modulation (PPM)

In PPM, the position of the pulse train is varied in accordance to the instantaneous value of the message

signal.

Advantages:

a. Since amplitude is constant, the noise affect is very less as compared to PAM

b. If noise affects the amplitude of the pulse, it can be easily removed by using a Schmitt trigger. (as in

case of PWM)


c. Amplitude and width of pulses are constant. Hence power required for transmitting pulses is also

constant.

d. As we know, narrower is the pulse width, more is the band width required. Ex. An impulse needs an

infinite bandwidth because it contains all the frequency components. If we increase the width of the

pulse, bandwidth requirement will be less. Therefore, if bandwidth available is narrow, we will use

wide pulse and if bandwidth available is large, we will use narrow pulse. This is called as bandwidth

optimization.

Disadvantages:

a. Time division multiplexing is difficult to achieve because of the varying pulse positions. The pulses

from different sample may overlap with each other.

6.2.6 Difference between PAM, PWM and PPM

S. No. PAM PWM PPM

1. Definition Amplitude of the pulses vary according to

the instantaneous value

of the message signal

Width of the pulses

vary according to the

instantaneous value of the

message signal

Position of the pulses vary

according to the

instantaneous value of

the message signal

2. Analogous to AM FM PM

3. Noise Large Less Less

4. Power Varies with the

amplitude of the pulses

Varies with the width

of the pulses

constant

7. Digital Modulation

Digital transmission It is the transmittal of digital information in the form of digits 0 and 1 between two or more points in a

communication system. If the signal is analog in nature, it is first converted to digital form and then

transmitted. The digital information can be transmitted by coaxial cables, optical fibers.

7.1 Why going from Analog to Digital?

Now we are shifting from analog modulation technique to digital modulation technique. The reasons

behind this are as follows:

a. The affect of noise is more on analog systems as compared to digital signals.


When the digital signal is transmitted, at the receiver it is not important to evaluate precise amplitude,

frequency or phase. A pulse during transmission may distort. A simple technique is used. If the pulse is

above reference level, it is taken as 1 and if the pulse is below reference level, it is taken as 0. Affect of

noise is greatly reduced in digital systems.

b. Since affect of noise is less for digital systems, it is suitable for long distance transmission.

c. Digital circuits are easy to handle.

d. Redundant information can be easily removed from digital transmission by using different

techniques.

e. Multiplexing techniques can be implemented in digital system easily.

f. In digital modulation, we make use of repeaters in the communication channel. Communication path

always add noise to the signal. Role of repeater is to remove the noise from the incoming signal and

regenerate a fresh noise less signal. These repeaters are repeatedly spaced in the entire communication

path.

Transmitter Signal +

Noise

(due to

channel)

Regenerator

repeater

signal

Receiver

Communication

Channel

g. In analog modulation, the signal to noise ratio i.e. SNR required at the receiver is 40-60dB for proper

detection of the message signal from the modulated one. In digital modulation, the SNR required is 10-

12dB. This is the advantage of digital modulation over analog modulation i.e. even if the noise factor is

large in the signal, then also we are able to recover the message signal easily.


7.2 Pulse Code Modulation

PCM is nothing but analog to digital conversion of the message signal i.e. the analog signal is

converted in the form of 0 and 1 and then transmitted through the communication channel to the

receiver.

A Pulse is used to represent 0 and 1. If pulse is present, it represents 1 and is pulse is absent, it

represents 0. These pulses are of constant width and amplitude.

Message

signal

sampler

Transmitter

quantizer encoder

Regenerative

repeater

Communication

crhannel Regenerative repeater

Receiver

decoder Hold

circuit

LPF Message

signal

Steps involved in PCM are

a. Sampling

This is the first step involved in PCM.

Analog signal which is continuous in both time and amplitude, is first converted to discrete time signal

by sampling (continuous in amplitude but discrete in time).

Instead of sending the complete signal, even if we send the samples of signal taken at discrete time then

also we are able to recover the signal from the samples at the receiver.


For example

The figure above shows if more number of samples are taken in a given signal, the reconstructed signal

will approach more towards the original signal.

b. Quantizer

Once the signal is sampled, it becomes discrete in time. Amplitude is still continuous in nature i.e. it

can have any of the infinite values in a finite range of amplitude and its not important to transmit exact

amplitude of the signal. So, the next step is to convert this infinite range to finite range i.e. to make this

amplitude discrete in nature. For this we use quantizer.


Continuous

sample

quantizer Discrete

sample

Quantized signal

The above quantized signal instead of having any value between 0 to 15V, will have only discrete

values ex. 0,1,2,3,4V etc.

c. Encoder

The above signal can have the amplitude from the range 0 to 15. The voltage can have any of these 16

values. But we cannot send these values directly. Before sending it through the communication

channel, it needs to get encoded in form of 0 and 1 which is suitable for the transmission over the

channel.

In above case, we need 4 bits to represent 16 levels.

Voltage level Bit

0V 0000

1V 0001

2V 0010


3V 0011

4V 0100

5V 0101

6V 0110

7V 0111

8V 1000

9V 1001

10V 1010

11V 1011

12V 1100

13V 1101

14V 1110

15V 1111

All these amplitude values are represented by the codes. In binary code, we use 0 and 1 and 1 is shown

by presence of pulse and 0 is shown by absence of pulse.

The encoded values for the above signal will be

These encoded signals now will be send through the channel.

d. Regenerative repeater

Role of repeater is to eliminate noise and distortion added by the communication channel and

regenerate a completely fresh noiseless signal. Many repeaters are used in the communication path

spaced closely with each other.

e. Decoder

Role of decoder is opposite that of encoder. It regroups the incoming bits and decodes it into a

quantized signal.


f. Filtering

This is the final operation in PCM. The signal is passed through a low pass filter and the original signal

is recovered.

7.3 Disadvantages of Pulse Code Modulation

a. Bandwidth requirement is more as compared to analog modulation technique.

b. Transmission noise: Role of repeater is to eliminate the noise added in the signal when it is

transmitted from transmitter to receiver. Since every electronic circuit generates its own noise. Hence

some transmission noise is always present which is random in nature.

c. Quantization noise

Looking at the second sample, the exact value of sample is 10.5 but in quantized signal it is estimated

by 11. The difference between the quantized value and the actual value is called as quantization noise.

8. Digital Carrier Modulation Technique

8.1 Introduction

In case of digital transmission, the message signal is transmitted in the form of 0 and 1's. If the signal is

digital, it is transmitted directly through the physical wires and if the signal is analog, it is first

converted to digital form using PCM and then transmitted through physical wires like coaxial cable or

optical fibers.

But such a message signal cannot be transmitted through free space in from of electromagnetic

radiations. The reason is that the message signal (whether digital or analog) has low frequency and the

required antenna height will be very large which is impractical. Another reason is less energy in the


PSK

ASK

message signal because of the less frequency, and it will get lost in the free space noise.

So solution is of course to modulate the digital signal with a high frequency carrier and then transmit

the modulated signal through free space using antennas.

We have different types of modulation techniques:

Digital Carrier

Modulation Techniques

FSK

QAM

8.2 Amplitude Shift Keying (ASK) or On-Off Keying (OOK)

The digital message signal is modulated with the high frequency carrier. The amplitude of the carrier is

changed according to the message signal. It is similar to AM


When input = high i.e. at logic 1, output's amplitude is same as high frequency carrier's amplitude.

When input = low i.e. at logic 0, output is 0.

Hence the information of the message signal is contained in the amplitude of the carrier signal i.e. if

input is 1, carrier is on and if input is 0. carrier is off. Therefore, it is also known as On Off keying.

8.3 Frequency shift keying (FSK)

The digital message signal is modulated with the high frequency carrier. The frequency of the carrier is

changed according to the message signal. It is similar to FM

When input = high i.e. at logic 1, carrier frequency is shifted up i.e. frequency increases. When

input = low i.e. at logic 0, carrier frequency is shifted down i.e. frequency decreases. Hence

the information of the message signal is contained in the frequency of the carrier signal.

8.4 Phase Shift Keying

The digital message signal is modulated with the high frequency carrier. The phase of the carrier is

changed according to the message signal. It is similar to PM

When input = high i.e. at logic 1, output wave is in phase with the carrier. i.e. 0 degrees phase

When input = low i.e. at logic 0, output wave is 180 degrees out of phase with the carrier.


9. Summary

Hence these are the various ways used in the communication systems to modulate the message

signal using high frequency carrier and transmit it from one point to another either through

copper wires, optical fibers or free space. There are other techniques as well like QPSK,

QAM, DM, ADM which are not discussed here.

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