UNIT-I

Introduction to Communication System

Communication is the process by which information is exchanged between individuals

through a medium.

Communication can also be defined as the transfer of information from one point in space
and time to another point.

The basic block diagram of a communication system is as follows.

 Transmitter: Couples the message into the channel using high frequency signals.
 Channel: The medium used for transmission of signals
 Modulation: It is the process of shifting the frequency spectrum of a signal to a
frequency range in which more efficient transmission can be achieved.
 Receiver: Restores the signal to its original form.
 Demodulation: It is the process of shifting the frequency spectrum back to the
original baseband frequency range and reconstructing the original form.

Modulation:

Modulation is a process that causes a shift in the range of frequencies in a signal.

• Signals that occupy the same range of frequencies can be separated.

• Modulation helps in noise immunity, attenuation - depends on the physical medium.

The below figure shows the different kinds of analog modulation schemes that are available
Modulation is operation performed at the transmitter to achieve efficient and reliable
information transmission.

For analog modulation, it is frequency translation method caused by changing the appropriate
quantity in a carrier signal.

It involves two waveforms:

 A modulating signal/baseband signal – represents the message.

 A carrier signal – depends on type of modulation.

•Once this information is received, the low frequency information must be removed from the
high frequency carrier. •This process is known as “Demodulation”.

Need for Modulation:

 Baseband signals are incompatible for direct transmission over the medium so,
modulation is used to convey (baseband) signals from one place to another.
 Allows frequency translation:
o Frequency Multiplexing
o Reduce the antenna height
o Avoids mixing of signals
o Narrowbanding
 Efficient transmission
 Reduced noise and interference

Types of Modulation:

Three main types of modulations:

Analog Modulation

 Amplitude modulation
 Example: Double sideband with carrier (DSB-WC), Double- sideband
suppressed carrier (DSB-SC), Single sideband suppressed carrier (SSB-SC), vestigial
sideband (VSB)
 Angle modulation (frequency modulation & phase modulation)
Example: Narrow band frequency modulation (NBFM), Wideband frequency
modulation (WBFM), Narrowband phase modulation (NBPM), Wideband phase
modulation (NBPM)

Pulse Modulation

 Carrier is a train of pulses

 Example: Pulse Amplitude Modulation (PAM), Pulse width modulation (PWM) ,
Pulse Position Modulation (PPM)

Digital Modulation

 Modulating signal is analog

o Example: Pulse Code Modulation (PCM), Delta Modulation (DM), Adaptive
Delta Modulation (ADM), Differential Pulse Code Modulation (DPCM),
Adaptive Differential Pulse Code Modulation (ADPCM) etc.
 Modulating signal is digital (binary modulation)
o Example: Amplitude shift keying (ASK), frequency Shift Keying (FSK),
Phase Shift Keying (PSK) etc

Frequency Division Multiplexing

Multiplexing is the name given to techniques, which allow more than one message to be
transferred via the same communication channel. The channel in this context could be a
transmission line, e.g. a twisted pair or co-axial cable, a radio system or a fibre optic system
etc.

FDM is derived from AM techniques in which the signals occupy the same physical ‘line’ but
in different frequency bands. Each signal occupies its own specific band of frequencies all the
time, i.e. the messages share the channel bandwidth.

 FDM – messages occupy narrow bandwidth – all the time.

 Fig.1. Frequency Division Multiplexing

Amplitude Modulation (AM)

Amplitude Modulation is the process of changing the amplitude of a relatively high

frequency carrier signal in accordance with the amplitude of the modulating signal
(Information).

The carrier amplitude varied linearly by the modulating signal which usually consists of a
range of audio frequencies. The frequency of the carrier is not affected.
  Application of AM - Radio broadcasting, TV pictures (video), facsimile transmission
 Frequency range for AM - 535 kHz – 1600 kHz
 Bandwidth - 10 kHz

Various forms of Amplitude Modulation

• Conventional Amplitude Modulation (Alternatively known as Full AM or Double

Sideband Large carrier modulation (DSBLC) /Double Sideband Full Carrier (DSBFC)

• Double Sideband Suppressed carrier (DSBSC) modulation

• Single Sideband (SSB) modulation

• Vestigial Sideband (VSB) modulation

Time Domain and Frequency Domain Description

It is the process where, the amplitude of the carrier is varied proportional to that of the
message signal.

Let m (t) be the base-band signal, m (t) ←→ M (ω) and c (t) be the carrier, c(t) = Ac
cos(ωct). fc is chosen such that fc >> W, where W is the maximum frequency component of
m(t). The amplitude modulated signal is given by

s(t) = Ac [1 + kam(t)] cos(ωct)

Fourier Transform on both sides of the above equation

S(ω) = π Ac/2 (δ(ω − ωc) + δ(ω + ωc)) + kaAc/ 2 (M(ω − ωc) + M(ω + ωc))

ka is a constant called amplitude sensitivity.

kam(t) < 1 and it indicates percentage modulation.

Fig.2. Amplitude modulation in time and frequency domain

Single Tone Modulation:

Consider a modulating wave m(t ) that consists of a single tone or single frequency
component given by

Expanding the equation (2), we get

 Fig.3. Frequency Domain characteristics of single tone AM

Power relations in AM waves:

Consider the expression for single tone/sinusoidal AM wave

The ratio of total side band power to the total power in the modulated wave is given by

This ratio is called the efficiency of AM system

Generation of AM waves:

Two basic amplitude modulation principles are discussed. They are square law modulation
and switching modulator.

Square Law Modulator

When the output of a device is not directly proportional to input throughout the
operation, the device is said to be non-linear. The Input-Output relation of a non-linear device
can be expressed as

When the output is considered up to square of the input, the device is called a square law
device and the square law modulator is as shown in the figure 4
 Fig.4. Square Law Modulator
Consider a non-linear device to which a carrier c(t)=Accos(2πfct) and an information
signal m(t) are fed simultaneously as shown in figure 4. The total input to the device
at any instant is

Therefore the square law device output 0 V consists of the dc component at f = 0.

The information signal ranging from 0 to W Hz and its second harmonics are signal at
fc and 2fc.
Spectrum is as shown below
Switching Modulator

Fig.5. Switching Modulator

The total input for the diode at any instant is given by
 When the peak amplitude of c(t) is maintained more than that of information
signal, the operation is assumed to be dependent on only c(t) irrespective of m(t).
When c(t) is positive, v2=v1since the diode is forward biased. Similarly, when
c(t) is negative, v2=0 since diode is reverse biased. Based upon above operation,
switching response of the diode is periodic rectangular wave with an amplitude unity
and is given by

The required AM signal centred at fc can be separated using band pass filter.
The lower cut off-frequency for the band pass filter should be between w and fc-w
and the upper cut-off frequency between fc+w and 2fc. The filter output is given by
the equation
Detection of AM waves

Demodulation is the process of recovering the information signal (base band) from the
incoming modulated signal at the receiver. There are two methods, they are Square law
Detector and Envelope Detector

Square Law Detector

Consider a non-linear device to which the AM signal s(t) is applied. When the level of s(t) is
very small, output can be considered upto square of the input.
The device output consists of a dc component at f =0, information signal ranging from 0-W
Hz and its second harmonics and frequency bands centered at fc and 2fc. The required
information can be separated using low pass filter with cut off frequency ranging between W
and fc-w. The filter output is given by

When the information level is very low, the noise effect increases at the receiver, hence the
system clarity is very low using square law demodulator.

Envelope Detector

It is a simple and highly effective system. This method is used in most of the commercial AM
radio receivers. An envelope detector is as shown below.
 Fig.7. Envelope Detector

During the positive half cycles of the input signals, the diode D is forward biased and
the capacitor C charges up rapidly to the peak of the input signal. When the input signal falls
below this value, the diode becomes reverse biased and the capacitor C discharges through
the load resistor RL.

The discharge process continues until the next positive half cycle. When the input
signal becomes greater than the voltage across the capacitor, the diode conducts again and the
process is repeated.

The charge time constant (rf+Rs)C must be short compared with the carrier period,
the capacitor charges rapidly and there by follows the applied voltage up to the positive peak
when the diode is conducting.That is the charging time constant shall satisfy the condition,

Where ‘W’ is band width of the message signal.

The result is that the capacitor voltage or detector output is nearly the same as the
envelope of AM wave.

Advantages and Disadvantages of AM:

Advantages of AM:
  Generation and demodulation of AM wave are easy.
 AM systems are cost effective and easy to build.

Disadvantages:
 AM contains unwanted carrier component, hence it requires more
transmission power.
 The transmission bandwidth is equal to twice the message
bandwidth.

To overcome these limitations, the conventional AM system is modified at the cost of

increased system complexity. Therefore, three types of modified AM systems are discussed.

DSBSC (Double Side Band Suppressed Carrier) modulation:

In DSBC modulation, the modulated wave consists of only the upper and lower side bands.
Transmitted power is saved through the suppression of the carrier wave, but the channel
bandwidth requirement is the same as before.

SSBSC (Single Side Band Suppressed Carrier) modulation: The SSBSC

modulated wave consists of only the upper side band or lower side band. SSBSC is suited for
transmission of voice signals. It is an optimum form of
modulation in that it requires the minimum transmission power and minimum channel band
width. Disadvantage is increased cost and complexity.

VSB (Vestigial Side Band) modulation: In VSB, one side band is completely passed
and just a trace or vestige of the other side band is retained. The required channel bandwidth
is therefore in excess of the message bandwidth by an amount equal to the width of the
vestigial side band. This method is suitable for the transmission of wide band signals.