Principles of communication systems for reference

Department of Electronics and Communication Engineering
Subject : Principles of Communication Systems
Course Code : BEC 403
Semester : 4th Sem
Dr Nataraj V
Associate Professor,
Department of Electronics and Communication Engineering
RVITM, Bengaluru - 560076

Module – I
• AMPLITUDE MODULATION: Introduction, Amplitude Modulation: Time &
Frequency – Domain description, Switching modulator, Envelop detector.
• DOUBLE SIDE BAND-SUPPRESSED CARRIER MODULATION: Time
and Frequency – Domain description, Ring modulator, Coherent detection,
Costas Receiver, Quadrature Carrier Multiplexing.
• SINGLE SIDE–BAND AND VESTIGIAL SIDEBAND METHODS OF
MODULATION: SSB Modulation, VSB Modulation, Frequency Translation,
Frequency- Division Multiplexing, Theme Example,VSB Transmission of
Analog and Digital Television

Course Learning objectives
• Design simple systems for generating and demodulating AM, DSB, SSB and
VSB signals
• Understand the concepts in Angle modulation for the design of
communication systems
• Design simple systems for generating and demodulating frequency
modulated signals
• Learn the concepts of random process and various types of noise.
• Evaluate the performance of the communication system in presence of noise.
Analyze pulse modulation and sampling techniques

Text Books and References
• Text Book:
• “Communication Systems”, Simon Haykins & Moher, 5th Edition, John Willey, India Pvt. Ltd,
2010, ISBN, 978 – 81 – 265 – 2151 – 7.
• Reference Books:
• Modern Digital and Analog Communication Systems, B. P. Lathi, Oxford University Press., 4th
edition.
• An Introduction to Analog and Digital Communication, Simon Haykins, John Wiley India Pvt.
Ltd.,2008, ISBN 978–81–265–3653–5.
• Principles of Communication Systems, H.Taub & D.L.Schilling, TMH,2011.
• Communication Systems, Harold P.E, Stern Samy and A.Mahmond, Pearson Edition, 2004.

Introduction
• A large number of information sources are analog sources such as speech, images and
videos. Today they are transmitted as analog signal transmission, especially in audio
and video broadcast. The transmission of an analog signal is either by modulation of
the amplitude, the phase or frequency of a sinusoidal carrier.

SIGNAL
INcompatible for direct transmission
strength has to be increased
modulating carrier sgl
one parameter change and others are constant

Modulation
• Message signal
• baseband signal
• modulating signal
• Modulation: is the process of putting information onto a higher frequency carrier for
transmission (frequency translation). Modulation occurs at the transmitting end of the
system.

Signals in the Modulation Process
• Message or Modulating Signal: The signal which contains a message to be
transmitted, is called as a message signal. It is a baseband signal, which has to
undergo the process of modulation, to get transmitted. Hence, it is also called as the
modulating signal.
• Carrier Signal: The high frequency signal, which has a certain amplitude, frequency
and phase but contains no information is called as a carrier signal. It is an empty
signal and is used to carry the signal to the receiver after modulation.
• Modulated Signal: The resultant signal after the process of modulation is called as a
modulated signal. This signal is a combination of modulating signal and carrier
signal.

Three Types Modulation
• Amplitude Modulation (AM) – the amplitude of the carrier
waveform varies with the information signal
• Frequency Modulation (FM)- the frequency of the carrier
waveform varies with the information signal.
• Phase Modulation (PM) – the phase of the carrier waveform
varies with the information signal.

Amplitude Modulation
• Amplitude modulation (AM) is a modulation technique used
in electronic communication, most commonly for transmitting
messages with a radio wave.

Expression for AM Wave
• The transmitted AM signal waveform is described by:
SAM(t) = Ac[1+ ka m(t)] cos(2πfct) …………….(1)
where m(t) = message signal, ka = constant
• Ac – is the carrier signal, cos(2πfct), where fc >>Wm where Wm
is the message bandwidth

Let us consider a single tone modulating signal as
m(t) = Am cos 2πfmt ……………….(1)
which contains a single frequency 2πfm.
Signal m(t) may be a voltage signal or a current signal. Here, we have assumed
that m(t) is a voltage signal with maximum amplitude equal to Am.
Mathematical expression for AM wave s(t)

Let the carrier signal be :
c(t) = Ac cos 2πfct …………… (2)
When c(t) is superimposed with m(t), c(t) is considered as a constant, therefore
A = Ac + m(t) or A = Ac + Am cos 2πfmt ……………….(3)
In this case, we have |m(t)|max = Am
Therefore, µ=Am / Ac Am = Ac µ or, µ = kaAm
……………………. (4)
where ka is amplitude sensitivity constant.
Equation (3) becomes s(t) = Ac [ 1+ µ Am cos 2πfmt] …………….(5)
This is the desired expression for single-tone modulated signal.
Mathematical expression for AM wave s(t)

The expression in equation (4) may further be simplified to observe the
frequency components present in AM signal.
s(t) = Ac cos 2πfct [ 1+ µ Am cos 2πfmt ] or
s(t) = Ac cos 2πfct + Ac . µ cos2πfct cos2πfmt

Frequency Spectrum of m(t) and s(t)
• With the help of Fourier transform 🡪
Eqn (6) written as,

Fig. 1 Effect of modulating factor | ka m(t)| for the requirements of
envelope to be satisfied

Fig. 2 Time domain and Frequency domain characteristics of message,
carrier and AM wave

Fig. 3 Frequency spectrum of message signal and AM wave

From the spectrum of S ( f ), fig.5, we note the following points:
• As a result of the modulation process the spectrum of the message signal m(t), for
negative frequencies extending from –W to 0 becomes completely visible for positive
frequencies, provided that the carrier frequency satisfies the condition fc >W.
• For positive frequencies: The spectrum of an AM wave above fc is referred to as the
upper side band, below fc is referred to as the lower sideband. For negative
frequencies: The spectrum of an AM wave above - fc is referred to as the upper side
band, below - fc is referred to as the lower sideband. Note that, the condition fc >W
ensures that the sidebands do not overlap.
• For positive frequencies, the highest frequency component of the AM wave equals fc
+W, and the lowest frequency component equals fc – W. The difference between these
two frequencies defines the transmission bandwidth (BW).

BW = f USB – f LSB = (fc + fm) - (fc - fm) = 2 fm(max) = 2W
∴ BW required for the AM is twice the frequency of the modulating
signal.

Modulation factor (Index)
• Modulation index is defined as the ratio of the amplitude of the modulating
signal to the amplitude of the carrier signal and is given by µ = Vm/Vc
• If modulation index is expressed in terms of percentage then it is called %
modulation.
i.e, % µ= Vm/Vc x 100%

Fig. 4 AM wave in time domain and frequency domain

Switching Modulator
• Switching modulator is the device used to generate an AM wave. The
switching modulator using diode is shown in fig 5(a). This diode is assumed
to be operating as an ideal switch. The modulating signal m(t) and the
sinusoidal carrier signal c(t) are connected in series with each other.
The input voltage to the diode is given by :
• V1(t) = c(t) + m(t) = Ac cos(2πfct) + m (t) ….. (1)

Switching Modulator
Fig. 5(a) Switching modulator Fig 5. (b) Idealized input-output Characteristics

The amplitude of carrier Ac >> |m(t)| and c(t) decides ON /OFF
status of the diode. Since modulating wave is weak compared with
the carrier wave, the nonlinear behavior of the diode can effectively
be replaced by an equivalent piecewise-linear model.

Working Operation and Analysis
• The diode acts as an ideal switch. In the positive half cycle of the
carrier, diode is forward biased by offering zero impedance acts as a
closed switch. Now the output voltage V2 (t) = V1(t).
• In the negative half cycle of the carrier, diode is reverse biased and
offers infinite impedance acts as an open switch, hence V2(t) = 0.
V2(t) = V1(t) for c(t) > 0
V2(t) = 0 for c(t) < 0 …….. (1)
• In other words, the load voltage V2(t) varies periodically between the
values V1(t) and zero at the rate equal to carrier frequency fc .

Working Operation and Analysis
Fig. 6 Periodic pulse train of duty cycle = one half cycle period.

Detection or Demodulation of AM Wave
• The process of recovering the original message signal m(t) from
the received modulated signal s(t), is known as demodulation.
This process of detection is exactly opposite to that of
modulation.

Envelope Detector
• The envelope detector is a simple and very efficient device which is suitable
for the detection of a narrowband AM signal. A narrowband AM wave is the
one in which the carrier frequency fc is much higher as compared to the
bandwidth of the modulating signal ( fc >> W). It produces an output signal
that follows the envelope of the input AM signal exactly.

Diode envelope demodulator
Fig. 7 Diode envelope demodulator

Working Operation
• The standard AM wave is applied at the input of the demodulator. In every positive
half cycle of the input, the diode is forward biased and quickly charge the filter
capacitor C connected across the load resistance R to the peak value of the input
voltage s(t).
• As soon as the input falls down this value, the diode stops conducting.
• The capacitor will now slowly discharge through load resistance R between the
positive peaks as shown in fig. The discharging process continues until the next
positive half cycle.
• When the input signal becomes greater than the capacitor voltage, the diode conducts
again and the process repeats itself.

Fig. 8 Input-output waveforms for the envelope demodulator.

Double Sideband Suppressed Carrier (DSB-SC) System
• For 100% AM modulation about 67% of the total power is required
for transmitting the carrier which does not contain any information.
Hence, if the carrier is suppressed, only the upper and lower sidebands
remain and saving of two-third power may be achieved.
But the channel bandwidth will be same as in AM. If the carrier is
suppressed and the saved power is distributed to the two sidebands, then
such a process is called as Double Sideband Suppressed Carrier
(DSBSC) system.

………….
Mathematical Expression of DSBSC wave
s(t)=m(t)c(t)
s(t)=AmAc cos(2πfmt) cos(2πfct)
s(t)=AmAc 2cos[2π(fc+fm)t]+AmAc 2cos[2π(fc−fm)t]
The equation of DSBSC wave represent the product of
modulating and carrier signals.

Fig. 9 Carrier is suppressed and sidebands are allowed for transmission

Fig. 10 The modulated signal s(t) undergoes a phase reversal whenever the
message signal m(t) crosses zero

Fig. 11 Frequency spectrum of DSBSC
The DSBSC modulated wave has only two frequencies. So, the maximum and minimum
frequencies are fmax = fc + fm and fmin = fc − fm
Bandwidth BW= fc + fm − (fc − fm) ⇒BW = 2 fm
Thus, the bandwidth of DSBSC wave is same as that of AM wave and it is equal to twice the
frequency of the modulating signal.

Ring Modulator
• Diode Ring modulator is one of the most useful product modulator, well
suited for generating a DSB-SC wave.
• Four diodes are arranged in ring form and they are controlled by a square-
wave carrier c(t) of frequency fc, which is applied longitudinally by means of
two center-tapped transformers.

Operation of the circuit
• Diodes are ideal, having constant forward resistance rf and backward resistance rb
• Both transformers are perfectly balanced for no leakage of the modulation frequency.
• |m(t)|<<|c(t)| and m(t) alone cannot forward bias the diodes.

Fig. 12 Circuit diagram of ring modulator and diode bias conditions.
When c(t) is positive, outer diodes are switched to their forward resistance rf (see fig 12 b) and
inner diodes are switched to their backward resistance rb (see fig 12 c)
On the other half-cycle of the carrier wave, the diodes operate in the opposite condition.
In fact, the ring modulator acts as a commutator. Idealized waveforms of the modulated signal s(t)
is shown in fig. 12 (c). Square-wave carrier c(t) can be represented by a Fourier series:

………………….. (1)
………………….. (2)

Fig. 13 Illustrating the operation of the ring modulator.

• It is sometimes referred to as a double-balanced modulator,
because it is balanced with respect to both the baseband signal
and the square wave carrier.
• From the equation (2) it is clear that output s(t) from the
modulator consists entirely of modulation products. If the
message signal m(t) is band limited to − W < f < W, the output
spectrum consists of side bands centered at fc and assume fc > W
so as to prevent side band overlaps

• The same carrier signal (which is used for generating DSBSC signal) is used
to detect the message signal. Hence, this process of detection is called as
coherent or synchronous detection. The block diagram of the coherent
detector is shown in the fig. 14.
Coherent Detector

Fig. 14 Block diagram of the coherent detector

• In this process, the message signal m(t) can be extracted from DSBSC wave by
multiplying it with locally generated carrier, having the same frequency and the phase
of the carrier used in DSBSC modulation. The resulting signal is then passed through
a Low Pass Filter. Output of this filter is the desired message signal.
• Let the DSBSC wave be s(t) = [Ac cos (2πfct)] m(t)
• The output of the local oscillator is c(t) =Ac cos (2πfct + ϕ)
• Where, ϕ is the phase difference between the local oscillator signal and the carrier
signal, which is used for DSBSC modulation.
• From the figure, we can write the output of product modulator as v(t) = s(t) c(t)
• Substitute, s(t) and c(t)values in the above equation.

• v(t)= {[Ac cos (2πfct)] m(t)} Ac cos (2πfct + ϕ)
• =Ac
2 cos(2πfct) cos(2πfct+ϕ)m(t)
• =½ Ac
2 [cos (4πfct+ϕ) + cosϕ] m(t)
• V(t)= ½ Ac
2 cos (4πfct+ϕ) m(t) + ½ Ac
2 cos ϕ m(t)
• In the above equation, the first term represents a DSB-SC modulated signal
with a carrier frequency 2fc. It can be removed by passing it through a low
pass filter. Second term represents message signal m(t) which is limited to the
interval − W < f < W. Therefore, the output of low pass filter is
• v0(t) =½ Ac
2 cos ϕ m(t)

Costas Receiver (Costas loop)
• Costas receiver is a synchronous receiver system, suitable for demodulating DSBSC waves. It consists of two
coherent detectors supplied with the same input signal, s(t). Costas loop is used to make both the carrier signal
(used for DSBSC modulation) and the locally generated signal in phase.
• Costas loop consists of two product modulators with common input s(t), which is DSBSC wave. The other
input for both product modulators is taken from Voltage Controlled Oscillator (VCO) with −900 phase shift to
one of the product modulator.
• The VCO is a sine-wave generator whose output
frequency is determined by the voltage applied to it’s input. The detector in the upper path is referred to as the
in-phase coherent detector or I channel, and that in the lower path is referred to as the quadrature-phase
coherent detector or Q-channel.

Fig. 15 Block diagram of Costas loop.

• The output of Q channel LPF has −900 phase difference with the output of I
channel LPF.
• The outputs of these I-channel and Q-channels are applied as inputs of the
phase discriminator (which consists of a multiplier followed by a LPF).
Based on the phase difference between these two signals, the phase
discriminator produces a DC control signal.
• This signal will be an input of VCO to correct the local phase errors.
Therefore, the carrier signal (used for DSBSC modulation) and the locally
generated signal (VCO output) are in phase.

Quadrature Carrier Modulation (QCM)
• QAM (quadrature amplitude modulation) or Quadrature carrier Modulation
(QCM) is a method of combining two amplitude-modulated (AM) signals
into a single channel, thereby doubling the effective bandwidth. QAM is used
with pulse amplitude modulation (PAM) in digital systems, especially in
wireless applications.
• QAM takes benefit from the concept that two signal frequencies; one shifted
by 90 degree with respect to the other can be transmitted on the same carrier.
One of them is multiplying it with cos ωct and the other multiplying it with
sin ωct. Finally the two signals are added to obtain the QAM signals. The
motivation for QAM comes from the fact that a DSBSC signal occupies 2fm
from which it is derived. This is considered wasteful of resources.

Fig.16 QCM transmitter and receiver
• The transmitter (see fig 16(a)) involves the use of two separate product
modulators that are supplied with two carrier waves of the same frequency but
differing in phase by -90o. The multiplexed signal s(t) consists of the sum of the
two product modulator outputs given by the equation.

Single Side Band Modulation (SSB)
• Transmission in which only one sideband is transmitted, carrier and one sideband are
completely suppressed is called single-sideband transmission or SSB.
• Ordinary AM is very inefficient from two points. 1) it occupies twice the bandwidth
of the maximum message signal frequency, and 2) it is inefficient in terms of the
power used. By removing some of the components of the ordinary AM signal it is
possible to significantly improve its efficiency.
• In the first instance, the carrier is removed – it can be re-introduced in the receiver,
and secondly one sideband is removed – both sidebands are mirror images of one
another and the carry the same information. Thus, SSB modulation requires half the
bandwidth of AM or DSBSC modulation.

Fig. 17 Generation of SSB signal. (a) Method: I (b) Method: II
Method: I shown in fig.17 (a) which employs a Hilbert transform filter
Method: II Frequency discrimination method shown in fig.17 (b) generates, a
DSB-SC AM signal and then employs a filter which selects either the upper sideband
or the lower sideband of the double-sideband AM signal.

Transmission Bandwidth of SSB-SC
• Since we are transmitting the frequencies only in the range (fc+ W) or ( fc –
W), the transmission bandwidth for the SSB-SC will be :
• Bandwidth B = (fc+ W) – fc = W Hz
• Or B= fc – ( fc – W) =W Hz

Demodulation of SSB AM Signals

Demodulation of SSB AM Signals.
• To recover the message signal m(t) in the received SSB AM signal, we
require a phase coherent or synchronous demodulator, as was the case for
DSB-SC AM signals. That means, output of SSB is multiplied with cos(2πfct)
and passed through low pass filter to get back the original message signal
m(t). A pilot carrier or a stable oscillator is needed to eliminate the
undesirable sideband signal components (Donald Duck effect).
• Drawbacks of SSB signal generation:
1. Generation of an SSB signal is difficult.
2. Selective filtering is to be done to get the original signal back.
3. Phase shifter should be exactly tuned to 900.
To overcome these drawbacks of SSB, VSB modulation is used. It can view
as a compromise between SSB and DSB-SC.

VSB Modulation
• VSB transmission is similar to single-sideband (SSB)
transmission, in which one of the sidebands is completely
removed. In VSB transmission, however, the second sideband is
not completely removed, but is filtered to remove all but the
desired range of frequencies.

• In VSB modulation: One sideband is transmitted fully along with a small part
(i.e. vestige) of the other sideband. Hence bandwidth here is represented as
B = (W + fv) Hz
• Where, fv is the vestige bandwidth and W is the message bandwidth. Typically,
fv is 25% of W, which means that the VSB bandwidth lies between the SSB
bandwidth W and DSB-SC bandwidth, 2W.
Generation of VSB modulated wave s(t), it needs to generate a DSB-SC signal
and then pass it through a bandpass filter H(f) as shown in fig. 18.

Fig. 18 Generation of VSB modulated wave

• s(t) = ½ Ac m(t) cos 2πfct - ½ Ac mQ(t) sin2πfct
…………………….. (1)
• In eqn. (1) the first term is in phase component and the second term is quadrature
component.
• We know that, DSB-SC signal u(t) = m(t) c(t)
• Spectrum of VSB modulated wave s(t) is S(f) (see fig. 19) and the transfer
function of the filter H(f)are related as
S(f) = U(f) H(f) = ½ Ac [M (f - fc )- M (f + fc)] H(f)

Demodulation of VSB signal
Fig. 19 Demodulation of VSB signal

VSB Transmission of Analog/Digital TV

Frequency-division multiplexing (FDM)
• The operation of transmitting a number of independent signals over the same
channel is called Multiplexing. The signal must be kept apart so that they do
not interfere with each other, and thus they can be separated at the receiving
end by separating the signals either in frequency or in time using the
techniques frequency-division multiplexing (FDM) or time-division
multiplexing (TDM).

Frequency Translation
• The basic operation involved in SSB modulation is in fact a form of
frequency translation. SSB modulation is sometimes referred to as
frequency changing, mixing, or heterodyning.
• Suppose that we have a modulated wave whose spectrum is centered on a
carrier frequency fc and the requirement is to translate it upward or
downward in frequency, such that fc is changed from to a new value.
• Band-pass filter bandwidth: equal to that of the modulated signal s1(t)
whose spectrum centered on f1 used as input

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