Overview of Some of The Topics We
Will Study in This Course
EE 341, Autumn’21
S.N. Merchant, Gaurav S. Kasbekar
Dept. of Electrical Engineering
IIT Bombay
Modulation and Demodulation
• Analog message signals (e.g, speech, temperature at a location as a
function of time) are often referred to as “baseband signals”
since they are typically lowpass in nature
• In rare cases, baseband signals can be directly transmitted over a channel
(e.g., telephone)
• However, usually, baseband signals produced by message sources are not
suitable for direct transmission over a communication channel
• “Modulation”: process of converting signal from message source into a
form that is suitable for transmission over a communication channel
performed at transmitter
• “Demodulation”: recovering the message signal from received signal
performed at receiver
• “Carrier” is a sinusoid of high frequency: 𝐴 cos 2𝜋𝑓𝑐 𝑡 + 𝜃
• During modulation, one of the carrier
sinusoidal parameters is varied as a function
of the message signal
e.g., amplitude, frequency or phase of carrier
signal (in amplitude modulation (AM), frequency
modulation (FM) and phase modulation (PM),
Ref: “Communication Systems” by S.
respectively) Haykin and M. Moher, 5th ed
Reasons for Modulation and Demodulation
• Ease of Radiation/ Transmission
In case of wireless signals, for efficient radiation of
electromagnetic energy, size of radiating antenna should be of
the order of a fraction (e.g., ½) of the wavelength of the signal
o E.g., power in a speech signal is concentrated at frequencies in the range
100 to 3000 Hz, which corresponds to wavelengths of 100 to 3000 km
Impractically large antenna would be needed
o Solution is to shift the signal to higher frequencies via modulation using a
high-frequency carrier
o E.g., if a 10 MHz carrier is used, then corresponding wavelength is 30 m
and an antenna size of the order of 3 m suffices
• Simultaneous Transmission of Multiple Signals (Multiplexing)
Modulation allows multiple signals to be transmitted
simultaneously in same geographical area without mutual
interference
E.g., signals from different TV stations can be modulated using
carriers of different frequencies
o This translates signal from each TV station to a different frequency range
o At receiver (e.g., TV set), a band-pass filter can select the desired TV
channel for viewing
Noise and Interference in Communication Systems
• Transmitted signal corrupted by noise
• Generated by a number of sources, e.g.:
random motion of electrons, vibration of atoms in receiver circuitry
thermal (“black-body”) radiation from earth, sun, objects in environment, etc.
• Apart from noise, “interference” from other communication systems in
vicinity also corrupts transmitted signal, e.g.:
in cellular networks, signals from neighboring cells
when frequency division multiplexing is used, power that leaks from
neighboring frequency bands
• Noise and interference are of central importance in communication systems
In principle, infinite bits per symbol can be transmitted in a (hypothetical)
channel with no noise or interference!
𝑃
Shannon capacity formula 𝐶 = B log 1 +
𝑁
• We will study:
how to model noise and interference in
communication systems
how to analyze the performance of
different modulation schemes in the
presence of noise
Ref: https://www.sciencedirect.com/topics/engineering/digital-communication-system
Analog and Digital Communications
• Digital communication systems have replaced or are replacing
analog communication systems
• E.g.:
first generation cellular phone technology was analog (Advanced Mobile
Phone Service (AMPS))
2G (Global System for Mobile communication (GSM)) and subsequent
cellular phone technologies (e.g., CDMA, LTE-Advanced and 5G) have
been digital
• E.g.:
Digital TV broadcasting replacing analog TV broadcasting worldwide
• “Analog communication” and “digital communication”:
in practice, all communication is via continuous signals and hence
analog in nature
the message signal that is to be transmitted is either analog or digital
E.g., if the source is speech, then:
o In analog communication, it is directly used to modulate a high-frequency
carrier signal
o In digital communication, it is sampled and quantized to obtain a bit stream,
which is then used to modulate a high-frequency carrier signal
Reasons Why Digital Communication
Outperforms Analog Communication
• Greater Immunity of Digital Signals to Noise and Interference
in digital communication, the message signal is a sequence of 0’s and
1’s
even if noise and interference get added to the modulated signal, it is
possible for the receiver to correctly distinguish a 0 from a 1 (assuming
that noise and interference power are within certain limits)
o so transmitted signal can usually be recovered exactly at receiver
in contrast, in analog communication, the message waveform shape
itself carries the required information, and even a slight amount of
noise or interference will show up in received signal
Ref: https://www.sciencedirect.com/topics/engineering/digital-communication-system
Reasons Why Digital Communication
Outperforms Analog Communication (contd.)
• Viability of Regenerative Repeaters in Digital
Communications
when the transmitter and receiver are separated by a large
distance (e.g., intercontinental link), repeaters are placed at
several points on the path (e.g., every 1 km)
In case of digital communications, at each repeater,
incoming message bits are detected and new, “clean” bits
are transmitted to the next repeater on path
Using this process, transmission over long distances with
great accuracy can be achieved using digital communication
However, in an analog communication system, when a link is
long, transmitted power gets significantly attenuated by the
time it reaches receiver and noise and interference power is
large compared to signal power
If a repeater is used in analog communication, it not only
amplifies useful signal, but also noise and interference
