A brief discussion of Introduction to communication systems. Prof. H.Amindavar complementary notes for the first session of "Advanced communications theory" course, Spring 2021

1. Advanced Communication’s Theory
Session 1
H.Amindavar
February 2021
Analog vs. Digital Communications:
1. In Analog communication analog signal is used for information transmission.
In Digital communication digital signal is used for information transmission.
2. Analog communication uses analog signal whose amplitude varies continuously with
time for example from 0 to 100.
Digital communication uses digital signal whose amplitude is of two levels either
Low i.e., 0 or either High i.e., 1.
3. In Analog communication , signal gets affected by noise highly during transmission
through communication channel.
In Digital communication signal gets affected by noise less during transmission
through communication channel.
4. In Analog communication only limited number of channels can be broadcasted
simultaneously.
Digital communication can broadcast large number of channels simultaneously.
5. In Analog communication error Probability is high. In Digital communication
error Probability is low.
6. In Analog communication noise immunity is poor. In Digital communication
noise immunity is good.
7. In Analog communication coding is not possible. In Digital communication
coding is possible. Different coding techniques can be used to detect and correct
errors.
8. Separating out noise and signal in Analog communication is not possible.
Separating out noise and signal in Digital communication is possible.
9. Analog communication system is having complex hardware and is less flexible.
Digital communication system is having less complex hardware and is more flexible.
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2. 10. In Analog communication for multiplexing Frequency Division Multiplexing (FDM)
is used.
In Digital communication for multiplexing Time Division Multiplexing (TDM) is
used.
11. Analog communication system is low cost. Digital communication system is
high getting cheaper by day.
12. Analog communication requires low bandwidth. Digital communication requires
high bandwidth.
13. In Analog communication Power consumption is high. In Digital communication
Power consumption is low.
14. In Analog communication is less portable.In Digital communication Portability
is high.
15. In Analog communication there is no privacy or privacy is less so it’s not highly
secured. In Digital communication Privacy is high so it is highly secured.
16. Analog communication does not assure an accurate data transmission. Digital
communication assures a more accurate data transmission.
17. Synchronization problem: Digital communication Synchronization problem is eas-
ier.
A Digital communication system consists of :
Formatting
Source
Encoding
Channel
Encoding
Encryption Multiplexing Modulation
Frequency
Spreading
Multiple
Access
Channel
Source of
Information
Demultiple
Access
Frequency
Despreading
Demodulation
Demultiplexing
Channel
Decoding
Decryption
Source
Decoding
deformatting
Sink of
Information
Synchronization
Figure 1: block diagram of a digital communication system
• Digital Signal Formatting is the process of transforming information from one
format into another.
This is often used in many digital devices and for communication processes. A digital
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3. system is a data technology that uses discrete (discontinuous) values. By contrast, non-
digital (or analog) systems use a continuous range of values to represent information.
Although digital representations are discrete, the information represented can be either
discrete, such as numbers, letters or icons, or continuous, such as sounds, images, and
other measurements of continuous systems.
Formatting
Lowpass Filterring
Clipping
Sampling
Quantization
Digitization
Figure 2: Formatting
• Encoding is the process of converting the data or a given sequence of characters,
symbols, alphabets etc., into a specified format, for the secured transmission of data.
• Decoding is the reverse process of encoding which is to extract the information
from the converted format.
• Encryption uses an algorithm to scramble, or encrypt data and then uses a key for
the receiving party to unscramble, or decrypt, the information. The message contained
in an encrypted message is referred to as plain-text. In its encrypted, unreadable form
it is referred to as cipher-text.
• The purpose of Channel Coding theory is to find codes which transmit quickly,
contain many valid code words and can correct or at least detect many errors. While
not mutually exclusive, performance in these areas is a trade off. So, different codes are
optimal for different applications. The needed properties of this code mainly depend
on the probability of errors happening during transmission. In a typical Compact Disc
(CD), the impairment is mainly dust or scratches.
• Modulation is a process of mixing a signal with a sinusoid to produce a new
signal. ... The signal that is used in modulating the carrier signal(or sinusoidal signal)
is known as the ”data signal” or the ”message signal”. It is important to notice that
a simple sinusoidal carrier contains no information of its own. The most fundamental
modulation schemes are:
F Amplitude modulation (AM) is a modulation technique used in electronic
communication, most commonly for transmitting messages with a radio carrier
wave. In amplitude modulation, the amplitude (signal strength) of the carrier
wave is varied in proportion to that of the message signal, such as an audio
signal.
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4. Modulation
Analog Digital
Amplitude
Modulation(AM)
Angle
modulation
frequency
modulation(FM)
phase
modulation(PM)
Constant
Envelope
Non constant
Envelope
FSK PSK ASK QAM
Figure 3: Analog and Digital modulation schemes
F Frequency modulation (FM) is the encoding of information in a carrier wave
by varying the instantaneous frequency of the wave. The technology is used in
telecommunications, radio broadcasting, signal processing, and computing.
F Phase modulation (PM) is a modulation pattern for conditioning commu-
nication signals for transmission. It encodes a message signal as variations in
the instantaneous phase of a carrier wave. Phase modulation is one of the two
principal forms of angle modulation, together with frequency modulation.
F Quadrature amplitude modulation (QAM) is the name of a family of digital
modulation methods and a related family of analog modulation methods widely
used in modern telecommunications to transmit information.
It conveys two analog message signals, or two digital bit streams, by changing
(modulating) the amplitudes of two carrier waves, using the amplitude-shift key-
ing (ASK) digital modulation scheme or amplitude modulation (AM) analog mod-
ulation scheme.
The two carrier waves of the same frequency are out of phase with each other by
90o
, a condition known as orthogonality or quadrature.
The transmitted signal is created by adding the two carrier waves together. At
the receiver, the two waves can be coherently separated (demodulated) because
of their orthogonality property.
Another key property is that the modulations are low-frequency/low-bandwidth
waveforms compared to the carrier frequency, which is known as the narrowband
assumption.
F In radio communications, single-sideband modulation (SSB) or single-sideband
suppressed-carrier modulation (SSB-SC) is a type of modulation used to
transmit information, such as an audio signal, by radio waves.
by a refinement of amplitude modulation, it uses transmitter power and band-
width more efficiently. Amplitude modulation produces an output signal the
bandwidth of which is twice the maximum frequency of the original baseband
signal.
Single-sideband modulation avoids this bandwidth increase, and the power wasted
on a carrier, at the cost of increased device complexity and more difficult tuning
at the receiver.
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5. • The Multiplexing divides the capacity of the communication channel into several
logical channels, one for each message signal or data stream to be transferred.
generally multiplexing combines several low-speed signals for transmission over a single
high-speed connection.
S1 S2 S3
R1 R2 R3
7
6
5
7
6
5
7
6
5
4 4 4 3 2 1
3 3 2 2 1 1
7 7 7 6 6
6 5 5 5 4 4 4
3
2
1
3
2
1
3
2
1
Channel
Figure 4: Comprehensive scheme of multiplexing and demultiplexing
A device that performs the multiplexing is called a multiplexer (MUX), and a device
that performs the reverse process is called a demultiplexer (DEMUX or DMX). some
multiplexing techniques are :
F Time-division multiplexing (TDM) is a method of transmitting and receiving
independent signals over a common signal path by means of synchronized switches
at each end of the transmission line so that each signal appears on the line only
a fraction of time in an alternating pattern.
F In telecommunications, Frequency-Division Multiplexing (FDM) is a tech-
nique by which the total bandwidth available in a communication medium is
divided into a series of non-overlapping frequency bands, each of which is used to
carry a separate signal.
F Space Division Multiplexing (SDM) transmits different information in dif-
ferent physical areas
F In fiber-optic communications, Wavelength-Division Multiplexing (WDM)
is a technology which multiplexes a number of optical carrier signals onto a single
optical fiber by using different wavelengths of laser light.
F Polarization-Division Multiplexing (PDM) is a physical layer method for
multiplexing signals carried on electromagnetic waves, allowing two channels of
information to be transmitted on the same carrier frequency by using waves of
two orthogonal polarization states.
F Orbital Angular Momentum multiplexing (OAM) is a physical layer method
for multiplexing signals carried on electromagnetic waves using the orbital angu-
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6. lar momentum of the electromagnetic waves to distinguish between the different
orthogonal signals.
Multiplexing
Analog Digital
Frequency
division
Multiplexing
Wavelength
division
Multiplexing
Time division
Multiplexing
Figure 5: Multiplexing techniques
• Multiple access just means many can access at one time.
Ethernet(Ethernet is a way of connecting computers and other network devices in a
physical space. This is often referred to as a local area network or LAN. The idea of an
Ethernet network is that computers and other devices can share files, information and
data between each other efficiently. Ethernet was released in 1980) is multiple access,
token ring(Token Ring is a computer networking technology used to build local area
networks. It was introduced by IBM in 1984, and standardized in 1989 as IEEE 802.5.
It uses a special three-byte frame called a token that travels around a logical ring of
workstations or servers) is not.
F The frequency-division multiple access (FDMA) channel-access scheme
is the most standard analog system, based on the frequency-division multiplex-
ing (FDM) scheme, which provides different frequency bands to different data
streams. In the FDMA case, the frequency bands are allocated to different nodes
or devices. An example of FDMA systems were the first-generation 1G cell-phone
systems, where each phone call was assigned to a specific uplink frequency chan-
nel, and another downlink frequency channel. Each message signal (each phone
call) is modulated on a specific carrier frequency.
F The time-division multiple access (TDMA) channel access scheme is
based on the time-division multiplexing (TDM) scheme. TDMA provides differ-
ent time slots to different transmitters in a cyclically repetitive frame structure.
For example, node 1 may use time slot 1, node 2 time slot 2, etc. until the last
transmitter when it starts over. An advanced form is dynamic TDMA (DTDMA),
where an assignment of transmitters to time slots varies on each frame.
F The code division multiple access (CDMA) scheme is based on spread
spectrum, meaning that a wider radio channel bandwidth is used than the data
rate of individual bit streams requires, and several message signals are transferred
simultaneously over the same carrier frequency, utilizing different spreading codes.
Per the Shannon–Hartley theorem(C = B log2(1 + SNR)), the wide bandwidth
makes it possible to send with a signal-to-noise ratio of much less than 1 (less than
0 dB), meaning that the transmission power can be reduced to a level below the
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7. level of the noise and co-channel interference from other message signals sharing
the same frequency range.
F space-division multiple access (SDMA) is a channel access method based
on creating parallel spatial pipes (focused signal beams) using advanced antenna
technology next to higher capacity pipes through spatial multiplexing and/or
diversity, by which it is able to offer superior performance in radio multiple access
communication systems (where multiple users may need to use the communication
media simultaneously).
• In telecommunication and radio communication, Spread-Spectrum techniques
are methods by which a signal (e.g., an electrical, electromagnetic, or acoustic signal)
generated with a particular bandwidth is deliberately spread in the frequency domain,
resulting in a signal with a wider bandwidth. These techniques are used for a variety of
reasons, including the establishment of secure communications, increasing resistance to
natural interference, noise, and jamming, to prevent detection, and to enable multiple-
access communications. some multiplexing techniques are :
F In digital communications, chirp spread spectrum (CSS) is a spread spectrum
technique that uses wideband linear frequency modulated chirp pulses to encode
information.
F In telecommunications, direct-sequence spread spectrum (DSSS) is a spread-
spectrum modulation technique primarily used to reduce overall signal interfer-
ence. The direct-sequence modulation makes the transmitted signal wider in
bandwidth than the information bandwidth.
F Frequency-hopping spread spectrum (FHSS) is a method of transmitting
radio signals by rapidly changing the carrier frequency among many distinct fre-
quencies occupying a large spectral band. The changes are controlled by a code
known to both transmitter and receiver. FHSS is used to avoid interference,
to prevent eavesdropping, and to enable code-division multiple access (CDMA)
communications.
• Time-hopping (TH) is a communications signal technique which can
be used to achieve anti-jamming (AJ) or low probability of intercept
(LPI). It can also refer to pulse-position modulation, which in its sim-
plest form employs 2k
discrete pulses (referring to the unique positions
of the pulse within the transmission window) to transmit k bit(s) per
pulse.
To achieve LPI, the transmission time is changed randomly by varying
the period and duty cycle of the pulse (carrier) using a pseudo-random
sequence. The transmitted signal will then have intermittent start and
stop times. Although often used to form hybrid spread-spectrum (SS)
systems, TH is strictly speaking a non-SS technique. Spreading of the
spectrum is caused by other factors associated with TH, such as using
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8. pulses with low duty cycle having a wide frequency response. An exam-
ple of hybrid SS is TH-FHSS or hybrid TDMA (time division multiple
access).
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