Lecture 1. References In no particular order Modern Digital and Analog Communication Systems, B. P....
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Communication Systems I Lecture 1
Lecture 1. References In no particular order Modern Digital and Analog Communication Systems, B. P. Lathi, 3 rd edition, 1998 Communication Systems Engineering,
References In no particular order Modern Digital and Analog
Communication Systems, B. P. Lathi, 3 rd edition, 1998
Communication Systems Engineering, J. G. Proakis, M. Salehi, 2 nd
edition, 2002 Communication Systems, S. Haykin, 4 th edition, 2001
Any
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Course Contents Communication Systems I (SC4104) covers: Signal
analysis (revision from signal and systems) Fourier transform
(revision from signal and systems) Power spectrum (also revision)
Principles of modulation AM FM Pulse Modulation Comparison between
Analog and Digital transmission
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Course Contents Time Division Multiplexing Frequency Division
Multiplexing Random Signals and Noise (will be discussed again for
digital communication) Transmission systems for cable, radio,
satellite and optical systems (just a very brief introduction for
later courses)
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Assessment Final exam (probably 70%) Test (probably 30%) H.W,
CW, etc (probably 0%, but you will not pass if you dont do
them!!!!!) Laboratory sessions (another subject!!)
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What is Telecommunication Telecommunication is the process of
transferring a usually unknown message from a source to a
destination separated by some distance (tele) The data/signal sent
from the source should be delivered to the destination accurately
enough to be identified by the destination
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Communication System Source of information generates messages
Transducer converts them to suitable electrical signal (baseband or
message signal) Transmitter modifies the baseband signal for
efficient transmission over the channel
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Communication System The signal is sent from the transmitter
via the channel (coaxial cable, fiber, wireless, ) The channel
distorts the signal and adds noise to it Receiver recovers the
baseband signal by undoing what the transmitter did and trying to
undo what the channel did The output transducer converts the
baseband signal to its original form and it is delivered to the
destination
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The Source We always assume the source have something to send
This can be: Voice (continuous analog signal varying with time)
Image (2D signal, converted to 1 or 3 analog time varying signals)
Video (continuous 2D signal, converted to 1 or 3 analog time
varying signals) Text and symbols Transducers convert these into
electrical signals which are fed to the transmitter To understand
why we need the transmitter (and receiver), we need to see the
channel first
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The Channel Plays key role in choosing the elements of the
system Causes attenuation that increases with the distance Its
attenuation is frequency dependent hence, it distorts the signal
Usually modeled as a low pass filter Also random signals, e.g. from
other transmitters (interference) or other natural or human sources
(noise), are added to the signal
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Sample Signal
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Frequency Domain
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Channel Response
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Freq. Domain After the Channel
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Input and Output Signals
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Channel with twice the bandwidth
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Channel with 4 times the BW
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Ten times the original BW
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Noise Noise power is 1/10 that of signal power (SNR =
10dB)
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So, the channel Attenuates the signal -> limits the distance
Acts as a low pass filter -> distorts the signal Adds noise
-> adds a random signal
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What Does The Transmitter Do? Transmitter Modifies the message
to generate a transmission signal The objective is to avoid/reduce
effects of the bad characteristics of the channel Possible
modifications include: Amplification Band limiting (usually for
digital signals another course) Signal shaping Modulation E.g. if
channel has high attenuation in 0-1 MHz, avoid this bandwidth if
possible
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What Does The Receiver Do? Receiver should undo the changes
made by the transmitter and the channel Possible modifications made
by the receiver: Amplification Demodulation Signal shaping
Filtering The ultimate goal is to make the combined response of the
transmitter, channel and receiver = 1, i.e. transparent system
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Modulation The message from the source, typically has a
bandwidth that starts from (or near) 0 Hz Such a signal is called a
baseband signal These frequencies are not always suitable for
transmission in the channel To solve this, the transmitter modifies
the signal to move it to a higher frequency more suitable for the
channel This process is known as modulation
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Modulation Modulation requires the existence of a high
frequency carrier This carrier is usually a cosine and/or sine
signal in the form: The modulation process alters the amplitude
(A), frequency (f c ) or phase ( ) of the carrier under the control
of the input message This leads to Amplitude Modulation (AM),
Frequency Modulation (FM) or Phase Modulation (PM)
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Demodulation The receiver monitors the amplitude, frequency or
phase of the received signal Based on the changes in these values,
it recovers the message transmitted
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Benefits of Modulation Ease of radiation Simultaneous
transmission of several signals Avoid bad bands in the channel Can
make the signal more resistant to noise
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Frequencies for Various Wired Channels
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Frequency Bands for Wireless Channels
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Required Course Tools To study communication systems we need
knowledge of the following: Analysis of signals and systems Fourier
transforms (signal BW, channel response) Laplace transform (channel
response) Convolution (output signal) Background about filters
Characteristics of trigonometric functions Probability and
statistics (study of noise)
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Revision What is Fourier transform? For periodic signals (x(t))
the signal is given by: Where is any suitable constant Fourier
transform is discrete (multiples of fundamental frequency f 0 = 1/T
0 )
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Revision For aperiodic signals (x(t)), the Fourier spectrum
(X(f)) is continuous For x(t) to have a Fourier transform,
Dirichlet conditions are met: x(t) is absolutely integrable in time
Finite number of maxima and minima within a finite time interval
Number of discontinuities within a finite time interval is finite
All applicable for real signals
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Revision The energy across 1 Ohm of an aperiodic signal is
given by (power = 0): |G(f)| 2 is the energy spectral density
(energy unit/Hz) Periodic signals have infinite energy, so we
calculate the energy in one cycle and average it over the cycle
duration to give the power (energy = ):