Communication
Systems
Basic of Signals
By
Engr. Dr. Jawwad Ahmad
1
Today’s Goal
Fundamental Types of Signals
Basic Parameters of Communication Systems
Limitation of Transmission
Shannon's Equation
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Analog & Digital
The information carrying signals are divided into two broad
classes;
Analog
Digital
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Analog Signals
Analog signal is a signal that varies with “time”, usually in a
smooth or continuous fashion and their values are taken from an
infinite set.
Analog signals are continuous electrical signals that vary with
time as shown in figure.
Most of the time, the variations follow that of the non-electric
(original) signal. Therefore, the two are analogous hence the
name analog.
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Analog Signals
Not all analog signals vary as smoothly as the waveform shown
in the Figure. Analog signals represent some physical quantity
and they are a ‘MODEL’ of the real quantity.
Example
Telephone voice signal is analog. The intensity of the voice
causes electric current variations. At the receiving end, the
signal is reproduced in the same proportion. Hence the electric
current is a ‘MODEL’ but not one’s voice since it is an electrical
representation or analog of one’s voice.
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Digital Signals
An ordered sequence of symbols selected from a finite set of
discrete elements.
Digital signals are non-continuous, they change in individual
steps. They consist of pulses or digits with discrete levels or
values. The value of each pulse is constant, but there is an abrupt
change from one digit to the next.
Digital signals with just two discrete values are known as Binary
Signals.
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Basic Parameters of Communication Systems
Spectrum
Spectrum of a signal is the collection of all component
frequencies it contains and is shown using a frequency domain
plot.
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Basic Parameters of Communication Systems
Band-Width
Bandwidth is the width of the frequency spectrum, to
calculate bandwidth, subtract the lowest from the highest of
the range.
The most significant limitation on performance of
communication system.
Bandwidth of Information signal / Bandwidth.
Bandwidth of Communication channel / Channel Capacity.
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Basic Parameters of Communication Systems
Band-width of Information Signal / Bandwidth
It is simply the difference between highest and lowest
frequencies contained in information signal.
Band-width of Communication Channel / Channel Capacity
It is the difference between highest and lowest frequencies
that a channel allows to pass.
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Basic Parameters of Communication Systems
Information Capacity
Information theory is
the study of efficient use
of bandwidth to
propagate information
through electronics
communication system.
It is the measure of how
much information can
be propagated through a
communication system.
It is necessary that the
Channel Capacity must
be greater then the
Bandwidth of the
information signal. Engr. Dr. Jawwad Ahmad 11
Basic Parameters of Communication Systems
Remember!!!
There is a bit difference between AC & Analog and DC
& Digital.
AC is referred to a signal whose polarity is changing with
respect to time.
DC is referred to a signal whose polarity is constant.
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Sampling and Sampling Theorem (Nyquist
Theorem)
The sampling frequency determines the limit of audio
frequencies that can be reproduced digitally.
One of the most important rules of sampling is called the
Nyquist Theorem, which states that “the highest frequency
which can be accurately represented is less than one-half of
the sampling-rate”.
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Sampling and Sampling Theorem (Nyquist
Theorem)
So, if we want a full 20 kHz audio bandwidth, we must
sample at least twice that fast, i.e. over 40 kHz.
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LIMITATION OF TRANSMISSION
The fundamental limitations of information transmission by
electrical means are Bandwidth and Noise.
The concept of bandwidth applies to both signals and
systems as a measure of Speed.
When a signal changes rapidly with time, its frequency
content, or spectrum, extends over a wide range and the
signal has a large bandwidth.
Now the ability of a system to follow signal variations is
reflected in its usable frequency response or Transmission
Bandwidth.
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LIMITATION OF TRANSMISSION
But all electrical systems contain energy storage elements,
and stored energy cannot be changed instantaneously.
Consequently, every communication system has a finite
bandwidth B that limits the rate of signal variations.
In simple words “Communication under real time conditions
requires sufficient transmission bandwidth to accommodate
the signal spectrum”; otherwise, severe distortion will result.
Noise imposes a second limitation on information, which is
unavoidable. And noise can be measure relative to an
information signal in terms of the signal-to-noise-power ratio
S/N.
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LIMITATION OF TRANSMISSION
These problems become most severe on long distance links
when the transmission–loss reduces the received signal
power down to the noise level.
Taking both limitations into account, Shannon (1948) stated
that the rate of information transmission cannot exceed the
Channel Capacity.
This relationship, known as the Hartley-Shannon Law, sets
an upper limit on the performance of a communication
system with a given bandwidth B and signal-to-noise ratio S/N.
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Thank you
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