Download Communication Channels and Noise

King Saud University
College of Applied studies and Community Service
1301CT
Chapter#6
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
the communication channel is the medium of
transmission that provides the connection
between the transmitter and the receiver.

It can be a physical or non-physical link

A communications channel moves
electromagnetic energy from one or more
source to one or more receiver.
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The channels can be classified as :
 Analog Channels:
These channels can carry analog signals.
Examples: telephone system, commercial
radio system
 Digital Channels:
These channels can carry digital signals.
Example: computer communications
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Channels also can be classified as either bounded or
unbounded.

In bounded (guided) channels, signals are confined to
the medium and do not leave it.
Examples: twisted pair, coaxial cable, and optical fiber

In unbounded (unguided) channels, the signals
originated by the source travel freely into the medium
and spread throughout the medium.

Unguided media employ an antenna for transmitting
through air or water
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The three commonly used guided media are
twisted pair, coaxial cable, and optical fiber
A twisted pair consists of two insulated
copper wires twisted together in a helical
form
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
Coaxial cable consists of two conductors. The
inner conductor is held inside an insulator
with the other conductor woven around it
providing a shield. An insulating protective
coating called a jacket covers the outer
conductor.
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
Fiber optic cables: These cables carry the
transmitted information in the form of a
fluctuating beam of light in a glass fiber.
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
An antenna can be defined as an electrical conductor
or system of conductors used either for radiating
electromagnetic energy or for collecting
electromagnetic energy.

For transmission of a signal, electrical energy from the
transmitter is converted into electromagnetic energy
by the antenna and radiated into the surrounding
environment (atmosphere, space, water).

For reception of a signal, electromagnetic energy
impinging on the antenna is converted into electrical
energy and fed into the receiver.
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Isotropic antenna radiates power in all directions
equally. The actual radiation pattern for the
isotropic antenna is a sphere with the antenna at
the center.
 Omni-directional antenna radiates power in a
circle.
 Dish and Yagi are two common types of directional
Antennas.

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
A signal radiated from an antenna travels along one of three
routes: ground wave, sky wave, or line of sight (LOS).

Ground waves: The signal follows the curvature of the
earth’s surface

Sky waves: The signal bounces back and forth between the
earth’s surface and the earth’s ionosphere (for the higher HF
frequencies).

Because it depends on the Earth's ionosphere, it changes
with the weather and time of day.
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Line of sight propagation transmits exactly in the line of
sight. The receive station must be in the view of the transmit
station.
It is limited by the curvature of the Earth for ground-based
stations (100 km, from horizon to horizon).
To facilitate beyond-the-horizon propagation, satellite or
terrestrial repeaters are used
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
We will talk about
two ranges of
frequencies:
microwave range
and radio range
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Microwave signals are higher frequency signals.
Due to a higher frequency of operation,
microwave systems carry large quantities of
information.
It is highly directional so it follow line-of-sight
(LOS) propagation.
The required antenna is smaller due to shorter
wavelength (due to higher frequencies). Take
note that the size of the antenna required to
transmit a signal is proportional to the
wavelength (λ) of the signal.
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Microwave is quite suitable for point-to-point
transmission and it is also used for satellite
communications.
Radio frequency is lower suitable for
omnidirectional applications.
It follow ground or sky wave propagation
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In unguided channels, signals are not only
transmitted directly from source to
destination but also a lot of paths from
source to destination by reflection,
diffraction , …etc.
So the receiver receive multiple copies
(components) of transmitted signal.
Line of sight (LOS) is the fastest component
reaching to destination.
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When a signal is transmitted over a communication
channel, it is subjected to different types of impairments
because of imperfect characteristics of the channel.
 As a consequence, the received and the transmitted
signals are not the same.
 These impairments introduce random modifications in
analog signals leading to distortion. On the other hand, in
case of digital signals, the impairments lead to error in the
bit values.
 Impairments:
- Attenuation and attenuation distortion
- Delay distortion
- Noise
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
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Irrespective of whether a medium is guided or
unguided, the strength of a signal falls off
with distance.
The attenuation leads to several problems:
1.
- To be able to detect correctly the signal, the
signal strength should be sufficiently high .
- If the strength of the signal is very low, the
signal cannot be detected and interpreted
properly at the receiving end.
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- An amplifier can be used to compensate the
attenuation of the transmission line.
- So, attenuation decides how far a signal can be sent
without amplification through a particular medium.
2. Attenuation Distortion
- Attenuation of all frequency components is not
same.
- Some frequencies are passed without attenuation,
some are weakened and some are blocked.
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- As an example, after sending a square wave
through a medium, the output is no longer a
square wave because of more attenuation of
the high-frequency components in the
medium.
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A composite signal made of different frequencies
components

Each signal component has its own propagation speed
through a medium and, therefore, its own delay in
arriving at the final destination.

Differences in delay may create a difference in phase if
the delay is not exactly the same as the period duration.

In other words, signal components at the receiver have
phases different from what they had at the sender.

The shape of the composite signal is therefore not the
same.
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As signal is transmitted through a channel,
undesired signal in the form of noise gets
mixed up with the signal, along with the
distortion introduced by the transmission
media.
Noise is any unwanted energy tending to
interfere with the signal to be transmitted.
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The noise either be:

External Noise. This is noise originating from
outside the communication system

Internal Noise: This is noise originating from
within the communication system.
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
Thermal Noise: This noise is due to the
random and rapid movement of electrons in
any resistive component. Electrons “bump”
with each other.

Impulse noise is irregular pulses or noise
spikes of short duration
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Cross talk is a result of bunching several
conductors together in a single cable. Signal
carrying wires generate electromagnetic
radiation, which is induced on other
conductors because of close proximity of the
conductors.
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In the study of noise, it is not important to
know the absolute value of noise.
Even if the power of the noise is very small, it
may have a significant effect if the power of
the signal is also small.
What is important is a comparison between
noise and the signal.
The signal-to-noise ratio (SNR) is the ratio of
signal power to noise power.
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SNR = Ps / Pn
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Ideally, SNR = ∞ (when Pn = 0). In practice, SNR
should be high as possible.

A high SNR ratio means a good-quality signal.

A low SNR ratio means a low-quality signal.

The SNR is normally expressed in decibels,
that is:
SNR = 10 log10 (Ps / Pn)
dB
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
The power of a signal is 10 mW and the power
of the noise is 1 μW; what are the values of
SNR and SNRdB
SNR = 10 × 10-3 / 10-6 = 10,000
SNRdB = 10 log10 (10 × 10-3 / 10-6)
= 10 log10 (10,000) = 40 dB
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
The maximum rate at which data can be correctly
communicated over a channel in presence of
noise and distortion is known as its channel
capacity.

The capacity of an analog channel is its bandwidth

The capacity of a digital channel is the number of
digital values the channel can convey in one
second. It is usually measured in bits per second
(bps)
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
The bandwidth of a channel is the difference
between the lowest and highest frequency an
analog channel can convey to a receiver .

A channel with a wide bandwidth is called
broadband channel

A channel with a narrow bandwidth is called
baseband channel.
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
Digital signals consist of a large number of frequency
components. If digital signals are transmitted over a
channel with a limited bandwidth, only those
components that are within the bandwidth of the
transmission medium are received.

The faster the data rate of a digital signal, the higher
the bandwidth will be required since the frequency
components will be spaced farther apart.

Therefore, a limited bandwidth will also limit the data
rate that can be used for transmission
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
For a noiseless channel, the Nyquist bit rate
formula defines the theoretical maximum bit
rate of a transmission medium as a function
of its bandwidth
C = 2 x B x log2 m
bits/sec,
C is known as the channel capacity,
B is the bandwidth of the channel
and m is the number of signal levels used.
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
Consider a noiseless channel with a bandwidth
of 3kHz transmitting a signal with two signal
levels. What is the Nyquist bit rate?
C=2 x 3000 x log2 2 =6000 bps

Consider the same noiseless channel
transmitting a signal with four signal levels.
What is the Nyquist bit rate?
C=2 x 3000 x log2 4 = 12,000 bps
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
In reality, we cannot have a noiseless channel;
the channel is always noisy. In 1944, Claude
Shannon introduced a formula, called the
Shannon capacity, to determine the
theoretical highest data rate for a noisy
channel
C = BW x log2 (1 +SNR)
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Consider an extremely noisy channel in which the
value of the SNR is almost zero. In other words, the
noise is so strong that the signal is faint. For this
channel the capacity C is calculated as
C=B log2 (1 + SNR) =B log2 (1 + 0) =B log2 1 =B x 0 = 0
 This means that the capacity of this channel is zero
regardless of the bandwidth.
 In other words, we cannot receive any data through
this channel.

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A telephone line normally has a bandwidth of 3000
Hz (300 to 3300 Hz) assigned for data
communications. The SNR is usually 3162. For this
channel the capacity is calculated as
C =B log2 (1 + SNR) =3000 log2(1 + 3162) =
3000 log2 3163 = 3000 x 11.62 = 34,860 bps
 This means that the highest bit rate for a telephone
line is 34.860 kbps.
 If we want to send data faster than this, we can
either increase the bandwidth of the line or
improve the SNR
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