Prepared by ENG. A.SAMY
TM355
Communication Technologies
Block 1 Part (1) Cont.
Channels for Communications
3.4 limitations of optical fiber
1- Attenuation and decibels
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Attenuation is the process by which the signal gradually loses power as it travels along a transmission
medium.
Decibels (dB) are a way of comparing two powers.
The two powers to be compared may represent a loss, as in fibre attenuation, or a gain, as when a
signal is amplified.
The decibel is a logarithmic measure of the ratio between two powers.
Decibels are defined such that:
• Increasing a power by 10 dB multiplies the power (in W) by 10
• Attenuating a power by 10 dB divides the power (in W) by 10.
The transmitter connects at the left-hand end of the fibre, so this point is labelled as 0 dB, meaning ‘no
loss’.
Another useful fact about decibels is that:
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Increasing a power by 3 dB doubles the power
Attenuating a power by 3 dB halves the power.
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If we were to work out the effect of a series of losses using simple ratios, then you would need to
multiply the ratios.
• Example: suppose a fibre-optic link consists of three sections. Half the power is lost in the first
section, another half of the remaining power is lost in the second section, and 95% of the
remaining power is lost in the third section (so 5% is left). Then the total fraction remaining is:
0.5 × 0.5 × 0.05 = 0.0125.
• Note : ( 1/2 * 1/2 * 1/20 ) 2=3db 10 =10db ; 20 = 10*2 = 10+3 = 13db
• But working in decibels, instead of multiplying the ratios, you just add the decibels. So in this
example: 3 dB + 3 dB + 13 dB = 19 dB total loss.
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Multimode fiber has a higher figure for attenuation, it is generally preferred for short-distance
applications because of lower component costs and greater ease of use than single-mode fiber.
2- Pulse spreading
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As with attenuation, the effects are cumulative ➔ the longer the fiber, the worse it gets.
Figure 1.21 illustrates the problem:
• The signal transmitted is called a pulse, so the effect is known as pulse spreading.
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One reason for pulse spreading in multimode fibers: different path lengths result in different
timings for the trip through the fiber ➔ this is called multimode distortion and is the main cause of
pulse spreading in multimode fibers.
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This effect is eliminated in single-mode fibers, other mechanisms can still cause pulse spreading:
• Dispersion‫تشتت‬, or ‘chromatic ‫لوني‬dispersion’, is caused by light of different wavelengths
travelling at different speeds.
• Polarization mode distortion affects single-mode fibers and is caused by variation in the speed
of light.
3- Optical transmitters and detectors
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Recall: a basic optical-fiber link has three main components:
• a transmitter that includes a light source (controlled by input data),
• the optical fiber itself,
• And a receiver that includes a detector.
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Optical transmitter: converts input data in the form of an electrical signal into a light signal that is sent
along the fiber.
There are two main types, both semiconductor devices: the light-emitting diode (LED) and the laser
diode
• LEDs emits light in the infrared region of the spectrum, where optical fibers are most
transparent.
• Laser diodes are also found in CD, DVD and Blu-ray drives, where they read and write data
from the disc.
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LEDs are inexpensive compared to laser diodes and is used in multimode fiber systems.
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However LEDs, they have a number of disadvantages:
• They are lower in power and emit over a range of wavelengths, leading to dispersion.
• LEDs emit a relatively broad cone ‫مخروط عريض‬of radiation,
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Advantages of laser diode it emits a strongly aligned beam‫حزمه مركزه‬.
Laser diode is much more efficient at transferring its energy to the fiber.
Laser diode has an advantage over the LED in the speed at which it can switch.
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The data rate of the transmitter depends on how fast the beam can be modulated.
With both LEDs and laser diodes, the beam can be modulated directly by varying the electrical power
supplied to them.
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A Optical detector converts the light signal back into an electrical signal.
The type of detector commonly used is called a photodiode.
It provides a current output that varies with the intensity‫ شدة‬of the light it receives.
4- Optical amplifiers
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Optical fibers attenuate over long distances.
One way of increasing the range of an optical-fiber link is to use a repeater or regenerator.
These are devices that counteract the effects of attenuation by restoring an optical signal to its
original form.
The optical signal is converted back to an electrical signal, which is then processed electronically and
retransmitted optically.
A repeater amplifies the signal to bring it back to its original amplitude, but at the same time it may
also amplify any noise that is mixed with the signal.
A regenerator does further processing, so that the degraded received pulse would be reshaped and
restored to its original amplitude without any noise.
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Long distance can be covered using a chain of regenerators, but there are disadvantages: these devices
have to be powered and maintained, and many of them may be needed to cover long distances.
This is a particular problem for international cables, which often run under the ocean, making
power provision and maintenance very difficult.
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Optical amplifiers have been developed as a better solution for long-haul links.
They amplify the optical signal directly, without converting it back to an electrical signal.
5- Copper cable
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The sender and receiver in most communications systems operate with electrical signals
Copper cable is a simple solution because it does not involve any conversions to other types of energy
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A current flow around the circuit: from the source to the load along the top wire, through the load and
back along the bottom wire.
In the case of the battery and bulb, the current circulates in the same direction all the time called Direct
current (DC)
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The Alternating current (AC) changes direction at regular intervals
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Voltage is the name of the force that sends the current around the circuit and is measured by Volts (V).
Current is measured in amperes or amps (A): due to the movement of atomic particles called electrons.
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The speed of a signal in a copper cable depends on the construction of the cable
A typical figure is 2 ×108 m/s, which is comparable to the speed of signals in optical fibers.
Magnetic and electric fields
• A conductor carrying current has both magnetic and electric fields associated with it.
• A magnetic field encircles a conductor carrying an electric current.
• Conductors also have associated electric fields
• Many cables have multiple pairs of conductors rather than just one, so that several independent signals
can be carried.
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There is a potential problem, the electric and/or magnetic fields associated with one pair of conductors
may couple with the conductors next to them to some extent
This is called crosstalk and can be minimized by appropriate design of the cable.
Resistance: electrons do not flow along conductors entirely freely, but are subject to frequent
collisions.
This results in loss of electrical energy, which is converted to heat.
Solution: The two conductors in a pair are kept apart by a non-conducting material, usually
plastic, known as an insulator or dielectric.
Distortion can occur in cables when signals of different frequencies travel at different speeds
Types of cables
• Unshielded twisted pair (UTP) and coaxial cable.
• In a UTP cable, a pair of conductors is twisted together along its length.
The effect of the twisting is that any interference entering the cable will affect both conductors equally, and can
be cancelled out by using a receiver
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So the twisting gives some protection against crosstalk.
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In coaxial cable, the two conductors take the form of a center conductor with a conducting shield
around it.
An advantage of this construction is that the electric and magnetic fields are confined within the shield.
This gives the cable good immunity to interference and minimizes losses due to radiation.
• A common use of coaxial cable is for connection to TV antennas.
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6- Radio
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Radio waves are another form of electromagnetic radiation but at a much lower frequency than light or
infrared, 300 GHz often being regarded as the upper limit.
Their electric and magnetic fields are generated directly from electrical signals in structures known as
antennas, or sometimes aerials.
A receiving antenna converts a radio signal back to an electrical signal.
An antenna simply consists of one or more conductors
A major challenge in radio communications is to ensure that a receiver picks up only the desired
transmission.
A filter in a receiver allows a narrow band of frequencies and attenuates all others.
Bandwidth and reception
• The bandwidth is equal to the difference between the highest and lowest frequencies, 𝒇𝟐 − 𝒇𝟏 .
• The larger the bandwidth, the more information the signal can convey.
• The centre frequency of this transmission, halfway between 𝒇𝟏 and𝒇𝟐 .
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The range of frequencies that the receiver responds best to is called the passband:
• Extends from a lower cut-off frequency to a higher cut-off frequency.
Some properties of radio waves
• Radio waves are forms of electromagnetic radiation.
• Radio waves can be reflected or refracted, like light, and they can lose power as they travel through a
medium.
• Radio waves can be guided in tubular structures called waveguides, which are used at microwave
frequencies in applications such as radar.
• As a radio wave moves away from the transmitter the power conveyed in the wave is spread over a
wider and wider area, the power received goes down
The inverse square law
• The inverse square law describes the reduction in power with distance from the transmitter, due to
spreading.
𝟏
• A receiver that is n times as far from the transmitter will receive 𝒏𝟐 of the power.
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Reflection‫االنعكاس‬
• Radio waves can be both reflected and scattered from any surface and especially metal.
Scattering ‫انتشار‬
• When reflecting objects or features are small compared to the wavelength, can result in a loss of
useful energy between the transmitter and receiver
Absorption‫االمتصاص‬
• Radio waves can be absorbed by the medium they travel through. Absorption is dependent on
frequency
Diffraction‫االنحراف‬
• Diffraction: is the spreading or bending of an electromagnetic wave when it passes through a gap or
encounters a sharp corner.
• Diffraction is very dependent on the dimensions of a gap or the sharpness of an edge.
Propagation models
• Propagation: at low frequencies, long-distance transmission depends on the state of the ionosphere,
• while at high frequencies multipath propagation causes fading.
• The inverse fourth-power law is often invoked as a first approximation for propagation at VHF (very
high frequency) and above in typical terrestrial environments‫بيئه ارضيه‬.
• With the inverse fourth-power law the received power decreases in proportion to1⁄𝑑4 .
7- Analogue modulation
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A single cable is made to carry several messages simultaneously by having a set of carriers at
different frequencies and modulating each independently,
Modulation is usually a matter of varying one or more properties of the sine wave in a way that
represents the information to be conveyed.
AM radio broadcasting is often called medium-wave radio due to the frequency range it mostly uses
(around 530 to 1700 kHz).
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FM usually operates in the VHF band between 87.5 and 108 MHz.
Amplitude modulation
• In amplitude modulation (AM) the amplitude of the carrier waveform is altered in proportion to the
information signal, referred to from now on as the modulating signal.
• The term envelope is used to describe the varying strength, or shape, of the modulating signal
• The biggest advantage of AM, compared to other types of modulation, is its simplicity.
• The modulated signal can be created simply by multiplying the modulating signal and the carrier
signal together.
• Done using a device commonly used in radio systems, known as a mixer.
• A mixer is used to shift power at one frequency to power at another frequency.
• The carrier waveform is usually generated using a local oscillator
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The AM-modulated signal has a frequency spectrum that comprises components, known as sidebands
Bandwidth of the modulated signal, 𝐵𝐴𝑀 , is twice that of the original information signal, 𝐵𝑚
➔ 𝐵𝐴𝑀 = 2𝐵𝑚 .
An AM signal is highly susceptible to noise. This will affect the signal’s envelope, making it
impossible to extract the exact signal.
Frequency modulation
• In frequency modulation (FM) the frequency of the carrier waveform is altered in proportion to the
envelope of the modulating signal, so the amplitude and the phase remain the same.
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The modulated signal is usually created using a voltage-controlled oscillator (VCO).
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This is an electronic circuit that takes a voltage signal as an input and produces a periodic
electronic signal
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Two terms that are closely related and that help determine the bandwidth of an FM signal are the
frequency deviation and the modulation index.
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The frequency deviation, Δf: defined as the maximum deviation of the FM-modulated frequency
from the carrier frequency.
The modulation index, β: ratio of the frequency deviation to the highest frequency component in
the modulating signal (modulating frequency), 𝑓𝑚 :
∆𝑓
𝛽=
𝑓𝑚
Bandwidth of frequency modulated signal is:
𝐵𝐹𝑀 = 2(∆𝑓 + 𝑓𝑚 ) = 2(1 + 𝛽)𝑓𝑚
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