Download Allocation of electromagnetic spectrum

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Allocation of electromagnetic spectrum
λ=
=f
In the figure, λ = c/f,
where:
● λ is the wavelength in
meters;
● c is the propagation
speed of light (identical to
that of a radio wave) in
meters per second
(approximately 300,000
km/s);
● f is the frequency in Hz
(defined as 1 cycle per
second).
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Radio propagation modes
Radio waves at different frequency bands propagate in different ways. Some of
these propagation modes are as follows:
• Ground wave: The radio wave follows the surface of the Earth, and thus
communication over the horizon is possible.
• Skywave: The radio wave is reflected from the ionosphere back to Earth. The
wave is reflected back from the Earth’s surface and back to the Earth again
making long-distance communication possible. The communication quality is not
stable because the characteristics of the ionosphere vary with time.
• Line of sight: The radio wave propagates along the straight line from the
transmitter to the receiver. A general requirement for good performance is that
the receiving antenna be visible from the transmitter. The radio frequencies
above 100 MHz that propagate in line-of-sight (LOS) mode are used in most
modern communication systems.
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Optical Communications
At the infrared light frequencies just below visible light (wavelength 400–700 nm)
a transmission medium, optical fiber, provides very low attenuation. Optical
fiber is the most important media for high-capacity long-distance transmission. It
is used in national long-distance networks as well as in international and
intercontinental submarine cable systems.
Commercial optical communication systems use binary light pulses. When the
transmitted information is in binary form, the receiver either detects light or
does not: this is also called intensity modulation (similar to BPSK: constellation
symbols “0” and “1”).
Modern optical systems are also able to use transmitted light as a carrier wave in
the same way that radio systems do. Radio systems are able to change phase
and frequency of the carrier wave, not just intensity.
Traditionally, one optical signal occupies the whole fiber although a small portion
of its very wide frequency is usable.
However, development of narrowband optical transmitters and optical filters has
made it possible to increase the data transmission capacity by inserting multiple
optical channels into the same fiber with the help of the dense wavelength
division multiplexing (DWDM) system.
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The free space attenuation formula
Attenuation, or loss, in free space:
Same expression as above in decibel:
The free-space loss formula may give results that are too optimistic in actual
conditions (i.e. real terrestrial propagation). Supplementary attenuation (Ls)
is introduced if there is a hill, a building, or a wall on or close to the straight
line between the transmitting and receiving antennas.
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Antennas and LOS propagation
Link loss was calculated assuming that antennas are isotropic, i.e. they transmit and
receive equally to and from all directions. However, practical antennas have a focusing
effect that can be expressed through an antenna gain, gT or gR depending if the antenna is
the transmit antenna or the receive antenna, respectively.
The maximum transmitting and receiving gain (along the direction of maximum
radiation) of an antenna with effective aperture area Ae is:
gT or gR :
The value of Ae for a dish or horn antenna approximately equals its physical area.
Received power and overall radio link
loss when antenna gains are considered:
(LTot )
Ls
LTot = L Ls = gT gR
Decibel expression:
Ltot, dB
Schematic of LOS radio link
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Transmission Media
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In modern communications ,to interconnect far-end and near-end
equipment, Transmission systems may use (mainly):
copper cables
optical cables
radio channels
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Copper cable as a transmission medium
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Copper Cables
Attenuation in copper cable increases with frequency approximately according to
the following formula:
AdB is attenuation in decibels, f is the frequency, and k is a constant specific
for each cable
Twisted Pair
Two wires are twisted together to reduce external electrical interference and
interference from one pair to another in the same cable
Used in the telecommunications networks in subscriber lines, in 2-Mbps digital
transmissions with distances up to 2 km between repeaters, in DSLs up to several
megabits per second
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Optical Fiber Cables
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Fiber optic links are used as the
major media for long-distance
transmission, now traditionally in
the access/metro networks, and
more and more in the access
network. Main characteristics are:
• High transmission capacity: very large bandwidth, to carry very high data rates, up
to several hundred Gbps (transport/metro networks) and to several hundred Mbps
(access network).
• Low cost: cost of the fiber has decreased to the level of a twisted-pair cable
• Tolerance against external interference: e.m. disturbances have no influence on
the light signal inside the fiber.
• Small size and low weight: Fiber material weighs little and the fiber diameter is
only of the order of a hundred micrometers instead of a millimeter or more for
copper wire.
• Unlimited material resource: Quartz used in glass fibers is one of the most
common materials on Earth.
• Low attenuation: Attenuation in modern fibers is less than half a decibel per
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kilometer and it is independent of the data rate.
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Optical fiber attenuation
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Radio transmission
Advantages
Radio transmission does not require any physical medium.
Radio systems are quick to install
Because no digging of cable into the ground is required, the investment costs are
much lower than cable transmission
Disadvantages
Spectrum is a limited resource.
Interference between systems
Bandwidth is generally much lower than that carried with cable systems
The use of radio frequencies is regulated by the ITU-R at the global level and,
for example, by ETSI at the European level and the FCC in the United States.
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