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Lecture 3
Introduction to Wireless Communication

2. MULTIPLEXING
•Multiplexing means sending many signals at the same time on one wire or
channel instead of one by one.
•Most wires or channels can carry more than one signal.
•Multiplexing lets us use the wire in a smart way.
•It makes sending signals faster and more efficient.
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3. REASONS FOR WIDESPREAD USE OF MULTIPLEXING
•Low Cost per kbps: When the data rate increases, the cost of transmission
per kbps becomes cheaper.
•Cheaper Equipment: Transmission and receiving devices become less costly
when used at higher data rates.
•Small Device Needs: Most devices don’t need very high data rates, so
many devices can share the same channel efficiently.
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4. MULTIPLEXING TECHNIQUES
•Frequency-division multiplexing (FDM)
•Uses different frequency ranges for each signal. It works because the
channel’s total bandwidth is larger than what one signal needs.
•Time-division multiplexing (TDM)
•Divides time into slots and gives each signal a time slot. It works because
the channel’s bit rate is higher than what one signal requires.
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5. CLASSIFICATIONS OF TRANSMISSION MEDIA
•Transmission Medium: The path through which data travels from sender to
receiver.
•Guided Media (Wired): Signals move through a physical medium.
Examples: Twisted pair cable, Coaxial cable, Optical fiber.
•Unguided Media (Wireless): Signals travel freely without physical path.
Transmission and reception are achieved by means of an antenna.
Examples: Air (atmosphere), Outer space.
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6. GENERAL FREQUENCY RANGES
•Microwave (1 GHz – 40 GHz):
•Can send signals in a straight, narrow beam.
•Best for point-to-point links.
•Commonly used in satellite communication.
•Radio (30 MHz – 1 GHz):
•Spreads signals in all directions.
•Good for broadcasting and mobile communication.
•Infrared (≈ 3×10¹¹ – 2×10¹⁴ Hz):
•Works well in short-range, local, or indoor communication.
•Used for point-to-point or multipoint links.
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7. TERRESTRIAL MICROWAVE
•Description:
•It uses large parabolic dish antennas (about 3 meters wide). These
antennas send a narrow, focused signal in a straight line, so the transmitter
and receiver must see each other. They are placed on tall towers to avoid
obstacles. Rain can weaken the signal, so amplifiers or repeaters are
installed every 10–100 km to boost it.
•Applications:
•Used for long-distance communication like telephone and internet services.
It also delivers TV signals to local cable stations and connects nearby
buildings, offices or campuses through short-distance point-to-point links.
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8. SATELLITE MICROWAVE
•Description:
•A satellite acts as a relay station in space. It connects two or more ground
stations. The satellite receives signals from Earth on one frequency (called
uplink), strengthens or repeats them, and sends them back to Earth on
another frequency (called downlink). It works in a broadcast way, so it can
cover very large areas around the world.
•Applications:
•Used for sending TV signals, making long-distance phone calls, linking
telephone exchanges, and connecting private business networks for data
and communication.
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9. SATELLITE MICROWAVE
•Characteristics:
•Works best in the 1–10 GHz frequency range. Below 1 GHz, signals get too
much noise from the sun, atmosphere, and other sources. Above 10 GHz,
signals weaken quickly because air absorbs them. It has a delay of about
250 milliseconds, which can be noticed during phone calls.
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10. BROADCAST RADIO
•Description:
•Uses antennas that send signals in all directions. They don’t need to be
adjusted or shaped like a dish. This makes them simple and effective for
wide coverage.
•Applications:
•Used in FM radio and television transmission.
•Characteristics:
•Has low signal loss. Not affected by rainfall. Can face multi-path
interference due to signal reflections from buildings, water or land.
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11. PROPAGATION MODES
1.Ground-wave propagation
2.Sky-wave propagation
3.Line-of-sight propagation
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12. GROUND WAVE PROPAGATION
•These waves can cover long distances.
•They work at low frequencies (up to 2 MHz).
•They bend slightly downward, following Earth’s curve.
•They don’t go into the upper atmosphere because they scatter.
•Example: AM radio uses ground wave propagation.
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13. SKY WAVE PROPAGATION
•Signals reflect between Earth and the ionosphere in multiple hops.
•This reflection is actually refraction in the ionized layer.
•It allows signals to travel very far, even across countries.
•Examples: Amateur radio, CB radio.
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14. LINE-OF-SIGHT PROPAGATION
•Antennas face each other without any large obstacles in between.
•Signals above 30 MHz travel straight and are not bounced back by the
ionosphere.
•When waves move through different materials, their speed changes,
causing them to bend at the edges.
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15. LINE-OF-SIGHT EQUATIONS
•Optical line of sight:
•Effective, or radio, line of sight:
•d = distance between antenna and horizon (km)
•h = antenna height (m)
•K = adjustment factor, rule of thumb K = 4/3
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16. LINE-OF-SIGHT EQUATIONS
•Maximum distance between two antennas:
•h
1
= height of antenna one
•h
2 = height of antenna two
•Example:
•Let h1 = 100m, h2 = 0
•D = 3.57 ( 4/3x100)^1/2 + 0 = 41 km.
•Now suppose that h2 = 10m, To achieve same distance:
•h1 = 3.57(Kh1)^1/2+(13.3)^1/2
•h1 = 46.2m
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17. PROPAGATION FACTORS
•Microclimate
•Frequency of the signal
•Power of the transmitter
•Conductivity of the Earth
•Shape of the Earth between transmitter and receiver
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18. MULTIPATH PROPAGATION
•Reflection: The signal bounces back when it hits a large surface (like
buildings, mountains or walls) that is bigger than its wavelength.
•Diffraction: The signal bends around the sharp edge of a large object (like a
building corner or hill) instead of being completely blocked.
•Scattering: The signal gets spread in many directions when it hits small
objects (like trees, lamp posts, street signs, or rough surfaces) that are
about the same size or smaller than the signal’s wavelength.
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19. THE EFFECTS OF MULTIPATH PROPAGATION
•When signals travel through many paths, the receiver gets multiple copies
of the same signal. These copies may arrive at different times and with
different phases (shifts).
•If the copies add up out of phase, the signal becomes weak and harder to
detect.
•Intersymbol Interference (ISI) delayed copies of one bit may overlap with
the next bit, causing errors in data.
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20. TYPES OF FADING
•Fast Fading: Means the signal becomes strong and weak very quickly when
you move a small distance. It happens because signals bounce off buildings
or objects and mix together. This makes the signal change even if you move
just a few centimeters. For example, at 900 MHz, fast fading can happen
when you are moving fast, like in a car or bus.
•Slow Fading: Means the signal strength changes slowly over a long
distance. It happens when large objects like buildings, hills, or trees block
or reflect the signal. When you move behind a big building, the signal
becomes weak, and when you come out, it becomes strong again. This
happens mostly in cities or hilly areas.
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21. TYPES OF FADING
•Flat Fading: Means the whole signal becomes weak equally. There is no
distortion, just a lower signal strength. For example, during a phone call,
the voice may sound weak but still clear.
•Selective Fading: Means only some parts or frequencies of the signal
become weak while others stay strong. This happens when signals reach
the receiver at different times after bouncing off objects. It can cause
problems like distortion or data errors. For example, in Wi-Fi or 4G, it may
cause slow internet or data loss.
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22. FADING CHANNEL
•A fading channel is a communication channel where the signal becomes weak or
changes strength due to distance, obstacles, or multiple signal paths. There are
three main types of fading channels:
•In Additive White Gaussian Noise (AWGN) channel the signal is only affected by
random thermal noise, not by fading or obstacles. It is mostly used in space
communication and some wired communication systems like coaxial cables
because signals travel in a straight line without interference.
•Rayleigh fading happens when there is no direct line between the sender and
receiver. The signal reaches the receiver after bouncing off buildings, trees or
other objects, creating many indirect paths. This type of fading is common in
outdoor areas like cities where tall structures block the direct signal path.
•Rician fading occurs when there is both a direct path along with many reflected
paths. The direct path makes the signal stronger and more stable than in Rayleigh
fading. This type of fading usually happens in small areas or indoor environments
where at least one clear signal path exists along with some reflections.