Download Cellular Communications

CELLULAR
COMMUNICATIONS
2. Radio Wave Propagation
Radio Waves
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Radio waves is a form of radiation known as
electromagnetic waves
Nature of radio waves and the way they travel are a
key elements in cellular system
Properties of radio waves affect
Modulation (encoding of a speech or data using radio
wave)
 Spectrum allocation (Transmission frequency)
 Network planning (location and characteristics of antennas)
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Electric fields
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Any electrically charged object (static or current) has an
electrical field associated with it
Like charges repel one another and opposite charges
attract
The electric field radiates out from any item with an
electric potential.
Strength of the field falls away as the inverse square of
the distance
Electric Potential around a charged sphere
Variation of potential with distance
Magnetic Field
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Field around the magnets and electric current
Have two types of poles (north and south)
analogous two 2 types of electric charge (positive
and negative)
Strength of the field falls away as the inverse
square of the distance
Magnetic Forces Around The Current
Radio Signals
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Form of the electromagnetic radiation (same as
light, ultraviolet or infrared)
Characterized by specific range of frequencies
(wavelength)
Inseparable magnetic and electric components
Radio Wave
Wave Frequency and Wavelength
Electromagnetic Wave Spectrum
Radio Wave Spectrum
UHF (450 MHz, 800/900 MHz, 1800/1900MHz and 2100 MHz)are
typically used in communications system:
Radio Waves Cont’d
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Ground wave (<2 MHz): Waves with low frequencies
 follow the earth’s surface and can propagate long distances
 AM(LF,MF) radio.
Sky wave (2–30 MHz): Short waves that are reflected at the
ionosphere.
 bounce back and forth between the ionosphere and the
earth’s surface, travelling around the world.
 International Radio Broadcasts
Line-of-sight (>30 MHz): Mobile phone systems, satellite
systems, cordless
Radio signals in “free space”
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Can travel over enormous distance
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Become weaker as have to cover greater area
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Area of surface (sphere) is proportional to the distance
square
Strength of the signal is inversely proportional to the
square of the distance
Radio waves in typical terrestrial environment
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Tree, buildings and other objects
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Signal strength dies away at much faster rate
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Around 1/d^4
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Typical formula used by network designer
 1/d^3.8
Reflection and Refraction
Sea and most areas are
good reflector
Sand absorb large
portion of the energy
Air with different
temperatures/pressure
might result in refraction
Snell’s law (refraction
index)
Refraction Demo
Some Consequences of refraction
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Slowly changing refraction index
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Steady bending of the wave
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Coverage of the antenna can be extended below
line of sight by 30%
Diffraction
Fading
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Signals to/from mobile handset vary greatly in strength
as the users moves from place to place
Variations is larger over the small distances , could not
be explained by distance from the antenna alone
Usually categorized into two types: slow fading and
fast fading
Slow fading: phone moves behind the building
Fast fading: Multipath
How fast is the fast fading?
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Assume only 2 paths
Frequency is 2Ghz
Wavelength c/f=3*10^8/2*10^9=0.15m.
If 2 paths of the same length , the received signal is
in-phase
Move handset to increase the distance of one path
by 7.5cm, the signals will become out of in phase
The two signals will almost completely cancel out
Combined effect of fading
Inter-Symbol Interference(ISI)
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In digital transmission bits of
information are transmitted
using radio wave
Radio wave is modulated to
carry binary information
Simple example :on-off
modulation
ISI cont’d
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Assume large variation between path distances
The receiver may be receiving a signal via the
direct path that may be one bit of data
The reflected signal may be delayed to a sufficient
extent that it is carrying the previous bit of data
ISI example
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Assume data transmission rate is 50 kbps
1 bit of data should be sent during 1/50*1024 about
20 micro second (10^-6)
During this time the radio signal will travel about 6km
Reflected signal from the building 3km behind the
handset will cause ISI
Fixing ISI and fading
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Equalization: Model the channel distortion by
sending known signal. Use model to fix real signal
Diversity: Send different data portion over multiple
carriers, multiple times , separated in time
Cells around the transmitter
Experimental Data