Transcript Radio Wave Propagation - Wireless | T/ICT4D Lab

Radio Wave Propagation
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Radio Wave Components
Wave component
Comments
Direct wave
Free-space propagation
Reflected wave
Reflection from passive antenna, ground,
wall, object, ionosphere <~100MHz, etc.
Refracted wave
Standard, Sub-, and Super-refraction,
ducting, ionized layer refraction <~100MHz
Diffracted wave
Ground-, mountain-, spherical earthdiffraction <~5GHz
Surface wave
<~30 MHz
Scatter wave
Troposcatter wave, precipitation-scatter
wave, ionized-layer scatter wave
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Absorption
• = the conversion of the transmitted EM
energy into another form, usually thermal.
• The conversion takes place as a result of
interaction between the incident energy
and the material medium, at the molecular
or atomic level.
• One cause of signal attenuation due to
precipitations (rain, snow, sand) and
atmospheric gases
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Diffraction
• = the mechanism the waves spread as they
pass barriers in obstructed radio path (through
openings or around barriers)
• Each point on a wave front acts as a source of
secondary spherical wavelets. When the wave
front approaches an opening or barrier, only the
wavelets approaching the unobstructed section
can get past. They emit new wavelets in all
directions, creating a new wave front, which
creates new wavelets and new wave front, etc.
- the process self-perpetuates.
•
[Huygens, 1629-1695].
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Reflection
• = the abrupt change in direction of a wave front
at an interface between two dissimilar media so
that the wave front returns into the medium from
which it originated. Reflecting object is large
compared to wavelength.
• Reflection may be specular (i.e., mirror-like) or
diffuse (i.e., not retaining the image, only the
energy) according to the nature of the interface.
• The phase of the reflected wave may change
depending on the nature of the media and
interface and wave polarization.
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Refraction
• = redirection of a wavefront passing
through a medium having a refractive
index that is a continuous function of
position (e.g., a graded-index optical fiber,
or earth atmosphere) or through a
boundary between two dissimilar media or
• For two media of different refractive
indices, the angle of refraction is closely
approximated by Snell's Law.
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Scattering
• - of a wave propagating in a material medium, a
phenomenon in which the direction or
polarization of the wave is changed when the
wave encounters discontinuities in the medium.
• Involves objects smaller than the wavelength
(e.g. foliage, street signs, …)
• Scattering results in a disordered or random
change in the incident energy distribution.
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Fading
• In a received signal, the variation (with
time) of the amplitude or relative phase, or
both, of one or more of the frequency
components of the signal.
• Fading is caused by changes in the
characteristics of the propagation path
with time.
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Power density
Outdoor Propagation
Min. acceptable level (wanted signal)
Coverage (useful, service) range
Max. tolerable level (unwanted signal)
Denied (occupied, sterile, excluded) range
Distance
n ~ 2, dominates
LOS & Rice statistics
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n ~ 4, dominates Diffraction
& Rayleigh statistics
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Propagation Models
• Different dominating propagation mechanism
–
–
–
–
For various frequencies
For various applications
For various environments
For the wanted or interfering signals
• Variability due to randomly changing factors
• Probabilistic approach
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Some Popular Models
• Longley-Rice Model (ITS Irregular Terrain Model)
– Point-to-Point and Point-to-Area modes,
40MHz-100GHz
• Okumura Model
– 150MHz-3GHz, urban areas, 1-100km
• Hata Model
– Based on Okumura model
• ITU Model
– Atlas of curves
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ITU Propagation Models
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Signal strength (log)
Outdoor Propagation
Free space
Received power
PR = Kd-n
n = 2 in free space
Typically 3 n 4
Open area (LOS)
Urban
Suburban
Distance (log)
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LOS - Fresnel Zone
R
T
d1
 d1d 2
d2
1
d
d
2
r1 : radius of the 1st Fresnel zone, m
r1 

d  d1  d 2 : distance T-R, m
 : wavelength, m
d1 , d 2 : distance to R and to T, m
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• Fresnel zones are loci of
points of constant pathlength difference of /2
(=constant phase
difference of 1800)
• The 1st Fresnel zone
corresponds to /2.
The n-th zone is the
region enclosed between
the 2 ellipsoides giving
path-length differences
n(/2) and (n-1)(/2)
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Fresnel Zone 2
• Energy transmission from T to R concentrates
in the 1st Fresnel zone. If this zone is not
obstructed, the energy transmitted
approximates energy transmitted in free-space.
• An obstruction may lie to the side, above, or
below the path. Ridges, bridges, cliffs, buildings,
and trees are examples of obstructions.
• It means, path obstructions that do not obstruct
the 1st Fresnel zone can be ignored. Sometimes
one ignores obstructions up to ½ of the 1st
Fresnel zone.
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PFD  PT
Free-Space Model
 4 d 
2
E0  120 PFD  30 PFD d
PFD : free-space power flux density, W/m 2
PT : power radiated (isotropic antenna), W
d : distance between antennas, m
E0 : free space field strength (isotropic antenna), V/m
Note: With real antennas, use e.i.r.p. instead of power
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Troposphere
• Troposphere - the lower layer of atmosphere,
between the earth surface and the stratosphere,
in which the change of temperature with height
is relatively large. It is the region where
convection is active and clouds form.
• The thickness of the troposphere varies with
season and latitude. It is usually 16 km to 18 km
thick over tropical regions, and less than 10 km
thick over the poles.
• This layer contains ~80% of the total air mass.
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LOS – Radio Horizon
Radio horizon
• Radio waves go
behind the
geometrical horizon
Geometrical horizon
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Refraction in Troposphere
• The EM waves travel in atmosphere with slightly
lower velocity (v) than in a vacuum (c).
• Refractive index: n = c/v – (~1)
• Modified refractive index: m = n + h/a
• Refractivity N = (n-1)x106
Atmospheric pressure, mbar
Vapor pressure, mbar
77.6 
e
N
 p  4810 
T 
T
Temperature of the atmosphere, Kelvins
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K- Factor
• M = N + (h/a)x106 –
Refractive modulus
•Optics: Snell’s law
Hight, h
Superrefraction
Duct
Subrefraction
• In standard conditions the radio wave
• travels approximately along an arc
bent slightly downward.
K-factor is a scaling factor of the ray
path curvature. K=1 means a
straight line. For the standard
atmosphere K=4/3
• Departure from the standard conditions
• may led to subrefraction, superrefraction
or duct phenomena.
• Strong dependence on meteorological
phenomena.
M
0.12 (M x 10-6)/m – Standard atmosphere
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Examples
K=4/3
K=2
Long LOS paths over water or
desert may show ducting
phenomena, - surface ducts
or elevated ducts.
K=1
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Specific Attenuation dB/km
Atmospheric Absorption
• At frequencies
above 10 GHz the
atmosphere
introduces
attenuation due to
interaction of radio
wave at molecular/
atomic level
10
10
H2O
0.1
10
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O2
100 GHz
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Multipath Propagation
• Reflection
coefficient
RHP 
RVP 
sin   c  cos 2 
sin   c  cos 2 
 c sin   c  cos 2 
 c sin   c  cos 2 
 c   r  j 60 (complex dielectric const.)
 : grazing angle (complementary angle of incidence)
 r : dielectric const. of reflection surface
 : conductivity of reflection surface, 1/ohm.m
 : wavelength, m
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Reflected signal
• The reflected and direct signals received
differ due to
– Reflection process: it changes the magnitude
and phase of the reflected signal
– Path-lengths difference of the reflected and
direct rays: it introduces phase delay
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Reflected signal 2
• The reflected and direct signals received also
differ due to
– Directive transmitting antenna: the magnitudes and
phases of the signals radiated in the receiver direction
and the reflection point direction are different
– Directive receiving antenna: the magnitudes and
phases of the signals received from the transmitter
direction and the reflection point direction are different
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Ray Tracing
• SISP – Site Specific propagation models based
on deterministic analysis of all possible rays
between the transmitter and receiver to account
for reflection, diffraction & scattering
• Requires exact data on the environment
– Indoor: detailed 3D data on building, room, equipment
– Outdoor: 3D data on terrain infrastructure, streets,
buildings, etc.
– Large databases
– Satellite/ aerial photographs or radar images,
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2 Rays: Path-length Difference
h1
h1
h2
D
 h h 
Direct ray: d d  D  (h1  h2 )  D 1   1 2 
 D 
2
2
2
h h 
Reflected ray: d r  D 2  (h1  h2 ) 2  D 1   1 2 
 D 
1
11 2 113 3
(1  x)  1  x 
x 
x  ...
2
24
246
  dr  dd
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h  h 
 1 2
2D
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h  h 
 1 2
2D
2

2
2h1h2
h h
if 1 2
D
D
1
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2 Ray Propagation Model
2
2
E  Edir
 Erefl
 2 Edir Erefl cos(  R   )
Edir

 Edir 1  R 2  2 R cos(  R   )
R
E
Erefl
Edir
e  jR
  2  (  4 h1h2  D if D  )
R
= lagging angle due to path-length difference
 = reflected path - direct path-length difference
Erefl
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Amplitude, relative to Free-space
Distance Dependence, 2 Rays
6 dB
0 dB relative to free-space
Slope: 40 dB/decade
Field-strength ~d-2
Power ~d-4
Distance
(h1h2)/
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Simulated Experiments
• Distance dependence
• Height dependence
• Frequency dependence
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Time – Frequency Characteristics
•
Radio channel can be treated as a linear two-terminal-pair transmission
channel (input port: transmitting antenna; output port: receiving antenna).
Y ( )  X ( ) H ( )

y (t ) 
 x(t )h(t   )d  x(t )  h(t )

H ( ) 


h(t )e  jt dt (frequency transfer function of the channel)

1
h(t ) 
2

 H ( )e
jt
d  (impulse response of the channel)

  2 f
x(t ), X ( ) : input signal time and spectral representation
y (t ), Y ( ) : output signal time and spectral representation
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Time Response, 2 Rays
Amplitude
Amplitude
Transmitted signal
Time
x(t)
a2
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+
a1
Received signal
Direct ray
Reflected ray

y(t)
Time
 = c(dref – ddir)
Path-length difference
Light velocity

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Direct RF Pulse Sounding
Detector
Key
Propagation
Channel
Digital Storage Oscilloscope
Pulse Generator
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BPF
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Frequency Domain Sounding
Vector Network Analyzer &
Swept Frequency Osillator
X()
Port 1
S-Parameter Test Set
Y()
Port 2
S21()  H() = [X()] / [X()]
Inverse DFT Processor
h(t)
h(t) = Inverse Fourier Transform of H()
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Power Delay Profile
Relative Power
N
 aver 
 
k 1
N
k
2
k
2

 k
k 1
N
Time
 rms 
  k   aver 
k 1
2
 k2
N

k 1
2
k
• The dispersion of the channel is normally
characterized using the RMS Delay Spread, or
standard deviation of the power delay profile
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Delay Spread
• If an impulse is sent from transmitter in a
multiple-reflection environment, the
received signal will consist of a number of
impulse responses whose delays and
amplitudes depend on the reflecting
environment of the radio link. The time
span they occupy is known as delay
spread.
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Inter-symbol Interference
• The delay spread limits the maximum data
rate: no new impulse should reach the
receiver before the last replica of the
previous impulse has perished.
• Otherwise the symbol spreads into its
adjacent symbol slot, the two symbols mix,
the receiver decision-logic circuitry cannot
decide which of the symbols has arrived,
and inter-symbol interference occurs.
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Inter-symbol Interference
Symbols Received
Symbols Sent
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Microcell vs Macrocell
Item
Cell radius
Tx power
Fading
Microcell
0.1-1 km
0.1-1 W
Ricean
RMS delay spread 10-100 ns
Max. Bit Rate 1 Mbps
Macrocell
1-20 km
1-10 W
Rayleigh
0.1-10us
0.3 Mbps
After R.H.Katz CS294-7/1996
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Error Bursts
• When the delay spread becomes a
substantial fraction of the bit period, error
bursts may happen.
• These error bursts are known as
irreductible since it is not possible to
reduce their value by increasing the
transmitter power.
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Error Reduction
• Antenna diversity (~10 dB)
– Dual antennas placed at /2 separation
• Automatic Repeat Request (ARQ)
– Retransmission protocol for blocks in error
• Error- resistant
– modulation,
– code,
– protocol
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Summary
• Propagation presents a number of
problems we do not control
• Dependence on environment, including
meteorological phenomena, difficult to
predict
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References
• Many good books, e.g.
– Freeman RL: Radio System Design for
Telecommunications, J Wiley
– Coreira LM: Wireless Flexible Persdonalised
Communications, J Wiley
– Shigekazu Shibuya, A Basic Atlas of RadioWave Propagation, J Wiley
– ITU-R Recommendations, SG 3
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