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27/01/2003 Property of R. Struzak 1
Radio Wave Propagation
27/01/2003 Property of R. Struzak 2
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 earth- diffraction <~5GHz
Surface wave <~30 MHz
Scatter wave Troposcatter wave, precipitation-scatter wave, ionized-layer scatter wave
27/01/2003 Property of R. Struzak 3
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
27/01/2003 Property of R. Struzak 4
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].
27/01/2003 Property of R. Struzak 5
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.
27/01/2003 Property of R. Struzak 6
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.
27/01/2003 Property of R. Struzak 7
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.
27/01/2003 Property of R. Struzak 8
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.
27/01/2003 Property of R. Struzak 9
Outdoor Propagation
Distance
Pow
er d
ensi
ty
Max. tolerable level (unwanted signal)Denied (occupied, sterile, excluded) range
Min. acceptable level (wanted signal)Coverage (useful, service) range
n ~ 2, dominatesLOS & Rice statistics
n ~ 4, dominates Diffraction & Rayleigh statistics
27/01/2003 Property of R. Struzak 10
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
27/01/2003 Property of R. Struzak 11
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
27/01/2003 Property of R. Struzak 12
ITU Propagation ModelsFig. 1a Field strength; Land; 1kW; 50%T, 50%L
-50
-40
-30
-20
-10
0
10
20
30
40
50
60
70
80
90
10 100 1000
Distance, km
h1=37.5m
h1=75m
h1=150m
h1=300m
h1=600m
h1=1200m
27/01/2003 Property of R. Struzak 13
Outdoor Propagation
Distance (log)
Sig
nal s
tren
gth
(log)
Free space
Open area (LOS)
Urban Suburban
Received power PR = Kd-n
n = 2 in free space
Typically 3 n 4
27/01/2003 Property of R. Struzak 14
LOS - Fresnel Zone• Fresnel zones are loci of points of constant path-length 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)
T R
1 21
1
1 2
1 2
1
2: radius of the 1st Fresnel zone, m
: distance T-R, m
: wavelength, m
, : distance to R and to T, m
d dr d
dr
d d d
d d
d1 d2
27/01/2003 Property of R. Struzak 15
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.
27/01/2003 Property of R. Struzak 16
Free-Space Model
2
0
2
0
4
120 30
: free-space power flux density, W/m
: power radiated (isotropic antenna), W
: distance between antennas, m
: free space field strength (isotropic antenna), V/m
Note:
T
T
PFD P d
E PFD PFD d
PFD
P
d
E
With real antennas, use e.i.r.p. instead of power
27/01/2003 Property of R. Struzak 17
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.
27/01/2003 Property of R. Struzak 18
LOS – Radio Horizon
• Radio waves go behind the geometrical horizon
Geometrical horizon
Radio horizon
27/01/2003 Property of R. Struzak 19
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
77.64810
eN p
T T
Atmospheric pressure, mbar
Temperature of the atmosphere, Kelvins
Vapor pressure, mbar
27/01/2003 Property of R. Struzak 20
K- Factor
• M = N + (h/a)x106 – Refractive modulus
•Optics: Snell’s law
Superrefraction
Duct Subrefraction
Hig
ht,
h
M0.12 (M x 10-6)/m – Standard atmosphere
• 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.
27/01/2003 Property of R. Struzak 21
Examples
K=1
K=2K=4/3 Long LOS paths over water or
desert may show ducting phenomena, - surface ductsor elevated ducts.
27/01/2003 Property of R. Struzak 22
Atmospheric Absorption
• At frequencies above 10 GHz the atmosphere introduces attenuation due to interaction of radio wave at molecular/ atomic level
10 100 GHz
Spe
cific
Att
enua
tion
dB
/km
0.1
10
10
H2O
O2
27/01/2003 Property of R. Struzak 23
Multipath Propagation
• Reflection coefficient
2
2
2
2
sin cos
sin cos
sin cos
sin cos
60 (complex dielectric const.)
: grazing angle (complementary angle of incidence)
: dielectric const. of reflection surface
:
cHP
c
c cVP
c c
c r
r
R
R
j
conductivity of reflection surface, 1/ohm.m
: wavelength, m
27/01/2003 Property of R. Struzak 24
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
27/01/2003 Property of R. Struzak 25
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
27/01/2003 Property of R. Struzak 26
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,
27/01/2003 Property of R. Struzak 27
2 Rays: Path-length Difference
h2
h1
h1
D
22 2 1 2
1 2
22 2 1 2
1 2
2 3
2 2
1 2 1 2 1 2 1 2
Direct ray: ( ) 1
Reflected ray: ( ) 1
1 1 1 1 1 3(1 ) 1 ...
2 2 4 2 4 6
2 if 1
2 2
d
r
r d
h hd D h h D
D
h hd D h h D
D
x x x x
h h h h h h h hd d
D D D D
27/01/2003 Property of R. Struzak 28
2 Ray Propagation Model
R
Edir
Erefl
E
2 2
2
1 2
2 cos( )
1 2 cos( )
2 ( 4 if D )
= lagging angle due to path-length difference
= reflected path - direct path-length difference
R
dir refl dir refl R
dir R
refl j
dir
E E E E E
E R R
ER e
E
h h D
27/01/2003 Property of R. Struzak 29
Distance Dependence, 2 Rays
Distance
Am
plitu
de,
rela
tive
to F
ree-
spac
e
Slope: 40 dB/decadeField-strength ~d-2
Power ~d-4
(h1h2)/
0 dB relative to free-space6 dB
27/01/2003 Property of R. Struzak 30
Simulated Experiments
• Distance dependence
• Height dependence
• Frequency dependence
27/01/2003 Property of R. Struzak 31
Time – Frequency Characteristics
• Radio channel can be treated as a linear two-terminal-pair transmission channel (input port: transmitting antenna; output port: receiving antenna).
( ) ( ) ( )
( ) ( ) ( ) ( ) ( )
( ) ( ) (frequency transfer function of the channel)
1( ) ( ) (impulse response of the channel)
2
2
( ), ( ) : input signal time
j t
j t
Y X H
y t x t h t d x t h t
H h t e dt
h t H e d
f
x t X
and spectral representation
( ), ( ) : output signal time and spectral representationy t Y
27/01/2003 Property of R. Struzak 32
Time Response, 2 Rays
Am
plitu
de
Time
Reflected ray
Direct ray
= c(dref – ddir)
Light velocityPath-length difference
a1
a2
+x(t) y(t)
Am
plitu
de
Time
Transmitted signal Received signal
27/01/2003 Property of R. Struzak 33
Direct RF Pulse Sounding
Key BPFPropagationChannel
Pulse Generator
Detector
Digital Storage Oscilloscope
27/01/2003 Property of R. Struzak 34
Frequency Domain Sounding
S-Parameter Test Set
Vector Network Analyzer &Swept Frequency Osillator
Inverse DFT Processor
X() Y()
S21() H() = [X()] / [X()]
Port 1 Port 2
h(t)
h(t) = Inverse Fourier Transform of H()
27/01/2003 Property of R. Struzak 35
Power Delay Profile
• The dispersion of the channel is normally characterized using the RMS Delay Spread, or standard deviation of the power delay profile
Time
Rel
ativ
e P
ower
2
1
2
1
2 2
1
2
1
N
k kk
aver N
kk
N
k aver kk
rms N
kk
27/01/2003 Property of R. Struzak 36
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.
27/01/2003 Property of R. Struzak 37
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.
27/01/2003 Property of R. Struzak 38
Inter-symbol Interference
Symbols Sent Symbols Received
27/01/2003 Property of R. Struzak 39
Microcell vs Macrocell
Item Microcell MacrocellCell radius 0.1-1 km 1-20 kmTx power 0.1-1 W 1-10 WFading Ricean RayleighRMS delay spread 10-100 ns 0.1-10usMax. Bit Rate 1 Mbps 0.3 Mbps
After R.H.Katz CS294-7/1996
27/01/2003 Property of R. Struzak 40
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.
27/01/2003 Property of R. Struzak 41
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
27/01/2003 Property of R. Struzak 42
Summary
• Propagation presents a number of problems we do not control
• Dependence on environment, including meteorological phenomena, difficult to predict
27/01/2003 Property of R. Struzak 43
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 Radio-
Wave Propagation, J Wiley– ITU-R Recommendations, SG 3