Radio Wave Propagation

27/01/2003 Property of R. Struzak 1

Radio Wave Propagation


Radio Wave Components

Wave component CommentsDirect 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


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


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].


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.


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.


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.


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.


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


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


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


ITU Propagation ModelsFig. 1a Field strength; Land; 1kW; 50%T, 50%L

-50-40-30-20-100102030405060708090

10 100 1000Distance, km

h1=37.5mh1=75mh1=150mh1=300mh1=600mh1=1200m


Outdoor Propagation

Distance (log)

Sig

nal s

treng

th (l

og)

Free space

Open area (LOS)

Urban Suburban

Received power PR = Kd-n

n = 2 in free spaceTypically 3 n 4


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

12

: radius of the 1st Fresnel zone, m: distance T-R, m

: wavelength, m, : distance to R and to T, m

d dr dd

rd d d

d d

d1 d2


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.


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

PFDPdE

With real antennas, use e.i.r.p. instead of power


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.


LOS – Radio Horizon

• Radio waves go behind the geometrical horizon

Geometrical horizon

Radio horizon


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

77.6 4810 eN pT T

Atmospheric pressure, mbar

Temperature of the atmosphere, Kelvins

Vapor pressure, mbar


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.


Examples

K=1

K=2K=4/3 Long LOS paths over water or

desert may show ducting phenomena, - surface ductsor elevated ducts.


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

Atte

nuat

ion

dB

/km

0.1

10

10

H2O

O2


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


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


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


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,


2 Rays: Path-length Difference

h2

h1

h1

D

22 2 1 2

1 2

22 2 1 2

1 2

2 3

2 21 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 12 2

d

r

r d

h hd D h h DD

h hd D h h DD

x x x x

h h h h h h h hd dD D D D


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


Distance Dependence, 2 Rays

Distance

Am

plitu

de, r

elat

ive

to F

ree-

spac

e

Slope: 40 dB/decadeField-strength ~d-2

Power ~d-4

(h1h2)/

0 dB relative to free-space6 dB


Simulated Experiments

• Distance dependence • Height dependence• Frequency dependence


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

fx t X

and spectral representation( ), ( ) : output signal time and spectral representationy t Y


Time Response, 2 Rays

Am

plitu

de

Time

Reflected ray

Direct ray

= c(dref – ddir)Light velocity

Path-length difference

a1

a2

+x(t) y(t)

Am

plitu

de

Time

Transmitted signal Received signal


Direct RF Pulse Sounding

Key BPFPropagationChannel

Pulse Generator

Detector

Digital Storage Oscilloscope


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()


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


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.


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.


Inter-symbol Interference

Symbols Sent Symbols Received


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


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.


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


Summary

• Propagation presents a number of problems we do not control

• Dependence on environment, including meteorological phenomena, difficult to predict


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