EXPLAIN M02 - 1 Radio Propagation Channel

1 © NOKIA 6-60726/ RADIO PROPAGATION CHANNEL/ v 2.0

RadioRadioPropagationPropagationChannelChannel


Module objectives

DESCRIBE THE BASIC PROPAGATION MECHANISMS

DESCRIBE MULTIPATH PROPAGATION

DESCRIBE THE DIFFERENCE BETWEEN FAST AND SLOW FADING

DESCRIBE THE FACTORS OF PATH LOSS

At the end of this module you will be able to …


Content

REFLECTIONS, DIFFRACTIONS AND SCATTERING

MULTIPATH AND FADING

PROPAGATION SLOPE AND DIFFERENT ENVIRONMENTS


Radio Propagation Channel





deciBelDefinition

Power

Voltages

dB PP

Plin

P dB=

⎛

⎝⎜

⎞

⎠⎟ =10 10

0

10log [ ].( )

dB EE

Elin

E dB=

⎛

⎝⎜

⎞

⎠⎟ =20 10

0

20log [ ].( )

Plin.=⏐Elin.⏐² / 2


deciBelConversion

• Calculations in dB (deciBel)• Logarithmic scale

• Always with respect to a reference• dBW = dB above Watt• dBm = dB above mWatt• dBi = dB above isotropic• dBd = dB above dipole• dBµV/m = dB above µV/m

• Rule-of-thumb: • +3dB = factor 2• +7 dB = factor 5• +10 dB = factor 10

-30 dBm = 1 µW-20 dBm = 10 µW-10 dBm = 100 µW-7 dBm = 200 µW-3 dBm = 500 µW

0 dBm = 1 mW+3 dBm = 2 mW+7 dBm = 5 mW

+10 dBm = 10 mW+13 dBm = 20 mW+20 dBm = 100mW

+30 dBm = 1 W+40 dBm = 10W+50 dBm = 100W


Radio ChannelMain Characteristics

• Linear• In field strength

• Reciprocal

• Dispersive• In time (echo, multipath propagation)• In spectrum (wideband channel)

amplitude

delay time

direct path

echoes


Propagation Mechanisms(1/2)

Free-space propagation• Signal strength decreases exponentially

with distance

Reflection• Specular reflection

amplitude A ⇒ a*A (a < 1)phase f ⇒ - fpolarisation material dependant

phase shift

• Diffuse reflectionamplitude A ⇒ a *A (a < 1)phase f ⇒ random phasepolarisation random

specular reflection

diffuse reflection

D


Propagation Mechanisms(2/2)

Absorption• Heavy amplitude• Attenuation material• Dependant phase shifts• Depolarisation

Diffraction• Wedge - model• Knife edge• Multiple knife edges

A A - 5..30 dB


ScatteringMacrocell

Scattering local to mobile• Causes fading • Small delay and angle spreads• Doppler spread causes time varying effects

Scattering local to base station• No additional Doppler spread• Small delay spread• Large angle spread

Remote scattering• Independent path fading• No additional Doppler spread• Large delay spread• Large angle spread

Scattering to mobile

Scattering to base station

Remote scattering


ScatteringMicrocell

• Many local scatterers: Large angle spread

• Low delay spread

• Medium or high Doppler spread







Doppler Spread• A moving source or receiver, the

frequency observed by the receiver will rise if the distance is decreasing or fall if the distance is increasing

• A carrier transmitted on a single frequency will propagate via multiple reflections and each reflection will arrive at the receiver shifted in frequency by a different amount. The difference between the highest shift and the lowest shift will give the Doppler spread.

• Doppler spectrum:


Angular Spread• Angular spread arises due to

multipath, both from local scatterersnear the mobile, near the base station and remote scatterers

• Angular spread is a function of base station location, distance and environment


Macrocellular Environment= Macrocell Coverage Area

Microcellular Environment= Microcell Coverage Area

Microcell Antenna

Macrocell Antenna

α

Angular Spread

• 5 - 10 degrees in macrocellular environment

• >> 10 degrees in microcellular environment

• < 360 degrees in indoor environment

• Effect on the performance of diversity reception and adaptive antennas


Time dispersion

• Echoes due to multipath propagation• 1 µs ≅ 300 m path difference

• GSM → equalizer in the receivers• Time window of 16 µs (~ 4.8 km path difference)• 2-path-model as “worst case” situation• Standardized delay profiles in GSM specs:

• TU3 typical urban at 3 km/h (pedestrians)• TU50 typical urban at 50 km/h (cars)• HT100 hilly terrain (road vehicles)• RA250 rural area (highways)

• No hard limitation at 250 km/h


t

P

4.3.2.

1.

”GSM window” = 16 µsMaximum delay,based on equaliser

1.

2.=>

f1

f1

f1

f1

BTS

1st floor

2nd floor

3rd floor

4th floor

Delay Spread

Multipath propagation Channel impulse response

<= Equaliser enables the use of DAS (Distributed antenna systems)


Delay Spread

Typical values

Environment Delay Spread (µs)

Macrocellular, urban 0.5-3

Macrocellular, suburban 0.5

Macrocellular, rural 0.1-0.2

Macrocellular, HT 3-10

Microcellular < 0.1

Indoor 0.01...0.1


Fading

• Average trend ~ 35 – 50 dB / decade (path loss)

• Slow fading: Caused by shadowing. Typically log-normal distributed (σ around 8 – 11 dB)

• Fast fading: Caused by local scatters near mobile. Typically Rayleigh distributed

• Time-selective fading: Short delay + Doppler

• Frequency-selective fading: Long delay

• Space-selective fading: Large angle


FadingSlow & Fast

Slow fading (Log-normal fading)• Shadowing due to large obstacles on the way

Fast fading (Rayleigh fading)• Destructive interference of several signals•“fading dips”, “radio holes”

+10

0

-10

-20

-300 1 2 3 4 5 m

level (dB)

920 MHzv = 20 km/h


time

power

2 sec 4 sec 6 sec

+20 dB

mean value

- 20 dB

lognormal fading

Rayleighfading

FadingSlow & Fast (2)


FadingGaussian Distribution

• Most general form of distribution• Superposition of several processes with any distribution function will always

converge towards a Gaussian distribution• Applicable to all natural processes, also to slow fading

• Mean value m, standard deviation σ


• Applicable to fast fading in obstructed paths

p r r r( ) exp( )= −σ σ2

2

22

FadingRayleigh Distribution


• Sliding transition between Gauss and Rayleigh• Applicable to fast fading with strong direct partial waves• “Rice-factor” K = r/A: direct / indirect signal energy

K = 0 ⇒ RayleighK >>1 ⇒ Gaussian

p r r r A I r A( ) exp *= −+⎛

⎝⎜

⎞⎠⎟

⎛

⎝

⎜⎜⎜

⎞

⎠

⎟⎟⎟σ σ σ2

2 2

2 0 22

K = 0(Rayleigh)

K = 1

K = 5

FadingRician Distribution







Propagation MechanismsFree Space Loss

Free space loss proportional to 1/d2

• Simplified case: isotropic antenna• Which part of total radiated power is found within surface s?• Power density = P/S

⇒ total power within surface s : P´ = P/S *s • Power density reduces with square of distance

⇒ received power per area unit reduces at same rate

RSurface S = 4π * R2

assume surfaces = 1m2

2d4d

A´ = 4*AA´´ = 16*A

A

d


2

4⎟⎠⎞

⎜⎝⎛

πλ

• Power density at the receiving end

• Effective antenna area

• Received power Pr = S Aeff

Mobile environments Pr = PsGsGrCd-γ (with γ = 2.5 ... 5)

Reff GAπλ4

2

=

SP G

ds s=

4 2πPP

G Gd

r

ss r=

⎛

⎝⎜

⎞

⎠⎟

λπ4

2

Ps

As

Gs

Pr

Ar

Gr

d

Propagation MechanismsSignal Propagation


• Basic loss formula

• Clutter loss factors• Land-usage classes (in dB/decade)• e.g.:

free space 20 dB/dec

open countryside 25 dB/dec

suburban areas 30 dB/dec

urban area 40 dB/dec

historic city centre >45 dB/dec

L = L0 + α*log(d)

loss at reference point (e.g. 1km)

losses are exponential with distance

0,1 km 10 km1 km

EIRP level

coupling loss= L0

referencedistance

20 dB/dec

30 dB/dec40 dB/dec

Path Loss


Path LossSignal Attenuation

25 dB/dec

30 dB/dec20 dB/dec

40 ..50 dB/decpath loss


Path LossMixed Path Loss

urban: 40 ..50 dB/decopen: 25 dB/dec open: 25 dB/dec

open area curveurban curve

actual signal level

signallevel

distance

• Mixed land usage types on propagation path


Exercises / Questions

Why is an equalizer needed in GSM terminal?

Explain the difference between slow and fast fading!


References

1. ETSI, Digital cellular telecommunications system (Phase 2+), Mobile radio interface layer 3 specification, GSM 04.08

2. D. Parsons, “The Mobile Radio Propagation Channel,” Pentech Press, 1992.

3. W.C. Jakes, Jr., (ed.), “Microwave Mobile Communications,” Wiley-Interscience, 1974

4. W.C.Y. Lee, “Mobile Communications Design Fundamentals,” John Wiley & Sons, 1993.

5. W.C.Y. Lee, “Mobile Cellular Telecommunication Systems,” McGraw-Hill Book Company, 1990

6. S. Saunders, “Antennas and Propagation for Wireless Communication Systems,” John Wiley & Sons, 1999.

7. J. Lempiäinen, M. Manninen, ”Radio Interface System Planningfor GSM/GPRS/UMTS,” Kluwer Academic Publishers 2001.

EXPLAIN M02 - 1 Radio Propagation Channel

Documents