03) Radio Signal Propagation

8/11/2019 03) Radio Signal Propagation

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Radio Signal Propagation

ZTE University




Content

1. Radio wave propagation

2. Propagation models

3. Antenna systems

4. Diversity technique

5. Interference and interference reduction

6. Link budget




Radio Link Propagation

Multi-path propagation

Radio path is a complicated propagation medium

Limited transmitting energy

The service range is determined by thetransmission power of mobiles

Battery life-time

Limited spectrum

Set upper limitation for data rate (Shannon´s

theorem)

Additional effort needed for channel coding

Frequency reused result in self- interference






Reflections

Strong echoes can cause excessive transmission delay

No impact If the delay falls in the equalizer window

Cause self-interference if the delay falls out of the

equalizer windowdirect signal

strong reflected signal

equalizer window 16 s

amplitude

delay time

long echoes, out of equalizer window:

self-interference




Fading(1)

Slow fading (Lognormal

Fading)

Shadowing due to large

obstacles on propagationdirection

Fast fading (Rayleigh

fading)

Serious interference from

multi-path signals

+10

0

-10

-20

-300 1 2 3 4 5 m

Level (dB)

920 MHz

v = 20 km/h


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Fading(2)

time

power

2 sec 4 sec 6 sec

+20 dB

mean

value

- 20 dB

lognormal

fading

Rayleigh

fading



Signal Variations

Rayleighfading Lognormalfading Large scalevariation

Cause Superposition ofmultiplepropagationpaths withdifferent phase

Shadowing orreflection bycars, trees,buildings

Prop. path profile, terrain& clutter structure, Earthcurvature

Correlation< 10 ... 100m > 100m

Prediction unpredictable mostlypredictable(buildings!!)

predictable (maps, terraindatabase)

Planningmethod

apply statisticalthresholds forRayleigh fading

signals

considerlognormaldistribution

around localmean (use =

3 ... 10dB)

use maps or digitalterrain & clutterdatabases to predict

(50 ..200m pixelresolution)



Propagation

Free- space propagation

Signal strength decreases with distance increases

Reflection

Specula R.

• Amplitude : A --> α*A (α< 1)

• Phase : --> -Ф

• Polarization : material determining phase shift

Diffuse R.

• Amplitude : A --> α*A (α<< 1)

• Phase : random•

specula reflection

diffuse reflection

D



Propagation

Absorption

Heavy amplitude attenuation

Material determining phase shift

Diffraction

Wedge-model

Knife edge

Multiple knife edges

A A - 5..30 dB




Mobile Radio Link



3. Antenna systems



6. Link budget




Propagation Model

Historical CCIR- Model for Radio station

Not very accurate nor serious

Okumura- Hata

Empirical model

Measure and estimate additional attenuations

Applied for larger distance estimation (range: 5 ..

20km) Not suitable for small distance ( < 1km)




Hata Model

Model used for 900 MHz

L A B f h a h

h d L

b m

b morpho

log . log ( )

( . . log ) log

1382

44 9 655with

f frequency in MHz

h BS antenna height [m]

a(h) function of MS antenna height

d distance between BS and MS [km] and

A= 69.55, B = 26.16 (for 150 .. 1000 MHz)

A= 46.3 , B = 33.9 (for 1000 ..2000MHz)

additional attenuation due

to land usage classes




Land Usage Types

Urban small cells, 40..50 dB/Dec attenuation

Forest heavy absorption; 30..40 dB/Dec;

differs with season (foliage loss)

Open, farmland easy, smooth propagation conditions

Water propagates very easily ==> dangerous !

Mountain surface strong reflection, long echoes

Glaciers very strong reflection; extreme delay ,strong interferences over long distance

Hilltops can be used as barriers between cells, do

not use as antenna or site location




Walfish- Ikegami Model

Model used for urban micro-cell propagation. Assume

regular city layout (“Manhattan grid”). Total path loss

consists of three parts:

Line-of-sight loss LLOS

Roof-to-street loss LRTS

Mobile environment loss LMS

h

w

b

d




Mobil Radio Link


2. Propagation model

3. Antenna system



6. Link budget






Coupling Between Antennas

main lobe

5 .. 10

Horizontal separation

Sufficient decoupling distance: 5-10λ

Antenna patterns superimposed if

distance too close

Vertical separation

Decoupling distance:1λ can provide good RX /TX

decoupling Minimum coupling loss




Recommended decoupling

TX - TX: ~20dB

TX - RX: ~40dB

Horizontal decoupling distance depends on

Antenna gain

Horizontal rad. pattern

Omni-directional antenna

Use vertical separation for RX and TX

Use vertical separation (“fork”) for RX and diversity

RXVertical decoupling is much more effective

0,2m

Omni-directional.: 5 .. 20m

directional : 1 ... 3m

Installation Examples




Installation Examples

Directional antenna

Antenna downtilt

Improve hotspot coverage

Reduce interference

5..8 deg




Feeder

Feeder parameterType Diameter 1800MHz 900MHz

(mm) dB/100m

dB/100m

3/8” 10 14 10

5/8” 17 9 6

7/8” 25 6 4

1 5/8” 47 3 2

Use the short feeder whenever possible




Distributed Antennas

Leaking feeder

Cables with very high loss per length unit

“distributed antenna” often used for tunnel coverage.

This kind of feeder is expensive

Optic fiber distribution system

Distribute RF signal radiate from discrete antenna

points at remote locations via (very thin) optic fiber.

50 Ohm

Propagation loss: 4 ... 40 dB/100m

coupling loss: ~ 60 dB (at 1m dist.)




Repeaters

Repeater type

Narrow-band Repeater

Wide-band Repeater

The Repeater is used to relay signal into shadowed

area

Behind hill

Into valley

Into building

Note: The Repeater needs a host cell

decoupling ~40 dB needed




Mobile Radio Link



3. Antenna systems



6. Link budget




Diversity

Time diversity

Coding, interleaving

Frequency diversity

Frequency hopping

Space diversity

Multiple antennas

Polarization diversity

Dual-polarized

antennas

Multi-path diversity

Equalizer

t

f






Mobile Radio Link



3. Antenna systems



6. Link budget




Interference

Signal quality =

sum of all expected signals carrier (C )

sum of all unexpected signal interference (I)=

Notes: GSM specification : C / I >= 9 dB (Co-Channel)

expected signal

atmospheric

noise

other signals




Effects of Interference

Affect signal quality

Cause bit error

Repairable errors : channel coding, error

correction

Irreducible errors : phase distortions

Interference situation is

Non- reciprocal : uplink <> downlink

Unsymmetrical : different situation at MS and

BTS

C/I

Co-Channel C/I : 9dB




Signal Quality in GSM

RX QualityRXQUAL class : 0 ... 7

RXQUAL Mean BER BER range

class (%) from... to0 0.14 < 0.2%

1 0.28 0.2 ... 0.4 %

2 0.57 0.4 ... 0.8 %

3 1.13 0.8 ... 1.6 %

4 2.26 1.6 ... 3.2 %

5 4.53 3.2 ... 6.4 %6 9.05 6.4 ... 12.8 %

7 18.1 > 12.8 %

usable signal

unusable

signal

good

acceptable




Interference sources

Multi-path (long echoes)

Frequency reuse

External interferenceNote : Interference has the same effect as poor coverage.

Reduce the interference

as possible.

M th d f d i




Methods for reducing

Interference

Frequency planning

Suitable site location

Antenna azimuth, downtilt and height

good location

bad location

M th d f d i





Interference

Frequency hopping

A diversity technique, frequency diversity include:

Less fading loss

De-coding gain

Interference averaging

Power control based on quality Evaluate signal level and quality

DTX

Silent transmission in speech pauses

M th d f d i





Interference

Adaptive antenna

According to subscriber distribution, concentrate

signal energy to certain direction.

Adaptive channel allocation

Always assign the best available channel during call

setup.




Frequency Hopping

Diversity technique

Frequency diversity can reduce fast fading effects

Useful for static or slow-moving mobiles

Cyclic base-band hopping

TRX hops cyclic between its allocated frequencies

RF hopping

Either cyclic or random hopping

Needs wideband combiner

Can use any frequency included in the MA




Power Control

Save battery life-time

Minimize interferenceGSM : 15 steps and 2 dB for each

Use power control in both uplink and downlink

triggered by level or quality

time

signal

level target level

e.g. -85 dm

Power control isn’t allowed

on BCCH




DTX

DTX (Discontinuous transmission)

Switch transmitter off in speech pauses and silence

periods, both sides transmit only silence

updates (SID frames) comfort noise generated bytranscoder.

VAD: voice activity detection

Transcoder is informed the use of DTX/ VAD

Battery saving and

interference reducing




Mobile Radio Link



3. Antenna systems



6. Link budget




Link Budget Calculation

Why we need a link budget?

Which will decide the coverage range?

The coverage range is limited by the weaker one.

Two-way communication needed

link usually limited by mobile transmitting power

Desired result: downlink = uplink

Link budget shouldbe balanced



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03) Radio Signal Propagation

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