ΑΣΥΡΜΑΤΕΣ-ΔΟΡΥΦΟΡΙΚΕΣΕΠΙΚΟΙΝΩΝΙΕΣ

Διδάσκων

Λούβρος Σπυρίδων

Επίκουρος Καθηγητής - ΤΕΣΥΔ

ΔΙΑΛΕΞΗ 9

ΕΠΙΓΕΙΑ ΜΙΚΡΟΚΥΜΑΤΙΚΗ ΖΕΥΞΗ (MW LINK) – ΣΧΕΔΙΑΣΜΟΣ 3ο Μέρος

Σχεδιασμός Μικροκυματικής Ζεύξης κοντινής απόστασης- Short-haul microwave radio design

Μικροκυματική μετάδοση υπό συνθήκες καναλιού ελεύθερες από κατακρυμνήσεις σήματος ισχύος (non-Fading Conditions)

The prediction cycle (1)

• Free-space loss• Gas attenuation

Link budget

• Rain attenuation• Diffraction – Refraction loss

• Multipath propagation

Quality and Availability

• Interference

”Slow”

”Fast”+

• Obstacle loss• Reflection loss

•BBER •ESR •SESR •UATR

Frequency planningLoss & attenuation Fading prediction

Fade marginObjectivesfulfilled?

No

Yes

Thresholddegradation

Fading probabilityPath loss

The prediction cycle (2)

• Rain attenuation• Diffraction – Refraction loss

• Multipath propagation

Quality and Availability

• Interference

”Slow”

”Fast”

•BBER •ESR •SESR •UATR

Frequency planningLoss & attenuation Fading prediction

Fade marginObjectivesfulfilled?

No

Yes

Thresholddegradation

Fading probabilityPath loss

Link budget

HoplengthFrequencyTerrainAntenna heightClimate ...

Tx powerRx thresholdAntenna gainBranching loss

Link budget The link budget is a generic way of calculating the received RF power in any radio communication system.

Branching loss can in the MW radio case be for example power splitter and/or flexible wave guide(s). Without such the bransching loss is set to zero.

The Fading margin is the difference in dB between nominal received power and nominal threshold level for the receiver. Normaly the receiver threshold level for is used. The fading margin is the mean to meet the quality and availability objectives in fading conditions.

Link budget diagram

M = Fading margin [dB]

Receiver threshold level (BER=10-3, BER=10-6)

Distance

Basic free space lossGas absorptionObstacle loss

Power level[dBm]

PTX

PRX

Bra

nchi

ng lo

ss,

ATX

Br

Free space loss

Ant

enna

gai

nG

TX

Ant

enna

gai

n, G

RX

Tran

smitt

er p

ower

PTX

Rec

eive

d po

wer

, PR

X

Bra

nchi

ng lo

ss, A

RX

Br

Basic free space loss

d [km]

f [GHz]

A [dB]= 92.4 + 20 log d + 20 log f

Where d = distance in km f = frequency in GHz

0

A = 92.4 + 20 log 30 + 20 log 15 = 145 dB0

Refer to ITU-R Rec. P.525

Example: f = 15

Example: d = 30

Gas absorption

H2O

O2

O2

H2OH2O

Radio Frequency (GHz)10 50 100 1000500

1000

100

10

1

0,1

0,01

+15 CH2O 7,5 g/m3

1013 hPa

Gases to be considered• Dry air (oxygen) • Water vapour

Atte

nuat

ion

Coe

ffici

ent

d

B /

km

Gas absorption

Total specific atmospheric attenuation for different values of temperature and relative humidity (g water/m3 atmosphere)

H2O 41 g/m3

H2O 25 g/m3

H2O 15 g/m3

For further studies:ITU Rec. P.676 and P.836

The warmer the air is the more water it can hold and the higher the microwave absorbtion will be.

Gas absorptionStatistical mass of water in air

For further studies:ITU Rec. P.676 and P.836

Equator

South 22°

North 22°

South 45°

North 45°

Annual statistics

Summer (worst case

Summer (worst case

Summer (worst case

Summer (worst case

Statistics used forcalculation of loss due togas absorbtion

Ground clearence

Diffracted

Design height of antennas so that thedirect ray is sufficiently clear of obstacles

Atmospheric refractionGeometrical line of sight

Radio optical line of sightRefracted

• In a “standard atmosphere” the radio beam is bent slightly downwards.• Especially for “long” hops this can help to overcome obstacles• The question is to establish how much the radio beam is bent and how

the bending changes over day and year• The statistical mean value for this bending effect will be an input to

designing the antenna mounting height.

Διαθλόμενη δέσμη

Radio refractivity - ΔιάθλασηThe bending effect is caused by changes to the atmospheric refractivity with altitude.Refraction relates to the speed of light:

Refractive index for light: n = c/vc = speed of light in vacuumv = speed of light in the media of interest, gas, liquid…

Radio refractivity: N = (1-n)106

expressed in: N-unitsRadio refractivity for vacuum = 0 N-units

”Low speed”

”High speed”

Gives

Vacuum

”Thin” atmosphere

”Dense” atmosphere

Differences by altitude in:• Temperature• Pressure• Humidity

”Low speed”

”High speed”

Radio refractivity of the atmosphere depends of • temperature, • pressure• air humidity.where the main effecting component is the humidity [Mass of water/cubicmeter air].Hot air can hold more water, giveslargest changes of Radio refractivity by altitude in hot humid climate.

Radio refractivity - Διάθλαση

Radio refractivity

Differences by altitude in:• Temperature• Pressure• Humidity

”Low speed”

”High speed”

In a ”well mixed” atmosphereRadio refractivity changesconstantly by altitude.• Lower value of radio refractivity

at higher altitude• Radio waves propagates faster

at higher altitude

• In a ”standard atmosphere”, commonly used for planning of antenna mounting height,radio refractivity is consideredto be constantly changing by altitude with: dN/dh = -40N-units/km

NAltitude

NGround

dN/dh

dN/dh, k-factordN/dh is a value describing how fast the radio refractivity changes with altitude.

In standard atmosphere dN/dh = -40 N units/km (or to be precise -39). The minus indicates that the atmosphere gets thinner with increasing height, and thereby radio waves travels faster at high altitudes.

dN/dh > 0 then k < 1, higher radio refractivity by altitude, up-bending of the wave front. Sub refractivity.

dN/dh = 0 then k = 1, no radio refractivity change by altitude, no bending of the wave front.

-157 < dN/dh < 0 the k > 1, radio refractivity declines by altitude, down bending of the wave front

dN/dh = -157 then k = , radio refractivity declines with altitude so the wave front will be bent same as the earth curvature which will give a “flat earth”

If dN/dh < -157 then k is negative, the wave front will be bent more than the earth curvature and will “hit the earth”. Super refractivity or ducting.

Μεταβολή dN/dh, k-factor• dN/dh will cause the wave front

to bend.• The bending effect is

described by the: Equivalent earth radius factor:

”Low speed”

”High speed”

NAltitude

NGround

dN/dh

Wav

e fro

nt

Wav

e fro

ntW

ave

front

Wav

e fro

nt

k= 157157 + [dN/dh]

which for a ”standard atmosphere” where dN/dh = -40N-units/km gives k~1.33 (4/3)

• Commonly dN/dh is a negative value (lower Radio refractivityat higher altitude) and the wave front is bent downwards

dN/dh, k-factor

0453-045

FIGURE 4

Monthly mean values of N: February

FIGURE 5

Monthly mean values of N: May

February

May

0453-067

FIGURE 6

Monthly mean values of N: August

FIGURE 7

Monthly mean values of N: November

August

November

ΔN=-40ΔN (dN/dh) charts• Used for calculation of

the k-factor • Gives the monthly mean

value for dN/dh• Expressed in N-units/km• Obtained from maps in

ITU-R Rec. P.453. ΔN=-40

ΔN=-40

ΔN=-40

k-factor by latitude

EquatorSouth 30°

North 30°

A study carried out by Ericssonshows that the k-factor north of N 30° and south of S 30° decreases according to :k = 1.91 – 0.01x|latitude|Air humidity [g water/cu m air]is the most important factor.

”Difficult” refractivity propertiesare well known to be found in the Persian gulf and thewest coast of Africa.

Equivalent earth radiusk = 4/3

12/3

Earth bulge at true earth radius (r)

Ground clearance

2/3

4/31

k = Earth bulge at equivalent earth radius (r·k)

Ground clearance

What happens:

How antenna mounting height is calculated:

Earth bulgeCalculation

Distanced1 d2

h : Earth bulge

d=20km

h =12.74 x kd1 x d2 h in meter

d1 and d2 in km

h =12.74 x 110 x 10

= 8 m= 6 mh = 10 x 1012.74 x 1.33

Earth bulge

1:st Fresnel zone

1:st Fresnel zone

• In all radio communication energy from transmitter to receiver is distributed in an elipzoid shaped zone around the line of sight.The zone has its focal points in the antennas:This is called: The 1:st Fresnel zone

• Its radius depends on frequency and path length• Design objective for radio links is basically to keep it free

from obstacles

Line of Sight

1:st Fresnel zoneDefinition

dadb

d

da + db - d = /2

dbda

1:st Fresnel zoneCalculation of radius

d

d1 x d2

f x drF = 17.3 x

rF in meterd, d1, d2 in kmf in GHz

rFrF

d1 d2

d1 d2

Antenna height design objective

1 The 1:st Fresnel zone shall be free from obstacles when k=4/3

2 On paths over water surfaces or desert areas, it is recommended to have the 1:st Fresnel zone free from obstacles when k=1(See also ITU-R Rec.P.530)

k = 4/3

1st Fresnel zone

Antenna heightExamples

d

rF

h

Fq.[GHz]

d[km]

h midpath [m]k=1 k=1.33

rF midpath [m]

615

38

5030

5

25.012.2

3.1

36.949.017.7 13.3

0.40.5

Ant. height k=1.33 [m]

~ 62~ 26

~4

Ant

enna

Hei

ght >

Δh

+ rF

Σχεδιασμός Μικροκυματικής Ζεύξης κοντινής απόστασης- Short-haul microwave radio design

Μικροκυματική μετάδοση υπό συνθήκες καναλιού με κατακρυμνήσεις σήματος

ισχύος (Fading Conditions)

The prediction cycle (1)

• Free-space loss• Gas attenuation

Link budget

• Rain attenuation• Diffraction – Refraction loss

• Multipath propagation

Quality and Availability

• Interference

”Slow”

”Fast”• Obstacle loss• Reflection loss

•BBER •ESR •SESR •UATR

Frequency planningLoss & attenuation Fading prediction

Fade marginObjectivesfulfilled?

No

Yes

Thresholddegradation

Fading probabilityPath loss

Definition of fading

• Variation in signal strength, phase or polarisation• Attenuation that is changing temporarily

Fading mechanismsFrequency dependancy

6 7 8 13 15 18 23 26 28 32 38GHz

UATR

SESR, BBER

”Cross-over”

• At ”low” frequencies the Quality objective from multipath fading is dimensioning.• At ”high” frequencies the Availability objective from rain fading is dimensioning.• A ”cross-over” region at around 15-18GHz where any of the objectives may

be dimensioning

The prediction cycle (2)

• Free-space loss• Gas attenuation

Link budget

Quality and Availability

• Interference

• Obstacle loss• Reflection loss

•BBER •ESR •SESR •UATR

Frequency planningLoss & attenuation Fading prediction

Fade marginObjectivesfulfilled?

No

Yes

Thresholddegradation

Fading probabilityPath loss

HoplengthFrequencyPolarizationTerrainClimateAtmosphere ...

Fading margin• The fading margin is designed into the system to meet

network quality and availability objectives.• The fading margin is the mean to meet the Q&A objectives, not the objective.

M = Fading margin [dB]

Receiver threshold level (BER=10-3, BER=10-6)

Distance

A0

Power level[dBm]

PTX

PRX

Fading mechanism

Flat multipath fading

Frequencyselective fading

Rain fading(precipitation)

Multipath fadingRefraction / Reflections

k-fadingDiffraction fading

FadingMechanisms

Σκέδαση - Precipitation

Precipitation can take the form of:• Snow

• Negligible for frequencies below 50 GHz• Ice coating can increase attenuation

• Hail• Depend on size• Consider as negligible

• Fog and haze• Consider as negligible

• Rain• Absorption and scattering

Rain fading

Falling raindropshape

Two types of attenuation mechanism:

Horizontally polarised waves are attenuated more than vertically polarised waves

Absorption

Scattering

Attenuation per km

ITU-R presents the cumulative distribution of rain intensity based on the rain intensity exceeded for 0,01 % of time.

Distance reduction factor has to be taken into consideration.

For calculations of outage due to rain, the percentage of time during which a given rain intensity is exceeded is of interest.

Rai

n A

ttenu

atio

n C

oeffi

cien

t [d

B /

km]

150100

5025

5

1.25

0.25

Frequency [GHz]

Inst

anta

neou

s R

ain

Inte

nsity

[mm

/h]

Effective rain path

The reduction factor transforms actual path lengthto effective rain path length, along which the rain is regarded as having a uniform distribution

Distance reduction factor:

effd = d r . d: Actual path length

r: Distance reduction factor

r = 1

1+0d

d 0d = 35 e. -0,015 R. 0,01where

R0,01= maximum rain intensityUse 100 for rain intensity 100 mm/h)

effd:Distance covered by the maximum rain intensity

effd

d

Rain intensityITU-R Rec. P.837-1

• ”Old way” of describing rain intensity.• The world divided into 15 Rain zones.• Data from maps. No data files.• Refer to ITU-R Rec. P.837-1

Rainfall exceeded for 0.01% of time

Rain zonemm/h

Rain zonemm/h

Rain intensityITU-R Rec. P.837-5

• Rain rate exceededfor 0.01% of theaverage year.

• Rain data available from the ITU-R asdata files or maps

• Resolution long. x lat. 1.125 x 1.125 degrees

• Integration time 1min• Refer to:

ITU-R Rec. P.837-5

Rain modelsComparison ITU-R vs Crane

0.00001

0.00010

0.00100

0.01000

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30

Path length (km)

UATR Example for 26 GHz, rain intensity 30 mm/h

ATR=99.000%

ATR=99.900%

ATR=99.990%

ITUCrane

ATR=99.999%

145dB 155dB 165dB

175dB

185dB

Improvement factors

+Higher system gain that is increased fade marginHigher output powerHigher antenna gainShorter path

+ Lower frequency band+ Vertical polarization

Fade margin

effd

d