01_03813-LZU113302 B

MINI-LINK HC Operation & Maintenance01/038 13 - LZU 113 302 B 1

Basic Microwave Propagation


Basic Microwave Propagation Agenda

Transmission quality

Link budget• Free Space Loss• Radio and Antenna properties

Frequency planning

Line of Sight

Fading


Link budget

A = Free Space Loss(incl. Gas Absorption)

GRX

Antenna Gain

PTX

Output Power

GTX

Antenna Gain

PRX Input Power

DistanceReceiver Threshold Level

PowerLevel[dBm]

Frequency

Fade Margin


d

f

Basic Free Space Loss

A = 92.4 + 20 log d + 20 log f

d = distance in km f = frequency in GHz

0

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

Basic Free Space Loss Calculation


“High” frequencyEasier to get licenseShort rangeUrban use in general

“Low” frequencyLong rangeGenerally used in rural areas

Free space loss

Generally frequency licenses shall be applied for from national administrations


Modulation Techniques, examples

C-QPSK (4QAM)

4 symbols2 bits/symbol

16-QAM


128-QAM


Channel spacing [MHz]3.57

1428

56 (2x 28)

C-QPSK48

1637

16QAM

1637

155

128QAM

155

Traffic capacity [Mbit/s]

Available combinations in MINI-LINK

C-QPSK

16QAM

128QAM

Distance [km]

Traffic capacityper bandwidth


AntennaAntenna gain

Lower radio frequency Larger antennaLonger path length Larger antenna

A “small” antenna gives: Less windload, less visibility and lower cost for antenna and installation

A “large” antenna gives: Higher gain, thereby longer hopand/or higher transmissionquality


Link Budget Antenna Gain

Transmitteroutput power Input pow

er tothe receiver

Receiver threshold level

Distance [km]

PowerLevel[dBm]

Antenna Gain[dBi]

0.3m antenna

0.6m antenna

1.2m antenna

0 km n km




Link budget

Frequency planning

Line of Sight• Atmospheric properties• Fresnell zone• Ground clearance

Fading


Line of sight

• Heights of masts must be designed so that there is a radio optical free line of sight and a sufficiently large ground clearance.

• Due to atmospheric properties the radio beam is normallybent slightly downwards

Radio optical line of sight

Geometrical line of sightGround clearence


Line of sight Refraction

• The bending effect is described by the k-factor

• k = 4/3 corresponds to a “standard” atmosphere

• The earth radii is multiplied with the k-factor and thereby, at standard atmosphere, making the earth flatter.

Radio optical line of sight

Geometrical line of sightGround clearence


Line of sight Earth bulge

Earth bulge

Flat earth surface

“Real” earth bulge, k = 1

Radio optical earth bulge at standard atmosphere, k = 4/3

Radio optical earth bulge at sub refractive atmosphere, k = 2/3

5km 15km 50km

0.4m

0.5m

0.7m

3.3m

4.4m

6.7m

37m

50m

75m

Hop length:


Line of sightThe 1:st Fresnell zone

The signal power is distributed in the space surrounding the direct line of sight

Line of sight

1st zone


Line of sightThe 1:st Fresnell zone, examples of radii at mid path

rF

15GHz

7GHz

38GHz

5km 15km 50km

7m

5m

3m

12m

8m

5m

23m

16m

10m

Hop length:


Line of sight Design Objective

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

• On paths over water surfaces or desert areas, it is recommended to have the 1st Fresnel zone free from obstacles when k = 1

rF


Line of sight Obstruction Loss - - Knife-edge Obstructions

6 dB 20 dB16 dB12 dB0 dB0 dB

Line of Sight




Link budget

Frequency planning

Line of Sight

Fading• Rain fading• Multipath fading


Rain fading

Rain drops real shape:

HV

Horizontally polarised waves are attenuated more than vertically

polarised waves


Radio frequency (GHz)

0.4

1

2

4

10

2230

50

100

150 TropicalDownpour

HeavyRain

MediumHeavyRain

LightRain

Drizzle

Inst

ant

ane

ous

Rai

n In

tens

ity (

mm

/h)

0.01

0.1

1

10

50

Rai

n A

bsor

ptio

n C

oeff

icie

nt

(dB

/km

)

5 10 20 50 100

Rain fading

7 GHz

0.15 dB/km

2.4 dB/km

38 GHz

37 dB/km150

6.0 dB/km

22


Rain fading Effect and what to do

effd

d

Rain fading will be seen as:• Low RF input power to the receiver

resulting in Unavailable time (UAT)

What to do?• Lower frequency band• Increased system gain (increased fade margin)

Larger antennas, increased transmitter output power.• Vertical antenna polarization• (Shorter hop…)


Multipath FadingDue to Atmospheric Layers


Multipath Fading

Flat fading• The loss is uniform across the

frequency spectrum

Selective fading • The loss varies across the

frequency spectrum


Multipath FadingFrequency Spectrum

Channel bandwidth (MHz)

Rec

eive

d po

wer

leve

l [dB

m]

Fading free Flat fadingSelective fading

Center fq


Multipath FadingEffect and what to do

Multipath fading will be seen as:Bit errors in the transmission. Resulting in • Errorred seconds (ES)• Severely errorred seconds (SES)

What to do?At flat multipath fading:• Increased system gain

Larger antennas, Increased transmitter output power.

• Space or Frequency diversity• (Shorter hop…)

What to do?At frequency selective multipath fading:• Space or Frequency diversity• (Shorter hop…)




Link budget

Frequency planning• Frequency plan• Sub-band allocation• Interfering signals

Line of Sight

Fading


Frequency PlanningChannel Spacing

3.5 MHz

3.5 MHz

7 MHz

7 MHz

14 MHz

14 MHz

28 MHz

28 MHz

By international regulations microwave radio-link frequency bands are divided into channels with different frequency bandwidths, defined as channel spacing.

Wide bandwidth: more information, traffic, can be sent over the path.

Narrow bandwidth: more paths can be present in a certain geographical areawithout disturbing each other, and each path may be longer.

Channel spacing


Frequency PlanningChannel arrangement example, 15GHz band, ITU-R Rec. F.636

3.5 MHz

28 MHz1D 15D

1A 3A 5A 7A 114A 116A 118A 120A

7 MHz1B 2B 3B 4B 57B 58B 59B 60B

14 MHz1C 2C 29C 30C

14500 M

Hz

14925 M

Hz

3.5 MHz

28 MHz1’D 15’D

1’A 3’A 5’A 7’A 114’A 116’A 118’A 120’A

7 MHz1’B 2’B 3’B 4’B 57’B 58’B 59’B 60’B

14 MHz1’C 2’C 29’C 30’C

14924 M

Hz

15343 M

Hz

Upper band

Lower band


LOWHIGH

LOW LOW

Frequency PlanningSub-band Allocation

NO YES


LOW HIGH LOWHIGHHIGHHIGHHIGH LOW

Frequency PlanningSub-band Allocation


Vertical polarization

Horizontal polarization

Frequency PlanningInterfering signals

H-pol.

V-pol.V-pol.

H-pol.

V-pol

H-pol H-pol

V-pol


GRX

Antenna Gain

PTX

Output Power

GTX

Antenna Gain

PRX

Input Power

PowerLevel[dBm]

Degraded Threshold Level

Nominal Threshold Level

Frequency PlanningInterfering signals


Frequency PlanningInterfering signals, effect and what to do

Interfering signals will be seen as:• Degraded path performances resulting

in ES / SES / UAT despite correct RF input power to the receiver

What to do?• Change of antenna polarization• High Performance antennas• Larger antennas / lower transmitter output power• Shadow the interfering signal

Lower the antenna / Move the site• Use another frequency


Microwave fundamentals Agenda


Link budget

Frequency planning

Line of Sight

Fading


Basic Microwave Propagation Quality targets

Quality and Availability Targets

P

D

• All links are designed to meet a certain transmission quality.

• Internationally accepted recommendations for transmission quality and how to predict it are published by the ITU (International Telecommunication Union).

• Quality is based on the ratio of errored bits.

• In microwave radio links it is, besides the distance, fading from rain or the fact that the signal can reach the receiver via different paths in the atmosphere, multipath fading, that commonly limits the performances.

01_03813-LZU113302 B

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