Basic Microwave Propagation

8/6/2019 Basic Microwave Propagation

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

Basic Microwave Propagation




Basic Microwave PropagationAgenda

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

Distance

Receiver

Threshold Level

Power

Level[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” frequency

Easier to get license

Short range

Urban use in general

“Low” frequency

Long range

Generally 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

14

28

56 (2x 28)

C-QPSK

48

16

37

16QAM

16

37

155

128QAM

155

Traffic capacity [Mbit/s]

Available combinations in MINI-LINK

C-QPSK

16QAM

128QAM

Distance [km]

Traffic capacity

per bandwidth


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AntennaAntenna gain

Lower radio frequency Larger antenna

Longer 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 hop

and/or higher transmission

quality



Link BudgetAntenna Gain

Transmitter

output power Inputpowerto

therece

iver

Receiver threshold level

Distance [km]

Power

Level

[dBm]

AntennaGain

[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 isa radio optical free line of sight and a sufficiently large

ground clearance.• Due to atmospheric properties the radio beam is normally

bent slightly downwards

Radio optical line of sight

Geometrical line of sightGround clearence



Line of sightRefraction

• 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 sightEarth 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

r F

15GHz

7GHz

38GHz

5km 15km 50km

7m

5m

3m

12m

8m

5m

23m

16m

10m

Hop length:




Line of sightDesign 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 fromobstacles when k = 1

r F




Line of sightObstruction 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 verticallypolarised waves




Radio frequency (GHz)

0.4

1

2

4

10

22

30

50

100

150

TropicalDownpour

Heavy

Rain

Medium

Heavy

Rain

Light

Rain

Drizzle

Instantan

eousRain

Intensity(

mm/h)

0.01

0.1

1

10

50

RainAbs

orptionCo

efficient (d

B/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

eff d

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)

R

eceived

powerle

vel[dBm]

Fading free Flat fading

Selective fading

Center fq






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 area

without 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

14500MH

z

14925MHz

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

14924MH

z

15343MH

z

Upper band

Lower band




LOWHIGH

LOW LOW

Frequency PlanningSub-band Allocation

NO YES




LOW HIGH LOWHIGHHIGHHIGHHIGH LOW

Frequency PlanningSub-band Allocation




Vertical polarization

H o r i z o nt al p o l ar i z at i o n

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

Power

Level[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 fundamentalsAgenda


Link budget

Frequency planning

Line of Sight

Fading



Basic Microwave PropagationQuality 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.

Basic Microwave Propagation

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