basic microwave propagation
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8/6/2019 Basic Microwave Propagation
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Basic Microwave Propagation
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Basic Microwave PropagationAgenda
Transmission quality
Link budget
• Free Space Loss• Radio and Antenna properties
Frequency planning
Line of Sight
Fading
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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
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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
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“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
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Modulation Techniques, examples
C-QPSK (4QAM)
4 symbols2 bits/symbol
16-QAM
16 symbols4 bits/symbol
128-QAM
128 symbols7 bits/symbol
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
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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
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Basic Microwave PropagationAgenda
Transmission quality
Link budget
Frequency planning
Line of Sight• Atmospheric properties• Fresnell zone• Ground clearance
Fading
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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
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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
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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:
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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
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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:
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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
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Line of sightObstruction Loss -- Knife-edge Obstructions
6 dB 20 dB16 dB12 dB0 dB0 dB
Line of Sight
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Basic Microwave PropagationAgenda
Transmission quality
Link budget
Frequency planning
Line of Sight
Fading• Rain fading• Multipath fading
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Rain fading
Rain drops real shape:
HV
Horizontally polarised waves are
attenuated more than verticallypolarised waves
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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
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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…)
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Multipath FadingDue to Atmospheric Layers
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Multipath FadingDue to Atmospheric Layers
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Multipath Fading
Flat fading• The loss is uniform across the
frequency spectrum
Selective fading• The loss varies across the
frequency spectrum
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Multipath FadingFrequency Spectrum
Channel bandwidth (MHz)
R
eceived
powerle
vel[dBm]
Fading free Flat fading
Selective fading
Center fq
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Basic Microwave PropagationAgenda
Transmission quality
Link budget
Frequency planning• Frequency plan• Sub-band allocation• Interfering signals
Line of Sight
Fading
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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
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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
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LOWHIGH
LOW LOW
Frequency PlanningSub-band Allocation
NO YES
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LOW HIGH LOWHIGHHIGHHIGHHIGH LOW
Frequency PlanningSub-band Allocation
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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
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GRX
Antenna Gain
PTX
Output Power
GTX
Antenna Gain
PRX
Input Power
Power
Level[dBm]
Degraded Threshold Level
Nominal Threshold Level
Frequency PlanningInterfering signals
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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
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Microwave fundamentalsAgenda
Transmission quality
Link budget
Frequency planning
Line of Sight
Fading
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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.