4. Proprietary and Confidential
RF Signal
Path Terrain
f1
f1’
• A Radio Link requires two end stations
• A line of sight (LOS) or nLOS (near LOS) is required
• Microwave Radio Link frequencies occupy 1-80GHz
Radio Relay Principles
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5. Proprietary and Confidential
High and Low frequency station
Local site
High station
Remote site
Low station
High station means: Tx(f1) >Rx(f1’)
Tx(f1)=11500 MHz Rx(f1)=11500 MHz
Rx(f1’)=11000 MHz Tx(f1’)=11000 MHz
Low station means: Tx(f1’) < Rx(f1)
Full duplex
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6. Proprietary and Confidential
Frequency reuse:
2,4V
1,3V
1,3H 1,3H 1,3H
Reduced risk for overshoot
Frequency shift:
1,3V
1,3H 2,4H
Reduced risk for overshoot
Only Low stations can interfere High stations
1,3H
Tx in upper part of band
Tx in lower part of band
1,3V
Low High Low High
Tx Tx Tx
Tx
Tx
Tx
Tx
Tx
Tx
Tx
Tx
Tx
Standard frequency plan patterns
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8. Proprietary and Confidential
RF Tx Filter
Branching
Network(*) Feeder
Z' B' C' D'
A'
Feeder
D
Branching
Network(*)
C B
RF Rx Filter
A
Receiver
E
Demodulator
Z
Modulator
E'
RECEIVER PATH
TRANSMITTER PATH
Transmitter
Digital
Line interface
Digital
Line interface Output
signal
Input
signal
Radio Principal Block Diagram
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9. Proprietary and Confidential
RF Principals
• RF - System of communication employing electromagnetic waves
(EMW) propagated through space
• EMW travel at the speed of light (300,000 km/s)
• The wave length is determined by the frequency as follows -
Wave Length
• Microwave – refers to very short waves (millimeters) and typically
relates to frequencies above 1GHz:
300 MHz ~ 1 meter
10 GHz ~ 3 cm
9
f
c
where c is the propagation velocity of electromagnetic
waves in vacuum (3x108 m/s)
10. Proprietary and Confidential
RF Principals
• We can see the relationship between colour, wavelength and amplitude
using this animation
10
14. Proprietary and Confidential
Parameters Affecting Propagation – Dispersion
• Electromagnetic signal propagating in a physical medium is degraded
because the various wave components (i.e., frequencies, wavelengths)
have different propagation velocities within the physical medium:
• Low frequencies have longer wavelength and refract less
• High frequencies have shorter wavelength and refract more
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15. Proprietary and Confidential
Parameters Affecting Propagation
Atmospheric Refraction
15
• Deflection of the beam towards the ground due to different electrical
characteristics of the atmosphere’s is called Dielectric Constant.
• The dielectric constant depends on pressure, temperature &
humidity in the atmosphere, parameters that are normally decrease
with altitude
• Since waves travel faster through thinner medium, the upper part of the
wave will travel faster than the lower part, causing the beam to bend
downwards, following the curve of earth
No Atmosphere
With Atmosphere
16. Proprietary and Confidential
Wave in atmosphere
Radio rays bends towards the more dense medium N – Radio refractivity
N = (n-1) * 106
= 77.6 / T *(p + 4810 * e /T)
n – refractivity ( speed in
vacuum / speed in media)
T – Temprature Kelvin
P – Total air pressure mbar
e – water vapour pressure hPa
Normal atmosphere:
km
units
N
40
dh
dN
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17. Proprietary and Confidential
Parameters Affecting Propagation – Multipath
• Multipath occurs when there is more then one beam reaching the receiver
with different amplitude or phase
• Multipath transmission is the main cause of fading in low frequencies
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Direct beam
Delayed beam
18. Proprietary and Confidential
Parameters Affecting Propagation – Duct
• Atmospheric duct refers to a horizontal layer in the lower atmosphere with
vertical refractive index gradients causing radio signals:
• Remain within the duct
• Follow the curvature of the Earth
• Experience less attenuation in the ducts than they would if the ducts were not
present
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Duct Layer
Terrain
Duct Layer
19. Proprietary and Confidential
Parameters Affecting Propagation - Polarization and Rain
19
• Raindrops have sizes ranging from 0.1 millimeters to 9 millimeters
mean diameter (above that they tend to break up)
• Smaller drops are called cloud droplets, and their shape is spherical.
• As a raindrop increases in
• size, its shape becomes more
• oblate, with its largest
cross-section facing the
• oncoming airflow.
Large rain drops become
Increasingly flattened on the
Bottom;
very large ones are shaped
like parachutes
20. Proprietary and Confidential
Parameters Affecting Propagation – Rain Fading
• Refers to scenarios where signal is absorbed by rain, snow, ice
• Absorption becomes significant factor above 11GHz
• Signal quality degrades
• Represented by “dB/km” parameter which is related the rain
density which represented “mm/hr”
• Rain drops falls as flattened droplet
V better than H (more immune to rain fading)
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22. Proprietary and Confidential
Parameters Affecting Propagation – Fresnel Zone
22
Terrain
Duct Layer0
1st
2nd
3rd
TX RX
1. EMW propagate in beams
2. Some beams widen – therefore, their path is longer
3. A phase shift is introduced between the direct and indirect
beam
4. Thus, ring zones around the direct line are created
23. Proprietary and Confidential
Parameters Affecting Propagation – Fresnel Zone
• Obstacles in the first Fresnel zone will create signals that will be 0 to 90 degrees out
of phase…in the 2nd zone they will be 90 to 270 degrees out of phase…in 3rd zone,
they will be 270 to 450 degrees out of phase and so on…
• Odd numbered zones are constructive and even numbered zones are destructive.
• When building wireless links, we therefore need to be sure that these zones are kept
free of obstructions.
• In wireless networking the area containing about 40-60 percent of the first Fresnel
zone should be kept free.
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27. Proprietary and Confidential
Main Parabolic Antenna Types
27
• Standard performance antennas (SP,LP)
• Used for remote access links with low capacity. Re-using frequencies on adjacent links is not
normally possible due to poor front to back ratio.
• High performance antennas (HP)
• Used for high and low capacity links where only one polarization is used. Re-using frequencies is
possible. Can not be used with co-channel systems.
• High performance dual polarized antennas (HPX)
• Used for high and low capacity links with the possibility to utilize both polarizations. Re-using
frequencies is possible. Can be used for co-channel systems.
• Super high performance dual polarized antennas (HSX)
• Normally used on high capacity links with the possibility to utilize both polarizations. Re-using
frequencies is possible with high interference protection. Ideal for co-channel systems.
• Ultra high performance dual polarized antennas (UHX)
• Normally used on high capacity links with high interference requirements. Re-using frequencies in
many directions is possible. Can be used with co-channel systems.
29. Proprietary and Confidential
Link Calculation – Basic Example (in vacuum)
Lfs
TSL Ga Lfsl Ga Lw
Lb
Lf
RSL
RSL=TSL+Ga-Lfsl+Ga-Lw-Lb-Lf
RSL - Received Signal Level
TSL – Transmitted Signal Level
Lfsl - Free-space loss = 92.45 + 20 log x(distance in km x frequency in GHz)
Lf - Filter loss
Lb - Branching loss
Lw - Waveguide loss
Ga – Antenna gain
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30. Proprietary and Confidential
Atmospheric attenuation
30
]
[dB
d
A a
a
Starts to contribute to the total attenuation above approximately 15GHz
Parameters in a:
Frequency
Temperature
Air pressure
Water vapour
31. Proprietary and Confidential
Objective examples
31
• Typical objectives used in real systems
• 99.999%
• Month: 25.9 sec
• Year: 5 min 12 sec
• 99.995 %
• Month: 2 min 10 sec
• Year: 26 min
• 99.99%
• Month: 260 sec
• Year: 51 min
• Performance requirements generally higher than Availability.
• ITU use worst month for Performance Average year for Availability
34. Proprietary and Confidential
Modem
1 0 1 1 0 1 1 0
1 0 1 1 0 1
1 0
Modem
1 1
1
1 1
0 0 0
0
1
1
1 0 1
1
F1
F2
F1 F1
F2
F1 F1
Modem
1 1 1 1 1
0 0 0
1 0 1 1 0 1 1 0
1800 phase shift
ASK modulation changes the amplitude to the analog
signale.”1” and “ 0” have different amplitude.
FSK modulation is a method of represent the two
binary states ”1” and ”0” with different
spcific frequencies.
PSK modulation changes the phase to the transmitted
signal. The simplest method uses 0 and 1800 .
Digital modulation
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35. Proprietary and Confidential
QAM Modulation
• Quadrature Amplitude Modulation employs both phase modulation
(PSK) and amplitude modulation (ASK)
• The input stream is divided into groups of bits based on the number
of modulation states used.
• In 8 QAM, each three bits of input, which provides eight values (0-7)
alters the phase and amplitude of the carrier to derive eight unique
modulation states
• In 64 QAM, each six bits generates 64 modulation states; in 128
QAM, each seven bits generate 128 states, and so on
4QAM 2bits/symbol 256QAM 8bits/symbol
8QAM 3bits/symbol 512QAM 9bits/symbol
16QAM 4bits/symbol 1024QAM 10bits/symbol
32QAM 5bits/symbol 2048QAM 11bits/symbol
64QAM 6bits/symbol
128QAM 7bits/symbol
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36. Proprietary and Confidential
Why QAM and not ASK or PSK for higher modulation?
• This is because QAM achieves a greater distance between adjacent points
in the I-Q plane by distributing the points more evenly
• The points on the constellation are more distinct and data errors are
reduced
• Higher modulation >> more bits per symbol
• Constellation points are closer >>TX is more susceptible to noise
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37. Proprietary and Confidential
Constellation diagram
• In a more abstract sense, it represents the possible symbols that may be
selected by a given modulation scheme as points in the complex plane.
Measured constellation diagrams can be used to recognize the type of
interference and distortion in a signal.
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38. Proprietary and Confidential
8 QAM Modulation Example
• We have stream: 001-010-100-011-101-000-011-110
38
Bit sequence Amplitude Phase (degrees)
000 1 None
001 2 None
010 1 pi/2 (90°)
011 2 pi/2 (90°)
100 1 pi (180°)
101 2 pi (180°)
110 1 3pi/2 (270°)
111 2 3pi/2 (270°)
How does constellation diagram look?
DIGITAL QAM (8QAM)
43. Proprietary and Confidential
10-3
10-4
10-5
10-6
10-7
10-8
-75 -72 -69 -66
Receiver input level [dBm]
BER change ratio vs. Noise is
dependent on Noise Power distribution
and coding
BER=
𝐵𝑎𝑑 𝑟𝑒𝑐𝑒𝑖𝑣𝑒𝑑 𝑏𝑖𝑡𝑠
𝑇𝑜𝑡𝑎𝑙 𝑡𝑟𝑎𝑛𝑠𝑚𝑖𝑡𝑡𝑒𝑑 𝑏𝑖𝑡𝑠
BER Impact on Transmission Quality
43
44. Proprietary and Confidential
RSL Vs. Threshold
44
Thermal Noise=10*log(k*T*B*1000)
S/N=23dB for 128QAM (37 MHz)
BER>10-6
RSL (dBm)
-20
-30 Nominal Input Level
-99
-96 Receiver amplifies thermal noise
-73 Threshold level BER=10-6
Fading Margin
K – Boltzmann constant
T – Temperature in Kelvin
B – Bandwidth
Time (s)
BER>10-6
