Engineering a Microwave
System
Microwave Communications | Module 3
Session 1
Microwave Design
and Calculations
At the end of this lesson,
the student is expected to:
• Design a microwave
system.
Photo Credit: authorjeniferchase.com
Source: Electronic Devices 9th edition by Thomas Floyd
Remaining Steps
• Step 3 – Attenuation due to rain and
multipath propagation
• Step 4 – Choice of Tx power and
antennas
• Step 5 – Link budget calculation
Step 3
Attenuation due to rain
and multipath
propagation
5 Factors For A Stable Microwave Link
Frequency
Selection
Interference &
Fading
LOS & Path loss
Capacity
Redundancy
Fading
A random increase in path loss
caused by unusual propagation
conditions
Multipath Fading the dominant fading factor for
below10 GHz.
Characteristics of multipath fading
in different regions:
➢Least amount of fading over dry, terrain areas.
➢Worst fading over hot and humid coastal areas.
➢Inland temperate regions are somewhere in
between.
➢Higher fading in low terrain (such as deserts &
lakes) due to increased incidence of reflections.
➢Less fading in irregular, hilly terrain or forests
(less reflections).
How to set a multipath fade margin (MFM)?
First, decide on a propagation reliability (PR) over time.
This is only one factor in overall system reliability
however.
From Barnett/Vigants model:
MFM = -10LOG[(1-PR)/(2.5x10-6abfD3)]
Where:
PR = fraction of time of path unavailability
a = terrain factor
b = climate factor
f = frequency in GHz
D = path length in miles
Other factors added to the fade margin
➢ Rain fade – significant for >6 GHz and long paths, very
complex problem to quantify
➢ Equipment aging – includes antenna misalignment, DoD
recommends 6 dB though more would be appropriate for
higher-gain dishes
➢ Atmospheric absorption – only consider water vapor
between 15 and 30 GHz, peaks at 22 GHz, estimate
0.2 dB/km
Use of multipath fade margin alone (plus allowance for rain > 10
GHz) is usually appropriate for gross planning below 10 GHz.
Estimated Rain Attenuation (RA)
at Specific Frequencies for 1”/hr
Rainfall
Freq. (GHz)
6
10
15
20
RA (dB/km)
0.1
0.6
1.6
2.6
Step 4
Choice of Tx power and
antennas
Three Considerations for the Choice of
Tx power and antennas
• For the main antenna, it is best to aim for a minimum of 50% efficiency at
the desired bandwidth. For supporting or supplemental antennas, a target
of 60% or better efficiency for GPS functions and 40% for Bluetooth is
reasonable. Diversity antennas can work well with efficiencies in the
range of 30 to 40%.
• It is absolutely critical to make sure that the antennas have the least
amount of interference amongst each other. Fifteen to 20 dB of isolation
should be a target for isolation between the multitudes of antennas in
today's devices. Chart 1 shows an example of isolation measurements
for a combination GPS/Bluetooth antenna with high isolation of 29dB for
GPS and 24dB for Bluetooth.
Three Considerations for the Choice of
Tx power and antennas
• The natural third factor engineers must consider when selecting an
antenna is the antenna's selectivity. In addition to increasing the
antenna's isolation, good selectivity also acts as a cost-effective method
to suppress interference between multiple applications. The selectivity
of an antenna must allow it to be fully-functional across the frequency
spectrum of interest without losing reception near the ends of the
bands.
Step 5
Link budget calculation
Consider a signal from a transmit point to a
receive point:
The free space path loss (FSL) is:
FSLdB = 92.44 + 20 log Dkm + 20 log fGHz
or FSLdB = 96.6 + 20 log Dmi + 20 log fGHz
If your path has obstacles in the
F.Z. and you can’t increase the
antenna heights to avoid them,
how do you estimate the
diffraction losses?
From the path profile, determine the
level of penetration of the 60% 1st F.Z.
of each obstacle and add the losses
according to the diagram.
Calculation of Received Signal (RSL) with Fading
RSL = Po - Ltx + Gatx - FSL - MFM - RA - Lrc + Grc – Lm
Po = Transmitter power output (dBm)
Ltx = All losses between transmitter and its antenna (dB)
Gatx = Gain of transmitting antenna (dBi)
FSL = Free space loss (dB)
MFM = Multipath fade margin (dB)
RA = Rain attenuation (dB)
Lrc = All losses between receiver and its antenna (dB)
Grc = Gain of receiving antenna (dBi)
Lm = Miscellaneous losses (obstacle, misalignment, aging) (dB)
A few things to watch out for
The reliability (or availability) calculation
does not include human failure or the
reliability of your telecom company or ISP
that delivers content to your microwave
site. These have to be taken into account.
A radio link path mostly over water is a received signal cancellation hazard due
to reflections from the surface. Antenna heights should be adjusted so that the
2nd F.Z. is below the surface. Also, use vertical polarization instead of horizontal
polarization.
5 Factors For A Stable Microwave Link
Interference &
Fading
Frequency
Selection
LOS & Path loss
Capacity
Capacity
Redundancy
Frequency Selection &
Capacity
➢The higher the frequency, the higher the available
capacity but at the same time, the effective range is
lowered and the link would be more susceptible to
rain or high humidity.
➢To use a frequency, a license should usually be
obtained from the legal authorities of the country.
➢There are also a few frequency bands that are
“license-free” – mainly 2.4 GHz, 3 GHz and 10 GHz.
Frequency Selection &
Capacity
• A channel’s capacity is directly proportional to the
width of the channel and the type of signal modulation
scheme used. Microwave backhaul generally uses a
frequency-division duplex (FDD) system, whereby
each hop is allocated a frequency channel pair known
as a go/return pair. This facilitates simultaneous
transmission in both directions across the link
Frequency Selection &
Capacity
• For point-to-point communications to function reliably and
efficiently, the energy directed from one antenna must arrive
precisely at the receiving antenna.
• The amount of received energy is called the carrier power (C).
Transmitted energy that is not received is called noise ratio.
• Shannon’s Law describes the relationship between capacity and
signal noise in this formula:
• Capacity = B x Log2 (1 + C/N)
• Where:
• B = Channel bandwidth
• C = Carrier power, or RSL
• N = Signal noise
Frequency Selection &
Capacity
• However,
the
contemporary
reality
of
wireless
communications has introduced a new variable—interference
caused by operators adding more links to their networks. So, if
we add the new variable (I) for interference and combine it
with signal noise, we see that the equation now reads:
• Capacity = B x Log2 (1 + C/(N+I))
• Where:
• B = Channel bandwidth
• C = Carrier power, or RSL
• N = Signal noise
Frequency Selection &
Capacity
•Shannon’s Law dictates that
capacity is limited by noise on the
system
Frenzel, L.E. (2003). Principles of
electronic communication
systems. New York: McGrawHill/Giencoe
Photo Credit: pininterest.com
Source: Electronic Devices 9th edition by Thomas Floyd
Session 2
Microwave
Engineering System
and Parameters
At the end of this lesson,
the student is expected to:
• Describe the microwave
engineering system and
parameters.
Photo Credit: authorjeniferchase.com
Source: Electronic Devices 9th edition by Thomas Floyd
TYPES OF LOSSES
Free Space Loss / Path
Attenuation (FSL / PA)
•
–
Computed from a formula. This
amount of loss, expressed in
dB, is how much the signal
density reduces as it travels in
free space.
Equipment Loss
•
–
losses due to the transmission
medium used in connecting radio
equipment to antenna.
Free-Space Loss (FSL)
• where D is measured in kilometers;
FSLdB = 32.45 + 20 log Dkm +20log FMHz
• where D is measured in statute miles;
FSLdB = 36.58 + 20 log D sm +20 log FMHz
• where D is measured in nautical miles;
FSLdB = 37.80 + 20 log D nm +20 log FMHz
Note: If F is stated in gigahertz, add 60 to the value of the constant term.
Waveguide Loss
Transmission Losses
COUPLING LOSS
RADOME LOSS
CONNECTOR LOSS
HYBRID LOSS
NET PATH LOSS
• Difference between the transmitter output
power and the RSL.
Frenzel, L.E. (2003). Principles of
electronic communication
systems. New York: McGrawHill/Giencoe
Photo Credit: pininterest.com
Source: Electronic Devices 9th edition by Thomas Floyd