Fundamentals of microwave link design

1. FUNDAMENTALS
OF
µ WAVE LINK
DESIGN
SHIV DUTT
RF Lead Engineer
Intarvo Technology Pvt Ltd

2.  Microwaves are electromagnetic radiations in the
frequency range 1 GHz to 30 GHz (generally for
Telecom).

4.  Small capacity systems generally employ the
frequencies less than 3 GHz while medium and
large capacity systems utilize frequencies ranging
from 3 to 15 GHz. Frequencies > 15 GHz are
essentially used for short-haul transmission.

5.  Less affected by natural calamities
 Less prone to accidental damage
 Links across mountains and rivers are more
economically feasible
 Single point installation and maintenance
 Single point security
 They are quickly deployed

7. As the frequency increases, the length of the link
decreases. Due to the high frequency range (2–
58GHz),the microwave links can be classified into
three main categories:
 (a) Long haul
 (b) Medium haul
 (c) Short haul

8.  The frequency of operation of these links is
usually 2–10 GHz. In the best of climatic
conditions and frequency of operation, the
distance covered by the links could range from 80
km to 45 km. These links are affected by multipath
fading.

9.  The frequency of operation of these links is
usually from 11 GHz to 20 GHz. Depending upon
the climatic conditions and frequency of operation,
the hop length can vary between 40 km and 20
km. These links are also affected by multipath
fading and rain fading.

10.  These links operate in high frequency ranges
(23–58 GHz) and thereby cover shorter
distances. At lowerfrequency ranges in this
band, links are affected by both multipath and
rain fading. At higher frequencies,when the hop
length is only a few kilometres, the multipath
phenomenon does not have a significant effect.
However, the impact of rain is quite severe.

12. Above figure shows a simple microwave link.
Major components that constitute a microwave
link are:
 (a) Indoor unit
 (b) Outdoor unit
 (c) Antenna
 (d) RF low loss cables/Waveguide
 (e) Microwave tower

13.  Also called IDU, it usually contains the radio modem; i.e.
it acts as a termination point which convert the signal
into a radio signal sent across the microwave link, using
modulation schemes.
 Indoor units are usually located in a protected
environment, i.e. within cabinets inside a building or
similar structure.
 They are not exposed to the environmental conditions
like the outdoor units.

14. Outdoor Unit (ODU)

15.  It converts the modulated low frequency signal into a
high frequency radio signal. The OU contains radio
frequency transmitters and receivers. Due this feature, it
is also known as a radio transceiver.
 The received signal usually passes the low noise
amplifier (LNA), which strengthens the weak signals.
This is followed by automatic gain control (AGC), which
ensures equality of the signal strength when entering the
radio receiver.
 The OU gets electrical power and the low frequency
modulated signal from the IU through coaxial cable/
waveguides connections

16.  An antenna interacts with free space; hence its
understanding becomes critical for the microwave link
planning engineers. An antenna is defined as the structure
that transfers the electromagnetic energy from the free
space into transmission lines and vice versa.
 There are many types of antennas: horn, parabolic, flat or
planar, lens, yagi, array, etc.
 The application of the antenna depends upon its electrical
and mechanical characteristics.

17.  RF low loss cables are used for 6/7GHz link.
Cables are limited to pass the high frequency
because its lumped component L and C form LPF
circuit.
 Waveguide are used for 15/18GHz link.
It has the capability to pass the high frequency
because its depends on the dimension of
waveguide.

18.  Earth’s atmosphere plays a vital role in the
propagation of microwaves.
 the atmosphere contains several minute layers,
each having its own pressure, temperature and
water vapour content.
 Refractive index of a particular media can be
defined as

19.  As pressure, temperature and humidity all change
with height, the refractive index also changes with
height, as refractivity varies with pressure,
temperature and humidity. This also means that
the radio refractivity N also changes with height:
N ∝ η (3.12)
η= 1.000 300

21. We can define K Factor using above information
K-Factor = R / R`
where
 R = Radius of ray beam curvature
 R` = Radius of earth
 K=4/3 for earth's atmosphere.
 Fig. shows value of K according to path traveled
by MW wave.

22. Fig: K-Factor in MW
Link

23. • Vertical Polarization is less affected by rain fading. Rain
droplets are generally flattened with increase in size and
thus Vertical polarization is less affected.
• However, horizontal polarization is very much used to
avoid interference but they are more prone to rain
fading, in case nearby areas are using Vertical
Polarization.

24.  So, vertical polarization is generally used for high
frequency links, because high frequencies are
less affected by rain fading and horizontal
polarization is generally used to avoid
interference. However, this cannot be treated as
rule. Each operator is free to decide.

25.  Diversity in MW Links is a sort of redundancy in
network. They also help overcome various factors
which affect MW links.
 In regions where multipath fading conditions exist,
it is necessary to incorporate diversity into the
system design.
 Two types of Diversity in MW links
1. Frequency Diversity
2. Space Diversity

26. • Frequency Diversity calls for use of two different
frequencies for same MW link. This is normally avoided
because two frequency allocation means double the
annual fee payable for frequency. Frequency diversity is
generally meant to overcome frequency interferences
and various other factors.

27. • Space Diversity uses two MW antennas at each side
and is best suited to overcome Reflection of MW
waves. Signal is received by both antennas called
Main Antenna and Diversity Antenna. This diversity
also helps a lot in areas of high wind because if one
antenna gets misaligned, network can function
without fail from another.
Note:-
Neither space diversity nor frequency diversity
provides any improvement or protection against rain
attenuation.

28. • From the figure above we can see that apart from
direct line of sight (LOS) we need to leave some
space above and below it to allow deviation of MW
wave from its original path. Fresnel zone is nothing
but distance below and above a point which should be
clear for LOS communication.
Fig :MW Communication

30.  where
 rn = radius of fresnel zone. Generally we consider
n=1 i.e. first fresnel zone clearance.
 d1 = distance of point from Point A
 d2 = distance of point from Point B

31. ( D=d1 +d2)

34.  Now we will see link budget of MW link i.e. we will
analyze gains and losses and calculate received
power at other end.
Fig : Link Budget for MW Link

35. • From Fig. it can be seen clearly that received
power at Point B can be calculated as
• RxA = TxA + GA - Lfs - Arain + GB
where
• TxA = Transmit Power
• GA = Gain of Antenna A
• Lfs = Free Space Loss
• Arain = Attenuation due to rain
• GB = Gain of Antenna B

36.  A microwave link frequently is used to transmit
signals in instances in which it would be
impractical to run cables.
 If you need to connect two networks separated by
a public road,
 for example, you might find that regulations
restrict you from running cables above or below
the road. In such a case, a microwave link is an
ideal solution.

37. • In areas with lots of rain, use the lowest frequency
band to handle such projects.
• Microwave hops over or in the vicinity of the large
water surfaces and flat land areas can cause
severe multipath fading. Reflections may be
avoided by selecting sites that are shielded from
the reflected rays.
• Hot and humid coastal areas