MICROWAVE LINK - FUNDAMENTALS
Definition of Microwave
Microwaves are electromagnetic radiations in the frequency range 1 GHz to 30 GHz
(generally for Telecom).
Various books uses various frequency ranges for identifying microwaves. Radio Frequency
or Microwaves are two different terms used to break monotony. This means both terms
convey similar meaning. Frequency from 300 MHz to 300 GHz are used in various ranges to
define range of RF / Microwaves.
It is to be noted that higher the frequency, higher the bandwidth. Thus using high
frequency gives us facility of transferring more data. However, everything comes with a
price. High frequency means high processing capabilities are required and thus higher the
cost. But use of frequency spectrum is very high and thus latter (i.e. high cost for high
capabilities) is generally adapted now a days.
MICROWAVE APPLICATIONS FOR TELECOM INDUSTRY
1. BTS connectivity
2. STM 1 (63 E1) ring closure
3. BTS on spur
4. Point of Interconnect (POI) connectivity.
(If you are not familiar with above telecommunication terms, refer tutorial on "Introduction
to basic fundamentals in telecom industry")
FREQUENCY - MW LINKS
Frequency used in MW Links
Microwave links of short distances are generally allocated with higher frequencies, because high
frequency means high losses in air and thus it is good to have short distances in these
cases. While for distances like 20-35 Kms or so we use lower frequencies. Please note that the
terms high and low used for frequencies are relative and the values for these terms can be
15/18 GHz or 6/7 GHz say.
Microwave Links can be of two types
Frequency allocated to MW link does not depend on the type of MW link. If the type of MW
link is to be explained in easiest possible manner, it may be as follows.
SDH link can carry optical signals i.e. each BTS falling in this MW link will have to have transport
equipment to convert optical signal into electrical signal. This is good if we wish to have MW
links of large no of hops and wish to use it for ring closure. In this case only what is required will
be dropped without disturbing the whole link. SDH link can carry maximum of STM 1 i.e. 64 E1s
as a whole for one MW ring.
PDH link can carry electrical signals i.e. all 16E1s (capacity of PDH link) will have to be dropped in
site falling in this link. Remaining E1s can then be retransmitted for next hop. (Hop means single
For 15 GHz link, Tx and Rx bandwidth is 28
MHz. Tx and Rx separation is 420 MHz. This
separation is defined by ITU and is there to
For 6 GHz link Tx and Rx separation is 152 MHz.
For 7 GHz link Tx and Rx separation is 154 MHz.
PRACTICAL VIEW - MW LINKS
If we wish to look at practical implementation of MW links in telecom industry, we can start from
Fig MW.4.1 General MW Link Setup in Field
In Door Unit (IDU) which resides in Shelter, acts as Modem i.e. Modulator and Demodulator. It
takes electrical / optical signal and convert it into analog (electromagnetic) which is sent to ODU
(Out Door Unit).
IF cable is a co-axial cable which carries Intermediate Frequency. Details of IF cable can be seen
in Fig MW.4.2. You can feel free to ignore this figure and continue. Generally, maximum
permissible length of IF cable from IDU to ODU is 300m and frequency do not exceed 2 GHz.
Fig MW.4.2 IF Cable
ODU is present just near MW antenna at height in tower. ODU performs upconversion (acts
as Mixer) to convert signal into required frequency allocated. For doing this ODU also have
high power amplifiers and filters. Since ODU output is high frequency cable connecting
ODU to antenna is "RF Low Loss Cable". Generally, for 6/7 GHz link this low loss cable is
used and for 15/18 GHz link waveguide is used to connect ODU to antenna.
Polarization defines the way of movement of MW waves in air. It can be either Linear or
Type of Polarization
1. Linear - can be sub-divided into Vertical and Horizontal
An electromagnetic wave is said to be following Vertical Polarization if its electrical component is
perpendicular to the horizon of earth as shown in Fig MW.5.1
Fig MW.5.1 Vertical Polarization
An electromagnetic wave is said to be following Horizontal Polarization if its electrical
component is parallel to the horizon of earth as shown in Fig MW.5.2
Fig MW.5.2 Horizontal Polarization
An electromagnetic wave is said to be following Circular Polarization if it radiates electric and
magnetic field in all directions i.e. they keep on rotating. Phase is the deciding factor
here. Don't worry about this... We generally do not use this in MW links.
WHICH POLARIZATION IS BETTER FOR MW LINKS?
There is no straight forward answer for this question. Definitely one can point out Vertical
Polarization as the best in first view because it is more prone to rain fading. Rain droplets are
generally flattened with increase in size (See Fig MW.5.3) and thus Vertical polarization is more
prone and less affected. However, horizontal polarization is very much used to avoid
interference, in case nearby areas are using Vertical Polarization. (See Fig MW.5.4)
So, vertical polarization is generally used for high frequency links, because high frequencies are
more prone to rain fading and horizontal polarization is generally used to avoid
interference. However, this cannot be treated as rule. Each operator is free to decide.
Fig MW.5.3 Rain Droplets Fig MW.5.4 Use of V and H Polarization to avoid interference
FACTORS AFFECTING MW LINK
Following major phenomenon affect MW Link
Factors affecting MW link - REFLECTION
Reflection is one of the major factors that affect MW link. Fig MW.7.1 explains this
Water is good reflector. Reflected Wave can have different phase and amplitude as compared
to LOS wave. Thus, this causes Fading of signal at receiver and this fading is called Multi Path
To overcome this problem, we either adjust antenna heights at two ends to avoid major source
of reflection or to reduce its intensity. Another solution is to use Space Diversity, about which
we will study later in this tutorial.
Trees are good absorbers. So, if trees are present in between MW link, chances of reflection
Fig MW.7.1 Reflection in MW Link
Factors affecting MW link - REFRACTION
DO YOU KNOW THIS ?
Theory says that MW / electromagnetic waves travel in a straight line and yes, they do so in
vacuum. But when it comes to atmosphere, it may come as surprise to most of us that MW
waves do not travel in a straight line. Phenomenon responsible for this is REFRACTION. Density
in atmosphere is not uniform. It varies from one place to another. As we all know that light ray
bends towards or away from normal as it moves from higher density medium to lower or vice
versa, we can easily understand why MW waves deviate from straight line path in atmosphere.
In homogeneous atmosphere vertical change in dielectric constant is gradual and hence bending
or refraction is continuous. Ray is bent from thinner density air towards thicker making it follow
earth curvature. This can be related with radii of spheres. First radius is of earth (6370 Km
approx) and second is formed by curvature of beam of ray with its center coinciding center of
We can define K Factor using above information
K-Factor = R / R`
R = Radius of ray beam curvature
R` = Radius of earth
K=4/3 for earth's atmosphere.
Fig MW.8.1 shows value of K according to path traveled by MW wave.
Fig MW.8.1 K-Factor in MW Link
Factors affecting MW link - Diffraction, Scattering & Absorption
Diffraction of wave occurs when bending takes place at sharp irregular edges. This diffracted
wave can interfere very much with desired signal.
Scattering of ray of light occurs when object it strikes is of smaller size that its own wavelength.
Above 10 GHz, absorption in atmosphere becomes dominant. Rain droplets become
comparable to wavelength.
This absorption can be 2 dB/Km or can be as high as 3 dB/Km in case of rain.
DIVERSITY IN MW LINKS
Diversity in MW Links is a sort of redundancy in network. They also help overcome various
factors which affect MW links.
Two types of Diversity in MW links
1. Frequency Diversity
2. Space Diversity
Fig MW.10.1 and MW.10.2 shows these diversities respectively.
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
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 and
it is IDU to decide which signal to receive. Generally IDU receives best possible signal. This
diversity also helps a lot in areas of high wind because if one antenna gets misaligned network
can function without fail from another. Thus this provides a sort of redundancy to our network.
FREE SPACE LOSS
Free Space Loss is defined as minimum loss an electromagnetic wave experiences if it travels in
atmosphere. It depends from place to place. Its value for Kerela and Rajasthan will be different
due to various factors one of which can be humidity. However, we may roughly define free
space loss for MW link as
Lfs = 92.45 + 20 log (dist * freq)
dist = MW hop length in Kms.
freq = Frequency of MW link in GHz.
For MW link of 15 GHz and hop length 10 Kms free space loss can roughly be calculated as
= 92.45 + 20 log ( 10 * 15)
= 135.97 dB
Antenna Gain is the gain antenna provides to the signal before transmitting it into air. For
parabolic antennas used for MW link, this gain is roughly
Antenna Gain = 17.8 + 20 log (f * dia)
f = Frequency in GHz
dia = Diameter of MW antenna.
For 18 GHz MW link and 0.3 m size MW antenna, Antenna Gain will be approx
= 17.8 + 20 log (18*0.3)
= 32.44 dBi
(Don't worry about unit dBi, refer tutorial "Introduction to dB" elsewhere on this website. To
learn more about antennas refer tutorial on it.)
To understand Fresnel zone we need to first refer Fig MW.12.1
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. This
deviation, as already studied, is due to refraction. Fresnel zone is nothing but distance below
and above a point which should be clear for LOS communication.
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
Lambda = Wavelength
Fig MW.12.1 MW Communication
Now we will see link budget of MW link i.e. we will analyze gains and losses and calculate
received power at other end.
Refer Fig MW.13.1 before moving further.
Fig MW.13.1 Link Budget for MW Link
From Fig MW.13.1 it can be seen clearly that received power at Point B can be calculated as
RxA = TxA + GA - Lfs - Arain + GB
TxA = Transmit Power
GA = Gain of Antenna A
Lfs = Free Space Loss
Arain = Attenuation due to rain
GB = Gain of Antenna B
Suppose we have 6.2 GHz MW link. Diameter of antenna at both sides is 1.8 m. Distance is 20
Kms. Calculate approx received power at point B, if transmitted power at point A is 25 dBm.
First we will calculate Gain of two antennas. Since diameter is same, both antennas will roughly
have gain of
= 17.8 + 20 log (freq * dia)
= 17.8 + 20 log (6.2 * 1.8)
= 38.753 dBi
Then, we will calculate rough free space loss as
= 98.45 + 20 log (dist * freq)
= 98.45 + 20 log (20 * 6.2)
= 140.318 dBm
Finally we will calculate received power at Point B from above given formula. We are assuming
rain attenuation as zero.
RxB = 25 + 38.753 - 140.318 - 0 + 38.753
= - 37.812 dBm Answer
Receiver sensitivity is generally around -65 dBm and hence the receive power we are getting is
good and also take care of rain attenuation margin during rainy season. It is good practice to
leave around 30 dB as rain margin.