Telecom Tutorials on Microwave Communication

Telecom Tutorials
Monday, June 03, 2013www.tempustelcosys.com
Training on Microwave Communication

Possible media for communication
Introduction to Communication Media
Introduction to Microwave communication
Manufacturers of Microwave
Why Microwave?
Characteristics of microwave
Types of Microwave communication
Types of Microwave Links
Requirements for the microwave communication
What is LOS?
Wave Propagation in the atmosphere
Multi path Propagation
LOS Purpose & requirements
Limitations of Line of Sight Systems
Design of Line of Sight Microwave Links
K- factor
Variations of the ray curvature as a function of k Monday, June 03, 2013www.tempustelcosys.com

Obstacles & Loses
Knife Edge Obstacles
Smooth Spherical Earth Obstacles
Path Loss
Other losses
Why vertical polarization favorable at high freq
Antenna type & Gain
RECEIVER SENSITIVITY, FADE MARGIN AND SIGNAL TO NOISE RATIO
Fading Margin
Reliability

Copper media
Microwave media
Optical fiber Media
Satellite Media

Microwave communication system is used to transfer data from one node to the other node using
the frequency ranging from 2GHz to 60GHz.
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.

Few well known Radio Manufacturers
Nokia
Nera
NEC
Siemens
Digital Microwave Corporation
Fujitsu
Ericsson
Alcatel
Hariss

Fast Deployment
Flexibility
Low implementation Cost
Link across Mountains and Rivers are economical & feasible
Quick maintenance of the system
GHz band has very low noise
LOW MTTR
Drawback of Microwave
Needs frequency license
Environment dependant link quality (e.g. rainfall)
LOS not always available

 Microwave are used for point to point and point to multipoint communication
 Microwaves are the electromagnetic waves comprises of electrical and magnetic field at
angle of 90 degree to each other.
 Normally communication on microwave is done between 3GHz to 30 GHz frequency

Point to point communication
Point to multi point
communication

 Long Haul Radios: ~ 30 - 80 km
2 GHz, 7 GHz
 Medium Haul Radios: ~ 25 - 45 km
10 GHz, 13 GHz, 15 GHz
 Short Haul Radios: ~ 5 - 30 km
18 GHz, 23 GHz, 26 GHz, 38 GHz,
 Nokia Metro hopper: < 1 km
57 GHz
(uses oxygen absorption in air to limit range)

 Microwave communication requires a clear line of sight between two nodes
 A Fresnel ellipsoids and their clearance criteria concept is used to calculate the radio Line
of sight
 Antenna height Calculation for clear LOS
 Parameters design like Power ,Frequency , Rx level and many more

 Radio signals, like all electromagnetic radiation, usually travel in straight lines. However, at low
frequencies (below approximately 2 MHz or so) diffraction effects cause significant ray bending,
allowing ray bundles to partially follow the Earth's curvature, thus enabling AM radio signals in low-
noise environments to be recieved well after the transmitting antenna has dropped below the
horizon. Additionally, frequencies between approximately 1 and 30 MHz, can be reflected by the
ionosphere, thus giving radio transmissions in this range a potentially global reach (see shortwave
radio).
 However, at higher frequencies, neither of these effects apply, and so any obstruction between the
transmitting and receiving antenna will block the signal, just like the light that the eye senses.
Therefore, as the ability to visually sight a transmitting antenna (with regards to the limitations of the
eye's resolution) roughly corresponds with the ability to receive a signal from it, the propagation
characteristic of high-frequency radio is called "line-of-sight" as per radiowave propagation is called
as "radio horizon".

 When rays reaches to the receiver from different paths then two possibilities are there
A) If they reaches in the same phase then the signal strength increases.
B) If they reaches in opposite phase then its cause fading called multipath fading

1. Purpose
For the establishment of short / long haul LOS links
Feasibility studies
Submission of tenders
Up gradation of existing links
2. Requirements of LOS links
• site locations
• planned antenna height
• direction to the other end of link
• restrictions to cherry-picker, etc.
Output
• LOS/NLOS
• minimum antenna height
• exact antenna location (rooftop)
• panorama picture with
landmarks and their directions
• extra observations
(forests,building sites etc.)

How far we can go: The range of LOS microwave systems is limited by:-
Curvature of earth-Actual
Technical radio characteristics (K-factor)-Modified Earth Curvature
Actual Obstructions en-route in each hop
RF effect of fresnel zone
Path loss
Transmitter power
Antenna gains
Transmission line looses
Frequency of operation
Received power
Receiver threshold
Signal to noise ratio
Fade margin required
Desired reliability of link

Link Design: The design of microwave links, involves three sets of calculations.
1. Working out antenna heights for the link.
K-factor is major dominant variable.
Earth bulge.
Fresnel zone radius.
Actual obstructions on the route
Path Loss
Operating frequency.
Path profile: it indicates the distance from one of the transmitter site where obstructions to the line
of sight radio link may occur.
The object of this calculation is to arrange tower heights along the entire route of the link, so that
an obstruction in the path does not enter into the fresnel zone by a specified amount for a
specified K-factor used.

Earth bulge and K-factor:
The propagation of radio beam is affected by atmospheric conditions and the obstructions on the way. It
can be subjected to:
Diffraction
Reflection
Refraction
Most important is refraction, which is caused by changes in the density of atmospheric layers
confronted by the radio beam front.
The curvature of earth and slight bending of waves as it is refracted downwards by the earth’s atmosphere
are two factors, that, must be considered while making path profiles.
The earth’s curvature and microwave beam refraction are combined to form fictitious earth curvature or
earth bulge.
EARTH CURVATURE (M) = 0.078 x d1 x d2 / K
WHERE K = EFFECTIVE EARTH RADIUS/TRUE EARTH RADIUS
EARTH BULGE = d1 x d2 / 12.75 x K
EARTH BULGE FOR K=4/3 = d1 x d2 / 17
EARTH BULGE FOR K=2/3 = d1 x d2 / 8.5

Different K values
True Earth’s curvature
= 6,371 Km
K=1
K=0.5
K=0.33

Fresnel zone:
The radio beam energy travels in an ellipsoidal wave front, the different components of which
maintains different path lengths.
The distance from microwave beam’s center is commonly measured in fresnel zones to take into
account both frequency and distance.
The first fresnel zone (FFZ) is the surface of the point along which the distance to the ends of the
path is exactly ½ wave length larger than the direct end to end path.
FFZ radius in meters=17.32√d1*d2/fD
Where d1 & d2 are in km’s, f is the frequency in GHz and D is the hop distance in Km’s.
In order to achieve a free space propagation condition for a radio beam at least 60 % of FFZ
should be cleared under the standard atmospheric condition of K=4/3.

Keep 1st Fresnel zone clear of obstacles
nth Fresnel zone: Ellipse around direct path, where path difference to direct line is n* /2.
d
b
1st Fresnel zone
2nd
3rd
Radius for n-th zone = b * sqrt(n)
b
d km
f MHz
m274
[ ]
[ ]
[ ]

1. If f=2.5 GHz and D=30 Km, then FFZ=32.99 M
Conclusion :FFZ radius decreases with increase in frequency.
Conclusion: FFZ radius increases with increase in distance

With 100% fresenal zone
NATURAL EARTH FEATURES
EARTH BULGE
“A” “B”
T
BUILDING
d1 d2
D
f

Types of Loss
Obstacle loss
Knife edge
obstacle loss
Smooth spherical
earth obstacle loss

Knife Edge Obstacles

Free Space Loss
Free space loss: consider a signal is traveling between transmitter at “A” to a receiver at “B”.
There is for a given frequency and distance, a characteristic loss. This loss increases with
both distance and frequency. It is known as free space loss.
Free space loss LdB=92.44+20 log10 F+20 log10 D
Where F is in GHz and D is in km's.
If D is 40 Km and F is 6 GHz, then free space in dB
LdB=92.44+20 log 40+20 log 6
=92.44+20*1.6021+20*0.7782
=92.44+32.042+15.564=140.046 dB

Example:- Free space loss if F=2.5 GHz and D=30 Km
FSL (dB) = 92.44 + 20 log 2.5 + 20 log 30
=92.44 + 20*0.398 + 20*1.478
=92.44 + 7.96 + 29.56 = 129.96 dB
Now, if F=7.5 GHz (changed) and D=30 Km (unchanged)
FSL (dB) = 92.44 + 20 log 7.5 + 20 log 30
=92.44 + 20*0.875 + 20*1.478
=92.44 + 17.5 + 29.56 = 139.5 dB
Now, if F=2.5 GHz (unchanged) and D=40 Km (changed)
FSL (dB) = 92.44 + 20 log 2.5 + 20 log 40
=92.44 + 20*0.398 + 20*1.602
=92.44 + 7.96 + 32.04 = 132.44 dB
It can be seen, that, free space loss increases both with distance and frequency

Precipitation
Transmission of microwave signal above 10 GHz is vulnerable to precipitation The energy
is attenuated due to radiation (scattering) and absorption (heating) Scattering
Radio waves are a time varying electromagnetic field, the incident field will
induce a dipole moment in the raindrop. The rain drop will also have the same time Variation
as the radio waves and will act as an antenna and reradiate the energy. As rain drop-antenna
have low directivity it will radiate energy arbitrary direction and add to loss.
Absorption
When the wavelength becomes small (High freq. < 18GHz) relative to the raindrop size more
energy is absorbed by heating of the raindrop.

As the rain-drop increases in size they depart spherical shape and extended in the
the horizontal direction.For freq. Higher than 18 GHz the wavelength is generally
in mm. So these rain-drops attenuate horizontally polarized waves than the vertical
Polarized.
Raindrop shapes
1mm 1.5mm 2mm 2.5mm

Transmit power : Transmit power is the power in dB that is required for the signal to travel from
one node to other.
The max and minimum transmission power for the equipment is vendor specific and changes
with the capacity of the E1 carried by the radio.

Digital Modem : To interface with customer equipment and to convert customer traffic to a
modulated signal
RF Unit : To Up and Down Convert signal in RF Range
Passive Parabolic Antenna : For Transmitting and Receiving RF Signal

Antenna type : There are different types of antenna used for the Microwave communication.
Mostly parabolic antennas are used.
Antenna can be put in vertical or horizontal mode. Also cross polarized antennas are available
that can carry both horizontal and vertical beams with a very low interference.
Gain of Antenna: Gain of the antenna is calculated by the approximate formula
Gain = 17.8 + 20 log (D.f) dBi
Where
D = Antenna diameter [m]
F = Frequency in GHz

Receiver Sensitivity: Sensitivity or Threshold Power of receiver is the level of signal which would
produce a 30 dB signal to noise ratio out of the base band of an analogue receiver, or a bit error
ratio (BER)=10-4 out of the base band of a digital receiver. Typically it is -70 to -90 dB.
Fading: Received Signal vary with time due to multipath fading and rain etc. Refractive index of
atmosphere varies with Temp. humidity and pressure which in turn cause the electromagnetic
waves to change direction. Another cause for Multipath fading is ground reflection. So a fade
margin is built in Link Designing.
Fade Margin: The fade margin is the power level, that, the unfaded received signal can fall to
until it reaches the receiver threshold. This margin will vary depending on geographic and climatic
conditions of different geographic areas and desired reliability of the system. Typically it is 20-40
dB.
Fade Margin dB=Prx-Pthresh
Signal to Noise Ratio: It’s the minimum power difference between the wanted received signal
and received noise.
Signal/Noise Ratio (dB)=10 log10 (Signal Power/Noise Power)
Typically it is > 50 dB, logically it should be more than the Fade Margin, so that it is
always below the threshold level.

 Fading margin:
“Safety” margin. Should be
large enough to guarantee
that quality and availability
objectives are met during
fading conditions.
Typical value ~ 40 dB
Received Power
Fading Margin
Receiver
threshold

Reliability of the link: Outage time for each hop and for the complete link is to be worked out,
which in turn will give the over all reliability of the link in terms of percentage.
Single hop reliability (%) Fade Margin
99.9 28 dB
99.99 38 dB
99.999 48 dB
CCIR defines its availability objective for radio relay systems over a hypothetical
reference circuit as 99.7 %. Resulting unavailability 0.3 % is of three components.
Outage due to power failure
Outage due to equipment failure
Outage due to propagation
It is reasonable to allot 50 % of the outage time to power and equipment failures and 50
% for propagation. Considering propagation alone, system should have an availability (reliability)
of 99.85 % apportioned across the 2500 Km route. This provide guide to establish a per hop
propagation reliability for a particular system.
Planner rather first set the limit for the reliability and for wide band links it is better than
99.99 %.

Telecom Tutorials on Microwave Communication

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