Transmission fundamentals

Telecom Tutorials
Monday, June 03, 2013www.tempustelcosys.com
TRANSMISSION FUNDAMENTALS……….

TRANSMISSION :
A PROCESS WHERE TRAFFIC (VOICE,DATA,VIDEO) IS DESPATCHED OVER
A MEDIUM BETWEEN THE SOURCE AND THE DESTINATION
TYPES OF TRANSMISSION MEDIA :
WIRED TRANSMISSION MEDIA
1.COPPER CABLE
2.OPTICAL FIBER
WIRELESS TRANSMISSION MEDIA
1.VSAT NETWORKS
2.MICROWAVE RADIO

COPPER CABLE : -
OLDEST KNOWN TRANSMISSION MEDIA
ADVANTAGES
1. CHEAP
2. EASILY AVAILABLE
DISADVANTAGES
1. PRONE TO LOSSES

VSAT NETWORKS : -
VSAT NETWORKS ARE A POPULAR TRANSMISSIOM MEDIA WHERE
FIBER OR MW CONNECTIVITY IS NOT POSSIBLE
NETWORK ELEMENTS IN A VSAT NETWORK
1.UPLINK ANTENNA (TRANSPONDER)
2.GEOSTATIONARY SATELLITE
3.DOWNLINK ANTENNA (RECIEVER)

FIBER OPTICS COMMUNICATIONS
Fiber-optic lines are strands of optically pure glass as thin as a human hair that
carry digital information over long distances
A FIBER Cable is essentially made up of
CORE
CLADDING
BUFFER COATING

Core - Thin glass center of the fiber where the light travels
Cladding - Outer optical material surrounding the core that
reflects the light back into the core
Buffer coating - Plastic coating that protects the fiber from damage and
moisture
Thousands of optical fibers are arranged in bundles to form an optical cable
.Optical cables are covered with protective jackets
TYPES OF OPTICAL FIBERS
SINGLE MODE FIBER :
Single-mode fibers have small cores (about 3.5 x 10-4 inches or 9 microns
in diameter) and transmit infrared laser light (wavelength = 1,300 to
1,550 nanometers).
MULTI MODE FIBER :
Multi-mode fibers have larger cores (about 2.5 x 10-3 inches or 62.5
microns in diameter) and transmit infrared light (wavelength = 850 to
1,300 nm)

When light passes from a medium with one index of refraction (m1)
to another medium with a lower index of refraction (m2), it bends or
refracts away from an imaginary line perpendicular to the surface
(normal line).
As the angle of the beam through m1 becomes greater with respect
to the normal line, the refracted light through m2 bends further
away from the line
At one particular angle (critical angle), the refracted light will not go
into m2, but instead will travel along the surface between the two
media (sine [critical angle] = n2/n1 where n1 and n2 are the indices
of refraction [n1 is greater than n2]).
If the beam through m1 is greater than the critical angle, then the
refracted beam will be reflected entirely back into m1 (total internal
reflection), even though m2 may be transparent
CRITICAL ANGLE = COS -1 ( N2 / N1)

TRANSMISSION OF LIGHT SIGNAL IN A FIBER OPTIC
The light in a fiber-optic cable travels through the core (hallway) by
constantly bouncing from the cladding (mirror-lined walls), a principle
called total internal reflection

Because the cladding does not absorb any light from the
core, the light wave can travel great distances
Some of the light signal degrades within the fiber, mostly
due to impurities in the glass
The extent that the signal degrades depends on the purity of
the glass and the wavelength of the transmitted light (For eg :
850 nm = 60 to 75 percent/km;
1,300 nm = 50 to 60 percent/km; 1,550 nm is greater than 50
percent/km).
Some premium optical fibers show much less signal
degradation -- less than 10 percent/km at 1,550 nm.
Fiber-Optic Relay System
TRANSMITTER
OPTICAL
RECEIVER
OPTICAL
REGENERATOR
OPTICAL FIBER OPTICAL FIBER

Transmitter - Produces and
encodes the light signals
Optical fiber - Conducts the
light signals over a distance
Optical regenerator - May be
necessary to boost the light
signal (for long distances)
Optical receiver - Receives
and decodes the light signals

ADVANTAGES :
1. RELIABLITY
2. HIGH DATA CARRYING CAPACITY
3. LOW SIGNAL LOSSES
4. NO INTERFERENCE DUE TO USE OF LIGHT
SIGNALS
5. FLEXIBLE AND LIGHTWEIGHT
DISADVANTAGES :
1. COSTLIER THAN MICROWAVE,COPPER CABLE
2. MORE REPAIR TIME
3. TRENCHING AND DUCTING INVOLVED HENCE
MORE DEPLOYMENT TIME

MIRCOWAVE TRANSMISSION : -
MICROWAVE MEDIA CAN BE USED FOR POINT–TO-POINT
AND POINT-MULTIPOINT TRANSMISSION
WHY MICROWAVE :-
1. Supports hop length from less than 50 meters to 60 k ms
2. Easy and fast deployment compared to any other media
3. Flexibility ,upgradeability ,capacity increase ,redeployment
4. High reliability and low maintenance cost
5. High MTBF and Low MTTR
6. Can reach farther remote inaccessible areas over water , forests and
mountains

 MICROWAVE PROPOGATION PRINCIPLES
 Microwave transmission occurs in the atmosphere
surrounding earth called troposphere which extends to an
average of 10 km from earth’s surface.
 Microwave is essentially a LINE OF SIGHT communication.
 Microwave travels at speed of light (3 x 10 power 8 m/s)
 Microwave transmission can occur between 2 Ghz to 30 Ghz
 Microwave frequency bands are 2,4,6,7,8,13,15,18,23 Ghz
 Microwave signal propogates through free space and suffers losses while
 travelling called FREE SPACE LOSS
 FSL =92.4 + 20 log10 (D x f)
 Where D=distance (kms) and f=frequency (Ghz)
 Total Loss suffered by a MW sig is given by
 Total Loss = FSL + Atmospheric Absorption Loss + Field Margin
 Net Path Loss suffered by a MW sig is given by
 =Total Loss – Gain of both antenna

 Components of a MW link
 A MW link consists of
 a. Radios (IDU) – 2nos
 b. ODU – 2nos
 c. Antennas – 2nos
 d. Inter-facility cables between IDU and ODU
 Function of components
a. Radio (IDU) : Coding and decoding digital data and converting digital data to IF
frequency
b. Inter-facility cable : Carries the IF frequency signal to
 ODU
 c. ODU : Converts IF signal to RF signal for propogation through medium
 d. Antenna : Transmitting and receiving RF signals

 MW RADIO :
 Which MW radios to be used
 a. Should meet ITU standards
 b. High Transmit Power
 c. High System Gain
 d. ATPC , XPIC
 e. High tolerance for co-channel and adjacent channel interference
 f. High dispersive fade margin to combat signal distortion
 g. Variable modulation schemes
 h. Rate independent
i. Should sustain severe climate
 f. Should have max traffic carrying capacity

 Types of MW Radios
 a. Plesiochronous Digital Heirarchy (PDH)
b. Synchronous Digital Heirarchy (SDH)
 Radio Configurations :
a. Space Diversity (SD)
b. Frequency Diversity (FD)
c. 1 + 0
d. 1 + 1
e. XPIC
f. MHSB
 INTER-FACILITY CABLE :
 Acts as a medium for transfer of signal to ODU
 Is a hollow waveguide covered by protecting sheath.
 Losses incurred are generally 0.5 dB for 100 mtrs cable

 Outdoor Unit (ODU) :
 Types of ODU
a. Antenna mount
b. Pole Mount
 MW Antenna :
 Can be omni-directional as well as directional
 Consists of following parts
 a. Reflector – Reflects MW energy towards the MW beam
 b. Antenna Mount – Mount for installing on pole
 c. Feed – Matches ODU and free space impedence and facilitates
 polarisation adjustment from H to V and vice versa
 d. Shield – Attached to reflector to improve radiation pattern of the antenna
 e. Radome –Protective cover from ice, rain and wind

 TYPES OF MW ANTENNA
 SPACE DIVERSITY
 HIGH GAIN HIGHLY DIRECTIONAL
 MW ANTENNA PARAMETERS
1. Antenna Gain : Gain is the figure of merit of its directivity and indicates how well it focuses MW
energy
 Antenna Gain = 17.8 + 20 log10 (d x f )
 where d= antenna diameter( mtrs) and f =frequency (Ghz)
 Also gain can be given as
 Antenna Gain = ( 4 x ∏ /λ ^ 2) x Aeff
 where Aeff = 0.65 x (∏ x D ^ 2)/4
 2. Beam Width : Width of the beam having 50 % of focused MW energy
 Beam Width = 70 x λ / D
 where λ = wavelength and D = Antenna Diameter(mtrs)

 Requirements for a MW link
 LOS – If a MW link has to be installed successfully there should be a proper line of
sight. In order to predict whether a MW link can be installed between two points a
LOS survey is required.
 LOS can be deduced by Toposheet study and LOS survey
 Toposheet study involves plotting the points on SOI maps and noting the AMSL
contours in the LOS path . The contour readings can be used to calculate the MW
antenna height required at both points.
 How to conduct a LOS Survey
 A surveyor needs to have the following equipments to successfully
 carry out a LOS survey
a. Two Altimeters
b. Compass
c. GPS ( min 12 channel)
d. Binoculars Monday, June 03, 2013www.tempustelcosys.com

Procedure :
1 .Calibrate the two altimeters by taking the AMSL at a railway station or at a previously
calibrated point.
2. Proceed to the far end by taking AMSL readings at regular intervals and also at places
where the AMSL changes drastically
3. Take the height of man made and natural obstructions ( bldgs, trees ,mountains) in
the LOS path
4. Calculate the MW antenna height required by using the formula
Antenna Height = Max obstruction height – First Obstruction height and Last Obstruction
height
Obstruction height is sum of actual obstruction height , Earth Bulge ,Fresnel Zone
Clearance
Points to remember while planning a MW link
The link should clear the first and second Fresnel Zones (min 60 % for first Fresnel Zone
and 30 % for second Fresnel zone)

D1 D2
Optical horizon

Optical Horizon is the straight line distance from the reciever and transmitter
antenna
Optical Horizon = 3.57 x √ (Ht + Hr)
Radio Horizon is due to the bending of the MW ray towards the earth.
Radio Horizon is 15 % bigger that the optical horizon
Radio Horizon = 4.12 x √ (Ht + Hr)

Fresenel Zones and LOS
As said earlier for a MW link to work successfully the first and second Fresnel
zones need to be cleared by minimum 60 % and 30 % resp.
Fresnel zones are ellipsoids around the MW link caused because of the differences in the
refractive indices of different medium
For calculating the Fresnel zone radius at any point P in the middle of the link
is the following:
Fn = √n λ d1 d2 / d1 + d2
where,
Fn = The nth Fresnel Zone radius in metres
d1 = The distance of P from one end in metres
d2 = The distance of P from the other end in metres
λ = The wavelength of the transmitted signal in metres

 If we take n =1 then the First Fresnel Zone can be given as
 F1 = 17.32 x √d1 d2 / f x d
 where
 d1 = The distance of P from one end in metres
 d2 = The distance of P from the other end in metres
 f = Frequency in Ghz
 d = Total path length ( d1 + d2)
 Earth Bulge and K Factor
 Earth’s curvature and microwave beam refraction combined to form fictitious earth curvature
or Earth Bulge
 Earth Bulge is given as
 EARTH BULGE at a dist d1 km
 = d1 * d2 / (12.75 * K) mtrs
 Where d2 = (d – d1) Km
 K = K Factor
 K Factor is given by r / ro
 Where r = true earth radius
 r0 = effective earth radius

 Losses due to obstacles
 MW signal is degenerated due to losses during propogation due
to two types of obstacles
 1.Knife Edge Obstacle : -
 Knife Edge Loss is given by
 A obst = 6.4 – 20 log ( V √ 2 + √ 1 + 2 V power 2 )
 V = hlos / r

Smooth surface losses are generally higher than knife edge losses
and can go high as 40 db.

1.Reflection :
There might be a free space loss calculated while designing a link and the actual
free space loss encountered by the link. This is because of multi-path propogations.
Multipath is caused because of smooth ground, water bodies,man made structures
etc
The signal received at the Rx antenna is a combination of the direct signal and the
multipath reflections.These reflected waves might cause losses if the reflected
signal is out of phase from the desired signal.These losses are called as down fade.
How to reduce multipath:
a.The Tx and Rx antennas should be adjusted in such a way that they are not at the
same height so that the angle of incidence is not same as the angle of reflection/
b.Use space diversity keeping a separation of atleast 200 λ between two antennas

2. Refraction :
The MW rays experience refractions due to the change in the refractive
indices of the propogating medium .These are due to various
atmospheric anomallies.
Temperature Inversion : Typicallly warm air in found near the earths surface
and the as the altitude increases the air becomes cooler. Sometimes the heat is
radiated from the ground and the air at the earths surface becomes cooler
whereas the upper layer of the atmosphere is cooler. This is known as a
atmospheric duct and the condition is called as temperature inversion.
When the MW signal passes through such a duct the refraction occurs in such a
way that the ray bends more than the normal and the radio horizon increases and
the ray travels beyond the LOS. This is called as the Super Refraction.
When the atmospheric density increases with height instead of decreasing it
causes a fog with warm air over cool air. When the MW link encounters this
atmospheric effect it causes the MW ray to bend less than the normal and the ray
falls short than the LOS. This leads to less Fresnel Zone clearance and
obstructions. This is called as the Sub Refraction

3. Diffraction :
Diffraction is seen due to the knife edge and smooth edge obstructions.
Typically good clearance of Fresnel zones nullifies the diffraction effect
Fading and their types:
Fading is generally of two types
a. Flat Fade : Flat fades are seen because of rain attenuation, ducting and beam bending.
Rain Fading : MW signal faces attenuation due to fading if the MW frequencies used are above
10 Ghz. Below 10 Ghz rain has no effect on a MW link.Rain drops act as poor di-electric absorbing
MW energy.
While designing a link the PL (50 or 90 ) factor and the rain file should be used as per the rainfall
rate in that particular region.
Also in regions where there is heavy rainfall links should be designed with vertical polarisation as
rain attenuation is considerably reduced as opposed to a horizontally polarised link.This is due to
the fact that as rain drops approach the earths surface because of the gravitational pull the drops
acquire a shape which is wider at its axis.
b. Frequency selective Fade : Frequency selective fade avries with frequency.It is seen in cases
where the reflected signal is received out of phase from the desired signal.

Parameter PDH SDH
Frequency Bands (GHz) 2,4,6,7,8,13,15,18,23 6,7,8,15,18,23
Traffic Capacity Max 32 E1 Max 63 E1
Modulation QPSK 128 QAM
Band Width Occupied 28 MHz 28 Mhz
System Gain 110 to 100 dB 90 dB
Good comb for 10 kms 1.2 m 1.8 m
MW cost with 1.2 m
antenna
4 lacs 6 lacs

Other parameters required to design a MW link
1. Received Signal (RSL) : The link is generally designed to get a receive signal of
around 30-36 dB
RSL = Tx power – Net Path Loss
= Pt – Lctx + Gatx –Lcrx + Garx – FSL
where Pt – Transmitted power in dBm
Lctx – Cable loss between Tx and its antenna
Gatx – Gain of transmitting antenna
Lcrx – Cable loss between Rx and its antenna
Garx – Gain of receiving antenna
FSL – Free Space Loss
Receiver threshold value is directly proportional to data rate or capacity
SDH ( 63 E1 ) threshold - -68 dBm
PDH ( 16 E1 ) threshold - -83 dBm
Receiver threshold is inversely proportional to BER( Bit error rate)
BER 10 -6 - Rx threshold -68 dBm
Ber 10 -3 - Rx threshold -69 dBm Monday, June 03, 2013www.tempustelcosys.com

Max Rx signal is equiment related generally - 20 dBm .If the received signal is more
that the equipment max Rx signal the equipment goes into saturation ( los of link).
2. Effective Isotropic Radiated Power ( EIRP ) :
EIRP = Tx Power (dBm) + Gain of single antenna (dBm)
3. Fade Margin :
1. Thermal Fade Margin :
Thermal Fade Margin = Rx s/g level – Rx threshold level
2. Dispersive Fade Margin : Due to inband distortions
3. Adjacent channel interference fade margin : Due to energy spill over in the adjacent
channel receivers
4. External interference fade margin : Due to inter system co-channel interference
5. Composite of Effective Fade Margin :
EFM = -10 log10 ( 10 – TFM /10 + 10 – DFM /10 + 10 – EIFM /10 )
4 . Path Inclination is given as
PI = Elevantion Diff / Path Length

5. Fade Occurrence Factor Po is given as
Po = C x (f/4) x d 3 x 10 – 5 dBm
Where C – speed of light ( 3 x 10 8 m/s)
f – Frequency in Ghz
d – Path distance
6. Fade Probability is given as
P = K x d 3.6 x f 0.89 x ( 1 + £p) -1.4 x 10 –F /10
Where K – Geo-climatic factor for worst month fading
d – path length in kms
f - Frequency in Ghz
£p – Path inclination in milli radians
F – Effect Fade Margin

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Transmission fundamentals