2_5 mwsatellite.pdf

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BHARAT SANCHAR NIGAM LIMITED


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Radio Bands - Propagation

VLF 3-30 kHz Surface waves

LF 30-300 kHz -do-

MF 300-3000 kHz -do-

HF 3-30 MHz Sky waves

VHF 30-300 MHz Space waves

UHF 300-3000 MHz -do-

SHF 3-30 GHz -do-

EHF 30-300 GHz -do-


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M/W Bands

UHF(>1GHz) and SHF bands are used for

Microwave communications

UHF and SHF bands are propagated by Spacewaves

Parabolic Dish is used for the antenna


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Microwave Impairments

Equipment, antenna, and waveguide

failures

Fading and distortion from multipathreflections

Absorption from rain, fog, and other

atmospheric conditions Interference from other frequencies


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Microwave Engineering

Considerations

Free space & atmospheric attenuation

Reflections

Diffractions

Rain attenuation


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Microwave Engineering

Considerations-contd

Skin Effect

Line of Sight (LOS)

Fading

Range Interference


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Free Space & Atmospheric

Attenuation

Free space & atmospheric attenuation is

defined by the loss the signal undergoes

traveling through the atmosphere.

Changes in air density and absorption by

atmospheric particles.



Reflections

Reflections can occur as the microwave

signal traverses a body of water or fog

bank; cause multipath conditions



Diffraction

Diffraction is the result of variations in the

terrain the signal crosses



Rain Attenuation

Raindrop absorption or scattering of the

microwave signal can cause signal loss in

transmissions.



Skin Effect

Skin Effect is the concept that high

frequency energy travels only on the

outside skin of a conductor and does not

penetrate into it any great distance.

Skin Effect determines the properties of

microwave signals.



Line of Sight

Fresnel Zone Clearance

Fresnel Zone Clearance is the minimum

clearance over obstacles that the signalneeds to be sent over.

Reflection or path bending will occur if

the clearance is not sufficient.



LOS & FZC-contd

Fresnel Zone

72.2

D1 X D2

F x D

formula



Microwave Fading

Normal Signal

Reflective Path

Caused by multi-path reflections and heavy rains



Range

The distance a signal travels and its

increase in frequency are inversely

proportional

Repeaters extend range

Back-to-back antennas

reflectors



Range-contd

High frequencies are repeated/received at

or below one mile

Lower frequencies can travel up to 100

miles but 25-30 miles is the typical

placement for repeaters


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Components of a Microwave

System

Baseband Digital Modem

Radio Frequency (RF) Unit

Antenna



Baseband Digital Modem

The digital modem modulates the

information signal (intermediate frequency

or IF).



RF Unit

IF is fed to the RF unit which is mounted

as close physically to the antenna as

possible (direct connect is optimal).



Antenna

The antenna is a passive device that

radiates the modulated signal.

It is fed by direct connect of the RF unit,

coaxial cable, or waveguides at higher

frequencies.


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Waveguides

Waveguides are hollow channels oflow-loss material used to direct the

signal from the RF unit to the

antenna.


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Modulation Methods

Primarily modulated today with digital FM or AM

signals

Digital signal remains quiet until failurethreshold bit error rate renders it unusable


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Diversity

Space Diversity

Frequency Diversity


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Space Diversity

Normal Signal

Transmitter Receiver


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Space Diversity-contd

Space Diversity protects against multi-path

fading by automatic switch over to another

antenna placed below the primary antenna.

This is done at the BER failure point or signal

strength attenuation point to the secondary

antenna that is receiving the transmitted signalat a stronger power rating.


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Frequency Diversity

Receiver

Normal XTMR

Frequency #1

Protect XTMR

Frequency #2

RCVR

Frequency #1

RCVR

Frequency #2

Transmitter


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Frequency Diversity-contd

Frequency Diversity uses separate frequencies

(dual transmit and receive systems);

It monitors primary for fail over and switches tostandby.

Interference usually affects only one range of

frequencies.

Not allowed in non-carrier applications because of

spectrum scarcity.


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Hot Standby*

Receiver

System XTMR

Primary #1

System XTMR

Standby #2

failure switch

Active RCVR

#1

Standby RCVR

#2

Transmitter

*Hot standby is designed for equipment failure only


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Availability Formula

Percent Availability equals:

1 (outage hours/8760 hours per year)

Private microwaves have 99.99% availability


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Microwave Path Analysis

Transmitter output power

Antenna gain

proportional to the physical characteristics ofthe antenna (diameter)

Free space gain

Antenna alignment factor

Unfaded received signal level


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Satellite-Related Terms

Earth Stations antenna systems on ornear earth

Uplink transmission from an earth

station to a satellite Downlink transmission from a satellite to

an earth station

Transponder electronics in the satellitethat convert uplink signals to downlinksignals


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Ways to CategorizeCommunications Satellites

Coverage area Global, regional, national

Service type

Fixed service satellite (FSS) Broadcast service satellite (BSS)

Mobile service satellite (MSS)

General usage

Commercial, military, amateur,experimental


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Classification of Satellite Orbits

Circular or elliptical orbit

Circular with center at earths center

Elliptical with one foci at earths center

Orbit around earth in different planes

Equatorial orbit above earths equator

Polar orbit passes over both poles

Other orbits referred to as inclined orbits

Altitude of satellites

Geostationary orbit (GEO)

Medium earth orbit (MEO)

Low earth orbit (LEO)


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Geometry Terms

Elevation angle - the angle from the horizontal to

the point on the center of the main beam of the

antenna when the antenna is pointed directly at

the satellite

Minimum elevation angle

Coverage angle - the measure of the portion of

the earth's surface visible to the satellite


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Minimum Elevation Angle

Reasons affecting minimum elevation angle of

earth stations antenna (>0o)

Buildings, trees, and other terrestrial objectsblock the line of sight

Atmospheric attenuation is greater at low

elevation angles

Electrical noise generated by the earth's heat

near its surface adversely affects reception


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GEO Orbit

Advantages of the the GEO orbit

No problem with frequency changes

Tracking of the satellite is simplified

High coverage area

Disadvantages of the GEO orbit

Weak signal after traveling around 36,000 km

Polar regions are poorly served

Signal sending delay is substantial


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LEO Satellite Characteristics

Circular/slightly elliptical orbit under 2000 km

Orbit period ranges from 1.5 to 2 hours

Diameter of coverage is about 8000 km

Round-trip signal propagation delay less than 20

ms

Maximum satellite visible time up to 20 min

System must cope with large Doppler shifts

Atmospheric drag results in orbital deterioration


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LEO Categories

Little LEOs Frequencies below 1 GHz 5MHz of bandwidth

Data rates up to 10 kbps

Aimed at paging, tracking, and low-ratemessaging

Big LEOs Frequencies above 1 GHz

Support data rates up to a few megabits persec

Offer same services as little LEOs in addition tovoice and positioning services


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MEO Satellite Characteristics

Circular orbit at an altitude in the range of5000 to 12,000 km

Orbit period of 6 hours

Diameter of coverage is 10,000 to 15,000km

Round trip signal propagation delay lessthan 50 ms

Maximum satellite visible time is a fewhours


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Satellite Bands

L - 1-2 GHz

S - 2-4 GHz

C - 4-8 GHz

X - 8-12 GHz

Ku - 12-18 GHz

K - 18-27 GHz

Ka - 27-40 GHz


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Satellite Link Performance Factors

Distance between earth station antennaand satellite antenna

For downlink, terrestrial distance between

earth station antenna and aim point ofsatellite

Displayed as a satellite footprint (Figure 9.6)

Atmospheric attenuation

Affected by oxygen, water, angle of elevation,and higher frequencies


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Satellite Footprint


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Satellite Network Configurations


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Capacity Allocation Strategies

Frequency division multiple access (FDMA)

Time division multiple access (TDMA)

Code division multiple access (CDMA)


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Frequency-Division Multiplexing

Alternative uses of channels in point-to-point configuration

1200 voice-frequency (VF) voice channels

One 50-Mbps data stream 16 channels of 1.544 Mbps each

400 channels of 64 kbps each

600 channels of 40 kbps each

One analog video signal Six to nine digital video signals


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Frequency-Division MultipleAccess

Factors which limit the number of

subchannels provided within a satellite

channel via FDMA

Thermal noise

Intermodulation noiseCrosstalk


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FAMA-FDMA

FAMA logical links between stations are

preassigned

FAMA multiple stations access the

satellite by using different frequency

bands

Uses considerable bandwidth


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DAMA-FDMA

Single channel per carrier (SCPC) bandwidthdivided into individual VF channels

Attractive for remote areas with few userstations near each site

Suffers from inefficiency of fixed assignment

DAMA set of subchannels in a channel istreated as a pool of available links

For full-duplex between two earth stations, apair of subchannels is dynamically assigned on

demand Demand assignment performed in a distributed

fashion by earth station using CSC


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Reasons for IncreasingUse of TDM Techniques

Cost of digital components continues to

drop

Advantages of digital components

Use of error correction

Increased efficiency of TDM

Lack of intermodulation noise


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FAMA-TDMA Operation

Transmission in the form of repetitive sequenceof frames

Each frame is divided into a number of timeslots

Each slot is dedicated to a particulartransmitter

Earth stations take turns using uplink channel

Sends data in assigned time slot

Satellite repeats incoming transmissions

Broadcast to all stations

Stations must know which slot to use fortransmission and which to use for reception

FAMA-TDMA Uplink


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p

FAMA-TDMA Downlink


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VSAT Equipment

ODU

IDU


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ODU

Transmit / Receive Dish (Antenna) (0.75m

- 1.8m)

Block Up-converter (BUC) (1W-2W)

Low Noise Block-Down-converter (LNB)

Feed Assembly


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IDU

The IDU may be a small desktop box, or it

may be a network module integrated with

a router providing VSAT network

connectivity just as any other network

module


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Features Broadband V-SAT system

Maximum Trans / Receive Data up to 2

Mbps / 4 Mbps with 10/100 Mbps Base-T

Ethernet interfaces.

Supports all IP V4 protocols


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Shared and customized Bandwidth for

customers requirements (i.e. dedicated or

shared Bandwidth)

Supports Video Conferencing

Built in GRE Tunneling


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VPNs can be created with VSAT Network,

MLLN nodes, MPLS nodes of BSNL

VoIP telephony with add on ATA (Analog

Telephone Adaptor)

Transparent to IP Sec protocols


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Embedded CPU Turbo Product Coding

Dynamic Link Allocation (DLA) Support (

unique to BSNL Gateway, which no other

VSAT service provider can support in India

as on date)

Connectivity to Broadband Internet


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Ku Band VSAT Architecture


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HUB at Bangalore

Satellite antenna of 8.1 m Cassegrain

feed type.

Ku Band RF equipment and its control

systems.

GATEWAY Networking Equipment with

interfaces to Terrestrial Networks like

MLLN, MPLS and NIB.


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V-SAT SERVICES OFFERED

LEASED LINES Through V-SAT on IP

PLATFORM: 4Kbps onwards

High speed Broadband Internet

VPN Networking

VOIP Telephony


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Facsimile

Telemedicine

E-learning

IP multicasting

Video conferencing

Architecture for High Speed BB


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Architecture for High Speed BBAccess


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