Slide 1

Introduction

Heart of satellite communication system is a satellite based antenna in a stable orbit above the earth. The antenna system on or near the earth are referred to as earth stations.

Uplink: transmission from earth station to satellite

Downlink: transmission from satellite to earth station

The electronics in the satellite that takes an uplink signal and converts it to a downlink signal is called a transponder.

What is satellite?

Satellite is a repeater in the sky.

Receives uplink frequencies and translates to downlink frequencies and transmits back to the earth station after amplification

The translation is done to avoid positive feedback .Trying to receive and transmit an amplified version of the same uplink waveform at the same satellite will cause unwanted feedback, or ring around, from the downlink

antenna back into the receiver

What is a communications satellite and how does it work?

A communications satellite is a radio relay station in orbit above the earth that receives, amplifies, and redirects analog and digital signals carried on a specific radio frequency.

In addition to communications satellites.

In satcom the satellite acts as a media for the data transfer between the source and destination.

Between the source and destination we use a pair of frequencies to communicate.

The Satellite acts as a frequency translator and amplifier.

Satellite

system

SW

Radio

MW

Radio

FM

Radio

Mobile

Telephony

WLANs

Bluetooth

Wireless Systems: Range Comparison

1,000 Km

100 Km

10 Km

1 Km

100 m

10 m

1 m

Different ways of categorizing communication satellites1. Coverage area: global, regional, or national. The larger the area of coverage, the more satellites must be involved in a single networked system.2. Service type: fixed service satellite (FSS), broadcast service satellite (BSS), and mobile service satellite (MSS).3. General usage: commercial, military, amateur and experimental.

What are the different kinds of orbits?

An orbit is the path that a satellite follows as it revolves around Earth. In terms of commercial satellites, there are three main categories of orbits:

500-2,000 km above the earth

These orbits are much closer to the Earth, requiring satellites to travel at a very high speed in order to avoid being pulled out of orbit by Earth's gravity

At LEO, a satellite can circle the Earth in approximately one and a half hours

LEO( Low Earth Orbit)

MEO( Medium Earth Orbit)

8,000-20,000 km above the earth

These orbits are primarily reserved for communications satellites that cover the North and South Pole

MEO's are placed in an elliptical (oval-shaped) orbit

GEO ( Geosynchronous Orbit)

35,786 km above the earth

Orbiting at the height of 22,282 miles above the equator (35,786 km), the satellite travels in the same direction and at the same speed as the Earth's rotation on its axis, taking 24 hours to complete a full trip around the globe. Thus, as long as a satellite is positioned over the equator in an assigned orbital location, it will appear to be "stationary" with respect to a specific location on the Earth.

A single geostationary satellite can view approximately one third of the Earth's surface. If three satellites are placed at the proper longitude, the height of this orbit allows almost all of the Earth's surface to be covered by the satellites.

Polar Orbiting Satellite

These satellites orbit the earth in such a way as to cover the north and south polar regions.

These satellites if in a low earth orbit have to travel at a very high speed.

These satellites can be kept in low earth orbit (800 -900 km) or at 36000km apart.

Geo-Synchronous Satellite

Orbit on the equatorial plane - appears stationary

Altitude of 36000 Kms.

Circular orbit around earth with period of 24 hours.

Coverage of about 1/3 of Earth.

2 deg. apart. Identified by Longitudinal position with ref. to Greenwich.

Advantage Of Geostationary Satellite

Simple ground station tracking requirements.

Removes Satellite hand-over problems.

Negligible Doppler shift

A disadvantage of Geostationary satellites is that points on Earth beyond about 80 deg latitude are not visible.

Inclined orbits, on the other hand can provide visibility to the higher northern and southern latitudes, although they require earth stations to continually track the satellite.

Inclined Orbit Satellite

Geosynchronous

Transponder

Footprint

Frequency

Polarization

Path Propagation

Timing

Satellite Access

Link Performance

Technical Terms

Geosynchronous

Geosynchronous means that the satellite is synchronized with the earth in time and direction. It means that is time taken by a satellite to complete its orbit around earth is equal to the time taken by to earth rotates around its own axis.

Transponder

Frequency band on the satellite is divided into several channels. Each channels are called transponder Each transponder have 40 MHz.

Satellite Footprint

Coverage of entire surface of earth that is visible by the satellite.

Frequency BandRange L-Band1 to 1.5 GHzS-Band1 to 3.9 GHzC-Band3.9 to 8 GHzX-Band8 to 12.5 GHzKu-Band10.95 to 18 GHzK-Band18 to 26.5 GHzKa-Band26.5 to 40 GHz

Frequency Bands use in Satellite Communication

Advantages And Disadvantages of Different Frequency Band

C-Band

Adv. : Broad Footprint, little rain fade

Disadv. : Weak signals, interference, large antenna sizes

and amplifiers

ExC-Band

Adv.. : Broad Footprint, little rain fade, less interference

Disadv. : Week signals, large antenna sizes and amplifiers

Ku-Band

Adv. : Focused Foot prints, no terrestrial interference

small antenna and amplifier

Disadv. : Interference to rain.

Ka-Band

Adv. : Focused Foot prints, large unused

bandwidths

Disadv. : Interference to rain.

FREQUENCY BANDUPLINKDOWNLINKC-Band5.925-6.4253.700-4.200Ex.C-Band6.725-7.0254.500-4.800Ku-Band14.00-14.5010.95-12.75Ka-Band30.0020.00

Satellites Uplink And Downlink Frequency in Different Bands

** all frequencies are in GHz.

VSAT Technology in Satellite Communication

VSAT stands for Very Small Aperture Terminal

It is a two way satellite ground station.

VSATs are most commonly used to transmit narrowband data or broadband data.

VSATs access satellites in geosynchronous orbit to relay data from small remote earth stations (terminals) to other terminals or master earth station "hubs.

Remote VSAT 1

Remote VSAT 2

Earth Station (HUB)

Uplink

Downlink

Satellite

Satellite Link

Capacity allocation1. Frequency division multiple access (FDMA)2. Time division multiple access (TDMA)3. Code division multiple access (CDMA)The number of sub-channels provided within a satellite channel via FDMA is limited by three factors:a. Thermal Noiseb. Intermediation Noisec. Cross talkAlthough FDM techniques are still quite common in satellite transmission, TDM techniques are in increasingly widespread use. The reasons include the following:a. The continuing drop in the cost of digital componentsb. The advantage of digital techniques, include the use of error correctionc. The increased efficiency of TDM due to lack of intermediation noise

Topologies

Star Topology

As the Hub is a powerful receiver and retransmitter , the remote VSAT sites are comparatively small.

A central uplink site, such as a Network Operation Center (NOC), to transport data back and forth to each VSAT terminal via satellite

VSATs communicate to Hub on Inroutes and the Hub Communicates to VSATs on

the Outroute

VSAT System Architecture

Mesh Topology

Each VSAT terminal relays data via satellite to another terminal by acting as a hub, minimizing the need for a centralized uplink site.

VSATs are comparatively larger as they have to communicate directly with each other

VSATs communicate directly with each other, Hub will be involved only for call setup and then fade out.( Hub will not be involved in permanent Circuits)

Parts Of VSATs

Antenna

Block Up Converter (BUC)

Low Noise Block Converter (LNB)

Orthomode Transducer (OMT)

Interfacility Link Cable (IFL)

Indoor Unit (IDU)

Types Of Antenna

Prime Focus Antenna

Single Reflector Antenna.

Feed horn is placed at the Focal point of the Reflector.

Antenna Electronics are placed on Feed.

More susceptible to Interference from Low elevation sources.

More Blockage because feed.

Antenna Efficiency is in the range of 60%.

Low Cost Antenna.

Primarily Used for Receive only applications.

Cassagrain Antenna

Main reflector is Paraboloid

Sub-Reflector is hyperboloid and placed at Prime Focus

Feed is Corrugated Horn and is placed at Center of the Main Reflectors.

The paraboloid converges towards the Sub Reflector ( prime focus), which is then reflected by Sub-Reflector to form a Spherical Wave converging on the Feed.

Gregarion Antenna

Main reflector is Paraboloid

Sub-Reflector is Paraboloid and placed at Prime Focus

Feed is Corrugated Horn and is placed at Center of the Main Reflectors.

The paraboloid converges towards the Sub Reflector ( prime focus), which is then reflected by Sub-Reflector to form a Spherical Wave converging on the Feed.

Offset Fed Antenna

Used for Smaller Earth Stations.

Main Reflector is a section of Parabolic, cutoff above the axis.

Feed is located below the axis giving a completely unblocked Aperture.

High Antenna efficiency.

A BUC (Block Up-Converter) takes an L-Band input and transmits it upstream to the satellite on Ka, Ku, or C band.

BUC's are rated according to their output power. A low power Ka-Band BUC can transmit with as little as 2 watts, while a high power C-Band BUC can transmit with as much as 200 watts.

Block Up-Converter (BUC)

Low Noise Block Converter(LNB)

It is typically mounted at the focal point of the

receiving parabolic dish.

It's primarily used to amplify and convert received satellite signals into frequencies compatible with the satellite receiver.

Orthomode Transducer (OMT)

OMT is 3-port microwave waveguide

system that split input power into two

parts, allow the simultaneous operation of

two outdoor radio units

OMT can be directly mounted on the

antennas that have circular waveguide

connector.

Interfacility Link Cable (IFL)

Interfacility Link (IFL) Distribution

Cable is an indoor rated, connectorized cable assembly that is used within a building to connect a fiber feeder splice point to an optical connector distribution panel.

The cable can be terminated with

connectors at one or both ends in

conjunction with various styles of fan-

out kits at any breakout length.

Indoor Unit (IDU)

Its Satellite Modem consists of Modulator and Demodulator.

The IDU also determines the access schemes under which the VSAT would operate.

The IDU also interfaces with various end user equipment, ranging from stand alone computers, LAN's, routers, multiplexes, telephone instruments, EPABX as per the requirement.