SATELLITE COMMUNICATION

&NETWORKING

Submitted By- Sanket Gupta

NTPC Electronics & CommunicationNational Thermal Power Corporation Raj Kumar Goel Engg. College

INDEX

1. Introduction a. How Do Satellites Work? b. Factors In Satellite

Communication2. Major Problems For Satellites

a. Advantages Of Satellites Communication

b. Disadvantages Of Satellites Communication

3. Types Of Satellites a. Geostationary Earth Orbit

(GEO)b. Low Earth Orbit (LEO) c. Medium Earth Orbit (MEO)

4. Frequency Bands Of Satellites

a. Satellite Services b. Frequency Bands

5. Terms Used In Satellite Communication

6. Components Of Satellite Communication

7. Satellite Communication System 8. Satellite Earth Station

Introduction

How Do Satellites Work? If two Stations on Earth want to communicate through

radio broadcast but are too far away to use conventional means, then these stations can use a satellite as a relay station for their communication. One Earth Station sends a transmission to the satellite. This is called an Uplink. The satellite Transponder converts the signal and sends it down to the second earth station. This is called a Downlink.

Factors In Satellite Communication Elevation Angle: The angle of the horizontal of the

earth surface to the center line of the satellite transmission beam. It affects the satellite’s coverage area.

Coverage Angle: A measure of the portion of the earth surface visible to a satellite taking the minimum elevation angle into account.

R/(R + h) = sin(π/2 – β - θ)/sin(θ + π/2) = cos(β + θ)/cos(θ) R = 6370 km (earth’s radius), h = satellite orbit heightβ = coverage angle, θ = minimum elevation angle

Major Problems For Satellites

Positioning:

• This can be achieved by using small rocket motors and fuel; over half of the weight of most satellites is made up of fuel.

• Commercial life of a satellite typically 10-15 years. Often it is the fuel availability which determines the lifetime of a satellite.

Stability:• It is vital that satellites are stabilised to ensure that

solar panels and communications antennae are aligned properly.

• Modern satellites use reaction wheel stabilisation, a form of gyroscopic stabilisation.

Power:• Modern satellites use solar panels, so solar power is

used to generate efficient electricity.• Batteries are needed as sometimes the satellites are

behind the earth; this happens about half the time for a LEO satellite.

Alignment:• There are a number of components which need

alignment like Solar panels and Antennae.• A parabolic dish at antennae can be used which is

pointing in the correct general direction.• Different feeder “horns” can be used to direct

outgoing and incoming beams more precisely.

Harsh Environment:

• Satellite components need to be especially “hardened”. Circuits which work on the ground will fail very rapidly in space.

• Temperature is also a problem, so satellites use electric heaters to keep circuits and other vital parts warmed up to control the temperature.

Atmospheric Attenuation:• It is caused by air and water that can impair the

transmission.• It is particularly bad during rain and fog.

Advantages & Disadvantages Of Satellites Communication

Advantages:• The coverage area of a satellite greatly exceeds that

of a terrestrial system.• Transmission cost of a satellite is independent of the

distance from the center of the coverage area.• Satellite to Satellite communication is very precise.• Higher Bandwidths are available for use.

Disadvantages:

• Launching satellites into orbit is costly.• Satellite bandwidth is gradually becoming used up.• There is a larger propagation delay in satellite

communication than in terrestrial communication.

Types Of Satellites

Geostationary Earth Orbit (GEO) These satellites are in orbit 35,863 km above the

earth’s surface along the equator with angular separation about 2 degrees; allows 180 satellites. Objects in Geostationary orbit revolve around the earth at the same speed as the earth rotates. This means

GEO satellites remain in the same position relative to the surface of earth.

Advantages: A GEO satellite’s distance from earth gives it a large coverage area, almost a one fourth of the earth’s surface. GEO satellites have a 24 hour view of a particular area. These factors make it ideal for satellite broadcast and other multipoint applications.

Disadvantages: A GEO satellite’s distance also cause it to have both a comparatively weak signal and a time delay in the signal, which is bad for point to point communication. GEO satellites, centered above the equator, have difficulty in broadcasting signals near Polar Regions.

Low Earth Orbit (LEO) LEO satellites are much closer to the earth than GEO

satellites, ranging from 500 to 1,500 km above the surface. These satellites don’t stay in fixed position relative to the surface, and are only visible for 15 to 20 minutes each pass. A network of LEO satellites is necessary for LEO satellites to be useful.

Advantages: A LEO satellite’s proximity to earth compared to a GEO satellite gives it a better signal strength and less of a time delay, which makes it better for point to point communication. LEO satellite has smaller area of coverage; less waste of bandwidth.

Disadvantages: A network of LEO satellites is needed, which is costly. LEO satellites have Doppler shifts cause

by their relative movement and atmospheric effects drag LEO satellites; causing gradual orbital deterioration.

Medium Earth Orbit (MEO) A MEO satellite is in orbit somewhere between 8,000

km and 18,000 km above the earth’s surface. MEO satellites are visible for much longer periods of time than LEO satellites, usually between 2 to 8 hours. MEO satellites are similar to LEO satellites in functionality.

Advantages: A MEO satellite’s longer duration of visibility and wider footprint, means fewer satellites are needed in a MEO network than a LEO network. MEO satellites have a larger coverage area than LEO satellites.

Disadvantages: A MEO satellite’s distance gives it a longer time delay and weaker signal than a LEO satellite, though not as bad as a GEO satellite.

Frequency Bands Of Satellites

Satellite Services

Fixed Service Satellites (FSS) Eg: Point to Point Communication

Broadcast Service Satellites (BSS) Eg: Satellite Television/Radio

Mobile Service Satellites (MSS) Eg: Satellite Phones

Frequency Bands L–Band: (1 to 2 GHz) used by MSS S-Band: (2 to 4 GHz) used by MSS, NASA, deep space

research C-Band: (4 to 8 GHz) used by FSS X-Band: (8 to 12.5 GHz) used by FSS, military and

meteorological satellites Ku-Band: (12.5 to 18 GHz) used by FSS and BSS (DBS) K-Band: (18 to 26.5 GHz) used by FSS and BSS Ka-Band: (26.5 to 40 GHz) used by FSS

Early Satellite Communications used C band in range 3.7 to 4.2 GHz and Could interfere with terrestrial communications. Its Beam width is narrower with higher frequencies.Recent Satellite Communications used Ku band mostly. Now Ka band is being used.

Terms Used In Satellite Communication

Leased Line:It connects two locations for private voice and/or data telecommunication service. It is not a dedicated cable, but actually a reserved circuit between two points. Leased lines are mostly rented by businesses to connect branch offices, because these lines guarantee bandwidth for network traffic. T1 leased lines are common and offer good data rate.

Dial-up Connection:It is established and maintained for a limited time duration. As an alternative, a dedicated connection continuously takes place. Dial-up lines are also called switched lines and dedicated lines are called non-switched lines. A dial-up connection can be initiated manually or automatically by the computer's modem or other device.

Integrated Services Digital Network (ISDN):It is a set of communications standards for simultaneous digital transmission of voice, video, data and other network services over the public switched telephone network. ISDN is a circuit-switched telephone

network system which also provides access to packet-switched networks, resulting in potentially better voice quality than an analog phone can provide.

Digital Subscriber Line (DSL):It is a family of technologies that provides digital data transmission over the wires of a local telephone network. In telecommunication marketing, the term Digital Subscriber Line is widely used as Asymmetric Digital Subscriber Line (ADSL). DSL service is delivered simultaneously with regular telephone on the same telephone line as it uses a higher frequency band that is separated by filtering.

VoIP (voice over IP):It is an IP telephony term used for a set of facilities to manage the delivery of voice information over the Internet. VoIP involves sending voice information in digital form in discrete packets rather than using the traditional circuit-committed protocols of the public switched telephone network (PSTN). A major advantage of VoIP is that it avoids the tolls charged by ordinary telephone service.

Multiprotocol Label Switching (MPLS):It is a mechanism in high-performance telecommunication networks which directs and carries data from one network node to the next. MPLS makes it easy to create "virtual links" between distant nodes. It can encapsulate packets of various network protocols.

MPLS operates at an OSI Model layer that lies between Layer 2 (Data Link Layer) and Layer 3 (Network Layer) and thus is often referred as a "Layer 2.5" protocol.

It provides a unified data-carrying service for both circuit-based clients and packet-switching. It is used to carry many different kinds of traffic, including IP packets, as well as native ATM, SONET, and Ethernet frames.

Very Small Aperture Terminal (VSAT):It is a two-way satellite ground station or a stabilized maritime VSAT antenna with a dish antenna that is smaller than 3 meters. VSAT antennas range from 75cm to 1.2m and data rates range from 56 Kbit/s upto 4 Mbit/s. VSAT access satellites in geosynchronous orbit to relay data from small remote earth stations to other terminals in mesh configurations or to master earth station "hubs" in star configurations.

They provide very efficient point-to-multipoint communication and are easy to install. VSAT networks offer immediate accessibility and continuous high-quality transmissions. They are adapted for any kind of transmission from data to voice, fax and video.

Most VSAT networks are configured in one of these topologies: A star topology, using a central uplink site such as a

network operations center (NOC) to transport data back and forth to each VSAT terminal via satellite.

A mesh topology, where each VSAT terminal relays data via satellite to another terminal by acting as a hub and minimizing the need for a centralized uplink site.

• A combination of both star and mesh topologies. Some VSAT networks are configured by having several centralized uplink sites connected in a multi-star topology with each star connected to each other in a mesh topology. Others are configured in only a single star topology having terminal connected to each other acting as a central hub. These configurations minimize the overall cost of the network and alleviate the amount of data that has to be relayed through a central uplink sites of a star or multi-star network.

Components Of Satellite Communication

Local Area Network (LAN):LAN is a computer network covering a small physical area like a home, office, or small groups of buildings such as a school or an airport. The defining characteristics of LANs in contrast to wide area networks (WANs), include their usually higher data-transfer rates, smaller geographic area, and lack of a need for leased telecommunication lines.

MODEM:The terminal and the computer both require data in digital format while the link is designed for analog signals. Some device is therefore necessary to convert data from digital to analog format and vice versa. Such a device is known as a MODEM. It does two operations: Modulation (at transmitting end) and Demodulation (at

receiving end). Thus, a modem is required at both ends in each line of a data communication network.

There are several techniques available to carry this modulation or demodulation process:

1. Amplitude Modulation2. Frequency modulation3. Phase Modulation

4. Multi-level Modulation (QAM)

Router:A router is a device that interconnects two or more computer networks and selectively interchanges packets of data between them. Each data packet contains address information that a router can use to determine if the source and destination are on the same network or if the data packet must be transferred from one network to another. The routers exchange information about target system addresses so that each router can build up a table showing the preferred paths between any two systems on the interconnected networks.

Multiplexer:A multiplexer takes a no. of communication channels and combines the signals into one common channel of a greater bandwidth or data rate in such a way that the original signals can be extracted again. A multiplexer is required at the end of each line to multiplex and de-multiplex the signals.

There are two types of Multiplexing:• Frequency division multiplexing (FDM):

It is the traditional way of separating radio signals from different transmitters. Frequency-division multiplexing is used as to allow multiple users to share a physical communications channel, hence it is called frequency-division multiple access (FDMA).

• Time division multiplexing (TDM):It is a type of digital or analog (rarely) multiplexing in which two or more signals or bit streams are transferred simultaneously as sub-channels in one communication channel but are physically taking

turns on the channel. Hence, it is called time-division multiple access (TDMA).

UP Converter:A block up-converter (BUC) is used in the transmission or uplink of satellite signals. It converts a band of frequencies from a lower frequency to a higher frequency. Modern BUCs convert from the L band to Ku band, C band and Ka band. Older BUCs convert from a 70 MHz intermediate frequency (IF) to Ku band or C band. An example of a system utilizing BUC is a VSAT system used for bidirectional internet access via satellite.

DOWN Converter:In digital signal processing, a digital down-converter (DDC) converts a digitized real signal centered at an

intermediate frequency (IF) to a baseband complex signal centered at zero frequency. In addition to down conversion, DDC’s typically decimate to a lower sampling rate allowing follow-on signal processing by lower speed processors.

Spectrum Analyzer:A spectrum analyzer is a laboratory instrument that displays signal amplitude or strength as it varies by signal frequency. The frequency appears on the horizontal axis and the amplitude is displayed on the vertical axis. A spectrum analyzer just looks like an oscilloscope.A spectrum analyzer interface is a device that can be connected to a wireless receiver or a personal computer to allow visual detection and analysis of electromagnetic signals over a defined band of frequencies. This is called Panoramic Reception and it can be used to determine the frequencies of sources of interference to wireless networking equipment.

Satellite Communication System

Many types of signals can be transmitted through satellite system such as audio message, video data etc. But in every case we have to convert data into digital form and after this we connect it to LAN as shown in fig.

Block Diagram Of Satellite Communication System:

Satellite Earth Station

The ground segment of the satellite system consists of all of the communicating earth stations which access the operational satellite. These earth stations consists of:

Audio

MessageESBX

LAN

Switch

LAN

Switch

Digital

Data

CodecVideo

MessageCamera

LAN

Switch

LAN MODEM ROUTERUP/DOWN

CONVERTER

HPASPECTRUM

ANALYZER

ANTENNA TRACKING CONTROLLER

SATELLITE

SITE

A

SITE

B

• Antenna (Plus tracking subsystem)• Feed system (Polarizers, Duplexers, orthanode

junctions etc.)• High power amplifier (HPAs)• Low noise amplifier (LNAs)• UP/DOWN Converters (between microwave to IF)• Ground communications equipment (GCE)

(Modems, coders, multiplexers etc.)• Control and monitoring equipment (CME)• Power supplies

A network may consist of a few to hundreds of earth stations and all have to access the satellite in an equitable manner. Finally the satellite system must be interfaced to the users either directly or via a network, for e.g. PSTN, ISDN or PLMN.

REMARKS

Duplexer

Baseband Signal (FROM USERS)

IF

Modulator

UP

Converter

RF High

Power

Amplifier

Baseband Signal (TO USERS)

IF

Demodulator

DOWN

Converter

RF Low

Noise

Amplifier