statellite communication

8/6/2019 Statellite Communication

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UNIT- I INTRODUCTION

SATELLITE SYSTEM


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History of satellite

communication


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Applications


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Satellite System for Global Mobile

Telecommunication

Communication with the satellite is possible for

the mobile users via a mobile user link (MUL).

The base station controlling the satellite and

acting as gateway to other networks via the

gateway link (GWL).

Satellites may be able to communicate directlywith each other via inter satellite links (ISL).


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Classical satellite systems


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Basics

Satellite orbit around the earth, depends on the application i.e., circular or

elliptical.


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Satellite period and orbits


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Dependency of satellite period and distance to

earth

When the satellite period equals 24 hours, i.e.

distance of 35,786 km, having an orbit time of 24

hours.


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Basics


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Inclination angle of a satellite

The inclination angle is defined as the angle between

the equatorial plane and the plane described by the

satellite orbit.

An inclination angle of 0 degrees means that thesatellite is exactly above the equator.

If the satellite doesnt have a circular orbit, the closes

point to the earth is called the perigee.


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Inclination


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Elevation angle of a satellite

Elevation angle I is defined as the angle

between the center of the satellite beam

and the plane tangential to the earths

surface.

Footprint : the area on earth where the

signals of the satellite can be received.


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Link budget of satellites


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Atmospheric attenuation


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Signal attenuation due to atmospheric

absorption

Its depends on the elevation, the signal

has to penetrate a smaller or larger

percentage of the atmosphere.

Generally ,an elevation less than 10

degrees is considered useless for

communication.


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Orbits I


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Orbits II


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Geostationary satellites


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MEO systems


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Routing


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Localization of mobile stations


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Handover in Satellite Systems

Several additional situation for handover in satellite systems compared

to cellular mobile phone networks caused by the movement of thesatellite.

1 . Intra-satellite handover :

- A user might move from one spot beam of a satellite

to another spot beam of the same satellite.

- using special antennas, satellite can create several

spot beams within its footprint.

2 . Inter-satellite handover:

- if a user leaves the footprint of a satellite or if the satellite moves

away, a handover to the next satellite takes place.- this type of handover can also takes palace between satellites if

they support ISLs.


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3. Gateway handover:

-While the mobile user and satellite might still have

good contact, the satellite might move away from the

current gateway. The satellite has to connect to the

another satellite.

4. Inter-system handover:

- The above three types of handover take place

within the satellite-based communication system, but thistype is concern with different systems.

- Here the handoverbetween satellite systems

and terrestrial cellular networks.


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Handover in satellite systems


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Overview ofLEO/MEO systems


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Unidirectional distribution systems or Broadcast

systems

Broadcast systems can be classified in two ways- DAB - Digital audio broadcast.

- DVB - Digital video broadcast.

It is an extreme version of asymmetric communication

systems. Its often to bandwidth limitations, differences in

transmission power, or cost factors prevent a

communication form being symmetrical.

Symmetrical communication systems offer the sametransmission capabilities in both the directions. i.e.

Channel transmission A -> B are the same as for B -> A.


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Unidirectional distribution systems


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Unidirectional distribution


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Cyclical Repetition of Data

The cyclical repetition of data blocks sent via broadcast is often

called a broadcast disk.

Different patterns are possible A,B, and C in a cycle.

Flat disk:-All blocks are repeated one after another. Every block is

transmitted for an equal amount of time, the average waiting time for

the receiving block is the same for A,B, and C.

Skewed disks :- One or more data blocks by repeating them once

or several times.

- Its raises the probability of receiving a repeated block (A) if the

block was corrupted the first time.

Multi-disks:- Distributes blocks that are repeated more often others

evenly the cyclic pattern.

this minimizes the delay if a user wants to access e.g. block A.


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Structuring transmissions -

broadcast disks


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DAB: Digital Audio Broadcasting


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OFDM


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DAB transport mechanisms


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Transport modes in MSC

Stream Mode:

Offers a transparent data transmission from the source

to the destination with a fixed bit rate in a sub channel.

A sub channel is part of the MSC and comprises the

several CUs within a CIF.

The fixed rate can be multiples of 8 kbits/s.

Packet Mode :

Transfers the data in addressable blocks (packets).These blocks are used to convey MSC data within a sub

channel.


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DABFrame Structure


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DAB Frame Structure

Each frame has a duration TF of 24, 48, or 96 ms depending

on the transmission mode. DAB define four different

transmission modes

Within each frame, 76 or 153 symbols are transmitted using

192,384,768, or 1536 different carriers for COFDM.

The guard intervals Td protecting each symbol can be

31,62,123, or 246 Qs.

Each frame consists of three parts.

The synchronization channel (SC) marks the start of a frame.

It consists of a null symbol and a phase reference to

synchronize the receiver.

The Fast Information Channel (FIC) follows, containing

control data in the FIBs.

Finally , the Main Service Channel (MSC) carries audio and

data service components.


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DAB sender


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Components of a DAB sender

Audio services are encoded and decoded for transmission. All data services are multiplexed and also coded with

redundancy.

The MSC multiplexer combines all user data streams and

forwards them to the transmission multiplexer. This unit creates the fame structure by interleaving the FIC.

Finally OFDM coding is applied and the DAB signal is

transmitted.


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DAB receiver


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Audio coding


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Dynamic reconfiguration of the DAB

multiplexer


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Dynamic reconfiguration of the DAB multiplexer

DAB transmits six audio programmes of different quality

together with in nine data services. Each audio program has its

PAD i.e. Program Associated Data can contain program

information and control information..

From the above diagram audio 1,2,and 3 have high quality, 4

and 5 have low quality, while 6 has the lowest quality.

Programmes 1 to 3 could be higher quality classic

transmissions, while program 6 could be voice transmission.

The DAB multiplexer dynamically interleaves data from all

different sources. To inform the receiver about the current configuration of the

MSC carrying the different data streams, the FIC sends

multiplex configuration information (MCI).


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Multimedia Object Transfer Protocol (MOT)

DAB are facing in which technology the receiver to receive the data?


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MOT Object Structure


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MOT object & segments (data carousel) Object repetition:

DAB can repeat objects several times. If an object A consists of four segments

(A1,A2,A3,A4) a simple repetition pattern would beA1A2A3A4A1A2A3A4A1A2A3A4

Interleaved objects:

DAB can also interleaving segments from different objects. Interleaving theobjects A, B, and C could result in the pattern A1B1C1A2B2C2A3B3C3.

Segment repetition :

DAB can repeat the these segments more often e.g. A1A1A2A2A2A3A4A4

Header repetition:

If a receiver cannot receive the header of an MOT, It will not be able to decode theobject. It can be useful to retransmit the header several times.

Then , the receiver can synchronize with the data stream as soon as it receives theheader and can start decoding.

A pattern could be HA1A2HA3A4HA5A6 with H being the header of the MOT objectA.


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Digital Video Broadcasting


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Components of DVB Architecture The center point is an integrated receiver-decoder (set-top-

box) connected to a high-resolution monitor. The set-of-box can receive DVB signals via satellites,

terrestrial local/regional senders like cable, B-ISDN, ADSL,etc.

Audio / video streams can be recorded, processed, andreplayed using digital versatile disk (DVD) or multimediaPCs.

DVB also transmits data using flexible containers. It sendsservice information contained in its data stream, whichspecifies the content of a container. Types of containers are

Network information table (NIT) : NIT lists the services of aprovider and contains additional information for the set-top-box.


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Service description table (SDT) : SDT lists names and

parameters for each service within an MPEG multiplex

channel. Event information table (EIT): EIT contains status

information about the current transmission and some

additional information for set-top-boxes.

Time and data table (TDT): finally,T

DT

contains updateinformation for set-top-boxes.


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DVBContainer


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Example: high-speed Internet

access


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DVB for high-speed Internet access An information provider e.g. video store, offers its data to potential

customers with the help of a service provider.

If a customer wants to download high-volume information, the informationprovider transmits this information to a satellite provider via a serviceprovider.

The satellite provider now multiplexes this data stream together with otherdigital TV channels and transmits it to the customer via satellite and asatellite receiver.

The customer can now receive the requested information with in the help ofa DVB adapter inside a multi-media PC.

The information for the customer will be encrypted to ensure that onlypaying customers can use the information.

The return channel for requests can be a standard TCP/IP connection via

the internet . The data rates per user are 5-30 Mbit/s for the downlink via satellite and

return channel with 33 kbits/s using a standard modem,64 kbits/s withISDN, or several 100 kbit/s using DSL.


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Convergence of broadcasting

and mobile comm.


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Convergence of digital broad casting systems and mobile

communication systems ETSI (2000) and ETSI (1999d) with

the interaction channels through GSM for DAB and DVB

respectively.

An interaction channel is not only common to DAB and DVB

but covers also different fixed and mobile systems (UMTS,

DECT, ISDN, PSTN , etc.).

The above fig shows high bandwidth audio and video is sent

together with IP data via the broadcast cannel. IP data could

use multi-casting, data carousels etc.


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Comparison ofUMTS,DAB and

DVB

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