Presentation Communication Satellite

8/7/2019 Presentation Communication Satellite

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Satellite Communications

Error Control ForDigital Satellite Links


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1-ErrorDetection And Correction

2-Channel Capacity3-ErrorControl Coding

4-Performance Ofblock ErrorCorrection Code

5-Convolution Codes

6-Implementation OfErrorDetection On Satellite Links7-Concatenated Coding AndInterleaving

8-Turbo Codes

Overview


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DIGITAL DATA TRANSMISSIO

N

The transmissionofbinary data across a link

can be accomplished either in parallel modeorserial mode.


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In parallel transmission, several bits ofdata,

organized intogroups are transmittedconcurrently through separatecommunication lines. The mechanismforparallel transmission is simple, use n wires to

sendn bits at a time insteadofone. In thisway each bit has its own wire, and all n bitsofone group can be transmittedfromonedevice to another.

1. Parallel Transmission


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isThe Advantage ofparallel transmission

speed.isThe Disadvantage ofparallel transmission

cost.

It is expensive; therefore parallel transmissionis usually limited toshort distances.


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2.Serial Transmission

In serial transmission,data is transmitted,one

bit at a time, in a continuous stream alongthe communication channels. Thus, in serialtransmission,one bit follows another, so weneedonly one communication channel rather

thann to transmit data between twocommunicationdevices


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The advantage ofserial overparallel

is that with only onetransmissioncommunication channel, serial

transmissionreduces the cost oftransmissionoverparallel by roughly a factorofn.


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The general definitions of the terms areas follows:

Error detection is the detectionoferrors caused

by noise orother impairments duringtransmissionfrom the transmitterto thereceiver.

Errorcorrection is the detectionoferrors andreconstructionofthe original, error-free data.


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Error Detection

Indigital transmission system, an erroroccurs when a bit is

altered between transmission andreception that is abinary 1 is transmitted and a binary 0 is received.

There can be two types oferrors:

1)Single-bit error: It is an isolated errorcondition that altersone bit but does not effects nearby bits.

2)Error-Burst: A group ofbits in which two successiveerroneous bits are always separated by less

than a givennumber x ofcorrect bits.


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Techniques for Error Detection

1)Parity Check : This simplest errordetecting scheme is toappend a parity bit to the endof the block ofdata

. A typical example is the character transmission, in which aparity bit is attached to each 7-bit character. The value ofthis bit is selected so that the characterhas an evennumberof1s (even parity) and anoddnumberof1s (odd

parity). Ifany evennumberofbits is inverteddue to error,evenan undetected erroroccurs. Therefore typically

parity is usedforsynchronous transmission andoddparity forasynchronous transmission.


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2) Cyclic Redundancy Check (CRC): It is the most

common & powerful errordetecting code


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3) ChecksumErrorDetection: Anothermethodof

errordetection uses a process known aschecksum togenerate an errordetection character.The characterresults from summing up all thebytes ofa message together,discarding the

carry-overfrom the addition. Again the process isrepeated at the receiverend and the twochecksums are compared. A mismatch indicatedthat an errorhas occurred.


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Error Correction

correctionoferrors using an errordetecting code

many cases like wireless data transmission thisapproach is in adequate fortworeasons-

1)The bit errorrate on the wireless link may behigherdue to signal strengths, which may resultin large numberofretransmissions.

2)In satellite links, the propagationdelay is verylong compared to the transmission time ofa

single frame.


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Error correction may generally berealized in two different ways:

y Automatic repeat request (ARQ) (sometimes also

referred to as backward error correction): This is anerrorcontrol technique whereby an errordetectionscheme is combined with requests forretransmissionoferroneous data.

y Every block ofdata received is checked using the

errordetection code used, and if the check fails,retransmissionofthe data is requested this may bedone repeatedly, until the data can be verified.


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ARQ and FEC may be combined, such that

minorerrors are corrected withoutretransmission, andmajorerrors arecorrectedvia a request forretransmission:

request-hybrid automatic repeatthis is called

(HARQ).


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y Forward error correction (FEC): The

senderencodes the data using anerror-correcting code (ECC) prior to transmission.The additional information (redundancy)added by the code is used by the receiver to

recover the original data. Ingeneral, thereconstructeddata is what is deemed the"most likely" original data.


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The difference between (analog) and (digital)

is: we can improve the quality ofdigital signalby the use oferrorcorrection techniques.

Analog:

We cannot ingeneral dfferentiation between

the signal and the noise electronically inanalog anddifficult toremove noise.

*The primary disadvantage ofanalog is noise.

Analog and Digital


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Analog signal required lesserbandwidth capacity thandigital capacity.

Digital:

Digital signal requiredgreaterbandwidth capacity thananalog signals.

**Indigital system we can add extra redundantbits toourdata stream which can tell us when an erroroccurs in the data and point to the particular bit orbits have been corrupted


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**Indigital system canonly detect erroruse

error detection make decision about whataction to take when error is detected.

**Indigital system candetect and correct errors

use forward error correction (FEC).


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Ignore the error to flag the error toSend block of information again

Estimate the error and replace theCorrupted data

OR

Which option is selecteddepends on the nature ofthe signal that is transmitted.

Options


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In advanceddigital satellite communication

links:1- The (FEC)may be switched in.

2- out ondemanddependingon bit errorrate(ER)or(C/N)ratio at a terminal.


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Coding use to change data fromone to anotherforexample (PCM) change analogdata in tobinary words fortransmissionovera digitallinks.


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Coding advantagesPn

Eb/N0 dB

10-8

10-3

8 19

Coding gain


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Coding disadvantages

More bandwidth due toredundant

Processing DelayDesign Complexity


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The efficiency ofa coding is numberof

redundant bits add to the detect orcorrect agivennumberoferrors.

Some of(FEC) system the numberofredundant bits = the numberofdata bits ,

result is halvingofdata rate ofgiven channeltransmissionrate, that calledhalf rate FEC.


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channel capacity is (the maximumdata rate

that can be attainedovera given channel).

Channel Capacity


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*A given communication system has a

maximumrate of informationCknown as the channel capacity.

*Ifthe informationrate Ris less thanC, thenone can approach

arbitrarily small errorprobabilities by usingintelligent coding

techniques.

Shannons theorem


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*Toget lowererrorprobabilities, the encoder

has to work on longerblocks ofsignal data. This entails longerdelaysand higher

computational requirements.

*Thus, ifR Cthen transmissionmay beaccomplished without errorin

the presence of noise.


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Error Control Coding

Errordetection coding:

A technique foraddingredundant bits to adata stream in such a way that one ormoreerrors in the data stream can be detected.

Simple Example: The single bit parity applied to the ASCII

code


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Data bits parity bit Sum(Modulo-2)

Even parity 0101101 0 0

Odd parity 0101101 1 1

Received codeword Sumofbits Errordetected

One error 01010010 1 yesTwo errors 01010110 0 noThree errors 11010110 1 yes


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The probability ofk bits being in error in a

block ofn bits is given by:

Pe(k)

P is the probability ofa single-bit occuring.


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Example 7.3.1

A data link transmits 7-bits ASCII words at a bit rateof1 Mbps with a single parity bit. The probability ofabit erroron the link is p=0.001

Find the probability ofundetected errorwhenuncodeddata is transmitted and when a single parity

bit is added to each 7-bit word. How manyundetected errors would be present ifa 500-pagetextbook were transmitted by this link using singleparity?


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LINEAR BLOCK CODESLINEAR BLOCK CODES

Almost all block codes used today belongAlmost all block codes used today belong

to a subset called (to a subset called (linear block codeslinear block codes).).

A linear block code is a code in which theA linear block code is a code in which the

exclusive OR (addition moduloexclusive OR (addition modulo--22) of two) of two

valid code words creates another validvalid code words creates another validcodeword.codeword.


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correcting-error, a linear code is ancoding theoryInofcodewordslinearcombinationforwhich anycode

is anothercodewordof the code. blockLinear codes are traditionally partitioned intoforward errorand Linear codes are used incodes

and are applied inmethods forcorrection)on abitstransmitting symbols (e.g.,

so thatcommunications channel

, iferrors occur in the communication, some errors canbe detected by the recipient ofa message block. The

are blocks ofsymbolslinear block code"codes" in awhich are encoded usingmore symbols than theoriginal value to be sent. A linearcode of length n

transmits blocks containingn symbols.


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Formal definition

linearis akand ranknA linear code of lengthvector spaceof thekdimensionwithCsubspace

elements.qwithfinite fieldwhere is theSuch a code with parameterqis called a q-ary code(e.g., when q= 5, the code is a 5-ary code).

Ifq= 2 orq= 3, the code is described as a binary

code, or a ternary code respectively.


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Remark: We want to give the usual standard basisbecause each coordinate represents a "bit"

which is transmitted across a "noisy channel"with some small probability of transmission error

).binary symmetric channel(a

If some other basis is used then this model cannotbe used and the Hamming metric (defined next)does not measure the number of errors intransmission, as we want it to.


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Generator matrix and parity checkmatrix

Because the linearcode could be considered as alinearsubspace Cof ) and therefore a codeword isa vector in this linearsubspace), any codewordcould be represented as a linear combinationofaset ofbasis vectors such that , where is themessage and is the generator matrix.

On anotherhand,forany linearsubspace , there isa dimensionn knull space such that . The basisvectors ofthe null space form anothermatrix such

that , where is calledparity check matrix


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Cyclic Code

In coding theory,cyclic codes are linear

block error-correcting codes that haveconvenient algebraic structures forefficienterrordetection and correction


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Definition

)ofq(GFfinite fieldoveralinearcodeLet be a,cyclic code. is called anblock length

, theCn)fromc,...,1c=(ccodewordifforeveryword (cn,c1,...,cn-1) inGF(qn)obtained by a

ofcomponents is again acyclic right shiftcodeword.

Same goes for left shifts. One right shift isequal ton 1 left shifts andvice versa.Therefore the linearcode is cyclic preciselywhen it is invariant underall cyclic shifts.


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Cyclic Codes have some additional structural

constraint on the codes.and becauseGalois fieldsThey are basedon

oftheirstructural properties they are veryuseful forerrorcontrols.

Theirstructure is strongly related to Galoisfields because ofwhich the encoding anddecoding algorithms forcyclic codes arecomputationally efficient.


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Algebraic structure

Cyclic codes can be linked to ideals in certainrings. Let R =A]x) / [xn 1( be a polynomial ring

over the finite fieldA = GF)q.(Identify theelements of the cyclic code Cwith polynomialsin Rsuch that maps to the polynomial : thusmultiplication byxcorresponds to a cyclic shift.Then Cis an ideal in R,and hence principal ,since Ris a principal ideal ring .The ideal is

generated by the unique monic element in Cofminimum degree, the generator polynomialg[1].This must be a divisor ofxn 1. It follows thatevery cyclic code is a polynomial code .If thegenerator polynomial ghas degree dthen therank of the code Cis n d.


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The idempotent ofCis a codeword e such

that e 2= e ) that is ,e is an idempotentelement ofC(and e is an identity for thecode, that is ec = cfor every codeword c.Such a word always exists and is

unique [2];it is a generator of the code.An irreducible code is a cyclic code in

which the code, as an ideal, is minimal inR,so that its generator is an irreducible

polynomial


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Example

For example, ifA=andn=3,the set ofcodewords contained in the (1,1,0)-cyclic code isprecisely

.

It corresponds to the ideal ingenerated by)1+x.(

Note that)1+x(is an irreducible polynomial inthe polynomial ring, and hence the code is anirreducible code.

The idempotent of this code is the polynomialx+

x2,corresponding to the codeword (0,1,1.(


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Convolutional Codes

In convolutional codes, each block ofkbits is

mapped into a block ofn bits but these n bitsare not only determined by the present kinformation bits but also by the previousinformation bits.

This dependence can be captured by a finitestate machine.


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ExampleA rate

convolutional coderk=1,n=2 withmemory length 2 and constraint length 3.


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The length ofthe shift register is 2, there are 4

different rates.The behaviorofthe convolutional codercan be

captured by a 4 state machine. States:00,01,10,11,


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Convolutional coding lends itself to very efficienttrellis based encoding and decoding.

They are very practical and powerful codes


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Implementation of error detection on

satellite links

Invariably the error detection is a user

defined serv

ice, which forms a partoperating protocol of any type ofcommunication system.

Implementation of error correction by use oferror detection and retransmissionrequires the use of protocols.


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The protocols are the set of actions, which

helps in accurate and ordered datatransmission through the link.

The usual technique is ARQ.


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But ARQ system works well on terrestrial

data links with relativ

ely low data ratesand short time delays but theirimplementation on satellite link is moredifficult due the long transmission delay

and thus forward error correction ispreferred for satellite paths.


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The following three basic techniques can beused, which are basedon the type of linkusedforretransmissionrequest:

1-In a one way simplex link.

2-In a stop and wait system.

3-In a continuous transmission systemusing the go-back-N techniques.


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With sufficient buffering at the both ends of

the link, only the corrupted blocks aretransmitted and the system is calledselective repeat(ARQ).


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Supervisor: Dr.majed Dwairi

Students:

1- Majd Khader

2- Saed Ayyash

3- Sufian Dare

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Presentation Communication Satellite