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Turbo codes

1. INTRODUCTION

TURBO CODESconstitute major development in the field of Forward Error

Correction (FEC).

In electrical engineering and digital communications turbo codes are a class of

high performance error correction code developed in 1! which are finding use in

deep satellite communication and other applications where designers see" to achieve

ma#imal information transfer over a limited bandwidth communication lin" in the

presence of data corrupting noise.

E#hibiting performance approaching the $hannon limit% Turbo Codes (TC)

have the TC bloc" set features efficient encoder and decoder designs seen rapid

adoption in the design of digital communication s&stems. 'esirable and 'esignable

introduces the basics of turbo codes in their different flavors (more specificall&%

parallel concatenated convolutional turbo codes and bloc" turbo codes). Through the

application of s&stemic design methodolog& that considers data transfer and storage as

top priorit& candidates for optimiation% the authors show how turbo codes can be

implemented and the attractive performance results that can be achieved in

throughput% latenc& .

The last ten &ears have seen the appearance of a new t&pe of correction code

the turbo code. This represents a significant development in the field of error

correcting codes. The principle of decoding is to be found in an iterative e#change of

information between elementar& decoders% called e#trinsic information% and it is this

principle from which the term turbo originates. The turbo concept is now applied to

bloc" codes as well as other parts of a digital transmission s&stem% such as detection%

demodulation. *pplications that integrate turbo codes into their standards are mobile

communications% wireless networ"s and local radio loops. Future applications could

include cable transmission% shortdistance communication or data storage includes

cable transmission% shortdistance communication or data storage.

+ehru College of Engineering *nd ,esearch Centre

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2. THE TURBO PRINCIPLE

*llows a single ver& comple# operation (decoding a turbo code) to be split

into two much simpler operations (decoding component codes).

-se soft information/ from output of one operation to assist with the other operation.

3. ERROR DETECTION AND CORRECTION

Errors can be categoried as follows0

a). $ingle bit0 one bit error per data unit.

b). urst0 two or more bit bit errors per data unit.

,edundanc& is the concept of sending e#tra bits for use in error detection.

For common methods of error detection are the following0

a). 2ertical redundanc& chec"(2,C)

b). 3ongitudinal redundanc& chec" (3,C)

c). C&clic redundanc& chec" (C,C)

d). Chec"sum

In 2,C an e#tra bit is added to the data unit.

2,C can detect onl& an odd number of errors0 it cannot detect an odd number

of errors.

In 3,C% a redundant data unit is follows n data units.

C,C% the most powerfull of the redundanc& chec"ing techni4ues% is based on

binar& division.

Chec"sum is used b& the higher la&er protocols ( TC56I5) for error

detection.

a). To divides the data into sections .

b). *dd the sections together using one7s complement arithmetic.

c). Ta"e the complement of the final sum8 this is the chec"sum.

The hamming code is a single bit error correction method using redundant

bits.


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4. TYPES OF ERRORS

:herever an electromagnetic signal flow one point to another %it is subject to

unpredictable inference from heat%magnetism and other forms of electricit&.This

inference can change the shape or timing of the signal.There are two t&pes of errors

0single bit error and burst error.In single bit error a ;

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Figure shows the effect of a single bit error as a data unit.For a

single bit error to occur %the noise must have a duration of ;1= microsecond.* single

bit error can happen if we are sending data using parallel transmission.

E#ample0

If B wires are used to send all of the B bits of a b&te at the sametime and one of

the wires is nois& one bit can be corrupted in each time.

-,$T E,,@,

* burst error means that two or more bits in the data unit have changed.

< < < 1 1 < 1 tio-

To carr& information% a s&stem must be able assure privac&. Encr&ption means

that the sender transforms the original information to another form and sends the

resulting message over the networ".

Co/>'e$$io-

'ata compression reduces the number of bits to be transmitted.

APPLICATION LAYER-

The application la&er enables the user% to access the networ"% it provides user

interface and support for mail such as electronic mail% file transfer% shared database

management s&stem etc.


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1. CONCLUSION

Toda&% man& modern s&stems use T-,@ C@'E$.

:e can use turbo codes for compression of binar& se4uences. +etwor"s must

be able to transfer data from one device to another with complete

accurac&.*n&time data are transmitted from source to destination8 the& can

become corrupted in passage. For reliable communication% errors must be

deleted and corrected. Error detect ion and correction are implemented either

at the data lin" la&er or the transport la&er of the @$I model.


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1. REFERENCES

=e$ite$-

1) www.wi"ipedia.org

9) www.ieee.org

!) www.turbocodes.com

>) www.libasc.e"m.org


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12863296-turbo-codes

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