Download - Turbo Codes Decoding Apps

7/29/2019 Turbo Codes Decoding Apps

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Turbo Codes Decoding and

ApplicationsBob Wall

EE 548


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The Turbo Decoder

Since a Turbo Code trellis would have avery large number of states (due to the

interleaver), a normal decoder would behuge Instead, perform an iterative decoding

process This requires some changes to the normal

decoder instead of making a hard decision,it needs to make a soft decision


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Decoder Decisions

A hard decision is choosing a sequence of 0 or 1output bits based on the input symbols. It does not

take into account how likely this string of bits was. A soft decision produces a value in [0, 1] that

corresponds to the likelihood (or log likelihood).This can be used as input to help make decisionsabout the transmitted sequence


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Decoder Algorithms

Normal decoding algorithms (i.e. Viterbialgorithm) find the most likely sequence of bitsthat was transmitted.

In a turbo decoder, want to find the likelihood of each bit. This serves as the a priori probability orthe reliability of each bit, to use as input to thenext decoder Optimal MAP (Maximum a Priori) BCJR (Bahl,

Cocke, Jelinek, Raviv) Simpler - SOVA (Soft Output Viterbi Algorithm) lose

roughly .7 dB coding gain


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Decoder Flow

Simplified flow diagram (fromhttp://www.xenotran.com/turbo_tech_error_turbo.html)


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Block Diagram

Generic Turbo Decoderextrinsic information

extrinsic information

x i yi

x k y k

DEC - DEC | Inter-leaver

De-inter-leaver

De-inter-leaver


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Turbo Decoding Delays

Delay introduced by interleaver depending on how it is designed, some orall of a block of data must be present beforeencoding or decoding can begin

Delay introduced by iteration

You have to have some drawbacks no freelunch!


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Performance


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Performance (cont.)


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Applications

Terrestrial Wireless Communications UMTS (Universal Mobile Telecommunication

Systems) Digital Video Broadcasting (DVB) CCSDS (Consultative Committee for Space

Data Systems) Telemetry Standard


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CCSDS Telemetry Standard

Upgraded to specify Turbo Code as analternative channel coding standard Alternative to older codes: (255, 223) Reed-

Solomon code, 64-state rate-1/2 convolutioncoder, and their serial concatenation

2.5 dB coding gain over old standard Also allows for symbol synchronization


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Space Telemetry

Symbol Synchronization recover clock information from the signal itself

Requires relatively frequent changes insignal (run lengths must be limited)

Specifications suggest max run length of 64symbols, and a minimum number of transitions in 1000 symbols of 275


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Symbol Synchronization

With turbo code specified in standard (N =8920), simulation has shown that the max

run lengths for the rate 1/2 and rate 1/3encoders are 42 and 53, respectively, andthe minimum number of transitions per

1000 symbols are 407 and 408, respectively Turbo code is very close to the ideal

random code

Download - Turbo Codes Decoding Apps

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