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

turbo codes decoding apps

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