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
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