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Turbo Codes – Decoding and Applications

Turbo Codes – Decoding and Applications. Bob Wall EE 548. The Turbo Decoder. Since a Turbo Code trellis would have a very large number of states (due to the interleaver), a normal decoder would be huge Instead, perform an iterative decoding process

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Turbo Codes – Decoding and Applications

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  1. Turbo Codes – Decoding and Applications Bob Wall EE 548

  2. The Turbo Decoder • Since a Turbo Code trellis would have a very large number of states (due to the interleaver), a normal decoder would be huge • 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

  3. Decoder Decisions • A “hard” decision is choosing a sequence of 0 or 1 output 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 decisions about the transmitted sequence

  4. Decoder Algorithms • Normal decoding algorithms (i.e. Viterbi algorithm) find the most likely sequence of bits that was transmitted. • In a turbo decoder, want to find the likelihood of each bit. This serves as the a priori probability or the reliability of each bit, to use as input to the next decoder • Optimal MAP (Maximum a Priori) – BCJR (Bahl, Cocke, Jelinek, Raviv) • Simpler - SOVA (Soft Output Viterbi Algorithm) – lose roughly .7 dB coding gain

  5. Decoder Flow • Simplified flow diagram (from http://www.xenotran.com/turbo_tech_error_turbo.html)

  6. Block Diagram • Generic Turbo Decoder extrinsic information Inter-leaver De-inter-leaver xk yk DEC- DEC| extrinsic information De-inter-leaver xi yi

  7. Turbo Decoding Delays • Delay introduced by interleaver – depending on how it is designed, some or all of a block of data must be present before encoding or decoding can begin • Delay introduced by iteration • You have to have some drawbacks – no free lunch!

  8. Performance

  9. Performance (cont.)

  10. Applications • Terrestrial Wireless Communications • UMTS (Universal Mobile Telecommunication Systems) • Digital Video Broadcasting (DVB) • CCSDS (Consultative Committee for Space Data Systems) Telemetry Standard

  11. CCSDS Telemetry Standard • Upgraded to specify Turbo Code as an alternative channel coding standard • Alternative to older codes: (255, 223) Reed-Solomon code, 64-state rate-1/2 convolution coder, and their serial concatenation • 2.5 dB coding gain over old standard • Also allows for symbol synchronization

  12. Space Telemetry • Symbol Synchronization – recover clock information from the signal itself • Requires relatively frequent changes in signal (run lengths must be limited) • Specifications suggest max run length of 64 symbols, and a minimum number of transitions in 1000 symbols of 275

  13. 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 1/3 encoders are 42 and 53, respectively, and the 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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