Error detection/correction and fault detection/recovery – Pulse or data error handling – Digital data error correction
Reexamination Certificate
1998-08-20
2001-04-24
Chung, Phung M. (Department: 2184)
Error detection/correction and fault detection/recovery
Pulse or data error handling
Digital data error correction
C714S786000
Reexamination Certificate
active
06223319
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates generally to error-correction coding and, more particularly, to parallel concatenated convolutional coding, i.e., turbo coding.
A new class of forward error control codes, referred to as turbo codes, offers significant coding gain for power limited communication channels. Turbo codes are generated by using two recursive systematic encoders operating on different orderings of the same information bits. A subset of the code bits of each encoder is transmitted to maintain bandwidth efficiency. Turbo decoding involves an iterative algorithm in which probability estimates of the information bits that are derived for one of the codes are fed back to a probability estimator for the other code. Each iteration of processing generally increases the reliability of the probability estimates. This process continues, alternately decoding the two code words until the probability estimates can be used to make reliable decisions.
The maximum a posteriori (MAP) type algorithm introduced by Bahl, Cocke, Jelinek, and Raviv in “Optimal Decoding of Linear Codes for Minimizing Symbol Error Rate”,
IEEE Transactions on Information Theory,
March 1974, pp. 284-287, is particularly useful as a component decoder in decoding parallel concatenated convolutional codes, i.e., turbo codes. The MAP algorithm is used in the turbo decoder to generate a posteriori probability estimates of the systematic bits in the code word. These probability estimates are used as a priori symbol probabilities for the second MAP decoder. Three fundamental terms in the MAP algorithm are: forward and backward state probability functions (the alpha and beta functions, respectively) and the a posteriori transition probabilities (the sigma function).
Various probability estimates comprise inputs at several points in a MAP decoding algorithm. It is desirable to modify or restrict these probability inputs in a MAP decoding algorithm in order to improve decoding performance, such as, for example, decrease the probability of error in the decoded bits.
SUMMARY OF THE INVENTION
A feedback control for a turbo decoder controls the feedback between component decoders of the turbo decoder by updated a priori probabilities calculated by one component decoder and used as inputs to another component decoder during the decoding process, resulting in a performance advantage. A feedback control switch selects either previously estimated a posteriori probabilities, modifications of these values, or neutral values as a priori probabilities utilized by the next component decoder. In one embodiment, unmodified, or standard, a posteriori probabilities from a prior decoding iteration of a component code word are inputted as a priori information is used in the alpha and beta calculations for all decoding iterations, but neutral values are switched in for all sigma calculations except for the final iteration for which the feedback control switch reverts back to full a priori information.
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“Illuminating the Structure of Code and Decoder of Parallel Concatenated Recursive Systematic (Turbo) Codes,” Patrick Robertson, IEEE, 1994, pp. 1298-1303.
“Optimal Decoding of Linear Codes for Minimizing Symbol Error Rate,” LR Bahl, J Cocke, F. Jelinek; J. Raviv, IEEE Transactions on Information Theory, Mar. 1974, pp. 284-287.
“Near Shannon Limit Error-Correcting Coding and Deconding: Turbo-Codes (1),” Claude Berrou, Alain Glavieux; Punya Thitimajshima, IEEE, 1993, pp. 1064-1070.
Anderson John Bailey
Fergus Ross John Anderson
Hladik Stephen Michael
Van Stralen Nick Andrew
Breedlove Jill M.
Chung Phung M.
General Electric Company
Stoner Douglas E.
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