Turbo code decoder with modified systematic symbol transition probabilities

• US 6,263,467 B1
• Filed: 08/20/1998
• Issued: 07/17/2001
• Est. Priority Date: 08/20/1998
• Status: Expired due to Term

• Current Assignee: General Electric Company
• Original Assignee: General Electric Company
• Inventors: Ross, John Anderson Fergus, Van Stralen, Nick Andrew, Hladik, Stephen Michael
• Primary Examiner(s): Decady, Albert
• Assistant Examiner(s): Ton, David
• Application Number:
  US09/137,256
• Time in Patent Office: 1,062 Days
• Field of Search: 375/341, 375/500.23, 375/231, 375/142, 375/349, 375/233, 375/755, 375/786, 375/749, 375/751
• US Class Current: 714/755
• CPC Class Codes: H03M 13/2957 Turbo codes and decoding
• CPC Class Codes: H03M 13/3927 Log-Likelihood Ratio [LLR] computation by combination of forward and backward metrics into LLRs
• CPC Class Codes: H03M 13/6502 Reduction of hardware complexity or efficient processing
• CPC Class Codes: H03M 13/658 Scaling by multiplication or division

Abstract

A feedback control for a turbo decoder controls the feedback between component decoders of the turbo decoder by substituting either a neutral value or a weighted value for the channel transition probabilities utilized by each component decoder during the iterative decoding process. A control switch selects either estimated channel transition probabilities, modifications of these values, or neutral values as the channel transition probabilities utilized by the next component decoder in the subsequent decoding iteration.

Claims

1. A method for decoding turbo code words, comprising:

iteratively calculating forward and backward state probabilities and a posteriori state transition probabilities for component code words of turbo code words according to a MAP decoding process;

for each received symbol output by a channel in response to the component code words, selecting either channel transition probability estimates or a modified channel transition probability estimates for calculating the forward and backward state probabilities and the a posteriori state transition probabilities, the selecting step for calculating forward and backward state probabilities being independent of the selecting step for calculating a posteriori state transition probabilities.

2. The method of claim 1, further comprising the step of utilizing an iteration index and providing modified channel transition probability estimates as a function of the iteration index.

3. The method of claim 1 wherein the modified channel transition probability estimates are calculated from modified received symbol values as follows:

$\mathrm{modified}<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{received}<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{symbol}<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{value}=\frac{\mathrm{received}<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{symbol}<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{value}\times <br><br>N+K\times <br><br>0.5}{N+K},$

where N is the current iteration index and K is a constant.

4. The method of claim 1 wherein the modified channel transition probability estimates are calculated from modified received symbol values as follows:

$\mathrm{modified}<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{received}<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{symbol}<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{value}=\frac{\mathrm{received}<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{symbol}<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{value}\times <br><br>N}{N+K},$

where N is the current iteration index and K is a constant.

5. The method of claim 1 wherein the modified channel transition probability estimates are calculated using a modified soft limiter function.

6. The method of claim 5 wherein the parameters of the modified soft limiter function are a function of a decoding iteration index.

7. The method of claim 1 wherein the MAP decoding process comprises a logarithmic MAP decoding process.

8. The method of claim 1 wherein the component code words comprise systematic bits and parity bits, the selecting step comprising selecting the channel transition probability estimates for the received symbols for calculating the forward state and backward state probabilities, the selecting step further comprising selecting either a channel transition probability estimate or a modified channel transition probability estimate for received symbols corresponding to the systematic bits and selecting the channel transition probability estimates for received symbols corresponding to the parity bits for calculating the a posteriori state transition probabilities.

9. A decoder for decoding turbo code words, comprising:

component decoders for iteratively calculating forward and backward state probabilities and a posteriori state transition probabilities for sequences of received symbols output by a channel in response to component code words of turbo code words according to a MAP decoding process;

a selection control which, for each received symbol in the component code words, selects either channel transition probability estimates or modified channel transition probability estimates for forward and backward state probability calculations and a posteriori state transition probability calculations, the selection control making independent selections for the forward and backward state probability calculations and the a posteriori state transition probability calculations.

10. The decoder of claim 9, wherein the component decoders utilize an iteration index and provide modified channel transition probability estimates as a function of the iteration index.

11. The decoder of claim 9 wherein the component decoders calculate the modified channel transition probability estimates from modified received symbol values as follows:

$\mathrm{modified}<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{received}<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{symbol}<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{value}=\frac{\mathrm{received}<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{symbol}<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{value}\times <br><br>N+K\times <br><br>0.5}{N+K},$

where N is the current iteration index and K is a constant.

12. The decoder of claim 9 wherein the component decoders calculate the modified channel transition probability estimates from modified received symbol values as follows:

$\mathrm{modified}<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{received}<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{symbol}<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{value}=\frac{\mathrm{received}<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{symbol}<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{value}\times <br><br>N}{N+K},$

where N is the current iteration index and K is a constant.

13. The decoder of claim 9 wherein the component decoders comprise a soft limiter function for calculating the modified channel transition probability estimates.

14. The decoder of claim 13 wherein the parameters of the modified soft limiter function are a function of a decoding iteration index.

15. The decoder of claim 9 wherein the MAP decoding process comprises a logarithmic MAP decoding process.

16. The decoder of claim 9 wherein the component code words comprise systematic bits and parity bits, the selection control selects the channel transition probability estimates for the received symbols for the forward state and backward state probability calculations, and the selection control selects either a channel transition probability estimate or a modified channel transition probability estimate for each received symbol corresponding to a systematic bit and a channel transition probability estimate for each received symbol corresponding to a parity bit for each a posteriori state transition probability.