Digital transmission system and method

• US 6,662,337 B1
• Filed: 10/26/1999
• Issued: 12/09/2003
• Est. Priority Date: 10/30/1998
• Status: Expired due to Term

• Current Assignee: Avago Technologies International Sales Pte Limited (Broadcom, Inc.)
• Original Assignee: Agere Systems Incorporated (Broadcom, Inc.)
• Inventors: Brink, Stephan ten
• Primary Examiner(s): Decady, Albert
• Assistant Examiner(s): Torres, Joseph D.
• Application Number:
  US09/427,331
• Time in Patent Office: 1,505 Days
• Field of Search: 714/774, 714/701, 714/792, 375/286
• US Class Current: 714/792
• CPC Class Codes: H04L 1/0001 Systems modifying transmission characteristics according to link quality, e.g. power backoff adaptive data allocation for multicarrier modulation H04L5/0044; controlling transmission power for radio systems H04W52/04
• CPC Class Codes: H04L 1/005 Iterative decoding, including iteration between signal detection and decoding operation
• CPC Class Codes: H04L 1/0065 Serial concatenated codes
• CPC Class Codes: H04L 27/34 Amplitude- and phase-modulated carrier systems, e.g. quadrature-amplitude modulated carrier systems

Abstract

A transmission system is disclosed in which a multilevel modulated signal is transmitted. The soft output information of a channel decoder is fed back and utilized by a soft demapping device in order to improve the decoding result by further iterative decoding steps. The receiver includes a demapper for generating a demapped signal, bit deinterleaver for generating a demapped and deinterleaved signal and a decoder for generating soft reliability values representative of the decoded signal. These soft reliability values are then bit interleaved and fed back to the demapper, as a priori knowledge, for use in further iterations of the decoding process. Two mappings are mixed adaptively dependent on the channel conditions and the number of iterations to be used.

Claims

1. A digital transmission system, comprising:

a coder for coding an input signal with an error checking or error correcting code, an interleaver for interleaving the bits of the coded signal, a mapper for mapping the interleaved coded signal into a multilevel signal, and a transmitter for sending the multilevel signal over a noisy channel in a transmission medium;

a receiver for receiving the multilevel signal distorted by noise (noisy multilevel signal), a demapper having a first input for receiving the noisy multilevel signal and operative to provide a soft demapped signal at an output a deinterleaver, for deinterleaving the demapped signal, a decoder to decode the deinterleaved demapped signal to provide a soft decoded signal, an interleaver to interleave the decoded signal, the demapper having a second input for receiving the interleaved decoded signal and being operative to recalculate the demapped signal iteratively using the noisy signal and the soft decoded signal,

characterized bye;

a mapping store for storing two different mappings having different mutual information;

means for deriving an indication of the channel conditions;

means for selecting t e mappings in a mixing ratio dependent on the derived indication of channel conditions, the mapper and demapper being operatively responsive to the selected mapping.

2. A digital transmission system, as claimed in claim 1, wherein the indication of the channel conditions is an estimate of the signal to noise ratio.

3. A digital transmission system, as claimed in claim 1, in which the soft output signals of the demapper and the decoder are likelihood ratios.

4. A digital transmission system, as claimed in claim 1, in which the soft output signals of the demapper and the decoder are log-likelihood ratios.

5. A digital transmission system, comprising:

a coder for coding an input signal with an error checking or error correcting code, an interleaver for interleaving the bits of the coded signal, a mapper for mapping the interleaved coded signal into a multilevel signal, and a transmitter for sending the multilevel signal over a noisy channel in a transmission medium;

a receiver for receiving the multilevel signal distorted by noise (noisy multilevel signal), a demapper having a first input for receiving the noisy multilevel signal and operative to provide a soft demapped signal at an output, a deinterleaver, for deinterleaving the demapped signal, a decoder to decode the deinterleaved demapped signal to provide a soft decoded signal, an interleaver to interleave the decoded signal, the demapper having a second input for receiving the interleaved decoded signal and being operative to recalculate the demapped signal iteratively using the noisy signal and the soft decoded signal,

characterized by;

a mapping store for storing two different mappings having different mutual information;

means for determining the number of iterations to be carried out from a plurality of possible numbers of iteration; and

means for selecting the mappings in a mixing ratio dependent on the number of iterations as determined, the mapper and demapper being operatively responsive to the selected mapping.

6. A digital transmission system, as claimed in claim 5, in which the soft output signals of the demapper and the decoder are likelihood ratios.

7. A digital transmission system, as claimed in claim 5, in which the soft output signals of the demapper and the decoder are log-likelihood ratios.

8. A digital transmission system, comprising:

a coder for coding an input signal with an error checking or error correcting code, an interleaver for interleaving the bits of the coded signal,

a mapper for mapping the interleaved coded signal into a multilevel signal, and a transmitter for sending the multilevel signal over a noisy channel in a transmission medium;

a receiver for receiving the multilevel signal distorted by noise (noisy multilevel signal), a demapper having a first input for receiving the noisy multilevel signal and operative to provide a soft demapped signal at an output, a deinterleaver, for deinterleaving the demapped signal, a decoder to decode the deinterleaved demapped signal to provide a soft decoded signal, an interleaver to interleave the decoded signal, the demapper having a second input for receiving the interleaved decoded signal and being operative to recalculate the demapped signal iteratively using the noisy signal and the soft decoded signal,

characterized by;

a mapping store for storing two different mappings having different mutual information;

means for deriving an indication of the channel conditions;

means for determining the number of iterations to be carried out from a plurality of possible numbers of iterations; and

means for selecting the mappings in a mixing ratio dependent on the derived indication of channel conditions and on the number of iterations as determined, the mapper and demapper being operatively responsive to the selected mapping.

9. A digital transmission system, as claimed in claim 8, wherein the indication of the channel conditions is an estimate of the signal to noise ratio.

10. A digital transmission system, as claimed in claim 8, in which the soft output signals of the demapper and the decoder are likelihood ratios.

11. A digital transmission system, as claimed in claim 8, in which the soft output signals of the demapper and the decoder are log-likelihood ratios.

12. A digital transmission system, as claimed in claim 8, wherein each mapping is stored with an indication of its mutual information, and including a mutual information store storing an approximation of the optimum mutual information with a plurality of respective combinations of channel conditions and number of iterations, the means for selecting being operative to identify an optimum mutual information from the number of iterations as determined and the nearest stored channel conditions to the derived indication, and to select the mapping in a mixing ratio to give a mutual information for the mix approximately equal to the optimum.

13. A digital transmission system, as claimed in claim 12, wherein the selecting means is operative to calculate the mixing ratio q according to the formula:

$q=\frac{I_{o,\max }-I_{o,\mathrm{opt}}}{I_{o,\mathrm{opt}}-I_{o,\min }}$

where

I_o,max is the larger mutual information of the stored mappings,

I_o,min is the smaller mutual information of the stored mappings, and

I_o,opt is the desired optimum mutual information.

14. A method of transmitting a digital signal, comprising:

coding an input signal with an error checking or error correcting code, interleaving the bits of the coded signal, mapping the interleaved coded signal into a multilevel signal, sending the multilevel signal over a noisy channel in a transmission medium;

receiving the multilevel signal distorted by noise (noisy multilevel signal), demapping the noisy multilevel signal to provide a soft demapped signal, deinterleaving the demapped signal, decoding the deinterleaved demapped signal to provide a soft decoded signal, interleaving the decoded signal, recalculating the demapped signal iteratively using the noisy signal and the soft decoded signal,

characterized by;

storing two different mappings having different mutual information;

deriving an indication of the channel conditions; and

selecting the mappings in a mixing ratio dependent on the derived indication of channel conditions, the mapping and demapping being operatively responsive to the selected mapping and demapping.

15. A method of transmitting a digital signal, as claimed in claim 14, wherein the indication of the channel conditions is the signal to noise ratio.

16. A method of transmitting a digital signal, as claimed in claim 14, in which the soft demapped and soft decoded signals are likelihood ratios.

17. A method of transmitting a digital signal, as claimed in claim 14, in which the soft demapped and soft decoded signals are log-likelihood ratios.

18. A method of transmitting a digital signal, comprising:

coding an input signal with an error checking or error correcting code, interleaving the bits of the coded signal, mapping the interleaved coded signal into a multilevel signal, sending the multilevel signal over a noisy channel in a transmission medium,

receiving the multilevel signal distorted by noise (noisy multilevel signal), demapping the noisy multilevel signal to provide a soft demapped signal at an output, deinterleaving the demapped signal, decoding the deinterleaved demapped signal to provide a soft decoded signal, interleaving the decoded signal, recalculating the demapped signal iteratively using the noisy signal and the soft decoded signal,

characterized by;

storing two different mappings having different mutual information;

determining the number of iterations to be carried out from a plurality of possible numbers of iteration; and

selecting the mappings in a mixing ratio dependent on the number of iterations as determined, the mapping and demapping being operatively responsive to the selected mapping.

19. A method of transmitting a digital signal, as claimed in claim 18, in which the soft demapped and soft decoded signals are likelihood ratios.

20. A method of transmitting a digital signal, as claimed in claim 18, in which the soft demapped and soft decoded signals are log-likelihood ratios.

21. A method of transmitting a digital signal, comprising:

coding an input signal with an error checking or error correcting code, interleaving the bits of the coded signal, mapping the interleaved coded signal into a multilevel signal, sending the multilevel signal over a noisy channel in a transmission medium;

receiving the multilevel signal distorted by noise (noisy multilevel signal), demapping the noisy multilevel signal to provide a soft demapped signal, deinterleaving the demapped signal, decoding the deinterleaved demapped signal to provide a soft decoded signal, interleaving the decoded signal, recalculating the demapped signal iteratively using the noisy signal and the soft decoded signal,

characterized by;

storing two different mappings having different mutual information;

deriving an indication of the channel conditions;

determining the number of iterations to be carried out from a plurality of possible numbers of iterations; and

selecting the mappings in a mixing ratio dependent on the derived indication of channel conditions and on the number of iterations as determined, the mapping and demapping being operatively responsive to the selected mapping.

22. A method of transmitting a digital signal, as claimed in claim 21, wherein the indication of the channel conditions is the signal to noise ratio.

25. A method of transmitting a digital signal, as claimed in claim 21, wherein each mapping is stored with an indication of its mutual information, and including storing an approximation of the optimum mutual information with a plurality of respective combinations of channel conditions and number of iterations, the step of selecting a mapping being operative to identify an optimum mutual information from the number of iterations as determined and the nearest stored channel conditions to the derived indication,

and selecting the mappings in a mixing ratio to give a mutual information for the mix approximately equal to the optimum.

26. A method as claimed in claim 25, wherein the mixing ratio q is calculated according to the formula:

$q=\frac{I_{o,\max }-I_{o,\mathrm{opt}}}{I_{o,\mathrm{opt}}-I_{o,\min }}$

where

i_o,max is the larger mutual information of the stored mappings,

i_o,min is the smaller mutual information of the stored mappings, and

I_o,opt is the desired optimum mutual information.

23. A method of transmitting a digital signal, as claimed in 21, in which the soft demapped and soft decoded signals are likelihood ratios.

24. A method of transmitting a digital signal, as claimed in 21, in which the soft demapped and soft decoded signals are log-likelihood ratios.