Iterative joint minimum mean square error decision feedback equalizer and turbo decoder
An iterative joint Minimum Mean Square Error (MMSE) decision feedback equalizer and turbo decoder includes a turbo decoder portion and a decision feedback equalizer (DFE) portion. The DFE receives input signals and processes these input signals to generate an estimated symbol sequence that is communicated to the turbo decoder portion. The estimated symbol sequence is also communicated to the feedback portion of the DFE. The feedback portion of the DFE includes a symbol-by-symbol detector and a feedback filter. The soft decoder output values generated by the turbo decoder portion are communicated to the feedback portion of the DFE and are combined with the coded output as log likelihood ratio values in a multipass, iterative fashion whereby the equalizer and decoder are linked via iterative feedback to provide communication performance enhancement.
1. A method for underwater acoustic telemetry utilizing an iterative joint minimum mean square error equalizer and turbo decoder, said method comprising the steps of:
processing input signals with a decision feedback equalizer;
generating a coded output comprising an estimated symbol sequence;
feeding the generated coded output to a turbo decoder with the turbo decoder outside of and separate from the decision feedback equalizer;
feeding the estimated symbol sequence into a feedback portion of the equalizer;
decoding the generated coded output with the turbo decoder;
generating soft decoder output values;
feeding the generated soft decoder output values to the feedback portion of the decision feedback equalizer;
combining the soft decoder output values with the estimated symbol sequence; and
using the combined soft decoder output values and estimated symbol sequence in a symbol-by-symbol detector to make symbol decisions in the decision feedback equalizer.
2. The method of claim 1 , wherein the soft decoder output values comprise log likelihood ratios and said step of combining the soft decoder output values with the estimated symbol sequence further comprising the step of calculating log likelihood values for the estimated symbol sequence.
3. The method of claim 2 , wherein said step of calculating log likelihood values further comprises the steps of:
computing a variance for the estimated symbol sequence; and
converting the estimated symbol sequence with the computed variance to log likelihood values.
4. The method of claim 2 , wherein said step of combining further comprises using a variance log likelihood ratio estimator to combine the soft decoder output values with the estimated symbol sequence.
5. The method of claim 1 , further comprising the step of using an output sequence of the symbol-to-symbol detector as inputs of a feedback finite impulse response filter in the decision feedback equalizer.