A new class of convolutional codes called Turbo codes, whose performances in terms of Bit Error Rate (BER) are close to the SHANNON limit. The Turho Code encoder is built using a parallel concatenation of two Recursive Systematic Convolutional codes and the associated decoder, using a feed hack decoding rule. The iterative operation of decoding blockinterleaved Turbo codes can be viewed as weakly coupled two-dimensional encoder constraints mutually interest in turn via posteriori probabirity of information hits. Based on this view point, we propose in each interation of MAP decoding to take into account the encoder constraint other than that the iteration follows. In order futher to strengthen the effect of the design, we replace the weakly coupled two-dimensional Turbo encoder with a closely coupled one. By employing an encoder optimized of its free distance, the proposed system was found to outperform the blockinterleaved Turbo codec.
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