On the EXIT Characteristics of Feed Forward Convolutional Codes

In our contribution we analyze the exit characteristics of feed forward convolutional codes employed in a parallel concatenated turbo scheme or as inner component in a serially concatenated scheme. It turns out that the exit characteristics reveal some general limitations. First we give an illustrative reason for the limited capabilities of feed forward and non-terminated recursive convolutional codes and secondly, we provide novel analytical means to determine the maximum mutual information available at the extrinsic output of feed forward convolutional decoders. This upper bound for the mutual information is only dependent on the channel quality and on the Hamming weight of the impulse response of the convolutional code. The paper concludes by determining the maximum mutual information for a wide range of channel conditions and code parameters, showing for which codes and channel conditions a maximum mutual information of ap1 bit can be reached.