Improving the constituent codes of turbo encoders

The performance of turbo codes can be improved by choosing the best constituent encoder. Previous lists of the best constituent codes were found based on maximising the input weight, two effective free distance d₂. We show that minimising the number of nearest neighbours N₂ is also an important requirement. If there are multiple codes having the same d₂ and N₂, we show that the minimum distance d_min can be a useful criteria to identify the best code. A list of generator polynomials suitable for turbo codes are presented in this paper which satisfy the criteria mentioned above. The previously published results are also extended to include larger memory