how to express the feedback of turbo codes in polynomial.

I want to express the feedback of turbo codes in polynomial, but I do not know the difference when i express the feedback of (1, 17/15) and (1, 17/13).Which is the expression of the polynomial 1+D+D^3. Thanks.

I guess the best forum to get an answer for this kind of question is the DSP forum. This is the forum which is for RF. This is as much as I can help.

I did turbo,trellis etc. codes last year but I honestly remember them vaguely.