On Inverses for Quadratic Permutation Polynomials over Integer Rings
  Quadratic permutation polynomial interleavers over integer rings have recently received attention in practical turbo coding systems from deep space applications to mobile communications. In this correspondence, a necessary and sufficient condition that determines the least degree inverse of a quadratic permutation polynomial is proven. Moreover, an algorithm is provided to explicitly compute the inverse polynomials. 
Jonghoon Ryu and Oscar Y. Takeshita
cs.IT math.IT