  This paper proposes and proves a theorem which stipulates sufficient
conditions the coefficients of two quadratic permutation polynomials (QPP) must
satisfy, so that the permutations generated by them are identical. The result
is used to reduce the search time of QPP interleavers with lengths given by
Long Term Evolution (LTE) standard up to 512, by improving the distance
spectrum over the set of polynomials with the largest spreading factor.
Polynomials that lead to better performance compared to LTE standard are found
for several lengths. Simulations show that 0.5 dB coding gains can be obtained
compared to LTE standard.
