Optimal importance sampling for some quadratic forms of ARMA processes

The determination of the asymptotically efficient importance sampling distribution for evaluating the tail probability P(L(n) > u) for large n by Monte Carlo simulations, is considered. It is assumed that L(n) is the likelihood ratio statistic for the optimal detection of signal with spectral density (s) over cap from noise with spectral density (c) over cap L(n) = (2n)(-1) X(n)(-t) {T-n ((c) over cap)(-1) - T-n((c) over cap + (s) over cap)(-1)} X(n) (c) over cap and (s) over cap being both modeled as invertible Gaussian ARMA processes, and X(n) being a vector of n consecutive samples from the noise process. By using large deviation techniques, a sufficient condition for the existence of an asymptotically efficient importance sampling ARMA process, whose coefficients are explicitly computed, is given. Moreover, it is proved that such an optimal process is unique.