﹩{C}_{h,N}^{(+)}(\ell )﹩ (forward) and ﹩{C}_{h,N}^{(-)}(\ell )﹩ (backward) ACF estimation scheme. ﹩{\tilde{x}}_{T}(n){\tilde{x}}_{T}(n+\ell )﹩ is averaged over the first half of the interval (n = 0, 1, ⋯  , N/2 − 1) for the forward estimate, while ﹩{\tilde{x}}_{T}(n){\tilde{x}}_{T}(n-\ell )﹩ is averaged over the second half (n = N/2, N/2 + 1, ⋯  , N − 1) for the backward estimate.