Asymptotic distributions and subsampling in spectral analysis for almost   periodically correlated time series
  The aim of this article is to establish asymptotic distributions and consistency of subsampling for spectral density and for magnitude of coherence for non-stationary, almost periodically correlated time series. We show the asymptotic normality of the spectral density estimator and the limiting distribution of a magnitude of coherence statistic for all points from the bifrequency square. The theoretical results hold under $\alpha$-mixing and moment conditions. 
{\L}ukasz Lenart
math.ST stat.TH