Abstract
Intrinsic wavelet transforms and wavelet estimation methods are introduced for curves in the non-Euclidean space of Hermitian positive definite matrices, with in mind the application to Fourier spectral estimation of multivariate stationary time series. The main focus is on intrinsic average-interpolation wavelet transforms in the space of positive definite matrices equipped with an affine-invariant Riemannian metric, and convergence rates of linear wavelet thresholding are derived for intrinsically smooth curves of Hermitian positive definite matrices. In the context of multivariate Fourier spectral estimation, intrinsic wavelet thresholding is equivariant under a change of basis of the time series, and nonlinear wavelet thresholding is able to capture localized features in the spectral density matrix across frequency, always guaranteeing positive definite estimates. The finite-sample performance of intrinsic wavelet thresholding is assessed by means of simulated data and compared to several benchmark estimators in the Riemannian manifold. Further illustrations are provided by examining the multivariate spectra of trial-replicated brain signal time series recorded during a learning experiment. Supplementary materials for this article are available online.
Supplementary Materials
Additional background material and detailed proofs of the presented results are available in the online supplementary materials, and the accompanying R-package pdSpecEst is publicly available on CRAN (Chau Citation2017).
Acknowledgments
We thank the UC Irvine Space-Time Modeling Group and Dr. Emad Eskandar (Massachusetts General Hospital) for the local field potential data to illustrate the methodology and the anonymous referees for their suggestions that helped improving the presentation of this work.