channels, the approximation given in has been shown to be accurate enough for a practical number of antennas, which usually ranges from 64 to 512 [@Wen15Channel; @Yin13coordinated; @Adhikary13Joint; @You15Pilot], and a detailed numerical example can be found in [@Wen15Channel]. Since the SFCCM model given in is a good approximation to the more complex physical channel model in when the number of BS antennas is sufficiently large, we will thus exclusively use the simplified SFCCM model in in the rest of the paper.

Realistic wireless channels are usually not wide-sense stationary [@Clerckx13MIMO], i.e., ${\mathbf{R}}_{k}$ varies as time evolves, although with a relatively large time scale.[^8] In practice, acquisition of the large dimensional ${\mathbf{R}}_{k}$ is rather difficult and resource-intensive for massive MIMO-OFDM. However, when we shift our focus from the space-frequency domain to the angle-delay domain, the problem can be significantly simplified. Motivated by the eigenvalue decomposition of the SFCCM given in , we decompose the SFCRM as follows $$\label{eq:gkl}
  {\mathbf{G}}_{k,\ell}={\mathbf{V}}_{M}{\mathbf{H}}_{k,\ell}{\mathbf{F}}_{{N_{\mathrm{c}}}\times{N_{\mathrm{g}}}}^{T}$$ where $$\label{eq:adcr}
  {\mathbf{H}}_{k,\ell}={\mathbf{V}}_{M}^{H}{\mathbf{G}}_{k,\ell}{\mathbf{F}}_{{N_{\mathrm{c}}}\times{N_{\mathrm{g}}}}^{*}\in{\mathbb{C}}^{M\times{N_{\mathrm{g}}}}$$ is referred to as the angle-delay domain channel response matrix (ADCRM) of UT $k$ at OFDM symbol $\ell$. In the following proposition, we derive a statistical property of the ADCRM.

\[prop:adcmsts\] For massive MIMO-OFDM channels, when the number 