over OFDM symbol $\ell$ and subcarrier $n$ as ${\left[{\mathbf{g}}_{k,\ell,n}\right]_{m}}$. Using a physical channel modeling approach (see, e.g., [@Clerckx13MIMO; @Liu03Capacity; @Barriac04Characterizing; @Auer12MIMO; @Fleury00First]), the channel response vector ${\mathbf{g}}_{k,\ell,n}\in{\mathbb{C}}^{M\times1}$ can be described as $$\begin{aligned}
\label{eq:wb_cha_mod}
  {\mathbf{g}}_{k,\ell,n}
  &=\sum_{q=0}^{{N_{\mathrm{g}}}-1}\int\limits_{-\infty}^{\infty}\int\limits_{-\pi/2}^{\pi/2}\!{{\mathbf{v}}_{M,\theta}}\cdot{\exp\left(-{\bar{\jmath}}2\pi \frac{n}{{T_{\mathrm{c}}}}\tau\right)}{\notag\\}&\quad\cdot{\exp\left({\bar{\jmath}}2\pi \nu\ell{T_{\mathrm{sym}}}\right)}\cdot
  g_{k}\left(\theta,\tau,\nu\right)\cdot{\delta\left(\tau-q{T_{\mathrm{s}}}\right)} {{\mathrm{d}}\theta}{{\mathrm{d}}\nu}{\notag\\}&=\sum_{q=0}^{{N_{\mathrm{g}}}-1}\int\limits_{-\infty}^{\infty}\int\limits_{-\pi/2}^{\pi/2}\!{{\mathbf{v}}_{M,\theta}}\cdot{\exp\left(-{\bar{\jmath}}2\pi \frac{n}{{N_{\mathrm{c}}}}q\right)}{\notag\\}&\quad\cdot{\exp\left({\bar{\jmath}}2\pi \nu\ell{T_{\mathrm{sym}}}\right)}\cdot
  g_{k}\left(\theta,q{T_{\mathrm{s}}},\nu\right){{\mathrm{d}}\theta}{{\mathrm{d}}\nu}\end{aligned}$$ where ${{\mathbf{v}}_{M,\theta}}$ is given in , $g_{k}\left(\theta,\tau,\nu\right)$ is the complex-valued joint angle-delay-Doppler channel gain function of UT $k$ corresponding to the incidence angle $\theta$, delay $\tau$, and Doppler frequency $\nu$. Note that the number of significant channel taps in the delay domain is usually limited, and smaller than ${N_{\mathrm{g}}}$; i.e., ${\left|g_{k}\left(\theta,q{T_{\mathrm{s}}},\nu\right)\right|}$ is approximately $0$ for most $q$. Since the locations of the significant channel taps in the delay domain are usually different for different UTs, we adopt in this paper to obtain a general channel representation applicable for all the UTs.

We write the [$k$th]{} UT’s channel at OFDM symbol $\ell$ over all subcarriers as $$\label{eq:chgkl}
  {\mathbf{G}}_{k,\ell}=
  \left[{\mathbf{g}}_{k,\ell,0}\quad{\mathbf{g}}_{k,\ell,1}\quad\ldots\quad{\mathbf{g}}_{k,\ell,{N_{\mathrm{c}}}-1}\right]
  \in{\mathbb{C}}^{M\times{N_{\mathrm{c}}}}$$ which will be referred to as the space-frequency domain channel response matrix (SFCRM). From , it is not hard to show that $$\begin{aligned}
\label{eq:phych}
  {\mathsf{vec}\left\{{\mathbf{G}}_{k,\ell}\right\}}&=\sum_{q=0}^{{N_{\mathrm{g}}}-1}\int\limits_{-\infty}^{\infty}\int\limits_{-\pi/2}^{\pi/2}\!
  \left[{\mathbf{f}}_{{N_{\mathrm{c}}},q}\otimes{{\mathbf{v}}_{M,\theta}}\right]\cdot{\exp\left({\bar{\jmath}}2\pi \nu\ell{T_{\mathrm{sym}}}\right)}{\notag\\}&\quad\cdot g_{k}\left(\theta,q{T_{\mathrm{s}}},\nu\right){{\mathrm{d}}\theta}{{\mathrm{d}}\nu}.\end{aligned}$$

We assume that channels with different incidence angles, delays, and/or Doppler frequencies are uncorrelated [@Clerckx13MIMO; @Auer12MIMO; @Fleury00First]. We also assume that 