% Please add the following required packages to your document preamble:
% \usepackage{multirow}
% \usepackage{graphicx}
\begin{table}[!htb]
\centering
\caption{Average results for using a single cascade ($N=1$). \textbf{Bold} numbers indicate better performance than using two cascades ($N=2$).}
\label{tab:N1-ssim-psnr-nmse}
\resizebox{\textwidth}{!}{%
\begin{tabular}{ccccccccccc}
\hline
\multicolumn{2}{c}{\multirow{2}{*}{\textbf{Method}}} &
  \multicolumn{3}{c}{\textbf{$4 \times$}} &
  \multicolumn{3}{c}{\textbf{$6 \times$}} &
  \multicolumn{3}{c}{\textbf{$8 \times$}} \\ \cline{3-11} 
\multicolumn{2}{c}{} &
  SSIM &
  pSNR &
  NMSE &
  SSIM &
  pSNR &
  NMSE &
  SSIM &
  pSNR &
  NMSE \\ \hline
\multirow{2}{*}{\begin{tabular}[c]{@{}c@{}}Frame-specific\\ (1D)\end{tabular}} &
  Adpt-ACSInit-N1 &
  \textbf{0.9879} &
  \textbf{46.43} &
  \textbf{0.0030} &
  \textbf{0.9848} &
  \textbf{45.11} &
  \textbf{0.0041} &
  \textbf{0.9815} &
  \textbf{43.89} &
  \textbf{0.0053} \\
 &
  Adpt-EqInit-II-N1 &
  0.9878 &
  46.62 &
  0.0029 &
  0.9841 &
  44.97 &
  0.0042 &
  0.9793 &
  43.45 &
  0.0059 \\ \hline
\begin{tabular}[c]{@{}c@{}}Frame-specific\\ (2D)\end{tabular} &
  Adpt-ACSInit-N1 &
  0.9937 &
  50.92 &
  0.0011 &
  0.9922 &
  \textbf{49.75} &
  \textbf{0.0014} &
  0.9902 &
  \textbf{48.54} &
  \textbf{0.0018} \\ \hline
\multirow{2}{*}{\begin{tabular}[c]{@{}c@{}}Unified\\ (1D)\end{tabular}} &
  Adpt-ACSInit-N1 &
  0.9744 &
  42.61 &
  0.0072 &
  0.9633 &
  39.89 &
  0.0134 &
  0.9503 &
  37.97 &
  0.0201 \\
 &
  Adpt-EqInit-II-N1 &
  0.9753 &
  42.85 &
  0.0070 &
  0.9628 &
  40.31 &
  0.0124 &
  0.9452 &
  37.92 &
  0.0212 \\ \hline
\begin{tabular}[c]{@{}c@{}}Unified\\ (2D)\end{tabular} &
  Adpt-ACSInit-N1 &
  0.9930 &
  50.45 &
  0.0012 &
  0.9910 &
  48.99 &
  0.0017 &
  0.9880 &
  47.33 &
  0.0024 \\ \hline
\end{tabular}%
}
\end{table}