如何在 \multicolumn 中容纳大文本

Question

我正在尝试设计一个表格来比较 3 个主题。只要主题 1 和 2 具有相同的特征，我就想合并它们的列\multicolumn（如图中第 1 行上的 Col. Ref.1 和 Col. Ref.2）。只要文本较小，它就可以正常工作。但是，当文本较大时，表格会超出页面限制，即不会发生换行。有什么帮助吗？

附件是 MWC 及其生成的 PDF。

\documentclass[journal]{IEEEtran}
\usepackage{booktabs}

\begin{document}
    
\begin{table*}[tp]
    \begin{tabular}{p{1cm}p{5.2cm}p{5.7cm}p{5cm}}
        \toprule
        Line no. &
        \multicolumn{1}{c}{Ref. 1}
         &
        \multicolumn{1}{c}{Ref. 2} &
        \multicolumn{1}{c}{The proposed}\\
        \toprule
    1 & \multicolumn{2}{c}{PUs cannot access the R-band directly.  }& PUs can directly access the R-band.  \\
    \hline
    2 & \multicolumn{2}{c}{The number of reserved channels dynamically changes. However, no more than a fixed number of channels can be reserved, which is kept fixed and static (i.e., half of the number of available channels in the network) } & The number of reserved channels dynamically changes. Moreover, any number of channels can be reserved depending on the number of active SU\textsubscript{H} in NB. \\ \hline
\end{tabular}
\end{table*}
\end{document}