i would not use diagbox. to my opinion is more clear and answer the following design of your table:

which is generated by:
\begin{document}
\begin{table}[htbp]
\centering
\caption{Add caption}
\label{tab:addlabel}%
\begin{tabularx}{\linewidth}{c*{7}{X}}
\toprule
\multirow{2.4}*{\textbf{Mobility Model}}
& \multicolumn{7}{c}{\textbf{Parameter}} \\
\cmidrule{2-8}
& \textbf{a} & \textbf{b} & \textbf{c} & \textbf{d} & \textbf{e} & \textbf{f} & \textbf{g} \\
\midrule
\end{tabularx}%
\end{table}%
\end{document}
i select tabularx table environment ad assume, that all columns except first have equal width. if you would provide some row of table body (that we can see size of other cells contents) than i will be able to suggest different column width.
addendum:
off course, proposed solution works at any columns type. for example, if you use p{...} columns, than the table definition can be:
\begin{tabular}{l*{7}{p{3em}}}
\toprule
\multirow{2.4}*{\textbf{Mobility Model}}
& \multicolumn{7}{c}{\textbf{Parameter}} \\
\cmidrule{2-8}
& \textbf{a} & \textbf{b} & \textbf{c} & \textbf{d} & \textbf{e} & \textbf{f} & \textbf{g} \\
\midrule
\end{tabular}
similary you can select column types as you need (as i mentioned bewof, the contents of table is not known to me, so i can't help you in the design columns types).
this also work with longtable. only you need accordingly design heads for those table
diagboxdoesn't work with rules frombooktabsyou should consider to use standard\hline– Zarko Mar 10 '19 at 07:29\begin{table}[htbp] \centering \caption{Add caption} \begin{tabular}{|p{9em}|p{9.89em}|p{16.165em}|p{12.28em}|p{8.165em}|p{9.945em}|p{6.665em}|p{8.165em}|} \hline\textbf{\backslash {Mobility Model}{Parameter}} & \textbf{a} & \textbf{b} & \textbf{c} & \textbf{d} & \textbf{e} & \textbf{f} & \textbf{g} \\end{tabular}% \label{tab:addlabel}% \end{table}% \end{document}
– monika Mar 10 '19 at 07:46booktabspackage, because they add some vertical space above and below rules. you have , as i see, three choices: (i) not use diagonally split cells (as i suggest in answer below), (ii) use ordinary\hlinefor horizontal lines in table and (ii) remove vertical space around rules frombooktabs. – Zarko Mar 10 '19 at 07:54