makecell package
\documentclass{article}
\usepackage{array, tabularx, caption, boldline}
\usepackage{graphicx}
\usepackage{cellspace}
\setlength\cellspacetoplimit{4pt}
\setlength\cellspacebottomlimit{4pt}
\begin{document}
\begin{table}[h!]
\caption{Main Notations}
\label{tab:table1}\centering
\begin{tabularx}{9cm}{|Sl|X|}
\hline
\textbf{Notation} & \textbf{Meaning} \\
\hline
$P_{md}$ & local probability of misdetection \\
\hline
$P_{d}$ & local probability of detection \\
\hline
$P_{fa}$ & probability of false alarm \\
\hline
$P_{pu}$ & signal \\
\hline
$P_b$ & broadcast \\
\hline
$N_{gd}$ & node 1 \\
\hline
$N_{ln}$ & Total number of node \\
\hline
$h_{gd1}$ & {h number when majority of the nodes in a topology gives a decision `1' when PU is present} \\
\hline
\end{tabularx}
\end{table}
\begin{table}[h!]
\caption{Main Notations}
\label{tab:table2}\centering
\begin{tabularx}{9cm}{SlV{2}X}
\textbf{Notation} & \textbf{Meaning} \\
\hlineB{1.2}
$P_{md}$ & local probability of misdetection \\
$P_{d}$ & local probability of detection \\
$P_{fa}$ & probability of false alarm \\
$P_{pu}$ & signal \\
$P_b$ & broadcast \\
$N_{gd}$ & node 1 \\
$N_{ln}$ & Total number of node \\
$h_{gd1}$ & {h number when majority of the nodes in a topology gives a decision `1' when PU is present}
\end{tabularx}
\end{table}
\end{document}