\begin{tikzpicture}

\begin{scope}
\node [anchor=south west,draw,thick,red,minimum width=0.9in,minimum height=0.7in] (space1) at (0,0) {};
\node [anchor=south west,fill=blue,minimum width=0.1in,minimum height=0.1in] (unit1) at (0.2,0.8) {};
\node [anchor=south west,fill=ugreen,minimum width=0.1in,minimum height=0.1in] (unit2) at (0.7,0.3) {};
\node [anchor=south west,fill=blue,minimum width=0.1in,minimum height=0.1in] (unit3) at (1.3,1.3) {};
\node [anchor=south west,fill=ugreen,minimum width=0.1in,minimum height=0.1in] (unit4) at ([xshift=0.1em]unit3.south east) {};

\begin{pgfonlayer}{background}
\node [rectangle,inner sep=2pt,rounded corners=1pt,draw,thick] [fit = (unit3) (unit4)] (unitbox) {};
\end{pgfonlayer}

\draw [->] ([yshift=1pt]unit1.north) .. controls +(north:0.23) and +(west:0.2) .. ([yshift=0.2em,xshift=-1pt]unitbox.west);
\draw [->] ([xshift=1pt]unit2.east) .. controls +(east:0.5) and +(south:0.2) .. ([xshift=0.2em,yshift=-1pt]unitbox.south);

\node [anchor=south] (spacelabel1) at (space1.north) {\scriptsize{离散表示空间}};
\node [anchor=north] (captain1) at ([yshift=-0.5em]space1.south) {\small{(a) \textbf{统计机器翻译}}};

\end{scope}

\begin{scope}[xshift=1.8in]
\node [anchor=south west,draw,thick,red,minimum width=0.9in,minimum height=0.7in] (space1) at (0,0) {};
\node [anchor=south west,fill=blue,minimum width=0.1in,minimum height=0.1in] (unit1) at (0.2,0.8) {};
\node [anchor=south west,fill=ugreen,minimum width=0.1in,minimum height=0.1in] (unit2) at (0.7,0.3) {};

\node [anchor=south west,draw,thick,red,minimum width=0.9in,minimum height=0.7in] (space2) at (1.3in,0) {};
\node [anchor=south west,circle,fill=orange,minimum width=0.1in,minimum height=0.1in] (unit3) at (1.7in,1.3) {};

\draw [->] ([yshift=1pt]unit1.north) .. controls +(north:0.4) and +(west:2) .. ([yshift=0.0em,xshift=-1pt]unit3.west);
\draw [->] ([xshift=1pt]unit2.east) .. controls +(east:1.5) and +(south:1) .. ([xshift=0.0em,yshift=-1pt]unit3.south);

\node [anchor=south] (spacelabel1) at (space1.north) {\scriptsize{离散表示空间}};
\node [anchor=south] (spacelabel2) at (space2.north) {\scriptsize{连续表示空间}};
\node [anchor=north] (captain1) at ([yshift=-0.5em,xshift=1em]space1.south east) {\small{(b) \textbf{神经机器翻译}}};

\end{scope}

\end{tikzpicture}