\def\layersep{1.5cm}
\def\outsep{0.7cm}
\def\dy{1}

\begin{tikzpicture}[->, >=stealth, shorten >= 0pt, draw=black!50, node distance=\layersep, font=\sffamily]
    \tikzstyle{node}=[circle,fill=black,minimum size=2pt,inner sep=0pt]
    \tikzstyle{block}=[draw=black,rectangle,fill=none,minimum size=1.5cm, inner sep=0pt]
    \tikzstyle{summer}=[draw=black,circle,fill=none,minimum size=1cm, inner sep=0pt]
    \tikzstyle{annot} = []

	\node[node] (sc) at (0, -\dy cm) {};
	\node[summer] (add) at (1*\layersep, -\dy cm) {\Large $+$};
	\node[node, below=\dy cm of add] (nc) {};
    \node[block] (DSP) at ($(add.east) + (\layersep, 0)$) {System};
	\coordinate (yc) at ($(DSP.center) + (\layersep, 0)$) {};
	
	%\coordinate (mid1) at ($(ADC.east)!0.5!(DSP.west)$) {};
	%\coordinate (mid2) at ($(DSP.east)!0.5!(DAC.west)$) {};
		
    \path (sc) edge (add);
    \path (add) edge (DSP);
    \path (nc) edge (add);
    \path (DSP) edge (yc);
    
    \node[left = 0mm of sc, text width = 1cm, align=center] {Signal};
    \node[right = 0mm of yc, text width = 1cm, align=center] {Output}; 
    \node[below = 0mm of nc, text width = 2cm, align=center] {Noise or \\ interference};
    

\end{tikzpicture}