I have four different tikzpictures merged in one single figure by means of the `subcaption` package and the `\subfigure` environment. However, the figures have different horizontal lengths, so I don't get a proper vertical alignment. 
![Immagine](https://i.stack.imgur.com/v5Nbt.jpg)

Instead of this, I would like to have the center of each box vertically aligned with the center of the other boxes and with the subcaptions, i.e. (a),(b)...

I wouldn't mind if something had to be rescaled (to fit the linewidth), but please note that each subfigure should be rescaled accordingly, in order for all the boxes to be of the same size.

Here is my code:

```
\documentclass[a4paper]{article}
\usepackage{tikz}
\usepackage{subcaption}
\usetikzlibrary{positioning}
\begin{document}
\begin{figure}
  \begin{subfigure}{\linewidth}
    \centering
    \begin{tikzpicture}[>=latex]
      \node (a) [] {$c_k\delta_{\Delta}(t-k\Delta)+c_j\delta_{\Delta}(t-j\Delta)$};
      \node [draw,thick,rectangle,minimum width=2cm,minimum height=1cm] (b) [right=of a] {\texttt{SISO-L}};
      \node (c) [right=of b] {$c_kh_{k\Delta}(t)+c_jh_{j\Delta}(t)$};
      \path [->,thick] (a) edge (b)
                     (b) edge (c);
    \end{tikzpicture}
  \caption{}
\label{fig:}
\end{subfigure}\\[.5cm]
\begin{subfigure}{\linewidth}
  \centering
  \begin{tikzpicture}[>=latex]
    \node (a) [] {$\displaystyle \sum_{k=-\infty}^{\infty}u(k\Delta)\delta_{\Delta}(t-k\Delta)\Delta$};
    \node [draw,thick,rectangle,minimum width=2cm,minimum height=1cm] (b) [right=of a] {\texttt{SISO-L}};
    \node (c) [right=of b] {$\displaystyle \sum_{k=-\infty}^{\infty}u(k\Delta)h_{k\Delta}(t)$};
    \path [->,thick] (a) edge (b)
                     (b) edge (c);
  \end{tikzpicture}
\caption{}
\label{fig:}
\end{subfigure}\\[.5cm]
\begin{subfigure}{\linewidth}
  \centering
  \begin{tikzpicture}[>=latex]
      \node (a) [] {$\delta(t-\tau)$};
    \node [draw,thick,rectangle,minimum width=2cm,minimum height=1cm] (b) [right=of a] {\texttt{SISO-L}};
    \node (c) [right=of b] {$h_{\tau}(t)$};
    \path [->,thick] (a) edge (b)
                     (b) edge (c);
  \end{tikzpicture}
\caption{}
\label{fig:}
\end{subfigure}\\[.5cm]
\begin{subfigure}{\linewidth}
\centering
  \begin{tikzpicture}[>=latex]
    \node (a) [] {$\displaystyle u(t)=\int_{-\infty}^{\infty}u(\tau)\delta(t-\tau)\,\mathrm{d}\tau$};
    \node [draw,thick,rectangle,minimum width=2cm,minimum height=1cm] (b) [right=of a] {\texttt{SISO-L}};
    \node (c) [right=of b] {$\displaystyle y(t)=\int_{-\infty}^{\infty}u(\tau)h_{\tau}(t)\,\mathrm{d}\tau$};
    \path [->,thick] (a) edge (b)
                     (b) edge (c);
  \end{tikzpicture}
\caption{}
\label{fig:}
\end{subfigure}
\caption{}
\label{fig:}
\end{figure}
\end{document}

```

Thanks in advance.