\documentclass{article}
\usepackage[pdftex,active,tightpage]{preview}
\setlength\PreviewBorder{2mm}
\usepackage{tikz}
\usetikzlibrary{fit,matrix,positioning}


\begin{document}
\begin{preview}
\begin{tabular}[c]{c| c c}
$U_1$ & $b_1^1$ & \\
$U_0$ & $\Omega(b_1^1)$ &  $b_1^0$
\end{tabular}
\end{preview}
\end{document}