How can we calculate the convolution of the following function $(M(f))$ with itself $(M(f) * M(f) )$
$$ M(f)=\begin{cases}\operatorname{sgn}(f) \,,~~~|f|\leq 3w \\ 0 \,,~~~|f|>3w \end{cases}$$
where $$ \operatorname{sgn}(f)= \begin{cases}1 \,, ~~~f>0 \\ 0 \,,~~~f=0 \\ -1 \,,~~~ f<0. \end{cases}$$