\titleformat{\chapter}{\normalfont\normalsize\bfseries}{\thechapter.}{1em}{} \titleformat{\section}{\normalfont\normalsize\bfseries}{\thesection.}{1em}{} \titleformat{\subsection}{\normalfont\normalsize\bfseries}{\thesubsection.}{1em}{} \titleformat{\subsubsection}{\normalfont\normalsize\bfseries}{\thesubsubsection.}{1em}{} \titlelabel{\thetitle.\quad}
I'm using titlesec,

\subsection{Outage probability for cooperative transmission using DF method under co-channel interference effect}