Caption away from the figure

Question

I have made a block diagram using the \framebox command. A large white space exists between the caption and bottom of the figure. How can I reduce it? Code:

\begin{figure*}[!t]
\setlength{\unitlength}{0.13in} % selecting unit length
\centering % used for centering Figure
\begin{picture}(48,8.2) % picture environment with the size (dimensions) % 32 length units wide, and 15 units high.
\put(3,4){\framebox(4,6){\parbox{1cm}{Symbol\\ Mapping} }}
\put(10,4){\framebox(5,6){\parbox{1cm}{Serial\\ to\\ Parallel\\ Converter} }}
\put(19,4){\framebox(4,6){\parbox{1cm}{IFFT} }}
\put(27,4){\framebox(5,6){\parbox{1cm}{Parallel to Serial \\Converter} }}
\put(36,4){\framebox(4,6){\parbox{1cm}{Cyclic Prefix} }}
\put(43,4){\framebox(4,6){\parbox{1cm}{DAC} }}
\put(0,7){\vector(1,0){3}}\put(7,7){\vector(1,0){3}}
\put(15,5){\vector(1,0){4}} \put(17,6.4){.} \put(17,7.15){.} \put(15,7.8){\vector(1,0){4}}\put(15,9.2){\vector(1,0){4}}
\put(23,5){\vector(1,0){4}} \put(25,6.4){.} \put(25,7.15){.} \put(23,8){\vector(1,0){4}}\put(23,9){\vector(1,0){4}}
\put(32,7){\vector(1,0){4}} \put(40,7){\vector(1,0){3}}\put(47,7){\vector(1,0){3}}
\put(0,8) {\parbox{1cm}{Input\\Data} } \put(8,8) {\textbf{X}}
\put(16.2,5.2){${\small X}_{{\small N-1}}$} \put(16.2,8.2){${\small X}_{{\small 1}}$} \put(16.2,9.4){${\small X}_{{\small 0}}$} 
\put(24.2,5.2){${\small x}_{{\small N-1}}$} \put(24.2,8.2){${\small x}_{{\small 1}}$} \put(24.2,9.4){${\small x}_{{\small 0}}$}
\put(47.4,8) {x(t)}
\end{picture}
\caption{Block Diagram of OFDM transmitter} % title of the Figure
\label{Model} % label to refer figure in text
\end{figure*}

Answer 1

For debugging put fbox around your picture (also, please always post complete documents)

Here you see that the y coordinates you have used for your \put are all too high, the items will over-print the text above the picture, and leave a big space below.

I also removed the [!t] which mainly acts to prevent page floats and makes the figure harder to place.

\documentclass[twocolumn]{article}
\begin{document}
\begin{figure*}
\setlength{\unitlength}{0.13in} % selecting unit length
\centering % used for centering Figure
\fbox{\begin{picture}(48,8.2) % picture environment with the size (dimensions) % 32 length units wide, and 15 units high.
\put(3,4){\framebox(4,6){\parbox{1cm}{Symbol\\ Mapping} }}
\put(10,4){\framebox(5,6){\parbox{1cm}{Serial\\ to\\ Parallel\\ Converter} }}
\put(19,4){\framebox(4,6){\parbox{1cm}{IFFT} }}
\put(27,4){\framebox(5,6){\parbox{1cm}{Parallel to Serial \\Converter} }}
\put(36,4){\framebox(4,6){\parbox{1cm}{Cyclic Prefix} }}
\put(43,4){\framebox(4,6){\parbox{1cm}{DAC} }}
\put(0,7){\vector(1,0){3}}\put(7,7){\vector(1,0){3}}
\put(15,5){\vector(1,0){4}} \put(17,6.4){.} \put(17,7.15){.} \put(15,7.8){\vector(1,0){4}}\put(15,9.2){\vector(1,0){4}}
\put(23,5){\vector(1,0){4}} \put(25,6.4){.} \put(25,7.15){.} \put(23,8){\vector(1,0){4}}\put(23,9){\vector(1,0){4}}
\put(32,7){\vector(1,0){4}} \put(40,7){\vector(1,0){3}}\put(47,7){\vector(1,0){3}}
\put(0,8) {\parbox{1cm}{Input\\Data} } \put(8,8) {\textbf{X}}
\put(16.2,5.2){${\small X}_{{\small N-1}}$} \put(16.2,8.2){${\small X}_{{\small 1}}$} \put(16.2,9.4){${\small X}_{{\small 0}}$} 
\put(24.2,5.2){${\small x}_{{\small N-1}}$} \put(24.2,8.2){${\small x}_{{\small 1}}$} \put(24.2,9.4){${\small x}_{{\small 0}}$}
\put(47.4,8) {x(t)}
\end{picture}}
\caption{Block Diagram of OFDM transmitter} % title of the Figure
\label{Model} % label to refer figure in text
\end{figure*}

\end{document}

Answer 2

I'm not really sure what causes this big gap between the picture and the caption (maybe the figure environment itself also adds a space) but what you could do if its just one case is putting following command before the \begin{figure}

\begingroup\setlength{\abovecaptionskip}{-6ex}

and after you end the figure you write

\endgroup

This sets the space above a caption to -6 ex and should push the caption within a reasonable distance of your figure.

But I'm sure there are cleaner solutions.

Also maybe worth noting to have a look at the tikz package which makes drawing a lot easier in my opinion.