\documentclass[a4paper,oneside,12pt]{report}
\usepackage{amsmath}
\usepackage{tikz}
\usetikzlibrary{positioning}
\usetikzlibrary{shapes,arrows,fadings, shadows}
\usepackage{fontspec}
\setmainfont{Times New Roman}
\begin{document}A major design parameter here is the accuracy of the factor $a$, and the residual Doppler the tolerated by an adaptive equalizer, where a residual Doppler of the order $10^{-4}$ is needed to a ... equalizer to track the changes, otherwise, it diverge ... In ... it was shown that the preprocessing resampling converts the wide-band signal to a narrow-band signal, where ... can be used to eliminate the residual Doppler effect resulting from the estimation error of the factor $a$.
\tikzstyle{block} = [draw, top color=blue!40,
bottom color=blue!5, rectangle, minimum height=3em, minimum width=6em, shade, rounded corners,drop shadow]
\tikzstyle{sum} = [draw, top color=blue!40, bottom color=blue!5, circle, node distance=1cm, shade,drop shadow]
\tikzstyle{input} = [coordinate]
\tikzstyle{output} = [coordinate]
\tikzstyle{pinstyle} = [pin edge={to-,thin,black}]
\begin{figure}[h!]
\begin{center}
\begin{tikzpicture}[auto, node distance=2cm,>=latex']
%\begin{tikzpicture}[scale = 0.6]
% We start by placing the blocks
\node [input, name=input] {};
\node [block, right of=input,node distance=3cm] (Map) {Mapping};
\node [block, right of=Map,
node distance=4cm] (IDFT) {IDFT \& $+$CP};
\node [block, right of=IDFT,
node distance=4cm] (ch) {$\{h_l\}$};
\node [block, below of=ch,
node distance=3cm] (DFT) {-CP \& DFT};
\node [output, right of=ch,node distance=3cm] (output1) {};
\node [sum, below of=output1,node distance=1.5cm] (sum) {$\Sigma$};
\node [block, left of=DFT,
node distance=4cm] (FDE) {FDE};
\node [block, left of=FDE,
node distance=4cm] (Dec) {Detection};
\node [output, left of=Dec,node distance=3cm] (output2) {};
\node [input, right of=sum,node distance=2cm] (input2) {};
%\node [output, name=output, right of=IDFT] {};
% We draw an edge between the controller and system block to
% calculate the coordinate u. We need it to place the measurement block.
%\draw [->] (controller) -- node[name=u] {$u$} (system);
%\node [block, below of=u] (measurements) {Measurements};
%
%% Once the nodes are placed, connecting them is easy.
\draw [draw,->] (input) -- node {$\{b_k\}$} (Map);
\draw [draw,->] (Map) -- node {$\{X_k\}$} (IDFT);
\draw [draw,->] (IDFT) -- node {$\{x_n\}$} (ch);
\draw [draw,->] (ch) -| node {} (sum);
%\draw [draw,->] (sum) -- node {} (output1);
\draw [draw,->] (sum) |- node [pos=0.8]{$\{y_n\}$} (DFT);
\draw [draw,->] (DFT) -- node {$\{Y_k\}$} (FDE);
\draw [draw,->] (FDE) -- node {$\{\tilde{X}_k\}$} (Dec);
\draw [draw,->] (Dec) -- node {$\{\hat{b}_k\}$} (output2);
\draw [draw,->] (input2) -- node {$\{z_n\}$} (sum);
%\draw [->] (sum) -- node {$e$} (controller);
%\draw [->] (system) -- node [name=y] {$y$}(output);
%\draw [->] (y) |- (measurements);
%\draw [->] (measurements) -| node[pos=0.99] {$-$}
%node [near end] {$y_m$} (sum);
\end{tikzpicture}
\caption{OFDM block diagram}
\label{Fig:OFDMBD}
\end{center}
\end{figure}
A major design parameter here is the accuracy of the factor $a$, and the residual Doppler the tolerated by an adaptive equalizer, where a residual Doppler of the order $10^{-4}$ is needed to a .. equalizer to track the changes, otherwise, it diverge ... In ... it was shown that the preprocessing resampling converts the wide-band signal to a narrow-band signal, where ... can be used to eliminate the residual Doppler effect resulting from the estimation error of the factor $a$.
\end{document}
