How to type different symbols/characters in math equation in LaTeX?

Question

How can I type/show the marking A(not just etalic) in math equation?

My working example is following and the mentioned A is appeared in the last of the code(last frame)-

\documentclass{beamer}
\usepackage{tikz}
\usepackage{amsmath}
\usetheme{Madrid}
\logo{%
  %\includegraphics[width=1cm,height=1.5cm,keepaspectratio]{DUlogo}%
  \includegraphics[width=1cm,height=1.5cm,keepaspectratio]{example-image-a}%
  \hspace{\dimexpr\paperwidth-2cm-5pt}%
  %\includegraphics[width=1cm,height=1cm,keepaspectratio]{GNR.png}%
  \includegraphics[width=1cm,height=1cm,keepaspectratio]{example-image-b}%
}
\begin{document}
  \title{Cross-Layer  Resource  Allocation with  elastic service scaling in Cloud Radio access network}
  \author{
  \parbox{2.5cm}{
\centering Md.Al-Helal\\Roll:SH-51}\hspace{1cm}
\parbox{2.5cm}{
{\centering Jobayed Ullah\\Roll:EK-107}}
}
\institute{Computer Science \& Engineering\\CSEDU}
\begin{frame}
  \maketitle
\end{frame}
  \title{Cross-Layer  Resource  Allocation with  elastic service scaling in Cloud Radio access network}
  \author{Jianhua Tang\\ Wee Pen Tay\\ Tony Q. S. Quek}
\institute{IEEE Transactions on Wireless Communications, vol 14, no. 9}
\date{September 2015}
\begin{frame}
  \maketitle
\end{frame}
  \author{alhelal}
\institute{CSEDU}
\begin{frame}
  \frametitle{Delay equation}
  \begin{exampleblock}{}
    \[
      d_{i} = \frac{1}{\mu_{i}- \lambda_{i}} + \frac{1}{c_{i} - \lambda_{i}}
  %    4545
  \]
  \end{exampleblock}
  \centering
  $d = \text{total delay}$\\
$\mu_{i} = \text{service rate}$\\
$\lambda_{i} = \text{arival rate}$\\
  $c_{i} = \text{transmission rate}$\\
\end{frame}
\begin{frame}
  \frametitle{Equation}
  \begin{exampleblock}{}
   \[
     \hat{x_{i}} = \sum_{j\in{A}}^{}h_{ij}^{H}w_{ij}x_{i} + \sum_{k\neq i}^{N}\sum_{j\in{A}}^{}h_{ij}^{H}w_{kj}x_{k} + \delta_{i}
   \]
  \end{exampleblock}
\end{frame}
%\begin{frame}
%  \frametitle{Equation}
%  \begin{exampleblock}{}
%    \[
%    %SINR(A) = \frac{\lvert\sum_{j\in{A}}^{}h_{ij}^{H}w_{ij}\rvert^2}{\sigma_{i}^2 + \sum_{k\neq i}^{N}\lvet\sum_{j\in{A}}^{}h_{ij}^{H}w_{kj}\rvert}
%%    SINR(A) = \sum_{j\in{A}}^{}h_{ij}^{H}w_{ij}
%      34
%    %{\sigma_{i}^2 + \sum_{k\neq i}^{N}\lvet\sum_{j\in{A}}^{}h_{ij}^{H}w_{kj}\rvert}
%    
%  \]
%  \end{exampleblock}
%\end{frame}
\begin{frame}
  \frametitle{Equation}
  \begin{exampleblock}{}
   \[
%     \text{SINR}_{i}(A) = \frac{\lvert\sum_{j\in{A}}^{}h_{ij}^{H}w_{ij}\rvert^2}{\sigma_{i}^2 + \sum{k\neq i}^{N}
    \text{SINR}_{i}(A) = \frac{\lvert\sum_{j\in{A}}^{}h_{ij}^{H}w_{ij}\rvert^2}{\sigma_{i}^2 + \sum_{k\neq i}^{N}\lvert\sum_{j\in{A}}^{}h_{ij}^{H}w_{kj}\rvert}
   \]
  \end{exampleblock}
\end{frame}
\end{document}