%-----------------------Homework------------------------------------
%-------------------Arman Shokrollahi---------------------------------
%---------------------Coding Theory-------------------------------
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\begin{center}
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\vbox{\vspace{2mm}
\hbox to 6.28in { {\bf EE567:~Coding Theory \hfill} }
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\begin{document}
\homework{Homework \#1}{due 10/02/14 }{Arman Shokrollahi}{}{Student's name}{}
{\begin{tikzpicture}[outline/.style={draw=#1,thick,fill=#1!50}]
\node [outline=red] at (0,1) {\bf Problem 1};
\end{tikzpicture}}
\textit{Problem $1.1$ of Ryan/Lin} (We have changed the received word):
A single error has been added (modulo $2$) to a transmitted $(7,4)$ Hamming codeword, resulting in the received word $r = (1100 \; 001)$. Using the decoding algorithm described in the chapter, find the error.
\vskip 1em
{\fbox{\it Solution}}
We want to solve this problem in two ways.
\underline{First Way}:
\begin{center}
\begin{tikzpicture}[thin,fill opacity=0.7]
\draw [draw=none, fill=red] (90:1.5) circle (2cm);
\draw [draw=none, fill=green] (-30:1.5) circle (2cm);
\draw [draw=none, fill=blue] (210:1.5) circle (2cm);
\begin{scope}
\clip (90:1.5) circle(2cm);
\draw [draw=none, fill=yellow] (-30:1.5) circle (2cm);
\end{scope} % blue + red = magenta
\begin{scope}
\clip (210:1.5) circle(2cm);
\draw [draw=none, fill=magenta] (90:1.5) circle (2cm);
\end{scope}
\node at (4.1,0) {Circle 2};
\node at (-4.1,0) {Circle 3};
\begin{scope} % green + blue = cyan
\clip (-30:1.5) circle(2cm);
\draw [draw=none, fill=cyan] (210:1.5) circle (2cm);
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\node at (0,4.1) {Circle 1};
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\clip (210:1.5) circle(2cm);
\draw [draw=none, fill=white] (-30:1.5) circle (2cm);
\end{scope}
\node at (1,.5) {$u_0$};
\node at (0,0) {$u_2$};
\node at (1.8,-1.1) {$p_1$};
\node at (-1,.5) {$u_3$};
\node at (0,-1.2) {$u_1$};
\node at (0,2) {$p_0$};
\node at (-2,-1.2) {$p_2$};
\end{tikzpicture}
\end{center}
%
We know that ${\mathbf v}=({\mathbf{u \; p}})=(u_0u_1u_2u_3 \; p_0p_1p_2)$ has been transmitted. We have
$r = (1100 \; 001)=(r_0r_1r_2r_3 \; r_4r_5r_6)$ as the received word. We rearrange the above Venn diagram as follows
%%
\begin{center}
\begin{tikzpicture}[thin,fill opacity=0.7]
\draw [draw=none, fill=red] (90:1.5) circle (2cm);
\draw [draw=none, fill=green] (-30:1.5) circle (2cm);
\draw [draw=none, fill=blue] (210:1.5) circle (2cm);
\begin{scope}
\clip (90:1.5) circle(2cm);
\draw [draw=none, fill=yellow] (-30:1.5) circle (2cm);
\end{scope} % blue + red = magenta
\begin{scope}
\clip (210:1.5) circle(2cm);
\draw [draw=none, fill=magenta] (90:1.5) circle (2cm);
\end{scope}
\node at (4.1,0) {Circle $2$};
\node at (-4.1,0) {Circle $3$};
\begin{scope} % green + blue = cyan
\clip (-30:1.5) circle(2cm);
\draw [draw=none, fill=cyan] (210:1.5) circle (2cm);
\end{scope}
\node at (0,4.1) {Circle $1$};
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\clip (90:1.5) circle(2cm);
\clip (210:1.5) circle(2cm);
\draw [draw=none, fill=white] (-30:1.5) circle (2cm);
\end{scope}
\node at (1,.5) {$r_0=1$};
\node at (0,0) {$r_2=0$};
\node at (1.8,-1.1) {$r_5=0$};
\node at (-1,.5) {$r_3=0$};
\node at (0,-1.2) {$r_1=1$};
\node at (0,2) {$r_4=0$};
\node at (-2,-1.2) {$r_6=1$};
\end{tikzpicture}
\end{center}
Clearly, Circles $2$ and $3$, in the figure above, have even numbers of $1$'s, but Circle $1$ does not. We conclude that the error cannot be in Circles $2$ and $3$, because their rules are satisfied. So it must be $r_4=0$ that is in error. Thus, $r_4$ must be $1$. Hence, the decoded codeword is
\begin{equation}
{\mathbf{\hat{v}}}=(1100 \; 101),
\label{equ1.1}
\end{equation}
from which the decoded data ${\mathbf{\hat{u}}}=(1100)$ may be recovered. \\
%%
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%
\underline{Second Way}:
We want to solve the problem by some techniques based on generator and parity-check matrices. The parity-check matrix $H$ is helpful in correcting single errors in transmission when
\begin{enumerate}
\item[(i)] $H$ has no column of $0$'s,
\item[(ii)] no two columns of $H$ are the same.
\end{enumerate}
Consider the following matrix
$$H=\left(
\begin{array}{cccccccc}
1 & 0 & 1 & 1 & \vdots & 1 & 0 & 0 \\
1 & 1 & 0 & 1 & \vdots & 0 & 1 & 0 \\
0 & 1 & 1 & 1 & \vdots & 0 & 0 & 1
\end{array}\right).$$
It is easy to check that $H$ satisfies these two conditions and that for the number of rows ($r=3$) in $H$, we have the maximum number of columns possible. If an additional column is added, $H$ will no longer be useful for correcting single errors.\\
The generator matrix $G$ associated with $H$ is
$$G=\left(
\begin{array}{cccccccc}
1 & 0 & 0 & 0 &\vdots & 1 & 1 & 0 \\
0 & 1 & 0 & 0 & \vdots& 0 & 1 & 1 \\
0 & 0 & 1 & 0 & \vdots& 1 & 0 & 1 \\
0 & 0 & 0 & 1 &\vdots & 1 & 1 & 1 \\
\end{array}\right).$$
Consequently we have a $(7,4)$ group code. The encoding function $E:\mathbb{Z}_2^4 \rightarrow \mathbb{Z}_2^7$ encodes four-bit messages into seven-bit code words. We realize that because $H$ is determined by three parity-check equations (that is, For all $w=w_1w_2w_3w_4\in\mathbb{Z}_2^4$, and $E(w)= wG = w_1w_2w_3w_4w_5w_6w_7 \in \mathbb{Z}_2^7$, now try to find $E(w)=wG$. We get some general equations which are called the {\textit{parity-check equations}}. For more details, see pages 97, 98 of Ryan/Lin), we have now maximized the number of bits we can have in the messages (of course, under our present coding scheme). In addition, the columns of $H$, read from top to bottom, are the binary equivalents of the integers from $1$ to $7$. In general, if we start with $r$ parity-check equations, then the parity-check matrix $H$ can have as many as $2^r-1$ columns and still be used to correct single errors. We denote the transposition of $B$ by $B^{tr}$. Under these circumstances $H=[B\ | \ I_r]$, where $B$ is an $r\times (2^r-1-r)$ matrix, and $G=[I_m\ | \ B^{tr}]$ with $m=2^r-1-r$. The parity-check matrix $H$ associated with a $(2^r-1, 2^r-1-r)$ group code. \\
We want to use some terminologies which can be found on pages 103, 104, and 105 of Ryan/Lin. We now have the matrix $H$ for a Hamming $(7,4)$ code. It is easy to check that the coset leader for the syndrome $(100)$ is $(0000 \; 100)$. Why we are talking about $(100)$? because it is the syndrome corresponding to our received word $r=(1100 \; 001)$; note that
$$H \cdot r^{tr}=
\left(
\begin{array}{c}
1 \\
0 \\
0 \\
\end{array}
\right).$$
Finally, if we assume that $c$ is the transmitted word, then $c=(0000 \; 100)+(1100 \; 001)= (1100 \; 101)\stackrel{\text{\tiny\ref{equ1.1}}}{=} \bf\hat{v}$. \\
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%%------------ Arman Shokrollahi--------------%%
