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\begin{document}
\input{preamble.tex}
\lecture{10}{October 13, 2004}{Madhu Sudan}{Jesse Kamp}
\section{Overview}
\begin{itemize}
\item Binary Codes
\begin{itemize}
\item Concatenation
\item Algorithmic Issues
\end{itemize}
\end{itemize}
\section{Recap}
So far, we have studied 3 classes of codes:
\benum
\item Hamming Codes
\bitem
\item $d=3$, $n\rightarrow \infty$, $\frac{d}{n}\rightarrow \infty$ (as $n \rightarrow \infty$).
\eitem
\item Reed-Solomon Codes
\bitem
\item $[n,k,n-k+1]_n$ codes.
\item Alphabet size $q=n\rightarrow \infty$ as $n \rightarrow \infty$.
\eitem
\item Reed-Muller
\bitem
\item $[n,\frac{n}{(2m)^m},\frac{n}{2}]_{n^\frac{1}{m}}$ codes (roughly).
\item Either $q\rightarrow \infty$ or $\frac{k}{n}\rightarrow \infty$ as $n\rightarrow \infty$.
\item Hadamard Codes (special case of Reed-Muller)
\bitem
\item $[n,\log n,\frac{n}{2}]_2$ codes.
\item $\frac{k}{n}\rightarrow 0$ as $n\rightarrow \infty$.
\eitem
\eitem
\eenum
\textbf{Big Question:} Can we get $q=2$ and $\frac{k}{n},\frac{d}{n}>0$ explicitly?
\section{Reed-Solomon Codes on CDs}
We now show how Reed-Solomon codes are used to encode the data on CDs.
First, the alphabet size is chosen to be $q=n=2^t$ for some integer
$t$. We start out with a $[n,\frac{n}{2},\frac{n}{2}]_n$ code. But
now, since $n$ is a power of $2$, we can write every element of $\F_n$
as a $t$-bit vector over $\F_2$ (using $\F_{2^t}\equiv \F_2^t$). This
transforms the original code into a $[nt,\frac{nt}{2},\frac{n}{2}]_2$
code. The distance comes from the fact that for each field element
which differs in the original code, we must have at least one location
differ in the corresponding $t$-bit vectors. Since $t=\log n$, this
code has parameters $[n\log n,\frac{n\log n}{2},\frac{n}{2}]_2$. Note
that in transforming the RS code into a binary code, we lose a lot in
the relative distance.
Now, instead of looking at distance $n/2$, we see what happens when we
perform the same procedure with constant distance $d$ (think of
$d=5$). We start out with a $[n,n-d+1,d]_n$ RS code and we end up
with a $[n\log n,n\log n-d\log n+\log n,d]_2$ code. Letting $N=n\log
n$ and thinking of $\log N ~ \log n$, this code is $[N,N-(d-1)\log
N,d]_2$.
For $d=3$, we can compare this with the Hamming code, which is
$[N,N-\log N,3]_2$. In comparison, we get $[N,N-2\log N,3]_2$, so
we're only off by the factor of $2$ in $k$.
We can also ask what we should be losing asymptotically. The best
known codes for constant $d$ are the BCH codes, which are
$[N,N-\frac{d-1}{2}\log N,d]_2$ codes. As in the Hamming case, we're
only off by a factor of $2$ in $k$.
Putting this all together, we see that for constant $d$, we get pretty
close to optimal using RS encoding even using the naive method of
encoding each field element as a $t$-bit vector. However, for $d$ a
constant fraction of $n$, we didn't do nearly as well. This loss
comes directly from this naive method of encoding field elements. To
do better, we can use error correcting codes to encode the field
elements. This leads to the idea of concatenated codes.
\section{Concatenated Codes}
\begin{figure}[h]
\begin{center}
\includegraphics[width=.9\textwidth]{concat}
\end{center}
\end{figure}
The above picture shows how a concatenated code works. First, we
encode with $E_1$ (RS, for example). Then viewing each of the field
elements in the codeword as $\log n$ bit vectors, we can encode each
of them separately with a second code $E_2$, which maps $\log n$ bits to $2\log n$ bits.
Now suppose $E_1$ is our $[n,\frac{n}{2},\frac{n}{2}]_n$ RS code (as
before) and that $\textrm{image}(E_2)=[2\log n,\log n,\frac{\log
n}{10}]_2$, which is about what we'd get randomly. The distance in
the concatenated code is the product of the distances, which is
$\frac{n}{2}\frac{\log n}{10}=\frac{n\log n}{20}$. Thus the
concatenated code is $[2n\log n,\frac{n\log n}{2},\frac{n\log
n}{20}]_2$. We lose $\frac12$ the rate and distance over $E_2$, but
we get the benefit of a much longer code.
We can search for $E_2$ efficiently. Since searching takes time
exponential in the length of the code, which is $O(\log n)$, it can be
done in polynomial time. This result was discovered by Forney in 1966.
\begin{theorem} [Forney] In polynomial time in $n$, we can find an ``asymptotically good'' code of length $n$, rate $Rn$, and distance $\delta n$, for constants $R,\delta>0$.
\end{theorem}
\begin{figure}
\begin{center}
\includegraphics{gvforney}
\caption{The above figure shows how the Forney codes compare to the Gilbert-Varshamov bound.}
\end{center}
\end{figure}
This is good, but the question is, can we be even more explicit than
this? We would like to be able to give the generator matrix $G$
implicitly by an algorithm to computer $G_{i,j}$ in time
$\poly(|i|,|j|)=\poly(\log n)$. However, finding the generator matrix
for $E_2$ takes time $\poly(n)$.
\section{Justesen Construction}
Forney's insight was that there exists a space of codes
$C_1,...,C_{\poly(n)}$ such that for some $i$, $C_i$ is a $[2\log
n,\log n,\frac{\log n}{10}]_2$ code.
Justesen later improved on this and showed that there
exists a space of codes $C_1,...,C_n$ such that for $90\%$ of all $i$'s
(roughly), $C_i$ is a $[2\log n,\log n,\frac{\log n}{10}]_2$ code.
(Note that it's not hard to get size exactly $n$ here.)
Now note that there's no reason that $E_2$ needs to be the same
everywhere. So we can apply $C_i$ from the Justesen construction to
the $i$th vector. Now $10\%$ of the columns have bad $C_i$, but we're
no worse of than if we'd started without these columns in the first
place, in which case we'd still have a $[n,\frac{n}{2},.4n]_n$ code.
Thus concatenating our $[n,\frac{n}{2},\frac{n}{2}]_n$ RS code with
$(C_1,...,C_n)$ gives us a $[2n\log n,\frac{n\log n}{2},\frac{.4n\log
n}{10}]_2$ code.
This gives us an improvement since unlike in the Forney case, we don't
have to search for a good $C_i$. Thus this is more explicit, and the
generator matrix can be implicitly computed. (Note that it's not
immediately clear how to compute in $\poly(\log n)$ time, but it
certainly can be done in logspace.)
\section{Justesen Ensemble $\Leftarrow$ Wozencraft}
The construction of Justesen uses a code earlier given by Wozencraft.
We index each of the codes by some $\alpha \in \F_q$, so we have
$\set{C_\alpha}_{\alpha \in \F_q}$. (Here $\F_{q=2^t}\equiv \F_2^t$.)
The encoding process is as follows:
\[
C_\alpha: x\in \F_2^t \rightarrow x'\in \F_{2^t} \rightarrow (x',\alpha\cdot x')\in \F_{2^t}^2 \rightarrow (x,\alpha(x))\in \F_2^{2t}
\]
It can be shown that for ``most'' $\alpha$, $C_\alpha$ is good. Combining $\set{C_\alpha}_{\alpha \in \F_q}$ with the $[q,\frac{q}{2},\frac{q}{2}]_q$ RS ``outer'' code, we get the Justesen bound $[2q\log q,\frac{q\log q}{2},...]_2$.
Figure 2 illustrates the final encoding process.
\begin{figure}[h]
\begin{center}
\psfrag{qbits}{{\LARGE $\frac{q}{2}\log q$ bits}}
\psfrag{pxdef}{{\LARGE $p(x)=\sum_{C_i} x^i$}}
\psfrag{p1}{$p(\alpha_1)$}
\psfrag{p2}{$p(\alpha_2)$}
\psfrag{a1p}{$\alpha_1p(\alpha_1)$}
\psfrag{a2p}{$\alpha_2p(\alpha_2)$}
\psfrag{pa}{{\LARGE $p(\alpha)$}}
\psfrag{phata}{{\LARGE $\hat{p}(\alpha)$}}
\psfrag{phatdef}{{\LARGE $\hat{p}(x)=xp(x)$}}
\resizebox{.9\textwidth}{!}{\includegraphics{encoding}}
\caption{Justesen encoding process}
\end{center}
\end{figure}
Now note that all we've really done is use two polynomials instead of
just one. Though this seems like a simple variation on the RS code,
we don't know how to reason about this directly without going through
the concatenation argument.
\section{Decoding Concatenated Codes}
The basic idea for decoding is to do a brute force search to find the
nearest codeword in the inner code. This allows us to decode the
inner code in time $\poly(n)$. Once we've done this, we can then
decode the outer (RS) code.
\subsection{Forney's Decoding}
\bitem
\item $[n,k,d]_q$ - outer code: RS $\rightarrow$ can correct $\frac{d}{2}$ errors.
\item $[n',k',d']_2$ - inner code (with $2^{k'}=q$) $\rightarrow$ can correct $\frac{d'}{2}$ errors in time $\poly(2^{k'})$.
\item $[nn',kk',dd']_2$ - concatenated code $\rightarrow$ can correct $\left(\frac{d}{2}\right)\left(\frac{d'}{2}\right) = \frac{dd'}{4}$ errors in ``poly'' time.
\item Would like to be correct $\frac{dd'}{2}$ errors.
\eitem
Forney's idea was to first observe that we can decode RS codes with
$e$ errors and $s$ erasures provided $e<\frac{d-s}{2}$. When decoding
the inner code, we can tell how many errors have happened (since there
must have been at least as many errors as the distance to the nearest
codeword, which we find by search). Let $d_i$ be the distance between
the $i$th vector in the recovered word and the closest codeword. Then
let $e_i=\min\{\frac{d'}{2},d_i\}$. Then instead of outputting the
actual decoded vector $v_i$ always, output $v_i$ with probability
$1-\frac{e_i}{d'/2}$ and output an erasure with probability
$\frac{e_i}{d'/2}$. The idea is that this forces the adversary to
throw more errors into each vector. For example, if the adversary
just put the minimum $\frac{d'}{2}$ errors into each vector, then we'd
always note this as an erasure instead of an error and thus be better
able to correct from it.
\end{document}