Decoding methods

In coding theory, decoding is the process of translating received messages into codewords of a given code. There have been many common methods of mapping messages to codewords. These are often used to recover messages sent over a noisy channel, such as a binary symmetric channel.

Notation

$C \subset \mathbb{F}_2^n$ is considered a binary code with the length $n$; $x,y$ shall be elements of $\mathbb{F}_2^n$; and $d(x,y)$ is the distance between those elements.

Ideal observer decoding

One may be given the message $x \in \mathbb{F}_2^n$, then ideal observer decoding generates the codeword $y \in C$. The process results in this solution:

$\mathbb{P}(y \mbox{ sent} \mid x \mbox{ received})$

For example, a person can choose the codeword $y$ that is most likely to be received as the message $x$ after transmission.

Decoding conventions

Each codeword does not have an expected possibility: there may be more than one codeword with an equal likelihood of mutating into the received message. In such a case, the sender and receiver(s) must agree ahead of time on a decoding convention. Popular conventions include:

1. Request that the codeword be resent -- automatic repeat-request
2. Choose any random codeword from the set of most likely codewords which is nearer to that.

Maximum likelihood decoding

Further information: Maximum likelihood

Given a received codeword $x \in \mathbb{F}_2^n$ maximum likelihood decoding picks a codeword $y \in C$ that maximizes

$\mathbb{P}(x \mbox{ received} \mid y \mbox{ sent})$,

that is, the codeword $y$ that maximizes the probability that $x$ was received, given that $y$ was sent. If all codewords are equally likely to be sent then this scheme is equivalent to ideal observer decoding. In fact, by Bayes Theorem,

$\begin{align} \mathbb{P}(x \mbox{ received} \mid y \mbox{ sent}) & {} = \frac{ \mathbb{P}(x \mbox{ received} , y \mbox{ sent}) }{\mathbb{P}(y \mbox{ sent} )} \\ & {} = \mathbb{P}(y \mbox{ sent} \mid x \mbox{ received}) \cdot \frac{\mathbb{P}(x \mbox{ received})}{\mathbb{P}(y \mbox{ sent})}. \end{align}$

Upon fixing $\mathbb{P}(x \mbox{ received})$, $x$ is restructured and $\mathbb{P}(y \mbox{ sent})$ is constant as all codewords are equally likely to be sent. Therefore $\mathbb{P}(x \mbox{ received} \mid y \mbox{ sent})$ is maximised as a function of the variable $y$ precisely when $\mathbb{P}(y \mbox{ sent}\mid x \mbox{ received} )$ is maximised, and the claim follows.

As with ideal observer decoding, a convention must be agreed to for non-unique decoding.

The maximum likelihood decoding problem can also be modeled as an integer programming problem.

The maximum likelihood decoding algorithm is an instance of the "marginalize a product function" problem which is solved by applying the generalized distributive law.

Minimum distance decoding

Given a received codeword $x \in \mathbb{F}_2^n$, minimum distance decoding picks a codeword $y \in C$ to minimise the Hamming distance :

$d(x,y) = \# \{i : x_i \not = y_i \}$

i.e. choose the codeword $y$ that is as close as possible to $x$.

Note that if the probability of error on a discrete memoryless channel $p$ is strictly less than one half, then minimum distance decoding is equivalent to maximum likelihood decoding, since if

$d(x,y) = d,\,$

then:

$\begin{align} \mathbb{P}(y \mbox{ received} \mid x \mbox{ sent}) & {} = (1-p)^{n-d} \cdot p^d \\ & {} = (1-p)^n \cdot \left( \frac{p}{1-p}\right)^d \\ \end{align}$

which (since p is less than one half) is maximised by minimising d.

Minimum distance decoding is also known as nearest neighbour decoding. It can be assisted or automated by using a standard array. Minimum distance decoding is a reasonable decoding method when the following conditions are met:

1. The probability $p$ that an error occurs is independent of the position of the symbol
2. Errors are independent events - an error at one position in the message does not affect other positions

These assumptions may be reasonable for transmissions over a binary symmetric channel. They may be unreasonable for other media, such as a DVD, where a single scratch on the disk can cause an error in many neighbouring symbols or codewords.

As with other decoding methods, a convention must be agreed to for non-unique decoding.

Syndrome decoding

Syndrome decoding is a highly efficient method of decoding a linear code over a noisy channel - i.e. one on which errors are made. In essence, syndrome decoding is minimum distance decoding using a reduced lookup table. This is allowed by the linearity of the code.

Suppose that $C\subset \mathbb{F}_2^n$ is a linear code of length $n$ and minimum distance $d$ with parity-check matrix $H$. Then clearly $C$ is capable of correcting up to

$t = \left\lfloor\frac{d-1}{2}\right\rfloor$

errors made by the channel (since if no more than $t$ errors are made then minimum distance decoding will still correctly decode the incorrectly transmitted codeword).

Now suppose that a codeword $x \in \mathbb{F}_2^n$ is sent over the channel and the error pattern $e \in \mathbb{F}_2^n$ occurs. Then $z=x+e$ is received. Ordinary minimum distance decoding would lookup the vector $z$ in a table of size $|C|$ for the nearest match - i.e. an element (not necessarily unique) $c \in C$ with

$d(c,z) \leq d(y,z)$

for all $y \in C$. Syndrome decoding takes advantage of the property of the parity matrix that:

$Hx = 0$

for all $x \in C$. The syndrome of the received $z=x+e$ is defined to be:

$Hz = H(x+e) =Hx + He = 0 + He = He$

Under the assumption that no more than $t$ errors were made during transmission, the receiver looks up the value $He$ in a table of size

$\begin{matrix} \sum_{i=0}^t \binom{n}{i} < |C| \\ \end{matrix}$

(for a binary code) against pre-computed values of $He$ for all possible error patterns $e \in \mathbb{F}_2^n$. Knowing what $e$ is, it is then trivial to decode $x$ as:

$x = z - e$

Partial response maximum likelihood

Main article: PRML

Partial response maximum likelihood (PRML) is a method for converting the weak analog signal from the head of a magnetic disk or tape drive into a digital signal.

Viterbi decoder

Main article: Viterbi decoder

A Viterbi decoder uses the Viterbi algorithm for decoding a bitstream that has been encoded using forward error correction based on a convolutional code. The Hamming distance is used as a metric for hard decision Viterbi decoders. The squared Euclidean distance is used as a metric for soft decision decoders.

See also

• Error detection and correction