# 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

Henceforth, could have been considered a code with the length ; shall be elements of ; and would be

## Ideal observer decoding

One may be given the message , then **ideal observer decoding** generates the codeword . The process results in this solution:

For example, a person can choose the codeword that is most likely to be received as the message 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:

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

## Maximum likelihood decoding

Given a received codeword **maximum likelihood decoding** picks a codeword to maximize:

i.e. choose the codeword that maximizes the probability that was received, given that was sent. Note that if all codewords are equally likely to be sent then this scheme is equivalent to *ideal observer decoding*.
In fact, by Bayes Theorem we have

Upon fixing , is restructured and is constant as all codewords are equally likely to be sent. Therefore is maximised as a function of the variable precisely when is maximised, and the claim follows.

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

The ML decoding problem can also be modeled as an integer programming problem.^{[1]}

The ML decoding algorithm has been found to be an instance of the MPF problem which is solved by applying the generalized distributive law. ^{[2]}

## Minimum distance decoding

Given a received codeword , **minimum distance decoding** picks a codeword to minimise the Hamming distance :

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

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

then:

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:

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. It is the linearity of the code which allows

Suppose that is a linear code of length and minimum distance with parity-check matrix . Then clearly is capable of correcting up to

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

Now suppose that a codeword is sent over the channel and the error pattern occurs. Then is received. Ordinary minimum distance decoding would lookup the vector in a table of size for the nearest match - i.e. an element (not necessarily unique) with

for all . Syndrome decoding takes advantage of the property of the parity matrix that:

for all . The *syndrome* of the received is defined to be:

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

(for a binary code) against pre-computed values of for all possible error patterns . Knowing what is, it is then trivial to decode as:

## Partial response maximum likelihood

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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

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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

## Sources

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