Constant-weight code
In coding theory, a constant-weight code, also called an m of n code, is an error detection and correction code where all codewords share the same . The code and the balanced code are two widely used kinds of constant-weight code.
The theory is closely connected to that of (such as and ). Most of the work on this very vital field of is concerned with binary constant-weight codes.
Binary constant-weight codes have several applications, including frequency hopping in networks. Most use a binary constant-weight code to simplify automatically setting the threshold. Most use either a constant-weight code, or a nearly-constant-weight . In addition to use as error correction codes, the large space between code words can also be used in the design of such as .
Constant-weight codes, like Berger codes, can detect all unidirectional errors.
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A(n,d,w)
The central problem regarding constant-weight codes is the following: what is the maximum number of codewords in a binary constant-weight code with length <math>n</math>, <math>d</math>, and weight <math>w</math>? This number is called <math>A(n,d,w)</math>.
Apart from some trivial observations, it is generally impossible to compute these numbers in a straightforward way. Upper bounds are given by several important theorems such as the and , and better upper bounds can sometimes be found in other ways. Lower bounds are most often found by exhibiting specific codes, either with use of a variety of methods from discrete mathematics, or through heavy computer searching. A large table of such record-breaking codes was published in 1990, and an extension to longer codes (but only for those values of <math>d</math> and <math>w</math> which are relevant for the GSM application) was published in 2006. used in delay insensitive circuits. For these codes, <math>n=N,~ d=2,~ w=1</math> and <math>A(n, d, w) = n</math>.
Some of the more notable uses of one-hot codes include uses a 1 of 2 code; uses a 1 of n code; , etc.
Balanced code
In coding theory, a balanced code is a forward error correction code for which each codeword contains an equal number of zero and one bits. Balanced codes have been introduced by ; they are a subset of so-called unordered codes, which are codes having the property that the positions of ones in a codeword are never a subset of the positions of the ones in another codeword. Like all unordered codes, balanced codes are suitable for the detection of all in an encoded message. Balanced codes allow for particularly efficient decoding, which can be carried out in parallel.
Some of the more notable uses of balanced-weight codes include uses a 1 of 2 code; uses a 4 of 8 code; the Hadamard code is a <math>2^k/2</math> of <math>2^k</math> code (except for the zero codeword), the code; etc.
m of n codes
An m of n code is a separable code with a code word length of n bits, where each code word contains exactly m instances of a “one.” A single bit error will cause the code word to have either m + 1 or m – 1 “ones”. An example m-of-n code is the used by the .
The simplest implementation is to append a string of ones to the original data until it contains m ones, then append zeros to create a code of length n.
Example:
| Original 3 data bits | Appended bits |
|---|---|
| 000 | 111 |
| 001 | 110 |
| 010 | 110 |
| 011 | 100 |
| 100 | 110 |
| 101 | 100 |
| 110 | 100 |
| 111 | 000 |
Some of the more notable uses of constant-weight codes, other than the one-hot and balanced-weight codes already mentioned above, include uses a 3 of 9 code; code uses a 2 of 7 code, the , etc.
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