A maximum distance separable code, or MDS code, is a way of encoding data so that the distance between code words is as large as possible for a given data capacity. This post will explain what that means and give examples of MDS codes.
A linear block code takes a sequence of k symbols and encodes it as a sequence of n symbols. These symbols come from an alphabet of size q. For binary codes, q = 2. But for non-trivial MDS codes, q > 2. More on that below.
The purpose of these codes is to increase the ability to detect and correct transmission errors while not adding more overhead than necessary. Clearly n must be bigger than k, but the overhead n-k has to pay for itself in terms of the error detection and correction capability it provides.
The ability of a code to detect and correct errors is measured by d, the minimum distance between code words. A code has separation distance d if every pair of code words differs in at least d positions. Such a code can detect up to d errors per block and can correct ⌊(d-1)/2⌋ errors.
The following example is not an MDS code but it illustrates the notation above.
The extended Golay code used to send back photos from the Voyager missions has q = 2 and [n, k, d] = [24, 12, 8]. That is, data is divided into segments of 12 bits and encoded as 24 bits in such a way that all code blocks differ in at least 8 positions. This allows up to 8 bit flips per block to be detected, and up to 3 bit flips per block to be corrected.
(If 4 bits were corrupted, the result could be equally distant between two valid code words, so the error could be detected but not corrected with certainty.)
There is a theorem that says that for any linear code
k + d ≤ n + 1.
This is known as the singleton bound. MDS codes are optimal with respect to this bound. That is,
k + d = n + 1.
So MDS codes are optimal with respect to the singleton bound, analogous to how perfect codes are optimal with respect to the Hamming bound. There is a classification theorem that says perfect codes are either Hamming codes or trivial with one exception. There is something similar for MDS codes.
MDS codes are essentially either Reed-Solomon codes or trivial. This classification is not as precise as the analogous classification of perfect codes. There are variations on Reed-Solomon codes that are also MDS codes. As far as I know, this accounts for all the known MDS codes. I don’t know that any others have been found, or that anyone has proved that there are no more.
Trivial MDS codes
What are these trivial codes? They are the codes with 0 or 1 added symbols, and the duals of these codes. (The dual of an MDS code is always an MDS code.)
If you do no encoding, i.e. take k symbols and encode them as k symbols, then d = 1 because different code words may only differ in one symbol. In this case n = k and so k + d = n + 1, i.e. the singleton bound is exact.
You could take k data symbols and add a checksum. If q = 2 this would be a parity bit. For a larger alphabet of symbols, it could be the sum of the k data symbols mod q. Then if two messages differ in 1 symbol, they also differ in added checksum symbol, so d = 2. We have n = k + 1 and so again k + d = n + 1.
The dual of the code that does no encoding is the code that transmits no information! It has only one code word of size n. You could say, vacuously, that d = n because any two different code words differ in all n positions. There’s only one code word so k = 1. And again k + d = n + 1.
The dual of the checksum code is the code that repeats a single data symbol n times. Then d = n because different code words differ in all n positions. We have k = 1 since there is only one information symbol per block, and so k + d = n + 1.
Reed Solomon codes
So the stars of the MDS show are the Reed-Solomon codes. I haven’t said how to construct these codes because that deserves a post of its own. Maybe another time. For now I’ll just say a little about how they are used in application.
As mentioned above, the Voyager probes used a Golay code to send back images. However, after leaving Saturn the onboard software was updated to use Reed-Solomon encoding. Reed-Solomon codes are used in more down-to-earth applications such as DVDs and cloud data storage.
Reed-Solomon codes are optimal block codes in terms of the singleton bound, but block codes are not optimal in terms of Shannon’s theorem. LDPC (low density parity check) codes come closer to the Shannon limit, but some forms of LDPC encoding use Reed-Solomon codes as a component. So in addition to their direct use, Reed-Solomon codes have found use as building blocks for other encoding schemes.