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**Coding theory** is the study of the properties of codes and their respective fitness for specific applications. Codes are used for data compression, cryptography, error detection and correction, data transmission and data storage. Codes are studied by various scientific disciplines—such as information theory, electrical engineering, mathematics, linguistics, and computer science—for the purpose of designing efficient and reliable data transmission methods. This typically involves the removal of redundancy and the correction or detection of errors in the transmitted data.

In communications and information processing, **code** is a system of rules to convert information—such as a letter, word, sound, image, or gesture—into another form or representation, sometimes shortened or secret, for communication through a communication channel or storage in a storage medium. An early example is the invention of language, which enabled a person, through speech, to communicate what they saw, heard, felt, or thought to others. But speech limits the range of communication to the distance a voice can carry, and limits the audience to those present when the speech is uttered. The invention of writing, which converted spoken language into visual symbols, extended the range of communication across space and time.

In signal processing, **data compression**, **source coding**, or **bit-rate reduction** involves encoding information using fewer bits than the original representation. Compression can be either lossy or lossless. Lossless compression reduces bits by identifying and eliminating statistical redundancy. No information is lost in lossless compression. Lossy compression reduces bits by removing unnecessary or less important information.

**Cryptography** or **cryptology** is the practice and study of techniques for secure communication in the presence of third parties called adversaries. More generally, cryptography is about constructing and analyzing protocols that prevent third parties or the public from reading private messages; various aspects in information security such as data confidentiality, data integrity, authentication, and non-repudiation are central to modern cryptography. Modern cryptography exists at the intersection of the disciplines of mathematics, computer science, electrical engineering, communication science, and physics. Applications of cryptography include electronic commerce, chip-based payment cards, digital currencies, computer passwords, and military communications.

- History of coding theory
- Source coding
- Definition
- Properties
- Principle
- Example
- Channel coding
- Linear codes
- Cryptographic coding
- Line coding
- Other applications of coding theory
- Group testing
- Analog coding
- Neural coding
- See also
- Notes
- References

There are four types of coding:^{ [1] }

- Data compression (or
*source coding*) - Error control (or
*channel coding*) - Cryptographic coding
- Line coding

Data compression attempts to remove redundancy from the data from a source in order to transmit it more efficiently. For example, Zip data compression makes data files smaller, for purposes such as to reduce Internet traffic. Data compression and error correction may be studied in combination.

**ZIP** is an archive file format that supports lossless data compression. A ZIP file may contain one or more files or directories that may have been compressed. The ZIP file format permits a number of compression algorithms, though DEFLATE is the most common. This format was originally created in 1989 and released to the public domain on February 14, 1989 by Phil Katz, and was first implemented in PKWARE, Inc.'s PKZIP utility, as a replacement for the previous ARC compression format by Thom Henderson. The ZIP format was then quickly supported by many software utilities other than PKZIP. Microsoft has included built-in ZIP support in versions of Microsoft Windows since 1998. Apple has included built-in ZIP support in Mac OS X 10.3 and later. Most free operating systems have built in support for ZIP in similar manners to Windows and Mac OS X.

In information theory, **joint source–channel coding** is the encoding of a redundant information source for transmission over a noisy channel, and the corresponding decoding, using a single code instead of the more conventional steps of source coding followed by channel coding.

Error correction adds extra data bits to make the transmission of data more robust to disturbances present on the transmission channel. The ordinary user may not be aware of many applications using error correction. A typical music CD uses the Reed-Solomon code to correct for scratches and dust. In this application the transmission channel is the CD itself. Cell phones also use coding techniques to correct for the fading and noise of high frequency radio transmission. Data modems, telephone transmissions, and the NASA Deep Space Network all employ channel coding techniques to get the bits through, for example the turbo code and LDPC codes.

In information theory and coding theory with applications in computer science and telecommunication, **error detection and correction** or **error control** are techniques that enable reliable delivery of digital data over unreliable communication channels. Many communication channels are subject to channel noise, and thus errors may be introduced during transmission from the source to a receiver. Error detection techniques allow detecting such errors, while error correction enables reconstruction of the original data in many cases.

In wireless communications, **fading** is variation of the attenuation of a signal with various variables. These variables include time, geographical position, and radio frequency. Fading is often modeled as a random process. A **fading channel** is a communication channel that experiences fading. In wireless systems, fading may either be due to multipath propagation, referred to as multipath-induced fading, weather, or shadowing from obstacles affecting the wave propagation, sometimes referred to as **shadow fading**.

The **NASA Deep Space Network** (**DSN**) is a worldwide network of U.S. spacecraft communication facilities, located in the United States (California), Spain (Madrid), and Australia (Canberra), that supports NASA's interplanetary spacecraft missions. It also performs radio and radar astronomy observations for the exploration of the Solar System and the universe, and supports selected Earth-orbiting missions. DSN is part of the NASA Jet Propulsion Laboratory (JPL). Similar networks are run by Russia, China, India, Japan and the European Space Agency.

In 1948, Claude Shannon published "A Mathematical Theory of Communication", an article in two parts in the July and October issues of the *Bell System Technical Journal*. This work focuses on the problem of how best to encode the information a sender wants to transmit. In this fundamental work he used tools in probability theory, developed by Norbert Wiener, which were in their nascent stages of being applied to communication theory at that time. Shannon developed information entropy as a measure for the uncertainty in a message while essentially inventing the field of information theory.

**Claude Elwood Shannon** was an American mathematician, electrical engineer, and cryptographer known as "the father of information theory". Shannon is noted for having founded information theory with a landmark paper, "A Mathematical Theory of Communication", that he published in 1948.

"**A Mathematical Theory of Communication**" is an article by mathematician Claude E. Shannon published in *Bell System Technical Journal* in 1948. It was renamed * The Mathematical Theory of Communication* in the 1949 book of the same name, a small but significant title change after realizing the generality of this work.

**Information** can be thought of as the resolution of uncertainty; it is that which answers the question of "what an entity is" and thus defines both its essence and nature of its characteristics. It is associated with data, as data represents values attributed to parameters, and information is data in context and with meaning attached. Information relates also to knowledge, as knowledge signifies understanding of an abstract or concrete concept.

The binary Golay code was developed in 1949. It is an error-correcting code capable of correcting up to three errors in each 24-bit word, and detecting a fourth.

In mathematics and electronics engineering, a **binary Golay code** is a type of linear error-correcting code used in digital communications. The binary Golay code, along with the ternary Golay code, has a particularly deep and interesting connection to the theory of finite sporadic groups in mathematics. These codes are named in honor of Marcel J. E. Golay whose 1949 paper introducing them has been called, by E. R. Berlekamp, the "best single published page" in coding theory.

Richard Hamming won the Turing Award in 1968 for his work at Bell Labs in numerical methods, automatic coding systems, and error-detecting and error-correcting codes. He invented the concepts known as Hamming codes, Hamming windows, Hamming numbers, and Hamming distance.

**Richard Wesley Hamming** was an American mathematician whose work had many implications for computer engineering and telecommunications. His contributions include the Hamming code, the Hamming window, Hamming numbers, sphere-packing, and the Hamming distance.

The **ACM A.M. Turing Award** is an annual prize given by the Association for Computing Machinery (ACM) to an individual selected for contributions "of lasting and major technical importance to the computer field". The Turing Award is generally recognized as the highest distinction in computer science and the "Nobel Prize of computing".

**Nokia Bell Labs** is an industrial research and scientific development company owned by Finnish company Nokia. With headquarters located in Murray Hill, New Jersey, the company operates several laboratories in the United States and around the world. Bell Labs has its origins in the complex past of the Bell System.

In 1972, Nasir Ahmed proposed the discrete cosine transform (DCT), which he developed with T. Natarajan and K. R. Rao in 1973.^{ [2] } The DCT is the most widely used lossy compression algorithm, the basis for multimedia formats such as JPEG, MPEG and MP3.

The aim of source coding is to take the source data and make it smaller.

Data can be seen as a random variable , where appears with probability .

Data are encoded by strings (words) over an alphabet .

A code is a function

(or if the empty string is not part of the alphabet).

is the code word associated with .

Length of the code word is written as

.

Expected length of a code is

The concatenation of code words .

The code word of the empty string is the empty string itself:

- is non-singular if injective.
- is uniquely decodable if injective.
- is instantaneous if is not a prefix of (and vice versa).

Entropy of a source is the measure of information. Basically, source codes try to reduce the redundancy present in the source, and represent the source with fewer bits that carry more information.

Data compression which explicitly tries to minimize the average length of messages according to a particular assumed probability model is called entropy encoding.

Various techniques used by source coding schemes try to achieve the limit of Entropy of the source. *C*(*x*) ≥ *H*(*x*), where *H*(*x*) is entropy of source (bitrate), and *C*(*x*) is the bitrate after compression. In particular, no source coding scheme can be better than the entropy of the source.

Facsimile transmission uses a simple run length code. Source coding removes all data superfluous to the need of the transmitter, decreasing the bandwidth required for transmission.

The purpose of channel coding theory is to find codes which transmit quickly, contain many valid code words and can correct or at least detect many errors. While not mutually exclusive, performance in these areas is a trade off. So, different codes are optimal for different applications. The needed properties of this code mainly depend on the probability of errors happening during transmission. In a typical CD, the impairment is mainly dust or scratches.

CDs use cross-interleaved Reed–Solomon coding to spread the data out over the disk.^{ [3] }

Although not a very good code, a simple repeat code can serve as an understandable example. Suppose we take a block of data bits (representing sound) and send it three times. At the receiver we will examine the three repetitions bit by bit and take a majority vote. The twist on this is that we don't merely send the bits in order. We interleave them. The block of data bits is first divided into 4 smaller blocks. Then we cycle through the block and send one bit from the first, then the second, etc. This is done three times to spread the data out over the surface of the disk. In the context of the simple repeat code, this may not appear effective. However, there are more powerful codes known which are very effective at correcting the "burst" error of a scratch or a dust spot when this interleaving technique is used.

Other codes are more appropriate for different applications. Deep space communications are limited by the thermal noise of the receiver which is more of a continuous nature than a bursty nature. Likewise, narrowband modems are limited by the noise, present in the telephone network and also modeled better as a continuous disturbance.^{[ citation needed ]} Cell phones are subject to rapid fading. The high frequencies used can cause rapid fading of the signal even if the receiver is moved a few inches. Again there are a class of channel codes that are designed to combat fading.^{[ citation needed ]}

The term **algebraic coding theory** denotes the sub-field of coding theory where the properties of codes are expressed in algebraic terms and then further researched.^{[ citation needed ]}

Algebraic coding theory is basically divided into two major types of codes:^{[ citation needed ]}

- Linear block codes
- Convolutional codes

It analyzes the following three properties of a code – mainly:^{[ citation needed ]}

- code word length
- total number of valid code words
- the minimum distance between two valid code words, using mainly the Hamming distance, sometimes also other distances like the Lee distance

Linear block codes have the property of linearity, i.e. the sum of any two codewords is also a code word, and they are applied to the source bits in blocks, hence the name linear block codes. There are block codes that are not linear, but it is difficult to prove that a code is a good one without this property.^{ [4] }

Linear block codes are summarized by their symbol alphabets (e.g., binary or ternary) and parameters (*n*,*m*,*d _{min}*)

- n is the length of the codeword, in symbols,
- m is the number of source symbols that will be used for encoding at once,
*d*is the minimum hamming distance for the code._{min}

There are many types of linear block codes, such as

- Cyclic codes (e.g., Hamming codes)
- Repetition codes
- Parity codes
- Polynomial codes (e.g., BCH codes)
- Reed–Solomon codes
- Algebraic geometric codes
- Reed–Muller codes
- Perfect codes

Block codes are tied to the sphere packing problem, which has received some attention over the years. In two dimensions, it is easy to visualize. Take a bunch of pennies flat on the table and push them together. The result is a hexagon pattern like a bee's nest. But block codes rely on more dimensions which cannot easily be visualized. The powerful (24,12) Golay code used in deep space communications uses 24 dimensions. If used as a binary code (which it usually is) the dimensions refer to the length of the codeword as defined above.

The theory of coding uses the *N*-dimensional sphere model. For example, how many pennies can be packed into a circle on a tabletop, or in 3 dimensions, how many marbles can be packed into a globe. Other considerations enter the choice of a code. For example, hexagon packing into the constraint of a rectangular box will leave empty space at the corners. As the dimensions get larger, the percentage of empty space grows smaller. But at certain dimensions, the packing uses all the space and these codes are the so-called "perfect" codes. The only nontrivial and useful perfect codes are the distance-3 Hamming codes with parameters satisfying (2^{r} – 1, 2^{r} – 1 – *r*, 3), and the [23,12,7] binary and [11,6,5] ternary Golay codes.^{ [4] }^{ [5] }

Another code property is the number of neighbors that a single codeword may have.^{ [6] } Again, consider pennies as an example. First we pack the pennies in a rectangular grid. Each penny will have 4 near neighbors (and 4 at the corners which are farther away). In a hexagon, each penny will have 6 near neighbors. When we increase the dimensions, the number of near neighbors increases very rapidly. The result is the number of ways for noise to make the receiver choose a neighbor (hence an error) grows as well. This is a fundamental limitation of block codes, and indeed all codes. It may be harder to cause an error to a single neighbor, but the number of neighbors can be large enough so the total error probability actually suffers.^{ [6] }

Properties of linear block codes are used in many applications. For example, the syndrome-coset uniqueness property of linear block codes is used in trellis shaping,^{ [7] } one of the best known shaping codes. This same property is used in sensor networks for distributed source coding, and in lossy compression of noisy sparse sources.^{ [8] }

The idea behind a convolutional code is to make every codeword symbol be the weighted sum of the various input message symbols. This is like convolution used in LTI systems to find the output of a system, when you know the input and impulse response.

So we generally find the output of the system convolutional encoder, which is the convolution of the input bit, against the states of the convolution encoder, registers.

Fundamentally, convolutional codes do not offer more protection against noise than an equivalent block code. In many cases, they generally offer greater simplicity of implementation over a block code of equal power. The encoder is usually a simple circuit which has state memory and some feedback logic, normally XOR gates. The decoder can be implemented in software or firmware.

The Viterbi algorithm is the optimum algorithm used to decode convolutional codes. There are simplifications to reduce the computational load. They rely on searching only the most likely paths. Although not optimum, they have generally been found to give good results in low noise environments.

Convolutional codes are used in voiceband modems (V.32, V.17, V.34) and in GSM mobile phones, as well as satellite and military communication devices.

Cryptography or cryptographic coding is the practice and study of techniques for secure communication in the presence of third parties (called adversaries).^{ [9] } More generally, it is about constructing and analyzing protocols that block adversaries;^{ [10] } various aspects in information security such as data confidentiality, data integrity, authentication, and non-repudiation ^{ [11] } are central to modern cryptography. Modern cryptography exists at the intersection of the disciplines of mathematics, computer science, and electrical engineering. Applications of cryptography include ATM cards, computer passwords, and electronic commerce.

Cryptography prior to the modern age was effectively synonymous with * encryption *, the conversion of information from a readable state to apparent nonsense. The originator of an encrypted message shared the decoding technique needed to recover the original information only with intended recipients, thereby precluding unwanted persons from doing the same. Since World War I and the advent of the computer, the methods used to carry out cryptology have become increasingly complex and its application more widespread.

Modern cryptography is heavily based on mathematical theory and computer science practice; cryptographic algorithms are designed around computational hardness assumptions, making such algorithms hard to break in practice by any adversary. It is theoretically possible to break such a system, but it is infeasible to do so by any known practical means. These schemes are therefore termed computationally secure; theoretical advances, e.g., improvements in integer factorization algorithms, and faster computing technology require these solutions to be continually adapted. There exist information-theoretically secure schemes that provably cannot be broken even with unlimited computing power—an example is the one-time pad—but these schemes are more difficult to implement than the best theoretically breakable but computationally secure mechanisms.

A line code (also called digital baseband modulation or digital baseband transmission method) is a code chosen for use within a communications system for baseband transmission purposes. Line coding is often used for digital data transport.

Line coding consists of representing the digital signal to be transported by an amplitude- and time-discrete signal that is optimally tuned for the specific properties of the physical channel (and of the receiving equipment). The waveform pattern of voltage or current used to represent the 1s and 0s of a digital data on a transmission link is called *line encoding*. The common types of line encoding are unipolar, polar, bipolar, and Manchester encoding.

This article or section may contain misleading parts.(August 2012) |

Another concern of coding theory is designing codes that help synchronization. A code may be designed so that a phase shift can be easily detected and corrected and that multiple signals can be sent on the same channel.^{[ citation needed ]}

Another application of codes, used in some mobile phone systems, is code-division multiple access (CDMA). Each phone is assigned a code sequence that is approximately uncorrelated with the codes of other phones.^{[ citation needed ]} When transmitting, the code word is used to modulate the data bits representing the voice message. At the receiver, a demodulation process is performed to recover the data. The properties of this class of codes allow many users (with different codes) to use the same radio channel at the same time. To the receiver, the signals of other users will appear to the demodulator only as a low-level noise.^{[ citation needed ]}

Another general class of codes are the automatic repeat-request (ARQ) codes. In these codes the sender adds redundancy to each message for error checking, usually by adding check bits. If the check bits are not consistent with the rest of the message when it arrives, the receiver will ask the sender to retransmit the message. All but the simplest wide area network protocols use ARQ. Common protocols include SDLC (IBM), TCP (Internet), X.25 (International) and many others. There is an extensive field of research on this topic because of the problem of matching a rejected packet against a new packet. Is it a new one or is it a retransmission? Typically numbering schemes are used, as in TCP. "RFC793". *RFCs*. Internet Engineering Task Force (IETF). September 1981.

Group testing uses codes in a different way. Consider a large group of items in which a very few are different in a particular way (e.g., defective products or infected test subjects). The idea of group testing is to determine which items are "different" by using as few tests as possible. The origin of the problem has its roots in the Second World War when the United States Army Air Forces needed to test its soldiers for syphilis.^{ [12] }

Information is encoded analogously in the neural networks of brains, in analog signal processing, and analog electronics. Aspects of analog coding include analog error correction,^{ [13] } analog data compression^{ [14] } and analog encryption.^{ [15] }

Neural coding is a neuroscience-related field concerned with how sensory and other information is represented in the brain by networks of neurons. The main goal of studying neural coding is to characterize the relationship between the stimulus and the individual or ensemble neuronal responses and the relationship among electrical activity of the neurons in the ensemble.^{ [16] } It is thought that neurons can encode both digital and analog information,^{ [17] } and that neurons follow the principles of information theory and compress information,^{ [18] } and detect and correct^{ [19] } errors in the signals that are sent throughout the brain and wider nervous system.

- Coding gain
- Covering code
- Error correction code
- Folded Reed–Solomon code
- Group testing
- Hamming distance, Hamming weight
- Lee distance
- List of algebraic coding theory topics
- Spatial coding and MIMO in multiple antenna research
- Spatial diversity coding is spatial coding that transmits replicas of the information signal along different spatial paths, so as to increase the reliability of the data transmission.
- Spatial interference cancellation coding
- Spatial multiplex coding

- Timeline of information theory, data compression, and error correcting codes

- ↑ James Irvine; David Harle (2002). "2.4.4 Types of Coding".
*Data Communications and Networks*. p. 18. ISBN 9780471808725.There are four types of coding

- ↑ Nasir Ahmed. "How I Came Up With the Discrete Cosine Transform". Digital Signal Processing, Vol. 1, Iss. 1, 1991, pp. 4-5.
- ↑ Todd Campbell. "Answer Geek: Error Correction Rule CDs".
- 1 2 Terras, Audrey (1999).
*Fourier Analysis on Finite Groups and Applications*. Cambridge University Press. ISBN 978-0-521-45718-7. - 1 2 Blahut, Richard E. (2003).
*Algebraic Codes for Data Transmission*. Cambridge University Press. ISBN 978-0-521-55374-2. - 1 2 Christian Schlegel; Lance Pérez (2004).
*Trellis and turbo coding*. Wiley-IEEE. p. 73. ISBN 978-0-471-22755-7. - ↑ Forney, G.D., Jr. (March 1992). "Trellis shaping".
*IEEE Transactions on Information Theory*.**38**(2 Pt 2): 281–300. doi:10.1109/18.119687o (inactive 2019-08-20). - ↑ Elzanaty, A.; Giorgetti, A.; Chiani, M. (2019). "Lossy Compression of Noisy Sparse Sources Based on Syndrome Encoding".
*IEEE Transactions on Communications*. doi:10.1109/TCOMM.2019.2926080. ISSN 0090-6778. - ↑ Rivest, Ronald L. (1990). "Cryptology". In J. Van Leeuwen (ed.).
*Handbook of Theoretical Computer Science*.**1**. Elsevier. - ↑ Bellare, Mihir; Rogaway, Phillip (21 September 2005). "Introduction".
*Introduction to Modern Cryptography*. p. 10. - ↑ Menezes, A. J.; van Oorschot, P. C.; Vanstone, S. A. (1997).
*Handbook of Applied Cryptography*. ISBN 978-0-8493-8523-0. - ↑ Dorfman, Robert (1943). "The detection of defective members of large populations".
*Annals of Mathematical Statistics*(14): 436–440. - ↑ Chen, Brian; Wornell, Gregory W. (July 1998). "Analog Error-Correcting Codes Based on Chaotic Dynamical Systems" (PDF).
*IEEE Transactions on Communications*.**46**(7): 881–890. CiteSeerX 10.1.1.30.4093 . doi:10.1109/26.701312. Archived from the original (PDF) on 2001-09-27. Retrieved 2013-06-30. - ↑ Hvala, Franc Novak Bojan; Klavžar, Sandi (1999). "On Analog Signature Analysis".
*Proceedings of the conference on Design, automation and test in Europe*. CiteSeerX 10.1.1.142.5853 . ISBN 1-58113-121-6. - ↑ Shujun Li; Chengqing Li; Kwok-Tung Lo; Guanrong Chen (April 2008). "Cryptanalyzing an Encryption Scheme Based on Blind Source Separation" (PDF).
*IEEE Transactions on Circuits and Systems I*.**55**(4): 1055–63. doi:10.1109/TCSI.2008.916540. - ↑ Brown EN, Kass RE, Mitra PP (May 2004). "Multiple neural spike train data analysis: state-of-the-art and future challenges".
*Nat. Neurosci*.**7**(5): 456–61. doi:10.1038/nn1228. PMID 15114358. - ↑ Thorpe, S.J. (1990). "Spike arrival times: A highly efficient coding scheme for neural networks" (PDF). In Eckmiller, R.; Hartmann, G.; Hauske, G. (eds.).
*Parallel processing in neural systems and computers*(PDF). North-Holland. pp. 91–94. ISBN 978-0-444-88390-2 . Retrieved 30 June 2013. - ↑ Gedeon, T.; Parker, A.E.; Dimitrov, A.G. (Spring 2002). "Information Distortion and Neural Coding".
*Canadian Applied Mathematics Quarterly*.**10**(1): 10. CiteSeerX 10.1.1.5.6365 . - ↑ Stiber, M. (July 2005). "Spike timing precision and neural error correction: local behavior".
*Neural Computation*.**17**(7): 1577–1601. arXiv: q-bio/0501021 . doi:10.1162/0899766053723069. PMID 15901408.

**Information theory** studies the quantification, storage, and communication of information. It was originally proposed by Claude Shannon in 1948 to find fundamental limits on signal processing and communication operations such as data compression, in a landmark paper titled "A Mathematical Theory of Communication". Its impact has been crucial to the success of the Voyager missions to deep space, the invention of the compact disc, the feasibility of mobile phones, the development of the Internet, the study of linguistics and of human perception, the understanding of black holes, and numerous other fields.

In telecommunication, a **convolutional code** is a type of error-correcting code that generates parity symbols via the sliding application of a boolean polynomial function to a data stream. The sliding application represents the 'convolution' of the encoder over the data, which gives rise to the term 'convolutional coding'. The sliding nature of the convolutional codes facilitates trellis decoding using a time-invariant trellis. Time invariant trellis decoding allows convolutional codes to be maximum-likelihood soft-decision decoded with reasonable complexity.

In telecommunication, **Hamming codes** are a family of linear error-correcting codes. Hamming codes can detect up to two-bit errors or correct one-bit errors without detection of uncorrected errors. By contrast, the simple parity code cannot correct errors, and can detect only an odd number of bits in error. Hamming codes are perfect codes, that is, they achieve the highest possible rate for codes with their block length and minimum distance of three. Richard W. Hamming invented Hamming codes in 1950 as a way of automatically correcting errors introduced by punched card readers. In his original paper, Hamming elaborated his general idea, but specifically focused on the Hamming(7,4) code which adds three parity bits to four bits of data.

In information theory, the **Hamming distance** between two strings of equal length is the number of positions at which the corresponding symbols are different. In other words, it measures the minimum number of *substitutions* required to change one string into the other, or the minimum number of *errors* that could have transformed one string into the other. In a more general context, the Hamming distance is one of several string metrics for measuring the edit distance between two sequences. It is named after the American mathematician Richard Hamming.

**Reed–Solomon codes** are a group of error-correcting codes that were introduced by Irving S. Reed and Gustave Solomon in 1960. They have many applications, the most prominent of which include consumer technologies such as CDs, DVDs, Blu-ray discs, QR codes, data transmission technologies such as DSL and WiMAX, broadcast systems such as satellite communications, DVB and ATSC, and storage systems such as RAID 6.

In information theory, a **low-density parity-check** (**LDPC**) **code** is a linear error correcting code, a method of transmitting a message over a noisy transmission channel. An LDPC is constructed using a sparse bipartite graph. LDPC codes are capacity-approaching codes, which means that practical constructions exist that allow the noise threshold to be set very close to the theoretical maximum for a symmetric memoryless channel. The noise threshold defines an upper bound for the channel noise, up to which the probability of lost information can be made as small as desired. Using iterative belief propagation techniques, LDPC codes can be decoded in time linear to their block length.

**Quantum error correction** (**QEC**) is used in quantum computing to protect quantum information from errors due to decoherence and other quantum noise. Quantum error correction is essential if one is to achieve fault-tolerant quantum computation that can deal not only with noise on stored quantum information, but also with faulty quantum gates, faulty quantum preparation, and faulty measurements.

In coding theory, **block codes** are a large and important family of error-correcting codes that encode data in blocks. There is a vast number of examples for block codes, many of which have a wide range of practical applications. The abstract definition of block codes is conceptually useful because it allows coding theorists, mathematicians, and computer scientists to study the limitations of *all* block codes in a unified way. Such limitations often take the form of *bounds* that relate different parameters of the block code to each other, such as its rate and its ability to detect and correct errors.

In coding theory, a **linear code** is an error-correcting code for which any linear combination of codewords is also a codeword. Linear codes are traditionally partitioned into block codes and convolutional codes, although turbo codes can be seen as a hybrid of these two types. Linear codes allow for more efficient encoding and decoding algorithms than other codes.

In mathematics and computer science, in the field of coding theory, the **Hamming bound** is a limit on the parameters of an arbitrary block code: it is also known as the **sphere-packing bound** or the **volume bound** from an interpretation in terms of packing balls in the Hamming metric into the space of all possible words. It gives an important limitation on the efficiency with which any error-correcting code can utilize the space in which its code words are embedded. A code that attains the Hamming bound is said to be a **perfect code**.

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.

In computing, telecommunication, information theory, and coding theory, an **error correction code,** sometimes **error correcting code,** (**ECC**) is used for controlling errors in data over unreliable or noisy communication channels. The central idea is the sender encodes the message with a redundant in the form of an ECC. The American mathematician Richard Hamming pioneered this field in the 1940s and invented the first error-correcting code in 1950: the Hamming (7,4) code. The redundancy allows the receiver to detect a limited number of errors that may occur anywhere in the message, and often to correct these errors without retransmission. ECC gives the receiver the ability to correct errors without needing a reverse channel to request retransmission of data, but at the cost of a fixed, higher forward channel bandwidth. ECC is therefore applied in situations where retransmissions are costly or impossible, such as one-way communication links and when transmitting to multiple receivers in multicast. For example, in the case of a satellite orbiting around Uranus, a retransmission because of decoding errors can create a delay of 5 hours. ECC information is usually added to mass storage devices to enable recovery of corrupted data, is widely used in modems, and is used on systems where the primary memory is ECC memory.

In coding theory, a **cyclic code** is a block code, where the circular shifts of each codeword gives another word that belongs to the code. They are error-correcting codes that have algebraic properties that are convenient for efficient error detection and correction.

In coding theory a **variable-length code** is a code which maps source symbols to a *variable* number of bits.

The **Hadamard code** is an error-correcting code named after Jacques Hadamard that is used for error detection and correction when transmitting messages over very noisy or unreliable channels. In 1971, the code was used to transmit photos of Mars back to Earth from the NASA space probe Mariner 9. Because of its unique mathematical properties, the Hadamard code is not only used by engineers, but also intensely studied in coding theory, mathematics, and theoretical computer science. The Hadamard code is also known under the names **Walsh code**, **Walsh family**, and **Walsh–Hadamard code** in recognition of the American mathematician Joseph Leonard Walsh.

In coding theory, **Hamming(7,4)** is a linear error-correcting code that encodes four bits of data into seven bits by adding three parity bits. It is a member of a larger family of Hamming codes, but the term *Hamming code* often refers to this specific code that Richard W. Hamming introduced in 1950. At the time, Hamming worked at Bell Telephone Laboratories and was frustrated with the error-prone punched card reader, which is why he started working on error-correcting codes.

In coding theory, **concatenated codes** form a class of error-correcting codes that are derived by combining an **inner code** and an **outer code**. They were conceived in 1966 by Dave Forney as a solution to the problem of finding a code that has both exponentially decreasing error probability with increasing block length and polynomial-time decoding complexity. Concatenated codes became widely used in space communications in the 1970s.

A **locally decodable code** (LDC) is an error-correcting code that allows a single bit of the original message to be decoded with high probability by only examining a small number of bits of a possibly corrupted codeword. This property could be useful, say, in a context where information is being transmitted over a noisy channel, and only a small subset of the data is required at a particular time and there is no need to decode the entire message at once. Note that locally decodable codes are not a subset of locally testable codes, though there is some overlap between the two.

Quantum block codes are useful in quantum computing and in quantum communications. The encoding circuit for a large block code typically has a high complexity although those for modern codes do have lower complexity.

- Elwyn R. Berlekamp (2014),
*Algebraic Coding Theory*, World Scientific Publishing (revised edition), ISBN 978-9-81463-589-9. - MacKay, David J. C..
*Information Theory, Inference, and Learning Algorithms*Cambridge: Cambridge University Press, 2003. ISBN 0-521-64298-1 - Vera Pless (1982),
*Introduction to the Theory of Error-Correcting Codes*, John Wiley & Sons, Inc., ISBN 0-471-08684-3. - Randy Yates,
*A Coding Theory Tutorial*.

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