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{{short description|Limit on data transfer rate}}
{{redirect|Shannon's theorem|text=Shannon's name is also associated with the [[sampling theorem]]}}
In [[information theory]], the '''noisy-channel coding theorem''' (sometimes '''Shannon's theorem''' or '''Shannon's limit'''), establishes that for any given degree of
The '''Shannon limit''' or '''Shannon capacity''' of a communication channel refers to the maximum [[Code rate|rate]] of error-free data that can theoretically be transferred over the channel if the link is subject to random data transmission errors, for a particular noise level. It was first described by Shannon (1948), and shortly after published in a book by Shannon and [[Warren Weaver]] entitled ''[[The Mathematical Theory of Communication]]'' (1949). This founded the modern discipline of [[information theory]].
== Overview ==
Stated by [[Claude Shannon]] in 1948, the theorem describes the maximum possible efficiency of [[error-correcting code|error-correcting methods]] versus levels of noise interference and data corruption. Shannon's theorem has wide-ranging applications in both communications and [[data storage device|data storage]]. This theorem is of foundational importance to the modern field of [[information theory]]. Shannon only gave an outline of the proof. The first rigorous proof for the discrete case is
The Shannon theorem states that given a noisy channel with [[channel capacity]] ''C'' and information transmitted at a rate ''R'', then if <math>R < C</math> there exist [[code]]s that allow the [[probability of error]] at the receiver to be made arbitrarily small. This means that, theoretically, it is possible to transmit information nearly without error at any rate below a limiting rate, ''C''.
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The channel capacity <math>C</math> can be calculated from the physical properties of a channel; for a band-limited channel with Gaussian noise, using the [[Shannon–Hartley theorem]].
Simple schemes such as "send the message 3 times and use a best 2 out of 3 voting scheme if the copies differ" are inefficient error-correction methods, unable to asymptotically guarantee that a block of data can be communicated free of error. Advanced techniques such as [[Reed–Solomon code]]s and, more recently, [[low-density parity-check code|low-density parity-check]] (LDPC) codes and [[turbo code]]s, come much closer to reaching the theoretical Shannon limit, but at a cost of high computational complexity. Using these highly efficient codes and with the computing power in today's [[digital signal processors]], it is now possible to reach very close to the Shannon limit. In fact, it was shown that LDPC codes can reach within 0.0045 dB of the Shannon limit (for binary [[
== Mathematical statement ==
[[Image:Noisy-channel coding theorem — channel capacity graph.png|thumb|right|300px|Graph showing the proportion of a channel’s capacity (''y''-axis) that can be used for payload based on how noisy the channel is (probability of bit flips; ''x''-axis)]]
The basic mathematical model for a communication system is the following:
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'''Theorem''' (Shannon, 1948):
: 1. For every discrete memoryless channel, the [[channel capacity]], defined in terms of the [[mutual information]] <math>I(X; Y)</math> as
:: <math>\ C = \sup_{p_X} I(X;Y)</math><ref>For a description of the "sup" function, see [[Supremum]]</ref>
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# The message W is sent across the channel.
# The receiver receives a sequence according to <math>P(y^n|x^n(w))= \prod_{i = 1}^np(y_i|x_i(w))</math>
# Sending these codewords across the channel, we receive <math>Y_1^n</math>, and decode to some source sequence if there exists exactly 1 codeword that is jointly typical with Y. If there are no jointly typical codewords, or if there are more than one, an error is declared. An error also occurs if a decoded codeword
The probability of error of this scheme is divided into two parts:
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Suppose a code of <math>2^{nR}</math> codewords. Let W be drawn uniformly over this set as an index. Let <math>X^n</math> and <math>Y^n</math> be the transmitted codewords and received codewords, respectively.
# <math>nR = H(W) = H(W|Y^n) + I(W;Y^n)</math> using identities involving entropy and
# <math>\le H(W|Y^n) + I(X^n(W);Y^{n})</math> since X is a function of W
# <math>\le 1 + P_e^{(n)}nR + I(X^n(W);Y^n)</math> by the use of [[Fano's Inequality]]
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=== Strong converse for discrete memoryless channels ===
A strong converse theorem, proven by Wolfowitz in 1957,<ref>{{cite book |first=Robert |last=Gallager
: <math>
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The technicality of [[lim inf]] comes into play when <math>\frac{1}{n}\sum_{i=1}^n C_i</math> does not converge.
== See also ==
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* [[Shannon–Hartley theorem]]
* [[Turbo code]]
== Notes ==
{{reflist}}
==References ==
*{{cite web |first=B. |last=Aazhang |title=Shannon's Noisy Channel Coding Theorem |date=2004 |work=Connections |publisher= |url=https://www.cse.iitd.ac.in/~vinay/courses/CSL858/reading/m10180.pdf}}
*{{cite
*{{cite book
*{{cite journal |last1=Feinstein |first1=Amiel |title=A new basic theorem of information theory |journal=Transactions of the IRE Professional Group on Information Theory |date=September 1954 |volume=4 |issue=4 |pages=2–22 |doi=10.1109/TIT.1954.1057459 |hdl=1721.1/4798 |bibcode=1955PhDT........12F|hdl-access=free }}
* [[David J.C. MacKay|MacKay, David J. C.]], ''[http://www.inference.phy.cam.ac.uk/mackay/itila/book.html Information Theory, Inference, and Learning Algorithms]'', [[Cambridge University Press]], 2003. {{ISBN|0-521-64298-1}} [free online]▼
*{{cite journal |first=Lars |last=Lundheim |title=On Shannon and Shannon's Formula |journal=Telektronik |volume=98 |issue=1 |pages=20–29 |date=2002 |doi= |url=http://www.cs.miami.edu/home/burt/learning/Csc524.142/LarsTelektronikk02.pdf}}
▲*{{cite
*{{cite journal|author-link=Claude E. Shannon | doi=10.1002/j.1538-7305.1948.tb01338.x | title=A Mathematical Theory of Communication | year=1948 | last1=Shannon | first1=C. E. | journal=Bell System Technical Journal | volume=27 | issue=3 | pages=379–423 }}
*{{cite book |author-link=Claude E. Shannon |first=C.E. |last=Shannon |title=A Mathematical Theory of Communication |publisher=University of Illinois Press |orig-year=1948 |date=1998 |pages= |url=http://cm.bell-labs.com/cm/ms/what/shannonday/paper.html}}
*{{cite journal |first=J. |last=Wolfowitz |title=The coding of messages subject to chance errors |journal=Illinois J. Math. |volume=1 |issue= 4|pages=591–606 |date=1957 |doi= 10.1215/ijm/1255380682|url=https://projecteuclid.org/download/pdf_1/euclid.ijm/1255380682|doi-access=free }}
{{DEFAULTSORT:Noisy-Channel Coding Theorem}}
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