BCH code: Difference between revisions

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One of the key features of BCH codes is that during code design, there is a precise control over the number of symbol errors correctable by the code. In particular, it is possible to design binary BCH codes that can correct multiple bit errors. Another advantage of BCH codes is the ease with which they can be decoded, namely, via an [[Abstract algebra|algebraic]] method known as [[syndrome decoding]]. This simplifies the design of the decoder for these codes, using small low-power electronic hardware.

BCH codes are used in applications such as satellite communications,<ref>{{cite web|title=Phobos Lander Coding System: Software and Analysis|url=http://ipnpr.jpl.nasa.gov/progress_report/42-94/94V.PDF |archive-url=https://ghostarchive.org/archive/20221009/http://ipnpr.jpl.nasa.gov/progress_report/42-94/94V.PDF |archive-date=2022-10-09 |url-status=live|access-date=25 February 2012}}</ref> [[compact disc]] players, [[DVD]]s, [[Disk storage|disk drives]], [[USB flash drive]]s, [[solid-state drive]]s,<ref>{{cite book|chapter=BCH Codes for Solid-State-Drives|doi=10.1007/978-981-13-0599-3_11 |chapter-url=https://link.springer.com/chapter/10.1007/978-981-13-0599-3_11|access-date=23 September 2023 |title=Inside Solid State Drives (SSDS) |series=Springer Series in Advanced Microelectronics |date=2018 |last1=Marelli |first1=Alessia |last2=Micheloni |first2=Rino |volume=37 |pages=369–406 |isbn=978-981-13-0598-6 }}</ref> and [[Bar codesBarcode|two-dimensional bar codes]].

== Definition and illustration ==

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====Peterson–Gorenstein–Zierler algorithm====

<!-- this confuses t (max number of errors that can be corrected) with ν (actual number of errors) -->

[[Peterson's algorithm]] is the step 2 of the generalized BCH decoding procedure. Peterson's algorithm is used to calculate the error locator polynomial coefficients <math> \lambda_1 , \lambda_2, \dots, \lambda_{v} </math> of a polynomial

: <math> \Lambda(x) = 1 + \lambda_1 x + \lambda_2 x^2 + \cdots + \lambda_v x^v .</math>

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:<math>R(x) = C(x) + x^{13} + x^5 = x^{14} + x^{11} + x^{10} + x^9 + x^5 + x^4 + x^2</math>

In order to correct the errors, first calculate the syndromes. Taking <math>\alpha = 0010,</math> we have <math>s_1 = R(\alpha^1) = 1011,</math> <math>s_2 = 1001,</math> <math>s_3 = 1011,</math> <math>s_4 = 1101,</math> <math>s_5 = 0001,</math> and <math>s_6 = 1001.</math>

Next, apply the Peterson procedure by row-reducing the following [[augmented matrix]].

:<math>\left [ S_{3 \times 3} | C_{3 \times 1} \right ] =

\begin{bmatrix}s_1&s_2&s_3&s_4\\

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==== Decoding with unreadable characters ====

Suppose the same scenario, but the received word has two unreadable characters [ 1 {{color|red|0}} 0 ? 1 1 ? 0 0 {{color|red|1}} 1 0 1 0 0 ]. We replace the unreadable characters by zeros while creating the polynomial reflecting their positions <math>\Gamma(x) = \left(\alpha^8x - 1\right)\left(\alpha^{11}x - 1\right).</math> We compute the syndromes <math>s_1=\alpha^{-7}, s_2=\alpha^{1}, s_3=\alpha^{4}, s_4=\alpha^{2}, s_5=\alpha^{5},</math> and <math>s_6=\alpha^{-7}.</math> (Using log notation which is independent on GF(2⁴) isomorphisms. For computation checking we can use the same representation for addition as was used in previous example. [[Hexadecimal]] description of the powers of <math>\alpha</math> are consecutively 1,2,4,8,3,6,C,B,5,A,7,E,F,D,9 with the addition based on bitwise xor.)

Let us make syndrome polynomial