## Description

An \([n,k,n-k+1]_q\) linear code based on polynomials over \(GF(q)\). Let \(\{\alpha_1,\cdots,\alpha_n\}\) be \(n\) distinct nonzero elements of \(GF(q)\) with \(q>n\). An RS code encodes \(\mu=\{\mu_0,\cdots,\mu_{k-1}\}\) into \(\{f_\mu(\alpha_1),\cdots,f_\mu(\alpha_n)\}\), with polynomial
\begin{align}
f_\mu(x)=\mu_0+\mu_1 x + \cdots + \mu_{k-1}x^{k-1}.
\end{align}
In other words, each codeword \(\mu\) is a string of values of the corresponding polynomial \(f_\mu\) at the points \(\alpha_i\).

## Protection

Since each polynomial \(f_{\mu}\) is of degree less than \(k\), it can be determined from its values at \(k\) points. This means that RS codes can correct erasures on up to \(n-k\) registers. The resulting distance, \(d=n-k+1\), saturates the Singleton bound.

## Decoding

Berlekamp-Massey decoder [3][4].Gorenstein-Peterson-Zierler decoder [5].Berlekamp-Welch decoder [6], assuming that \(t \geq (n+k)/2\).Gao decoder using extended Euclidean algorithm [7].List decoders try to find a low-degree bivariate polynomial \(Q(x,y)\) such that evaluation of \(Q\) at \((\alpha_i,y_i)\) is zero. By choosing proper degrees, it can be shown such polynomial exists by drawing an analogy between evaulation of \(Q(\alpha_i,y_i)\) and homogenous linear equation(interpolation). Once this is done, list roots of \(y\) that agree at \(\geq t\) points. The Sudan list decoding algorithm corrects up to \(1-\sqrt{2R}\) proportion of errors [8]. The Sudan–Guruswami algorithm improves that to \(1-\sqrt{R}\) [9].

## Realizations

Cross-interleaved RS code (CIRC) is adopted in Compact Discs (CDs) and RS Product Code (RSPC) in DVDs; see Ch. 4 of Ref. [10].In DSL technologies and its variants against impluse noise [11].RS codes as outer codes concatenated with convolutional codes are used indirectly in solar exploration programs; see Ch. 3 of Ref. [10].Coded sharding designs in blockchains to increase efficiency [12].Private Information Retrieval [13].Used in QR-Codes to retrieve damaged barcodes [14].Wireless communication systems such as 3G, DVB, and WiMAX [15].Correcting pooled testing results for SARS-CoV-2 [16].

## Notes

\([n,k,n-k+1]\) RS code requires an order \(O(n^2)\) operations while encoding if a straightforward matrix multiplication is employed and \(k=\mathcal{O}(n)\). Using the FFT algorithm, complexity of evaluating a polynomial at \(n\) roots of unity becomes \(O(n\log n)\). The FFT can be generalized to finite fields and rings, which is referred as Number-theoretic Transform (NTT). However, for some values of \(n\), which can not be factorized into small primes or do not have \(n\) roots of unity, the FFT algorithm fails. Independently developed by [17][18] and generalized in Ref. [19], the additive FFT solves this problem by evaluating the polynomial at \(n-1\) roots of unity when \(n\) is power of 2.Although using iFFT has its counterpart iNNT for finite fields, the decoding is usually standard polynomial interpolation in \(k=\mathcal{O}(n\log^2 (n))\). However, in erasure decoding, encoded values are only erased in \(r\) points, which is a specific case of polynomial interpolation and can be done in \(\mathcal{O}(n\log (n)\) by computing product of the received polynomial and an erasure locator polynomial and using long division to find an original polynomial. The long division step can be omitted to increase speed further by only dividing the derivative of the product polynomial, and derivative of erasure locator polynomial evaluated at erasure locations.

## Parents

- Generalized Reed-Solomon (GRS) code
- Maximum distance separable (MDS) code — Every RS code is MDS. If \(k \leq p\), then all linear MDS codes \( [n,k,n-k+1]_{p^m} \) are RS codes [20].

## Children

## Cousins

- Hamming code
- Bose–Chaudhuri–Hocquenghem (BCH) code — Cyclic RS codes of length \(q-1\) are a particular case of BCH codes; see Remark 15.3.21 of Ref. [21].
- Cyclic code — If the length divides \(q-1\), then it is possible to construct a cyclic RS code.
- Approximate secret-sharing code — The classical information in this code is encoded using a Reed-Solomon code.
- Convolutional code — Convolutional codes are often used in concatenation with Reed-Solomon codes for communication [22].
- Galois-qudit polynomial code (QPyC) — Polynomial codes are CSS codes constructed from Reed-Solomon codes.
- Justesen code — An RS code is the outer code of Justesen codes
- Maximum-rank distance (MRD) code — MRD rank-metric codes can be thought of as matrix analogues of MDS Reed-Solomon codes as both constructions utilize a Vandermonde matrix [23].
- Prime-qudit polynomial code (QPyC) — Polynomial codes are CSS codes constructed from Reed-Solomon codes.
- Rank-modulation code — RS codes can be used to design rank modulation codes [24].

## Zoo code information

## References

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## Cite as:

“Reed-Solomon (RS) code”, The Error Correction Zoo (V. V. Albert & P. Faist, eds.), 2022. https://errorcorrectionzoo.org/c/reed_solomon