IntegerOutput RS Decoder
Decode ReedSolomon code to recover integer vector data
 Library:
Communications Toolbox / Error Detection and Correction / Block
Description
The IntegerOutput RS Decoder block recovers a message vector from a ReedSolomon codeword vector. For proper decoding, the parameter values in this block must match those in the corresponding IntegerInput RS Encoder block.
The ReedSolomon code has message length K, and codeword length N – number of punctures. You specify N and K directly in the block dialog. The symbols for the code are integers in the range [0, 2^{M}1], which represent elements of the finite field GF(2^{M}). Restrictions on M and N are described in Restrictions on the M and the Codeword Length N below.
This icon shows optional ports.
The input and output are integervalued signals that represent codewords and messages, respectively. For more information, see Input and Output Signal Length in RS Blocks. The block inherits the output data type from the input data type. For information about the data types each block port supports, see Supported Data Types.
For more information on representing data for ReedSolomon codes, see the section Integer Format (ReedSolomon Only).
If the decoder is processing multiple codewords per frame, then the same puncture pattern holds for all codewords.
The default value of M is ceil(log2(N+1))
, that is, the smallest
integer greater than or equal to log2(N+1). You can change the value of M from the
default by specifying the primitive polynomial for GF(2^{M}), as
described in Specify the Primitive Polynomial below.
You can also specify the generator polynomial for the ReedSolomon code, as described in Specify the Generator Polynomial.
An (N, K) ReedSolomon code can correct up to
floor((NK)/2)
symbol errors (not bit
errors) in each codeword.
If decoding fails, the message portion of the decoder input is returned unchanged as the decoder output.
The sample times of the input and output signals are equal.
Ports
Input
In
— ReedSolomon codeword
integer column vector
ReedSolomon codeword, specified as an (N_{C}×(N – K + S – P)by1 integer column vector. N_{C} is the number of codewords, N is the Codeword length N, K is the Message length K, S is the Shortened message length S, P is the number of punctures per codeword.
For more information, see Input and Output Signal Length in RS Blocks.
Data Types: single
 double
 integer
Era
— Erasure vector
binary column vector
Erasure vector, specified as a binary column vector input signal with the same size as the input ReedSolomon codeword.
Erasure values of 1 correspond to erased bits in the same position in the codeword. Values of 0 correspond to bits that are not erased. For more information, see Puncturing and Erasures.
Dependencies
To enable this port, select Enable erasures input port.
Data Types: double
 Boolean
Output
Out
— Decoded message
integer column vector
Decoded message, returned as one of the following:
When there is no message shortening, a (N_{C}×K)by1 integer column vector.
When there is message shortening, a (N_{C}×S)by1 integer column vector.
N_{C} is the number of message words, K is the Message length K (symbols), and S is the Shortened message length S (symbols).
Note
The number of decoded message words equals the number of codewords.
For more information, see Input and Output Signal Length in RS Blocks.
Err
— Decoding errors
integer vector
Symbol decoding errors, returned as an integer vector with N_{C} elements, where N_{C} is the number of codewords. This port indicates the number of symbol errors detected during decoding of each codeword. A negative integer indicates that the block detected more errors than it could correct by using the specified coding scheme.
Note
An (N,K) ReedSolomon code
can correct up to floor((NK)/2)
symbol errors
(not bit errors) in each codeword. When a received codeword contains
more than (NK)/2 symbol
errors, a decoding failure occurs.
Dependencies
To enable this port, select Output number of corrected symbol errors.
Data Types: double
For more information, see Supported Data Types.
Parameters
Codeword length N
— Codeword length
7
(default)  integer
Codeword length, specified as an integer.
For more information, seeRestrictions on the M and the Codeword Length N and Input and Output Signal Length in RS Blocks.
Message length K
— Message word length
3
(default)  integer
Message word length, specified as an integer in the range [1, N–2], where N is the codeword length.
Shortened message length S
— Shortened message word length
3
(default)  integer
Shortened message word length, specified as an integer, such that S ≤ K. When Shortened message length S < Message length K, the ReedSolomon code is shortened.
You still specify N and K values for the fulllength (N, K) code but the decoding is shortened to an (N–K+S, S) code.
Dependencies
To enable this parameter, select Specify shortened message length.
Generator polynomial
— Generator polynomial
rsgenpoly(7, 3, [], [],
'double')
(default)  polynomial character vector  binary row vector  binary Galois row vector
Generator polynomial with values in the range [0, 2^{M}–1], in order of descending power, specified as one of the following:
A polynomial character vector. For more information, see Representation of Polynomials in Communications Toolbox.
An integer row vector that represents the coefficients of the generator polynomial in order of descending power.
An integer Galois row vector that represents the coefficients of the generator polynomial in order of descending power.
Each coefficient is an element of the Galois field defined by the primitive polynomial. For more information, see Specify the Generator Polynomial.
Example: [1 3 1 2 3]
, which is equivalent to
rsgenpoly(7,3)
Dependencies
To enable this parameter, select Specify generator polynomial.
Primitive polynomial
— Primitive polynomial
'X^3 + X + 1'
(default)  polynomial character vector  binary row vector
Primitive polynomial in order of descending power. This polynomial is of order M and defines the finite Galois field GF(2^{M}) corresponding to the integers that form message words and codewords. Specify the primitive polynomial as one of the following:
A polynomial character vector. For more information, see Representation of Polynomials in Communications Toolbox.
A binary row vector that represents the coefficients of the generator polynomial.
For more information, see Specify the Primitive Polynomial.
Example: 'X^3 + X + 1'
, which is the primitive
polynomial used for a (7,3) code, ppoly = primpoly(3,'nodisplay');
int2bit(ppoly,ceil(log2(max(ppoly))))'
Dependencies
To enable this parameter, select Specify primitive polynomial.
Puncture vector
— Puncture vector
[ones(2,1); zeros(2,1)]
(default)  binary column vector
Puncture vector, specified as an
(N–K)by1 binary column vector.
Element indices with 1
s represent data
symbol indices that pass through the block
unaltered. Element indices with 0
s represent data
symbol indices that get punctured, or removed,
from the data stream. For more information, see Puncturing and Erasures.
Note
If the encoder is processing multiple codewords per frame, then the same puncture pattern holds for all codewords.
Dependencies
To enable this parameter, select Puncture code.
Enable erasures input port
— Enable erasures input port
off (default)  on
Selecting this check box enables the erasures port, Era. For more information, see Puncturing and Erasures.
Output number of corrected symbol errors
— Enable port to output number of corrected symbol errors
off (default)  on
Selecting this check box enables an additional output port, Err, which indicates the number of symbol errors the block corrected in the input codeword.
Block Characteristics
Data Types 

Multidimensional Signals 

VariableSize Signals 

More About
Input and Output Signal Length in RS Blocks
The ReedSolomon code has a message word length, K, or shortened message word length, S. The codeword length is N – K + S – P, where N is the full codeword length and P is the number of punctures per codeword. When there is no message shortening, the codeword length expression reduces to N – P, because K = S. If the decoder is processing multiple codewords per frame, then the same puncture pattern holds for all codewords.
This table provides expressions for the input and output signal lengths for the ReedSolomon encoder and decoder.
The notation y = N_{C} × x denotes that y is an integer multiple of x.
Input, Erasure, and Output Vector Lengths  

RS Block Coder  No Message Shortening Used  Message Shortening Used 
IntegerInput RS Encoder 
Input Length (symbols): N_{C} × K Output Length (symbols): N_{C} × (N–P) 
Input Length (symbols): N_{C} × S Output Length (symbols): N_{C} × (N–K+S–P) 
IntegerOutput RS Decoder 
Input Length (symbols): N_{C} × (N–P) Erasures Length (symbols): N_{C} × (N–P) Output Length (symbols): N_{C} × K 
Input Length (symbols): N_{C} × (N–K+S–P) Erasures Length (symbols): N_{C} × (N–K+S–P) Output Length (symbols): N_{C} × S 
N is the codeword length.
K is the message word length.
S is the shortened message word length.
N_{C} is the number of codewords (and message words).
P is the number of punctures, and is equal to the number of zeros in the puncture vector.
M is the degree of the primitive polynomial. Each group of M bits represents an integer between
0
and2^{M}–1
that belongs to the finite Galois fieldGF(2^{M})
.
For more information on representing data for ReedSolomon codes, see Integer Format (ReedSolomon Only).
Restrictions on the M and the Codeword Length N
If you do not select Specify primitive polynomial, valid values for the codeword length, N, are from 7 to 65535. In this case, the block uses the default primitive polynomial of degree
M = ceil(log2(N+1))
. You can display the default primitive polynomial by runningprimpoly(ceil(log2(N+1)))
.If you select Specify primitive polynomial, valid values for the primitive polynomial degree, M, are from 3 to 16. The valid values for N in this case are from 7 to 2^{M}–1. Selecting Specify primitive polynomial enables you to specify the primitive polynomial that defines the finite field GF(2^{M}), which corresponds to the values that form message words and codewords.
Specify the Primitive Polynomial
You can specify the primitive polynomial that defines the finite field
GF(2^{M}),
corresponding to the integers that form messages and codewords. To do so, first select
Specify primitive polynomial. Then, in the Primitive
polynomial text box, enter a binary row vector that represents a primitive
polynomial over GF(2^{M}), in
descending order of powers. For example, to specify the polynomial
x^{3}+x+1, enter the vector [1 0 1 1]
.
If you do not select Specify primitive polynomial, the block uses the default primitive polynomial of degree M = ceil(log2(N+1)). You can display the default polynomial by entering primpoly(ceil(log2(N+1)))
at the MATLAB^{®} prompt.
Specify the Generator Polynomial
Select Specify generator polynomial to enable the Generator polynomial parameter for specifying the generator polynomial of the ReedSolomon code. Enter an integer row vector with element values from 0 to 2^{M}1. The vector represents a polynomial, in descending order of powers, whose coefficients are elements of GF(2^{M}) represented in integer format. For more information about integer and binary format, see Integer Format (ReedSolomon Only). The generator polynomial must be equal to a polynomial with this factored form:
g(x) = (x+α^{b})(x+α^{b+1})(x+α^{b+2})...(x+α^{b+NK1})
α is the primitive element of the Galois field over which the input message is defined, and b is an integer.
If you do not select Specify generator polynomial, the block uses the
default generator polynomial, corresponding to b=1, for ReedSolomon
encoding. You can display the default generator polynomial by running rsgenpoly
.
If you are using the default primitive polynomial (Specify primitive polynomial is not selected), the default generator polynomial is
rsgenpoly(N,K)
, whereN = 2^{M}1
.If you are not using the default primitive polynomial (Specify primitive polynomial is selected) and you specify the primitive polynomial as
poly
, the generator polynomial isrsgenpoly(N,K,poly)
.
Note
The degree of the generator polynomial is N − K, where N is the codeword length and K is the message word length.
Puncturing and Erasures
1
s and 0
s have
precisely opposite meanings for the puncture and erasure vectors.
In a puncture vector,
1
means that the data symbol is passed through the block unaltered.0
means that the data symbol is to be punctured, or removed, from the data stream.
In an erasure vector,
1
means that the data symbol is to be replaced with an erasure symbol.0
means that the data symbol is passed through the block unaltered.
These conventions apply to both the encoder and the decoder. For more information, see Shortening, Puncturing, and Erasures.
Supported Data Types
Port  Supported Data Types 

In 

Out 

Era 

Err 

Pair Block
Algorithms
This block uses the BerlekampMassey decoding algorithm. For information about this algorithm, see Algorithms for BCH and RS Errorsonly Decoding.
References
[1] Wicker, Stephen B., Error Control Systems for Digital Communication and Storage. Upper Saddle River, N.J.: Prentice Hall, 1995.
[2] Berlekamp, Elwyn R., Algebraic Coding Theory, New York: McGrawHill, 1968.
[3] Clark, George C., Jr., and J. Bibb Cain. ErrorCorrection Coding for Digital Communications, New York: Plenum Press, 1981.
Extended Capabilities
C/C++ Code Generation
Generate C and C++ code using Simulink® Coder™.
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