Reed-solomon decoder with a variable number of correctable errors

An error-correcting code (ECC) or forward error correction (FEC) code are codes where each data signal conforms to specific rules of construction so that errors in the received signal can be detected and corrected. These codes are often used in computer data storage and in data transmission.

A powerful class of error-correcting codes are Reed-Solomon (RS) codes. Reed-Solomon error correction is works by constructing a polynomial from the data. The coefficients of this polynomial are sent or recorded. As long as the receiver gets many of the coefficients correctly, the receiver can recover the original polynomial even in the presence of a few erroneous coefficients.

Reed-Solomon codes are used in a wide variety of commercial applications. For example, Reed-Solomon codes are used in CDs and DVDs. Reed-Solomon codes are also used in data transmission technologies such as Digital Subscriber Loop (DSL), WiMAX, Digital Video Broadcasting (DVB), and digital television.

An embodiment may therefore comprise a Reed-Solomon decoder, comprising: a syndrome calculator that receives an input codeword and calculates a first set of syndromes; a syndrome transform that receives the first set of syndromes having a first number of syndromes and determines a second set of syndromes based on the first set of syndromes, wherein the second set of syndromes has a second number of syndromes, and wherein the first number of syndromes is greater than the second number of syndromes; and, a key equation solver that receives the second set of syndromes and produces an indication of zero or more error locations and an indication of zero or more error values.

Another embodiment may therefore comprise a method of Reed-Solomon decoding, comprising: receiving a first codeword having a first number of information symbols and a second number of redundant symbols; determining a first set of syndromes having a third number of syndromes equal to the second number plus the first number; determining a second set of syndromes, wherein the number of syndromes in the second set of syndromes is equal to the second number; and, based on the second set of syndromes, determining an indication of zero or more error locations and an indication of zero or more error values.

Another embodiment may therefore comprise. A method of transforming syndromes, comprising: receiving a first indicator of a first number of redundant symbols having a non-zero symbol value; determining a set of transform coefficients based on the first number; receiving a first set of syndromes; and, determining a second set of syndromes based on the first set of syndromes and the set of transform coefficients, wherein the first set of syndromes has a first number of syndromes, the second set of syndromes has a second number of syndromes, and the first number is greater than the second number.

FIG. 1 is a block diagram illustrating a Reed-Solomon decoder.

FIG. 2 is a flowchart illustrating a method of Reed-Solomon decoding.

FIG. 3 is a flowchart illustrating a method of transforming syndromes.

FIG. 1 is a block diagram illustrating a Reed-Solomon decoder. Reed-Solomon decoder 100 comprises: syndrome calculator 110 ; syndrome transform 120 ; key equation solver 130 ; and error correction 140 . Syndrome calculator 110 receives input codeword 101 . Input codeword 101 is comprised of a set of information symbols and a set of redundant symbols. Redundant symbols may also be known as parity symbols.

In an embodiment, input codeword 101 may also includes a set of zero symbols in redundant symbol positions. This set of zero symbols are appended to a smaller input codeword in redundant symbol positions to bring the total number of symbols in input codeword 101 up to a size that syndrome calculator 110 or Reed-Solomon decoder 100 is expecting. For example, if Reed-Solomon decoder 100 is configured to receive at most a RS(255,239) code, and input codeword 101 is a codeword that was encoded with a RS(247,239) code, then eight (8) redundant symbol positions of input codeword 101 may be set to zero symbol values.

Syndrome calculator 110 produces a first set of syndromes 111 based on input codeword 101 . Syndrome transform 120 receives the first set of syndromes 111 . Syndrome transform 120 produces a second set of syndromes 121 based on the first set of syndromes 111 . Syndrome transform 120 also receives an indicator of a maximum error limit 102 . In an embodiment, syndrome calculator 110 produces the first set of syndromes 111 based on input codeword 101 that does not have a set of zero symbols appended in redundant symbol positions.

Key equation solver 130 receives the second set of syndromes 121 . Key equation solver 130 produces at least one indication of an error location 131 and at least one indication of an error value 132 . Key equation solver 130 also receives the indicator of maximum error limit 102 . Error correction 140 receives the indication of an error location 131 and the indication of an error value 132 . Error correction 140 also receives input codeword 101 . Error correction 140 produces corrected information symbols 141 based on the indication of an error location 131 , the indication of an error value 132 , and the input codeword 101 .

The indicator of maximum error limit 102 tells syndrome transform 120 and key equation solver 130 the maximum number of actual errors that can be corrected. From this information, syndrome transform 120 and key equation solver 130 may deduce how many of the redundant symbol positions in input codeword 101 contain non-zero redundant symbols, and how many contain zero symbol values. For example, if Reed-Solomon decoder 100 is configured to receive at most a RS(255,239) code, and input codeword 101 is a codeword that was encoded with a RS(247,239) code, then the indicator of maximum error limit 102 would tell syndrome transform 120 and key equation solver 130 that at most four ((247−239)÷2=4) errors can be corrected. Syndrome transform 120 and key equation solver 130 may deduce from this that eight (255−(4*2)−247=8) redundant symbol positions of input codeword 101 would be set to zero symbol values and eight (4*2=8) would contain non-zero values.

In general, syndrome calculator 10 , key equation solver 130 and error correction 140 function like a conventional Reed-Solomon syndrome calculator, key equation solver, and error correction blocks, respectively. Key equation solver 130 may use or implement either the Berlekamp-Massey algorithm or the Euclidean algorithm. However, because key equation solver 130 receives the indication of maximum error limit 102 , key equation solver 130 need not produce an indication of error location 131 or an indication of error value 132 for those redundant symbol positions that contained the appended zero symbol values.

Syndrome transform 120 transforms the first set of syndromes 111 into the second set of syndromes 121 . Syndrome transform 120 accomplishes this transformation by multiplying each of the first set of syndromes by a corresponding transform coefficient. These transform coefficients are based on the coefficients of the polynomial:

Ψ ⁡ ( x ) = ∏ i = 1 2 ⁢ ⁢ t - 2 ⁢ ⁢ k ⁢ ⁢ ( x - B i )

where t is the maximum number of errors that Reed-Solomon decoder 120 can be configured to correct, and k is the maximum number of errors that input codeword 101 is encoded to correct. The set of B i are indicators of the location of the additional zero value redundant symbols in input codeword 101 that were added. The Ψ(x) polynomial may be referred to as an erasure polynomial.

The value of k may be provided by, or calculated from, the indicator of maximum error limit 102 . The value of t may be calculated from the indicator of maximum error limit 102 and a maximum number of redundant symbols that Reed-Solomon decoder 100 may be configured to receive. For example, if the maximum number of redundant symbols that Reed-Solomon decoder 100 may be configured to receive is from an RS(255,239) code, then t=(255−239)/2=8.

The transform coefficients, denoted Ψ i , are determined according to the equation:

∏ i = 1 2 ⁢ ⁢ t - 2 ⁢ ⁢ k ⁢ ⁢ ( x - B i ) = ∑ i = 0 2 ⁢ ⁢ t - 2 ⁢ ⁢ k ⁢ ψ i ⁢ x 2 ⁢ ⁢ t - 2 ⁢ ⁢ k - i .

The first set of syndromes 111 is denoted S i where: S i =(S 0 , S 1 . . . S 2t-1 ). The second set of syndromes 121 is denoted {tilde over (S)}, where {tilde over (S)} i =({tilde over (S)} 0 , {tilde over (S)} 1 , . . . {tilde over (S)} 2k-1 ). The second set of syndromes 121 is created according to:

S ~ i = ∑ i = 0 2 ⁢ ⁢ t - 2 ⁢ ⁢ k ⁢ ψ r ⁢ S i + ( 2 ⁢ ⁢ t - 2 ⁢ ⁢ k ) - r ⁢ ⁢ for ⁢ ⁢ i = 0 , 1 , … ⁢ , 2 ⁢ ⁢ k - 1.

Note that because t>k, the number of syndromes in the second set of syndromes 121 is less than the number of syndromes in the first set of syndromes 111 .

FIG. 2 is a flowchart illustrating a method of Reed-Solomon decoding. The steps illustrated in FIG. 2 may be performed by one or more elements of Reed-Solomon decoder 100 .

A first codeword is received ( 202 ). For example, Reed-Solomon decoder 100 may receive an input codeword 101 that contains fewer redundant symbols than the maximum number of redundant symbols that Reed-Solomon decoder 100 may be configured to decode. Zero value symbols may be appended in redundant symbol positions ( 204 ). For example, Reed-Solomon decoder 100 , or syndrome calculator 110 , may append zero value symbols to a smaller received input codeword to produce an input codeword 101 that matches the length of the largest code that Reed-Solomon decoder 100 can decode. This block is shown in FIG. 2 with a dashed outline to explicitly indicate it is an optional step.

To illustrate optional step 204 , consider a case where the largest code that Reed-Solomon decoder 100 can decode is an RS(255,239) code. Consider further that Reed-Solomon decoder 100 receives a codeword that is encoded using a RS(251,239) code. Reed-Solomon decoder 100 or syndrome calculator 110 may append four (255−251=4) zero symbols to the received codeword. This allows syndrome calculator 110 to calculate the first set of syndromes 111 as if the received codeword was encoded with a RS(255, 239) code.

To further illustrate optional step 204 , consider a received codeword of the maximum size code RS(255,239) to be the set of symbols: M=(m 0 , m 1 , . . . , m 239 , m 240 , . . . , m 254 ) where symbols m 240 through m 254 are redundant (i.e., parity) symbols. Consider the received codeword of an RS(251,239) code to be the set of symbols: R=(r 0 , r 1 , . . . , r 239 , r 240 , . . . , r 250 ). Reed-Solomon decoder 100 or syndrome calculator 110 may append zero symbols to the received codeword such that: {tilde over (R)}=(r 0 , r 1 , . . . , r 239 , r 240 , . . . , r 250 , 0, 0, 0, 0) which is a size that matches the input of syndrome calculator 110 . Thus, syndrome calculator 110 may calculate the first set of syndromes as if the received codeword was encoded with a RS(255, 239) code.

A first set of syndromes is determined ( 206 ). For example, syndrome calculator 110 may produce a first set of syndromes 111 . Syndrome calculator 110 may produce the first set of syndromes 111 from a shortened input codeword 101 . Optionally, syndrome calculator 110 may produce the first set of syndromes 111 from an input codeword that has zero symbol values appended in redundant symbol positions.

A second set of syndromes with fewer syndromes than the first set of syndromes is determined ( 208 ). For example, syndrome transform 120 may produce the second set of syndromes 121 based on the first set of syndromes 111 . The second set of syndromes 121 may have a fewer number of syndromes than the first set of syndromes 111 .

In an embodiment, the second set of syndromes is determined by multiplying each of the first set of syndromes by a corresponding transform coefficient. These transform coefficients are based on the coefficients of the polynomial:

where t is one-half of the quantity of the number of redundant symbols in the received codeword plus the number of zero symbols that optionally may be appended to the received codeword, and k is one-half the number of redundant symbols in the received codeword. The set of B i are indicators of the location of optionally appended zero value redundant symbols. The Ψ(x) polynomial may be referred to as an erasure polynomial.

The transform coefficients, denoted ψ i , are determined according to the equation:

∏ i = 1 2 ⁢ ⁢ t - 2 ⁢ ⁢ k ⁢ ⁢ ( x - B i ) = ∑ i = 0 2 ⁢ ⁢ t - 2 ⁢ ⁢ k ⁢ ψ i ⁢ x 2 ⁢ ⁢ t - 2 ⁢ ⁢ k - i .

The first set of syndromes may be denoted S i , where: S i =(S 0 , S 1 , . . . S 2t-1 ). The second set of syndromes may be denoted {tilde over (S)} i where S i =(S 0 , S 1 , . . . {tilde over (S)} 2k-1 ). The second set of syndromes may be created according to:

S ~ i = ∑ i = 0 2 ⁢ ⁢ t - 2 ⁢ ⁢ k ⁢ ψ r ⁢ S i + ( 2 ⁢ ⁢ t - 2 ⁢ ⁢ k ) - r ⁢ ⁢ for ⁢ ⁢ i = 0 , 1 , … ⁢ , 2 ⁢ ⁢ k - 1.

Note that because t>k, the number of syndromes in the second set of syndromes is less than the number of syndromes in the first set of syndromes.

Based on the second set of syndromes, zero or more error locations and zero or more error values are determined ( 210 ). For example, based on the second set of syndromes 121 , key equation solver 130 may determine zero or more error locations and zero or more error values. The zero or more error locations and zero or more error values may be sent to an error correction function. The zero or more error locations and zero or more error values may be sent to the error correction function using zero or more indicators of error location and zero or more indicators of error value.

FIG. 3 is a flowchart illustrating a method of transforming syndromes. The steps illustrated in FIG. 3 may be performed by one or more elements of Reed-Solomon decoder 100 . For example, the steps illustrated in FIG. 3 may be performed by syndrome transform 120 .

An indicator of a first number of redundant symbols with a non-zero value is received ( 302 ). For example, syndrome transform 120 may receive indicator of maximum error limit 102 . The indicator of maximum error limit 102 may tell syndrome transform 120 the maximum number of actual errors that can be corrected. From this information, syndrome transform 120 may deduce how many of the redundant symbol positions in an input codeword contain non-zero redundant symbols, and how many contain zero symbol values.

To illustrate, consider a Reed-Solomon decoder that is configured to receive at most a RS(255,239) code. This decoder receives an input codeword that was encoded with a RS(247,239) code. The indicator of maximum error limit would tell the system that at most four ((247−239)÷2=4) errors can be corrected. From this result, it may be deduced that eight (255−(4*2)−247=8) redundant symbol positions of the input are set to zero symbol values and eight (4*2=8) would contain non-zero values.

An erasure polynomial is evaluated based on the first number to determine a set of transform coefficients ( 304 ). For example, the erasure polynomial

Ψ ⁡ ( x ) = ∏ i = 1 2 ⁢ ⁢ t - 2 ⁢ ⁢ k ⁢ ⁢ ( x - B i )

may be evaluated according to the equation:

∏ i = 1 2 ⁢ ⁢ t - 2 ⁢ ⁢ k ⁢ ⁢ ( x - B i ) = ∑ i = 0 2 ⁢ ⁢ t - 2 ⁢ ⁢ k ⁢ ψ i ⁢ x 2 ⁢ ⁢ t - 2 ⁢ ⁢ k - i

to produce a set of transform coefficients ψ=(ψ 0 , ψ 1 , . . . , ψ 2t-2k ) where t is error correcting capability of the largest code that the Reed-Solomon decoder can decode, and k is the error correcting capability of the Reed-Solomon code of a received codeword. The set of B i are indicators of the location of the zero value redundant symbols that are appended to a received codeword.

A first set of syndromes are received ( 306 ). For example, syndrome transform 120 may receive a first set of syndromes 111 . These syndromes may be the result of calculating syndromes for a shorter codeword that has zero value symbols appended to it to make a longer codeword.

A second set of syndromes is determined based on the first set of syndromes and the set of transform coefficients by multiplying at least one of the set of transformation coefficients by at least one of the first set of syndromes ( 308 ). For example, syndrome transform 120 may determine a second set of syndromes 121 based on the first set of syndromes 111 . Syndrome transform 120 may create the second set of syndromes by multiplying at least one of the set of transform coefficients created in step 304 by at least one of the first set of syndromes 111 .

The first set of syndromes may be denoted S i , where: S i =(S 0 , S 1 , . . . S 2t-1 ). The second set of syndromes may be denoted {tilde over (S)} i where: {tilde over (S)} i =({tilde over (S)} 0 , {tilde over (S)} 1 , . . . {tilde over (S)} 2k-1 ). The second set of syndromes may be determined according to the equation:

S ~ i = ∑ i = 0 2 ⁢ ⁢ t - 2 ⁢ ⁢ k ⁢ ψ r ⁢ S i + ( 2 ⁢ ⁢ t - 2 ⁢ ⁢ k ) - r ⁢ ⁢ for ⁢ ⁢ i = 0 , 1 , … ⁢ , 2 ⁢ ⁢ k - 1.

Note that because t>k, the number of syndromes in the second set of syndromes is less than the number of syndromes in the first set of syndromes.

The foregoing description of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and other modifications and variations may be possible in light of the above teachings. The embodiment was chosen and described in order to best explain the principles of the invention and its practical application to thereby enable others skilled in the art to best utilize the invention in various embodiments and various modifications as are suited to the particular use contemplated. It is intended that the appended claims be construed to include other alternative embodiments of the invention except insofar as limited by the prior art.