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A BCH (Bose, Ray-Chaudhuri, Hocquenghem) code is a much studied code within the study of coding theory and more specifically error-correcting codes. In technical terms a BCH code is a multilevel, cyclic, error-correcting, variable-length digital code used to correct multiple random error patterns. BCH codes may also be used with multilevel phase-shift keying whenever the number of levels is a prime number or a power of a prime number. A BCH code in 11 levels has been used to represent the 10 decimal digits plus a sign digit.
Construction BCH codes use field theory and polynomials over finite fields. To detect errors a check polynomial can be constructed so the receiving end can detect if some errors had occurred. The BCH code with designed distance δ over the field GF(qm) is constructed by first finding a polynomial over GF(q) whose roots include δ consecutive powers of γ, some root of unity. To construct a BCH code that can detect and correct up to two errors, we use the finite field GF(16) or Z2''x''/<x4 + x + 1>. Now if α is a root of m1(x) = x4 + x + 1, then m1 is minimal for α since m1(x) = (x - α)(x - α2)(x - α4)(x - α8)=x4 + x + 1. If we wish to construct a code to correct 1 error we use m1(x). Our codewords will be all the polynomials C(x) satisfying C(x) ≡ 0 (mod m1(x)) which has roots α, α2, α4, α8. This does not allow us to choose many codewords - so we look for the minimal polynomial for the missing power of α from above - α3, and then the minimal polynomial for this is m3(x) = x4 + x3 + x2 + x + 1. which has roots α3, α6, α12, α24=α9. We take codewords having all of these as roots, so we form the polynomial m1,3(x) = m1(x)m3(x) = x8 + x7 + x6 + x4+1 and take codewords corresponding to polynomials of degree 14 such that C(x) ≡ 0 (mod m1,3(x)) So now C(α) = C(α2) = ... = C(α8) = 0. ( Now in GF(16) we have 15 nonzero elements, and thus our polynomial will be of degree 14 with 8 check and 7 information bits - we have 8 check bits since we have ( Encoding Construct our information codeword as (c14, c13, ..., c8) so our polynomial will be c14+c13+...+c8 Call this CI. We then want to find a CR such that CR=CI (mod m1,3(x))=c7+c6+...+c0 So we have the following codeword to send C(x) = CI+CR (mod m1,3(x)) = 0 For example, if we are to encode (1,1,0,0,1,1,0) CI=x14+x13+x10+x9 and using polynomial long division of m1,3(x) and CI to get CR(x), in Z2 we obtain CR to be x3+1 So then the codeword to send is (1,1,0,0,1,1,0, 0,0,0,0,1,0,0,1) Decoding BCH decoding is split into the following 4 steps The following steps are illustrated below. Suppose we receive a codeword vector r (the polynomial R(x)). If there is no error R(α)=R(α3)=0 If there is one error, ie r=c+ei where ei represents the ith basis vector for R14 So then S1=R(α)=C(α)+αi=αi S3=R(α3)=C(α3)+(α3)i =(αi)3=S13 so we can recognize one error. A change in the bit position shown by α's power will aid us correct that error. If there are two errors r=c+ei+ej then S1=R(α)=C(α)+αi+αj S3=R(α3)=C(α3)+(α3)i+(α3)j = (α3)i+(α3)j which is not the same as S13 so we can recognize two errors. Further algebra can aid us in correcting these two errors. Original source (first two paragraphs) from Federal Standard 1037C The above text has been taken from: http://bch-code.foosquare.com/ BCH Decoding algorithms Popular decoding algorithms are, Assumptions Peterson's algorithm, is the step 2, of the generalized BCH decoding procedure. We use Peterson's algorithm, to calculate the error locator polynomial coefficients of a polynomial Now the procedure of the Peterson Gorenstein Zierler algorithm, for a given BCH code designed to correct errors, is Algorithm then declare an empty error locator polynomial stop Peterson procedure. end set continue from the beginning of Peterson's decoding Factoring Error Locator polynomial Now that you have polynomial, you can find its roots in the form using, the Chiens search algorithm. The exponential powers of the primitive element , will yield the positions where errors occur in the received word; hence the name 'error locator' polynomial. Correcting Errors For the case of binary BCH, you can directly correct the received vectors, at the positions of the powers of primitive elements, of the error locator polynomial factors. Finally, just flip the bits for the received word, at these positions, and we have the corrected code word, from BCH decoding. We may also use Berlekamp-Massey algorithm for determining the error locator polynomial, and hence solve the BCH decoding problem. Simulation Results The simulation results for a AWGN BPSK system using a (63,36,5) BCH code are shown in this figure. A coding gain of almost 2 dB is observed at a bit error rate . | ||||||||
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