Why isnt my equation evalutaing correctly?

So I'm making a BCH decoder for a uni project in Python. It takes a 10 digit codeword with or without errors and is supposed to detect and correct any errors it finds. For double error correction there is a mathmatical formula to detect the position and magnitute of these errors:
formula for error pos and mag

i = (-Q + ((Q^2-4*P*R)^(1/2))/2*P)<br/>
j = (-Q - ((Q^2-4*P*R)^(1/2))/2*P)

b = (i*s1-s2)/(i-j)<br/>
a = s1-b

I have already correctly calculated QPR and s1,s2.
So, I'm debugging with this example (2 error BCH example). It has the number 8888880747 being transmitted to 8899880747 so there are 2 errors in positions 3 and 4 both by a magnitute of 1. So far my program generates the correct syndromes(s1 to s4 - 2,7,3,3) and the correct PQR values (10,7,10) but when doing the calculation for i and j I get different values than the example - 10.7 and 10.6 as opposed to 3 and 4. Here is my code for i and j:

#work out error positions i and j
sqrt = ((Q**2 - 4*P*R) % 11)**(1/2)
i = (((-Q+sqrt)/(2*P))%11)
j = (((-Q-sqrt)/(2*P))%11)

Can anyone see what I'm doing wrong? Thanks.

1 answer

  • answered 2019-11-08 14:38 James Reinstate Monica Polk

    The operations are meant to be carried in GF(11), in other words they are meant to be carried out mod 11. Square roots and division mod 11 are different than they are in the real numbers.

    There are advanced algorithms for doing both square roots and modular inverses but for numbers as small as 11 you can just use brute force or precompute a table. Go with first principles here. For sqrt(x) try [0,1,2,...10] and see which one when squared is equal to x. That is the square root. For division (inverses), 1/x, try [0,1,2,...10] and see which one when multiplied by x is equal to 1 mod 11. Now we plugin these value to get i = 3, j = 4.

    So, going through the calculations mod 11, we have:

    P, Q, R = (10,7,10) (Q**2 - 4*P*R) = -351, -351 % 11 = 1 That's convenient since the square root of 1 is just 1 mod 11. Looking at the next two equations we see we are going to need to compute 1/(2*P) mod 11, in other words we need to find the inverse of 2*P mod 11, i.e. (2P)-1 mod 11. 2*P % 11 = 20 % 11 = 9. By trying all the possibilities we find that 9-1 mod 11 is 5.