This web page describes the logic inside the Check Node function. First consider a LDPC decoder as illustrated below which inputs probability values. A block diagram of the Check Node function is shown below... Gallager very cleverly derived the formula for the probability that the number of 1's in a set of probabilites is even on page 41 of his 1963 paper on LDPC codes. p(# 1's is even) = 1/2 + 1/2[(1-2p1)(1-2p2)(1-2p3)...] p(# 1's is odd) = 1 - p(# 1's is even) = -1/2 + 1/2[(1-2p1)(1-2p2)(1-2p3)...] In the above example, p1out = 1/2(1-2p2in)(1-2p3in) -1/2 p2out = 1/2(1-2p1in)(1-2p3in) -1/2 p3out = 1/2(1-2p1in)(1-2p2in) -1/2 The Check Node function could be implemented in a number of different ways...
Simulations may tell which method is the most effective. |
LDPC Codes > LDPC Decoder Examples >