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This short paper explores density evolution (DE) for low-density parity-check(LDPC) codes at signal-to-noise-ratios (SNRs) that are significantly above thedecoding threshold. The focus is on the additive white Gaussian noise channeland LDPC codes in which the variable nodes have regular degree.  Prior work, using DE, produced results in the error floor region which wereasymptotic in the belief-propagation decoder's log-likelihood ratio (LLR)values. We develop expressions which closely approximate the LLR growthbehavior at moderate LLR magnitudes. We then produce bounds on the meanextrinsic check-node LLR values required, as a function of SNR, such that thegrowth rate of the LLRs exceeds that of a particular trapping set's internalLLRs such that its error floor contribution may be eliminated. We find that ourpredictions for the mean LLRs to be accurate in the error floor region, but thepredictions for the LLR variance to be lacking beyond several initialiterations.

LDPC Code Density Evolution in the Error Floor Region