Construction of Irregular LDPC Code Using ACE Algorithm

The Low Density Parity Check (LDPC) codes have been widely acclaimed in recent years for their nearcapacity performance, they have not found their way into many important applications. For some cases, this is due to their increased decoding complexity relative to the classical coding techniques. For other cases, this is due to their inability to reach very low bit error rates at low signal-to-noise ratios (SNRs), a consequence of the error rate floor phenomenon associated with iterative LDPC decoders. The construction of finite-length irregular LDPC codes with low error floors is currently an attractive research problem A new metric, called extrinsic message degree (EMD), measures cycle connectivity in bipartite graphs of LDPC codes. Using an easily computed estimate of EMD, we propose a Viterbi-like algorithm that selectively avoids small cycle clusters that are isolated from the rest of the graph. , a simple but powerful heuristic design algorithm, the approximate cycle extrinsic message degree (ACE) constrained design algorithm, has recently been proposed. This algorithm is different from conventional girth conditioning by emphasizing the connectivity as well as the length of cycles. Keywords—Error floor, extrinsic message degree (EMD), graph cycles, irregular low-density parity-check (LDPC) codes, stopping sets, unstructured graph construction.