  Low-density parity-check (LDPC) codes are capable of achieving excellent
performance and provide a useful alternative for high performance applications.
However, at medium to high signal-to-noise ratios (SNR), an observable error
floor arises from the loss of independence of messages passed under iterative
graph-based decoding. In this paper, the error floor performance of short block
length codes is improved by use of a novel candidate selection metric in code
graph construction. The proposed Multipath EMD approach avoids harmful
structures in the graph by evaluating certain properties of the cycles which
may be introduced in each edge placement. We present Multipath EMD based
designs for several structured LDPC codes including quasi-cyclic and irregular
repeat accumulate codes. In addition, an extended class of diversity-achieving
codes on the challenging block fading channel is proposed and considered with
the Multipath EMD design. This combined approach is demonstrated to provide
gains in decoder convergence and error rate performance. A simulation study
evaluates the performance of the proposed and existing state-of-the-art
methods.
