  In this paper, we investigate novel strategies for generating rate-compatible
(RC) irregular low-density parity-check (LDPC) codes with short/moderate block
lengths. We propose three puncturing and two extension schemes, which are
designed to determine the puncturing positions that minimize the performance
degradation and the extension that maximize the performance. The first
puncturing scheme employs a counting cycle algorithm and a grouping strategy
for variable nodes having short cycles of equal length in the Tanner Graph
(TG). The second scheme relies on a metric called Extrinsic Message Degree
(EMD) and the third scheme is a simulation-based exhaustive search to find the
best puncturing pattern among several random ones. In addition, we devise two
layer-structured extension schemes based on a counting cycle algorithm and an
EMD metric which are applied to design RC-LDPC codes. Simulation results show
that the proposed extension and puncturing techniques achieve greater rate
flexibility and good performance over the additive white Gaussian noise (AWGN)
channel, outperforming existing techniques.
