  LDPC codes based on multiple-edge protographs potentially have larger minimum
distances compared to their counterparts, single-edge protographs. However,
considering different features of their Tanner graph, such as short cycles,
girth and other graphical structures, is harder than for Tanner graphs from
single-edge protographs. In this paper, we provide a novel approach to
construct the parity-check matrix of an LDPC code which is based on trades
obtained from block designs. We employ our method to construct two important
categories of LDPC codes; quasi-cyclic (QC) LDPC and spatially-coupled LDPC
(SC-LDPC) codes.
  We use those trade-based matrices to define base matrices of multiple-edge
protographs. The construction of exponent matrices corresponding to these base
matrices has less complexity compared to the ones proposed in the literature.
We prove that these base matrices result in QC-LDPC codes with smaller lower
bounds on the lifting degree than existing ones.
  There are three categories of SC-LDPC codes: periodic, time-invariant and
time-varying. Constructing the parity-check matrix of the third one is more
difficult because of the time dependency in the parity-check matrix. We use a
trade-based matrix to obtain the parity-check matrix of a time-varying SC-LDPC
code in which each downwards row displacement of the trade-based matrix yields
syndrome matrices of a particular time. Combining the different row shifts the
whole parity-check matrix is obtained.
  Our proposed method to construct parity-check and base matrices from trade
designs is applicable to any type of super-simple directed block designs. We
apply our technique to directed designs with smallest defining sets containing
at least half of the blocks. To demonstrate the significance of our
contribution, we provide a number of numerical and simulation results.
