  In this paper, we analyze the error floor of quasi-cyclic (QC) low-density
parity-check (LDPC) codes decoded by the sum-product algorithm (SPA) with row
layered message-passing scheduling. For this, we develop a linear state-space
model of trapping sets (TSs) which incorporates the layered nature of
scheduling. We demonstrate that the contribution of each TS to the error floor
is not only a function of the topology of the TS, but also depends on the row
layers in which different check nodes of the TS are located. This information,
referred to as TS layer profile (TSLP), plays an important role in the
harmfulness of a TS. As a result, the harmfulness of a TS in particular, and
the error floor of the code in general, can significantly change by changing
the order in which the information of different layers, corresponding to
different row blocks of the parity-check matrix, is updated. We also study the
problem of finding a layer ordering that minimizes the error floor, and obtain
row layered decoders with error floor significantly lower than that of their
flooding counterparts. As part of our analysis, we make connections between the
parameters of the state-space model for a row layered schedule and those of the
flooding schedule. Simulation results are presented to show the accuracy of
analytical error floor estimates.
