  In this paper we consider the generalization of binary spatially coupled
low-density parity-check (SC-LDPC) codes to finite fields GF$(q)$, $q\geq 2$,
and develop design rules for $q$-ary SC-LDPC code ensembles based on their
iterative belief propagation (BP) decoding thresholds, with particular emphasis
on low-latency windowed decoding (WD). We consider transmission over both the
binary erasure channel (BEC) and the binary-input additive white Gaussian noise
channel (BIAWGNC) and present results for a variety of $(J,K)$-regular SC-LDPC
code ensembles constructed over GF$(q)$ using protographs. Thresholds are
calculated using protograph versions of $q$-ary density evolution (for the BEC)
and $q$-ary extrinsic information transfer analysis (for the BIAWGNC). We show
that WD of $q$-ary SC-LDPC codes provides significant threshold gains compared
to corresponding (uncoupled) $q$-ary LDPC block code (LDPC-BC) ensembles when
the window size $W$ is large enough and that these gains increase as the finite
field size $q=2^m$ increases. Moreover, we demonstrate that the new design
rules provide WD thresholds that are close to capacity, even when both $m$ and
$W$ are relatively small (thereby reducing decoding complexity and latency).
The analysis further shows that, compared to standard flooding-schedule
decoding, WD of $q$-ary SC-LDPC code ensembles results in significant
reductions in both decoding complexity and decoding latency, and that these
reductions increase as $m$ increases. For applications with a near-threshold
performance requirement and a constraint on decoding latency, we show that
using $q$-ary SC-LDPC code ensembles, with moderate $q>2$, instead of their
binary counterparts results in reduced decoding complexity.
