  We consider the dynamics of belief propagation decoding of spatially coupled
Low-Density Parity-Check codes. It has been conjectured that after a short
transient phase, the profile of "error probabilities" along the spatial
direction of a spatially coupled code develops a uniquely-shaped wave-like
solution that propagates with constant velocity v. Under this assumption, and
for transmission over general Binary Memoryless Symmetric channels, we derive a
formula for v. We also propose approximations that are simpler to compute and
support our findings using numerical data.
