  We study the error probability performance of rotated lattice constellations
in frequency-flat Nakagami-$m$ block-fading channels. In particular, we use the
sphere lower bound on the underlying infinite lattice as a performance
benchmark. We show that the sphere lower bound has full diversity. We observe
that optimally rotated lattices with largest known minimum product distance
perform very close to the lower bound, while the ensemble of random rotations
is shown to lack diversity and perform far from it.
