  The properties of linear Alfv\'en, slow, and fast magnetoacoustic waves for
uniform plasmas in relativistic magnetohydrodynamics (MHD) are discussed,
augmenting the well-known expressions for their phase speeds with knowledge on
the group speed. A 3+1 formalism is purposely adopted to make direct comparison
with the Newtonian MHD limits easier and to stress the graphical representation
of their anisotropic linear wave properties using the phase and group speed
diagrams. By drawing these for both the fluid rest frame and for a laboratory
Lorentzian frame which sees the plasma move with a three-velocity having an
arbitrary orientation with respect to the magnetic field, a graphical view of
the relativistic aberration effects is obtained for all three MHD wave
families. Moreover, it is confirmed that the classical Huygens construction
relates the phase and group speed diagram in the usual way, even for the lab
frame viewpoint. Since the group speed diagrams correspond to exact solutions
for initial conditions corresponding to a localized point perturbation, their
formulae and geometrical construction can serve to benchmark current
high-resolution algorithms for numerical relativistic MHD.
