  We investigate broad-band emission from supernova ejecta powered by a
relativistic wind from a central compact object. A recent two-dimensional
hydrodynamic simulation studying the dynamical evolution of supernova ejecta
with a central energy source has revealed that outermost layers of the ejecta
are accelerated to mildly relativistic velocities because of the breakout of a
hot bubble driven by the energy injection. The outermost layers decelerate as
they sweep a circumstellar medium surrounding the ejecta, leading to the
formation of the forward and reverse shocks propagating in the circumstellar
medium and the ejecta. While the ejecta continue to release the internal energy
as thermal emission from the photosphere, the energy dissipation at the forward
and reverse shock fronts gives rise to non-thermal emission. We calculate light
curves and spectral energy distributions of thermal and non-thermal emission
from central engine powered supernova ejecta embedded in a steady stellar wind
with typical mass loss rates for massive stars. The light curves are compared
with currently available radio and X-ray observations of hydrogen-poor
superluminous supernovae, as well as the two well-studied broad-lined Ic
supernovae, 1998bw and 2009bb, which exhibit bright radio emission indicating
central engine activities. We point out that upper limits on radio luminosities
of nearby superluminous supernovae may indicate the injected energy is mainly
converted to thermal radiation rather than creating mildly relativistic flows
owing to photon diffusion time scales comparable to the injection time scale.
