  We compare the detection abilities for the relic gravitational waves by two
kinds of forthcoming cosmic microwave background radiation (CMB) experiments,
space-based Planck satellite and the various ground-based experiments.
Comparing with the ground-based experiments, Planck satellite can observe all
the CMB power spectra in all the multipole range, but having much larger
instrumental noises. We find that, for the uncertainty of the tensor-to-scalar
ratio $\Delta r$, PolarBear (II) as a typical ground-based experiment can give
much smaller value than Planck satellite. However, for the uncertainty of the
spectral index $\Delta n_t$, Planck can give the similar result with PolarBear
(II). If combining these two experiments, the value of $\Delta n_t$ can be
reduced by a factor 2. For the model with $r=0.1$, the constraint $\Delta
n_t=0.10$ is expected to be achieved, which provides an excellent opportunity
to study the physics in the very early universe. We also find the observation
in the largest scale ($\ell<20$) is very important for constraining the
spectral index $n_t$. So it is necessary to combine the observations of the
future space-based and ground-based CMB experiments to determine the relic
gravitational waves.
