  We present two hyperentanglement concentration schemes for two-photon states
that are partially entangled in the polarization and time-bin degrees of
freedom. The first scheme distills a maximally hyperentangled state from two
identical less-entangled states with unknown parameters via the Schmidt
projection method. The other scheme can be used to concentrate an initial state
with known parameters, and requires only one copy of the initial state for the
concentration process. Both these two protocols can be generalized to
concentrate N-photon hyperentangled Greenberger- Horne-Zeilinger states that
are simultaneously entangled in the polarization and time-bin degrees of
freedom. Our schemes require only linear optics and are feasible with current
technology. Using the time-bin degree of freedom rather than the spatial mode
degree of freedom can provide savings in quantum resources, which makes our
schemes practical and useful for long-distance quantum communication.
