  The millimeter wave is a promising technique for the next generation of
mobile communication. The large antenna array is able to provide sufficient
precoding gain to overcome the high pathloss at millimeter wave band. However,
the accurate channel state information is the key for the precoding design.
Unfortunately, the channel use overhead and complexity are two major challenges
when estimating the channel with high-dimensional array. In this paper, we
propose a two-stage approach which reduces the channel use overhead and the
computational complexity. Specifically, in the first stage, we estimate the
column subspace of the channel matrix. Based on the estimated column subspace,
we design the training sounders to acquire the remaining coefficient matrix of
the column subspace. By dividing the estimation task into two stages, the
training sounders for the second stages are only targeted for the column
subspace, which will save the channel uses and the computational complexity as
well.
