  This paper presents a new view of multi-user (MU) hybrid massive
multiple-input and multiple-output (MIMO) systems from array signal processing
perspective. We first show that the instantaneous channel vectors corresponding
to different users are asymptotically orthogonal if the angles of arrival
(AOAs) of users are different. We then decompose the channel matrix into an
angle domain basis matrix and a gain matrix. The former can be formulated by
steering vectors and the latter has the same size as the number of RF chains,
which perfectly matches the structure of hybrid precoding. A novel hybrid
channel estimation is proposed by separately estimating the angle information
and the gain matrix, which could significantly save the training overhead and
substantially improve the channel estimation accuracy compared to the
conventional beamspace approach. Moreover, with the aid of the angle domain
matrix, the MU massive MIMO system can be viewed as a type of non-orthogonal
angle division multiple access (ADMA) to simultaneously serve multiple users at
the same frequency band. Finally, the performance of the proposed scheme is
validated by computer simulation results.
