  This paper focuses on a new path division multiple access (PDMA) for both
uplink (UL) and downlink (DL) massive multiple-input multiple-output network
over a high mobility scenario, where the orthogonal time frequency space (OTFS)
is adopted. First, the 3D UL channel model and the received signal model in the
angle-delay-Doppler domain are studied. Secondly, the 3D-Newtonized orthogonal
matching pursuit algorithm is utilized for the extraction of the UL channel
parameters, including channel gains, directions of arrival, delays, and Doppler
frequencies, over the antenna-time-frequency domain. Thirdly, we carefully
analyze energy dispersion and power leakage of the 3D angle-delay-Doppler
channels. Then, along UL, we design a path scheduling algorithm to properly
assign angle-domain resources at user sides and to assure that the observation
regions for different users do not overlap over the 3D cubic area, i.e.,
angle-delay-Doppler domain. After scheduling, different users can map their
respective data to the scheduled delay-Doppler domain grids, and simultaneously
send the data to base station (BS) without inter-user interference in the same
OTFS block. Correspondingly, the signals at desired grids within the 3D
resource space of BS are separately collected to implement the 3D channel
estimation and maximal ratio combining-based data detection over the
angle-delay-Doppler domain. Then, we construct a low complexity beamforming
scheme over the angle-delay-Domain domain to achieve inter-user interference
free DL communication. Simulation results are provided to demonstrate the
validity of our proposed unified UL/DL PDMA scheme.
