  Millimeter wave (mmWave) communication by utilizing lens antenna arrays is a
promising technique for realizing cost-effective 5G wireless systems with large
MIMO (multiple-input multiple-output) but only limited radio frequency (RF)
chains. This paper studies an uplink multi-user mmWave single-sided lens MIMO
system, where only the base station (BS) is equipped with a full-dimensional
(FD) lens antenna array with both elevation and azimuth angle resolution
capabilities, and each mobile station (MS) employs the conventional uniform
planar array (UPA) without the lens. By exploiting the angle-dependent energy
focusing property of the lens antenna array at the BS as well as the multi-path
sparsity of mmWave channels, we propose a low-complexity path division multiple
access (PDMA) scheme, which enables virtually interference-free multi-user
communications when the angle of arrivals (AoAs) of all MS multi-path signals
are sufficiently separable at the BS. To this end, a new technique called path
delay compensation is proposed at the BS to effectively transform the
multi-user frequency-selective MIMO channels to parallel frequency-flat
small-size MIMO channels for different MSs, for each of which the
low-complexity single-carrier(SC) transmission is applied. For general
scenarios with insufficient AoA separations, analog beamforming at the MSs and
digital combining at the BS are jointly designed to maximize the achievable
sum-rate of the MSs based on their effective MIMO channels resulting from path
delay compensation. In addition, we propose a new and efficient channel
estimation scheme tailored for PDMA, which requires negligible training
overhead in practical mmWave systems and yet leads to comparable performance as
that based on perfect channel state information (CSI).
