  Millimeter wave (mmWave) communication is a promising technology for 5G
cellular systems. To compensate for the severe path loss in mmWave systems,
large antenna arrays are generally used to achieve significant beamforming
gains. However, due to the high hardware and power consumption cost associated
with radio frequency (RF) chains, it is desirable to achieve the large-antenna
gains, but with only limited number of RF chains for mmWave communications. To
this end, we study in this paper a new lens antenna array enabled mmWave MIMO
communication system. We first show that the array response of the proposed
lens antenna array at the receiver/transmitter follows a "sinc" function, where
the antenna with the peak response is determined by the angle of arrival
(AoA)/departure (AoD) of the received/transmitted signal. By exploiting this
unique property of lens antenna arrays along with the multi-path sparsity of
mmWave channels, we propose a novel low-cost and capacity-achieving MIMO
transmission scheme, termed \emph{orthogonal path division multiplexing
(OPDM)}. For channels with insufficiently separated AoAs and/or AoDs, we also
propose a simple \emph{path grouping} technique with group-based small-scale
MIMO processing to mitigate the inter-path interference. Numerical results are
provided to compare the performance of the proposed lens antenna arrays for
mmWave MIMO system against that of conventional arrays, under different
practical setups. It is shown that the proposed system achieves significant
throughput gain as well as complexity and hardware cost reduction, both making
it an appealing new paradigm for mmWave MIMO communications.
