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