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Multi-aperture phased arrays versus multi-beam lens arrays for mmW multiuser MIMO

Multi-beamforming and data multiplexing is a key functionality for realizing the spatial multiplexing gains in multiuser MIMO systems at millimeter-wave (mmW) frequencies. Due to prohibitive complexity of conventional digital beamforming, two main approaches to hybrid analog-digital beamforming are being investigated that use either phased arrays or lens arrays for analog beamforming. While the two approaches are equivalent from a purely communication-theoretic perspective, they offer very different tradeoffs in terms of performance, complexity and power consumption. In particular, all existing phased array-based prototypes are limited to a single beam per aperture, due to hardware constraints, thereby necessitating the use of multiple smaller sub-arrays for multi-beam operation. On the other hand, the beam switching network presents a hardware challenge in lens array-based systems, thereby limiting the coverage achieved by particular radio-frequency chain. In this paper, we apply the beamspace MIMO framework to compare the achievable sum-rates in a small cell served by an access point using a lens array-based or phased array-based architecture. We also discuss the hardware complexity of the two architectures to enable performance-complexity-energy optimization.