Performance of a 28 GHz Two–Stage Rotman Lens Beamformer for Millimeter Wave Cellular Systems

Phase shifter-based hybrid beamforming has received a lot of attention at millimeter-wave frequencies for cellular communications. Nevertheless, the implementation complexity of such beamformers is rather high due to the complexities involved in designing and fabricating the required radio- frequency (RF) circuits. In contrast, lens-based RF beamformers significantly reduce the implementation complexity, as all active circuits can be replaced by a passive device. In this paper, we present the sum spectral efficiency performance of an uplink multiuser multiple-input multiple-output (MU-MIMO) system with a 28 GHz Rotman lens. An asymmetric two-stage stacked design is fabricated with a 15 element (3×5) uniform rectangular array feeding 9 RF down-conversion chains towards baseband. Zero-forcing processing is employed at baseband for interference nulling and multistream recovery. Our results show that the MU- MIMO gains are substantially more pronounced for the two- stage architecture relative to a single-stage design due to the inclusion of the elevation multipath components. Moreover, we show that the asymmetric design can help to further reduce the implementation complexity, since the conventional beam selection network can be omitted from the RF front-end.