  Lens-based beam-forming antennas offer a low-power, low-cost alternative to
hybrid beamforming antenna arrays. They are ideally suited to millimeter-wave
massive MIMO systems due to their native beam-space operation and angular
selectivity and minimal dependence of high-speed data converters. We discuss
the design of a compact and low-cost lens-based beam-forming antenna for small
form-factor platforms such as small-cells and mobile devices in 5G wireless
networks. We discuss a gradient-index design method and low-cost fabrication
method based on perforated dielectrics. We discuss the need for high-contrast
permittivity ranges to achieve wide scan angles which are essential for
leveraging the full capability of massive MIMO systems (e.g., full stream
capacity). Finally, we show that by using an appropriately designed perforated
medium, gradient-index lenses with low minimum permittivity of 1.25 can achieve
a maximum beam-steering angle of 44 degrees. We suggest that such an approach
can enable practical low-loss, low-cost, and compact beam-steering lens
antennas for millimeter-wave MIMO with wide beam-steering angles.
