  This work demonstrates the sensitivity of lens antenna arrays operating at
millimeter-wave (mmWave) frequencies. Considering a Rotman lens array in
receive mode, our investigation focuses on its most imperative defect:
aberration of electromagnetic (EM) energy. Aberration leads to spillover of
electric fields to neighboring ports, reducing the lens' ability to focus the
EM energy to a desired port. With full EM simulations, we design a 28 GHz, 13
beam and 13 array port Rotman lens array to characterize its performance with
the aforementioned impairment. Our findings show that the impact of aberration
is more pronounced when the beam angles are close to the array end-fire. More
critically, the corresponding impact of aberration on the desired signal and
interference powers is also investigated for an uplink multiuser cellular
system operating at 28 GHz. The presented results can be used as a reference to
re-calibrate our expectations for Rotman lens arrays at mmWave frequencies.
