  We investigate the beam squint effect in uniform planar arrays (UPAs) and
propose an efficient hybrid beamforming (HBF) design to mitigate the beam
squint in multiple-input multiple-output orthogonal frequency-division
multiplexing (MIMO-OFDM) systems operating at terahertz band. We first analyze
the array gain and derive the closed-form beam squint ratio that characterizes
the severity of the beam squint effect on UPAs. The effect is shown to be more
severe with a higher fractional bandwidth, while it can be significantly
mitigated when the shape of a UPA approaches a square. We then focus on the HBF
design that maximizes the system spectral efficiency. The design problem is
challenging due to the frequency-flat nature and hardware constraints of the
analog beamformer. We overcome the challenges by proposing an efficient
decoupling design in which the digital and analog beamformers admit closed-form
solutions, which facilitate practical implementations. Numerical results
validate our analysis and show that the proposed HBF design is robust to beam
squint, and thus, it outperforms the state-of-the-art methods in wideband
massive MIMO systems.
