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.
翻译:我们研究了统一平面阵列(UPAs)中的波束偏移效应,并提出了一种有效的混合波束成形(HBF)设计来抑制波束偏移,用于操作太赫兹频段的多输入多输出正交频分复用(MIMO-OFDM)系统。我们首先分析了阵列增益,并推导出了闭式波束偏移比,该比值表征了波束偏移效应在UPAs上的严重程度。该效应表明,随着分数带宽的增加,效应更严重,当UPA的形状接近正方形时,可以显著抑制效应。然后,我们重点关注最大化系统谱效率的HBF设计。由于模拟波束成形器的频率平坦特性和硬件限制,设计问题具有挑战性。我们通过提出一种有效的解耦设计来克服这些挑战,在这种设计中,数字和模拟波束成形器均可以得到闭式解决方案,这有利于实际实现。数值结果验证了我们的分析,并显示所提出的HBF设计对波束偏移具有鲁棒性,并因此在宽带大规模MIMO系统中优于现有的方法。