# A Critical Examination of the Beam-Squinting Effect in Broadband Mobile Communication: Review Paper
## 3 Studies Related to the Beam Squint Effects
To further support the effectiveness of the proposed design, a numerical comparison with the computed Cramer Rao lower bound (CRLB) was performed. The wideband mmwave communication supported by a reconfigurable intelligent surface (RIS) suffering from the beam-squinting effect was first studied by [57]. Each component of the RIS is a nearly passive device, similar to the phase shifter [58], and it is a novel sort of \"array\" that has garnered considerable interest from both academics and the industry. Compared with a conventional antenna array, which has energy-hungry radio frequency chains, RIS is able to adaptively reflect the incident signals in the desired directions while using less power. Given that a passive reconfigurable intelligent surface (RIS) is applied in time domain and that mmwaves communications will have bandwidths up to several GHz, the phase shifts are assumed symmetrical for all frequencies. This will result in significant performance degradation. Several phase shifter design ideas were put forth to address the beam squint issue, taking into consideration both line of sight and non-line of sight instances. A near-optimal phase shift design approach was developed, which is dependent only on the long-term angle data after identifying the best phase shift for each frequency in the LoS scenario. To achieve this, the obtained upper bound of the cumulative attainable rate was maximized. For the non-line of sight (NLoS) scenario, a method based on the mean channel covariance matrix (MCCM), was presented in order to determine the common phase shift for all frequencies. When a large number of components or a high bandwidth were used in the RIS, the detailed numerical analyses showed that the beam squint would result in a performance loss of more than 3 \(\mathrm{bps}/\mathrm{Hz}\), and the effectiveness of the suggested approaches was proven to lessen the influence of the beam squint. In [59], two wideband hybrid beamforming techniques were proposed. The authors' first approach suggested dividing the array into virtual subarrays, as illustrated in Figure 12, to produce a bigger beam and provide an evenly distributed array gain across the entire operational frequency spectrum. Results showed that this technique can reduce beam squint to some extent. The second method modified the analog beamformer/combiner for a hybrid phased array transceiver on the basis of true time delay lines (TDD). Instead of replacing the phase shifters entirely, a small number of TTD lines are utilized to reduce the costs. The phase shifters were divided into groups and each group had one TTD line, as shown in Figure 13. The results of this method showed better results in removing beam squint while reducing the cost. In [60], the authors investigated the impact of beam squinting on single carrier frequency domain equalization (SC-FDE) transmission systems used in mmwave communications. In high-throughput mmwave LoS multiple-input single-output (LoS-MISO) systems, the response of a uniform linear array (ULA) antenna and the common analog beamforming vector was described as a spatial equivalent channel. It was discovered through analysis of the spatial equivalent channel's characteristics that it behaves similarly to frequency-selective fading. The spatial equivalent channel's deep fading points were eliminated using an improved analog beamforming technique based on the Zadoff-Chu (ZC) sequence. The receiver then uses low-complexity linear zero-forcing and minimal mean squared error equalizers to reduce ISI brought on by the beam squint. The simulation results show that the proposed analog beamforming based on ZC can successfully reduce the performance impact of beam squint. Figure 12: Diagram of the proposed subarray design [59]. Figure 13: Diagram of the proposed analog beamforming design with true time delay lines [59]. Beam squint in RF lens antenna was discussed in [61] for ultra-wideband millimeter-wave (mmwave) systems.