Hybrid Beamforming for Terahertz Wireless Communications with Beam Squint: A Survey

Benefiting from the ultra-wide bandwidth, terahertz (THz) communication is considered one of the core technologies of the 6G mobile communication. However, high path loss is a severe issue for THz band, which needs to be solved by the ultra-massive multiple-input multiple-output (MIMO) technology. For the sake of reducing the hardware cost and power comsumption, the hybrid beamforming technology is required, which has been adopted in the 5G communication. In the hybrid beamforming for the THz wideband system, the enormous antenna and ultra-wide bandwidth cause the spatial- and frequency-wideband effect, resulting in acute beam squint. Eliminating beam squint is the key to the engineering realization of ultra massive MIMO for the THz band. To circumvent this issue, this paper presents a comprehensive overview of the hybrid beamforming technology including the channel model, the traditional phase shifter based and the true time delay based architectures, the 3D hybrid beamforming for the uniform planar array. Finally, multiple significant open issues and potential challenges are pointed out and elaborated.