Triple-Layer TTD Hybrid Beamformer for Beam Split Mitigation in Wideband THz Systems

For future 6G networks, Terahertz (THz) communication is emerging as a key technology as it can provide ultra-wide bandwidth. To overcome the significant propagation losses of signals in $\mathbf{T H z}$ band, ultra-massive Multiple-Input Multiple-Output (MIMO) systems with hybrid beamforming are utilized because they employ highly directional beams to enhance array gains. However, due to the enormous number of antennas and the large bandwidth, the conventional hybrid beamforming approach, which relies on frequency-independent phase shifters, struggles with the beam split effect in the $\mathbf{T H z}$ band. It causes beams to deviate into different physical directions across different frequencies, resulting in a significant loss of array gain throughout the entire bandwidth. To address this issue, a novel triple-layer True-Time-Delay (TTD) scheme is proposed to mitigate the beam split effect while maintaining the performance level of a single-layer TTD. The proposed scheme significantly reduces the number of large-range delay devices as well as the hardware complexity by reducing the number of required digital TTD bits. As a result, the proposed scheme increases the overall energy efficiency without sacrificing the achievable sum rate. To get the optimal configuration(s) for the proposed scheme at each layer, an algorithm is proposed. The results demonstrate that the proposed triple-layer TTD has the same achievable sum rate as the double and single-layer TTD scheme but with fewer TTDs.