6G networks are expected to provide more diverse capabilities than their predecessors and are likely to support applications beyond current mobile applications, such as virtual and augmented reality (VR/AR), AI, and the Internet of Things (IoT). In contrast to typical multiple-input multiple-output (MIMO) systems, THz MIMO precoding cannot be conducted totally at baseband using digital precoders due to the restricted number of signal mixers and analog-to-digital converters that can be supported due to their cost and power consumption. In this thesis, we analyzed the performance of multiuser massive MIMO-OFDM THz wireless systems with hybrid beamforming. Carrier frequency offset (CFO) is one of the most well-known disturbances for OFDM. For practicality, we accounted for CFO, which results in Intercarrier Interference. Incorporating the combined impact of molecular absorption, high sparsity, and multi-path fading, we analyzed a three-dimensional wideband THz channel and the carrier frequency offset in multi-carrier systems. With this model, we first presented a two-stage wideband hybrid beamforming technique comprising Riemannian manifolds optimization for analog beamforming and then a zero-forcing (ZF) approach for digital beamforming. We adjusted the objective function to reduce complexity, and instead of maximizing the bit rate, we determined parameters by minimizing interference. Numerical results demonstrate the significance of considering ICI for practical implementation for the THz system. We demonstrated how our change in problem formulation minimizes latency without compromising results. We also evaluated spectral efficiency by varying the number of RF chains and antennas. The spectral efficiency grows as the number of RF chains and antennas increases, but the spectral efficiency of antennas declines when the number of users increases.

翻译：6G网络预计将提供比前代网络更多样化的能力，并可能支持超越当前移动应用的新场景，例如虚拟现实与增强现实（VR/AR）、人工智能（AI）以及物联网（IoT）。与典型的多输入多输出（MIMO）系统不同，太赫兹MIMO预编码无法完全在基带通过数字预编码器实现，原因是受限于成本和功耗，可支持的信号混频器和模数转换器数量有限。本文分析了采用混合波束成形的多用户大规模MIMO-OFDM太赫兹无线系统的性能。载波频率偏移（CFO）是OFDM系统中最常见的干扰之一。出于实际考虑，我们纳入了导致载波间干扰（ICI）的CFO影响。结合分子吸收、高稀疏性以及多径衰落的综合效应，我们分析了三维宽带太赫兹信道以及多载波系统中的载波频率偏移。基于该模型，我们首先提出了一种两阶段宽带混合波束成形技术：模拟波束成形阶段采用黎曼流形优化，数字波束成形阶段则采用迫零（ZF）方法。我们调整了目标函数以降低复杂度，不再以最大化比特率为目标，而是通过最小化干扰来确定参数。数值结果表明，在实际部署太赫兹系统时考虑ICI具有重要意义。我们还展示了问题公式化的改变如何在保证性能的前提下降低延迟。此外，通过改变射频链和天线数量，我们评估了频谱效率。频谱效率随射频链和天线数量的增加而提升，但天线频谱效率会随用户数增加而下降。