## 1 Introduction
In [14], a nonlinear hybrid transceiver design relying on Tomlinson-Harashima precoding was proposed. Their approach only considers FC architectures but can achieve a performance close to the fully-digital transceiver. Most of the hybrid solutions for mmWave systems aim to achieve near-optimal performance using FC structures, resorting to phase shifters or switches. However, the difficulty of handling the hardware constraint imposed by the analog phase shifters or by switches in the THz band is an issue that limits the expected performance in terms of SE. In [15], the authors proposed a low complexity design based on the alternating direction method of multipliers (ADMM) that can approximate the performance of a hybrid precoder to the fully-digital performance. It is targeted to the millimeter wave (mmWave)/THz bands and can incorporate different architectures at the analog component of the precoder, making it suitable for supporting ultra-massive MIMO (UM-MIMO) in severely hardware-constrained systems that are typical at these bands. There are some recent experimental demonstrations of hybrid precoding schemes for MIMO mmWave communications, as mentioned in [16; 17]. However, these implementations still have limitations in the size of the adopted arrays and the number of RF chains. Regarding the applications of these schemes in the THz band, the technology is still in the early stages and there are still no UM-MIMO implementations with hybrid precoding/beamforming. Nevertheless, there are already some simpler MIMO implementations in THz, such as the ones mentioned in [18; 19]. When transitioning from a link-level perspective to a system-level analysis, we must deal with several other issues, such as distance limitation, signal-to-noise ratio (SNR) degradation, weak coverage areas and blind zones, which are particularly relevant when working in the high-frequency spectrum [11; 20; 21]. This is the main reason why network and resource allocation planning are crucial when deploying these systems for cellular communications, independently of the scenario under study [11; 22]. There are few system-level evaluations of mmWave schemes in the literature, such as the ones mentioned in [23; 24], but beyond all the proposals in the field of system-level analysis very few examples have been extended to the 5G New Radio (5G NR) standard of the 3 rd Generation Partnership Project (3 GPP) and for the THz level (e.g., 100 GHz) as we cover in this paper. This standard suggests the use of deterministic cluster delay lines (CDLs) for link-level simulations, which requires the definition of average angles of departure (AODs) and arrival (AOAs) and also a tapped delay line (TDL). For system-level simulations, a full three-dimensional (3D) modeling of a radio channel is recommended since this type of analysis requires a statistical approach [25; 26]. Motivated by the work above, in this paper, we study a cloud radio access network (C-RAN) for 5G and beyond systems which is based on the adoption of low complexity hybrid precoding designs for massive and ultra-massive multi-user MIMO (MU-MIMO) schemes operating in the mmWave/THz bands. As far as the authors are aware there are no previous studies similar to the one presented in this paper focusing on the system-level evaluation of robust hybrid algorithms with different architectures (FC, AoSA, DaoSA, etc.) at the THz band. The C-RAN studied in this paper assumes that each access point (AP) uses the precoder to remove the multi-user interference (MUI) generated at the receivers, breaking the MU communication into equivalent small SU links, which enables a lower complexity at the receiver side. We consider a virtualized C-RAN, where the network determines which APs are to be associated with each terminal. The cell moves with and always surrounds the terminal in order to provide a cell-center experience throughout the entire network. Each terminal designated as user equipment (UE) is served by its preferred set of APs.