## 1 Introduction
The actual serving set for a UE may contain one or multiple APs and the terminal's data are partially or fully available at some cluster with potential serving APs. The AP controller (Central Processor) will accommodate each UE with its preferred cluster and transmission mode at every communication instance while considering load and channel state information (CSI) knowledge associated with the cluster of APs [27]. The main contributions of this paper can be summarized as follows:  
* Thorough system-level assessment of a virtualized C-RAN with two clusters sizes, namely, size 1 and 3, where the APs operate in the mmWave/THz bands with massive/ultra-massive antenna arrays combined with low-complexity hybrid precoding architectures. The system-level simulations were performed based on link-level results between the APs and multiple terminals, where it is considered that the ADMM algorithm from [15] is applied for hybrid precoding design at the transmitter side. * System-level evaluation of the proposed C-RAN in two 5G NR three-dimension (3D) scenarios, namely, the indoor-mixed office (InD-MO) and indoor-open office (InD-OO). * System-level evaluation demonstrates that low-complexity hybrid precoding-based C-RAN deployments in an indoor scenario can enable the practical implementation of those schemes, which rely on massive/ultra-massive antenna arrays to combat distance limitation and minimize the MUI. While these hybrid designs sacrifice some performance, significant throughput performance and coverage improvements can still be achieved over typical cellular networks. System-level assessment of the proposed based C-RAN with the fifth numerology of 5G NR in the 3D indoor-mixed office scenario with different parameters, such as the number of transmitting antennas per user and the number of subcarriers, with the results benchmarked against two alternative MU-MIMO schemes. In Table 1, we present a list of acronyms adopted along with the text in order to improve the readability of the paper. The paper is organized as follows: Section 2 presents the model for the low-complexity hybrid precoding system and the system-level scenario that is considered in the evaluation. Section 3 presents and discusses the system-level simulations results, whereas the conclusions are outlined in Section 4. \begin{table}
\begin{tabular}{c c} \hline \hline
**Acronym** & **Designation** \\ \hline
3D & Three-dimensional \\
3 GPP & The 3 rd Generation Partnership Project \\
5G & The fifth generation of wireless communications \\
5 GNR & 5G New radio \\
6G & The sixth generation of wireless communications \\ ADMM & Alternating direction method of multipliers \\ AM & Alternating minimization \\ AOA & Average angle of arrival \\ AOD & Average angle of departure \\ AcSA & Array of subarrays \\ AP & Access point \\ B5G & Beyond 5G \\ BER & Bit error rate \\ BLER & Block error rate \\ CDL & Cluster delay line \\ C-RAN & Cloud radio access network \\ CSI & Channel state information \\ DAoSA & Dynamic array of subarrays \\ DPS & Double phase shifters \\ EC & Energy consumption \\ EE & Energy efficiency \\ FC & Fully-connected \\ InD-MO & Indoor-mixed office \\ InD-OO & Indoor-open office \\ LASSO & Least absolute shrinkage and selection operator \\ LOS & Line-of-sight \\ MIMO & Multiple Input Multiple Output \\ MU & Multi-user \\ MUI & Multi-user interference \\ MU-MIMO & Multi-user MIMO \\ mmWave & Millimeter wave \\ NLOS & Non-line of sight \\ PC & Partially-connected \\ QPS & Quantized phase shifters \\ UM-MIMO & Ultra-massive MIMO \\ SE & Spectral efficiency \\ SNR & Signal-to-noise ratio \\ SPS & Single phase shifters \\ SU & Single-user \\ TDL & Tapped delay line \\ TRP & Transmission and reception point \\ THz & Terahertz \\ UE & User equipment \\ UPA & Uniform planar array \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Table of acronyms. Notation: Matrices and vectors are denoted by uppercase and lowercase boldface letters, respectively.