## 1 Introduction
In contrast, practical implementations of 5G/6G mobile networks utilize multi-tone modulations, typically Orthogonal Frequency-Division Multiple Access (OFDMA). Moreover, the time-frequency radio resources are often scheduled to users in 5G/6G networks as so-called resource blocks (RBs), which are also typically not considered in recent works. In addition, user throughput in practical 5G/6G communication systems depends on the selected mod  
Figure 1: The concept and architecture of a UCCF M-MIMO network. (MCS), not the Shannon rate, as claimed in most recent works, e.g., [11]. Another point is the utilization of simple statistical models of radio channels that neglect the spatial correlations between users, which are crucial for M-MIMO. For example, the independent and identically distributed Rayleigh channel (which, in our work, is shown to be inadequate for simulations of M-MIMO systems), e.g., in [10]. Finally, most current works on both network-centric and UCCF M-MIMO networks neglect the impact of the typically nonlinear Power Amplifier (PA) on users' throughputs. However, it has been shown that in the worst-case scenario of Line-of-Sight (LoS) communication, the nonlinear distortion is steered toward scheduled users with the same precoding gain as the desired signal [12]. Also, when multiple users are scheduled, spatial intermodulations can occur, further affecting network performance [13]. Only the authors of [14] considered nonlinear distortion in the UCCF M-MIMO network but evaluated it under the Rayleigh channel, which we show is not appropriate for such an analysis. Therefore, there is a need to investigate M-MIMO systems with a user-centric architecture in an environment closely resembling a real network, including the use of OFDMA and a precise radio channel model based on 3D ray tracing. Furthermore, the multi-layer nature of future 6G systems, consisting of various functional blocks responsible for resource allocation, precoding, and the selection of modulation and coding schemes, should be considered. Finally, studies on the UCCF M-MIMO network should take into account the possible nonlinear distortion generated by Power Amplifiers (PAs) installed at M-MIMO RUs. Compared to previous works, our contribution involves the proposition of an advanced simulation environment of an OFDMA-based UCCF M-MIMO network. This environment utilizes a realistic 3D ray-tracing radio channel model, radio resource allocation performed by a dedicated scheduler entity, throughput calculation based on the MCS, and, most importantly, a realistic model of a Power Amplifier (PA) that relies on the embedded link level to obtain a realistic impact of nonlinear distortion on the user throughput. This paper aims to provide the research community with an advanced system model that can be used to develop and test algorithms within an environment that is far closer to the real world compared to most other proposed system models [4; 7; 8; 9; 10]. The solution is described using mathematical formulas and algorithms for easy implementation by other researchers. Moreover, using the proposed advanced computer simulator, we examine the benefits of implementing a UCCF M-MIMO network architecture compared to the traditional network-centric architecture. This comparison is performed under an idealistic independent and identically distributed (i.i.d.) Rayleigh channel model and a realistic one obtained using ray-tracing software. Last but not least, we examine the UCCF M-MIMO network when utilizing nonlinear PAs. The simulation studies show that the results obtained under simplified models, e.g., under a Rayleigh channel or without nonlinear distortion, overestimate the achievable throughput compared to realistic 3D ray-tracing-based simulations. This is consistent with our observations during our previous studies [15], i.e., the antenna selection algorithm optimized under a simplified system model does not perform well under the more realistic one. In the following sections of this work, Section 2 presents the system model.