## 5 Results
### Throughput Analysis Considering Nonlinear Effects  
The last studies are related to system-level simulation considering nonlinearities. We followed the same setup and UE distribution used in Section 5.2, i.e., the number of UEs and their spatial distribution. However, here, all the steps of the main simulation loop were executed. We compared the results achieved without nonlinear effects, named _Perfect PA_, against the system with nonlinear distortion, with the IBO set to 0 dB and 6 dB, respectively. For the _Perfect PA_ scenario, we assumed that the average system transmission power was \(P_{0}=46\) dBm (macro-RU) and \(P_{l}=30\) dBm (micro-RUs, for \(l>0\)). While this means that the IBO was equal to 0 dB for all RUs, causing severe nonlinear distortion effects, in reality, we omitted these in the _Perfect PA_ scenario. As a result, \(\mathbf{A}_{l}=\mathbf{I}_{M_{l}}\) for each RU, and no nonlinear distortion appeared, i.e., \(\sigma_{\text{dis},jr}^{2}=0\). We are aware of the fact that in such a case, a comparison between _Perfect PA_ and  
Figure 7: Comparison of 10 th, 50 th (median), and 90 th percentiles from the distribution of the average UE rate for varying numbers of serving RUs and different radio channel models.  
real frontend cases is unfair, as the _Perfect PA_ case uses higher useful power and is not penalized with nonlinear distortion that would result in higher throughput. However, this allows for a qualitative comparison of both schemes. For comparison under the i.i.d. Rayleigh channel, we selected the network-centric approach and the UCCF approach with two RUs serving a single UE, as it exhibited the highest rate improvements in Figure 7. The results are presented in Figure 8. It can be seen that under a high operation point of the PA (IBO 0 dB), the user throughput significantly degraded compared to the _Perfect PA_ scenario and the results for the case of 6 dB IBO. This is due to the strong nonlinear distortion that was generated when the PA's operation point was low, i.e., close to 0 dB. On the other hand, lowering the operation point reduced the transmit power, but, as can be seen in this case, it only slightly affected the UE rates. This might be because in most cases, the lower transmit power did not degrade the SINR, resulting in the selection of a lower MCS for a given UE.  
The results obtained under the more realistic ray-tracer-based radio channel model are presented in Figure 9. For comparison, we selected the network-centric approach and the UCCF approach with three RUs serving a single UE. This configuration provided high average UE rate gains over the network-centric approach and did not deteriorate the rates achieved by any group of UEs (neither median nor 90 th percentile), as visible in Figure 7. The observations are similar to those under the Rayleigh channel. The high operation point of the PA resulted in the generation of significant nonlinear distortion. However, under the realistic ray-tracer radio channel, a small drop in UE rates compared to the perfect PA case can be observed, related to the fact that the transmit power decreased by lowering the operation point of the PA by 6 dB. It might be that while each RU had a lower power to distribute among scheduled UEs, the allocation of a new spatial layer did not meet the condition of sum-rate improvement.  
Figure 8: CDFs depicting the average UE rates for the network-centric and UCCF approaches, with varying numbers of RUs, under the Rayleigh radio channel, considering nonlinear effects.  
Figure 9: CDFs of average UE rates for network-centric and UCCF approaches, with varying numbers of RUs, under the Rayleigh radio channel, considering nonlinear effects.