## 3 Transmitter and Receiver Structure
### System-Level Simulations
This was expected due to lower ISD, in spite of a higher number of users per unit of area. The total number of TRP antennas is the same and equals 256. The number of receiving antennas per user is kept fixed and equals 5. It is interesting to find the \(N_{tx}/N_{u}\) that provides higher throughput. From Figure 11, we can conclude that the best \(N_{tx}/N_{u}\) depends on the C-RAN cluster size and modulation. For cluster size 1, the highest throughput performance is obtained for \(N_{tx}/N_{u}=12\) using 16 QAM, and the lowest throughput is achieved for 64 QAM with \(N_{tx}/N_{u}=5\). The high inter-interference of this cluster size makes high-order modulation with the maximum number of users served at the same time the worst case of achieved throughput. The case \(N_{tx}/N_{u}=12\) using 16 QAM has the minimum number of users served simultaneously but with the highest bpcu per user and continues to present the lowest interference, corresponding to the best case. Cluster size 3 has much less inter-interference between sites. In this case, the highest throughput performance is achieved for 64 QAM with \(N_{tx}/N_{u}=5\). Figure 11: Throughput vs. number of active TRP antennas, for UMi scenario, \(N_{tx}=5\), for two C-RAN cluster sizes, modulations and numbers of antennas per user, namely with \(N_{tx}/N_{u}=5\), 64 QAM(5) and 4 QAM(5); with \(N_{tx}/N_{u}=8\), 64 QAM(8) and 4 QAM(8); and with \(N_{tx}/N_{u}=12\), 16 QAM(12). The lowest throughput is achieved for \(N_{tx}/N_{u}=8\) using 4 QAM. This is explained by the low-order modulation and bpcu associated with a reduced number of served users compared to the case of \(N_{tx}/N_{u}=5\). Cluster size 3 has a small level of inter-interference, which allows the case of 64 QAM with \(N_{tx}/N_{u}=5\) to achieve higher throughput than 64 QAM with \(N_{tx}/N_{u}=8\) due to a higher number of users with almost the same bpcu. Considering what we have been observing throughout this study, we conclude that there is a value of \(N_{tx}/N_{u}\) that can achieve the highest throughput, but it depends on the cluster size and modulation. Higher \(N_{tx}/N_{u}\) with 16 QAM is recommended for C-RAN cluster size 1 with a high level of inter-interference and a lower number of users per site. 64 QAM with \(N_{tx}/N_{u}=5\) is recommended for C-RAN cluster size 3 with lower inter-interference but a higher number of users per site. Figure 12 corresponds to Figures 10 and 11, but in this case, we consider the InD scenario. When we compare the three figures, it is obvious that the best throughput performance is attained in the InD scenario. This was expected due to the 2-fold increase in the transmitted bit rate and lower ISD, in spite of the highest number of users per unit of area. The total number of TRP antennas is the same and equals 256. The number of receiving antennas per user is kept fixed and equals 5. In this case, we also wanted to find the value of \(N_{tx}/N_{u}\) that could provide the highest throughput in this InD scenario at 28 GHz. From Figure 12, we conclude that the best \(N_{tx}/N_{u}\) depends on the C-RAN cluster size and modulation. For cluster size 1, the highest throughput performance continues to be that of \(N_{tx}/N_{u}=12\) using 16 QAM, and the lowest throughput is obtained for 64 QAM with  
Figure 12: Throughput vs. number of active TRP antennas, for InD scenario, \(N_{tx}=5\), for two C-RAN cluster sizes, modulations and numbers of antennas per user, namely with \(N_{tx}/N_{u}=5\), 64 QAM(5) and 4 QAM(5); with \(N_{tx}/N_{u}=8\), 64 QAM(8) and 4 QAM(8); and with \(N_{tx}/N_{u}=12\), 16 QAM(12). \(N_{tx}/N_{u}=5\). The high inter-interference of cluster size 1 makes high-order modulation with the maximum number of users served at the same time the worst case of achieved throughput. The case of 16 QAM with \(N_{tx}/N_{u}=12\), in spite of having the minimum number of users served at the same time, has the highest bpcu per user and continues to present the lowest interference.