## 3 Transmitter and Receiver Structure
### System-Level Simulations
When we compare the maximum throughput values of Figures 8 and 9, we can see an increase of 14 for 4 QAM and 7 for 64 QAM. These numbers refer to 3C RAN clustering, confirming its lower inter-interference. In Figure 9, we can also observe the difference of 3 times more throughput achieved by RAN clustering 3C compared to the case of 1C. This occurs independently of the modulation. For 3C, there is a slight throughput gain of 64 QAM-modulated packets compared to 4 QAM packets. For 1C, there is a slight throughput gain of 4 QAM packets compared to 64 QAM. This corroborates the results of Figures 6 and 7, for the value of \(N_{u}\) = 20. In Figure 10, we only consider the UMa scenario, we compare the throughput for three cases of the number of transmitting antennas per user \(N_{tx}/N_{u}\), with RAN cluster sizes of 1 and 3. Based on the BLER results of Figure 5, the best BLER performance with the highest bpcu per user belongs to \(N_{tx}/N_{u}\) = 12, followed by \(N_{tx}/N_{u}\) = 8 and \(N_{tx}/N_{u}\) = 5. However, a higher number of transmitting antennas per user implies that a lower number of users are served, because the total number of TRP antennas is fixed and equals 256. The number of receiving antennas per user is kept fixed and equals 5. It should be pointed out that it is not clear what \(N_{tx}/N_{u}\) would provide higher throughput. From Figure 8, we conclude that the best \(N_{tx}/N_{u}\) ratio depends on the RAN cluster size and modulation. For cluster size 1, the highest throughput performance is for \(N_{tx}/N_{u}\) = 12 using 16 QAM, and the lowest throughput is for 64 QAM with \(N_{tx}/N_{u}\) = 5. Figure 8: Throughput performance (bps) vs. CDF of throughput for UMa scenario with \(N_{tx}\) = 5N\({}_{u}\), \(N_{f}\) = 4, \(N_{df}\) = 3, \(N_{s}\) = 5, \(N_{a}\) = 2, \(N_{rx}\) = 5, 64 QAM (11.75 bpcu per user) and 4 QAM (5.75 bpcu per user). Figure 10: Throughput vs. number of active TRP antennas, for UMa scenario, \(N_{xx}=5\), 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). Figure 9: CDF of TRP throughput vs. TRP throughput (bps) for UMa scenario with \(N_{tx}=5N_{u}\), \(N_{f}=4\), \(N_{off}=3\), \(N_{s}=5\), \(N_{a}=2\), \(N_{rx}=5\), 64-QAM (11.75 bpcu per user) and 4 QAM (5.75 bpcu per user). This result can be explained by the existence of high inter-interference in this cluster size, which affects mostly high-order modulations with the maximum number of users served at the same time. The case with \(N_{tx}/N_{u}=12\) using a constellation of 16 QAM has the minimum number of users served at the same time but has the highest bpcu per user, resulting in the lowest interference, which is the best case. However, for cluster size 3, there is much less inter-interference between sites. In this case, the highest throughput performance is achieved for the two 64 QAM cases, namely with \(N_{tx}/N_{u}=8\) or \(N_{tx}/N_{u}=5\). 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 limited number of served users compared to the case of \(N_{tx}/N_{u}=5\). With a small level of inter-interference of cluster size 3, high-order modulations present the best throughput performance. From these results, we conclude that there is an optimum \(N_{tx}/N_{u}\) to 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. Lower \(N_{tx}/N_{u}\) with 64 QAM is recommended for RAN cluster size 3 with lower inter-interference but a higher number of users per site. Figure 11 corresponds to Figure 10, but in this case, we consider the UMi scenario. When we compare both figures, it is obvious that the best throughput performance is achieved in the UMi scenario.