## 3 Transmitter and Receiver Structure
### System-Level Simulations
For cluster size 3, there is much less inter-interference between sites, and the highest throughput performance is obtained for 64 QAM with \(N_{tx}/N_{u}=8\). The lowest throughput is achieved for 4 QAM with \(N_{tx}/N_{u}=5\). This is explained by the low-order modulation and lower bpcu associated with this case in spite of the maximum number of served users per site. Cluster size 3 has a small level of inter-interference and allows 64 QAM with \(N_{tx}/N_{u}=8\) to achieve higher throughput than 64 QAM with \(N_{tx}/N_{u}=5\), due to a higher bpcu despite a lower number of users per site. As before, 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 RAN cluster size 1 with a high level of inter-interference and a lower number of users per site. 64 QAM with \(N_{tx}/N_{u}=8\) is recommended for RAN cluster size 3 with lower inter-interference but a higher number of users per site. Table 2 presents the SE of the three scenarios, namely UMa, UMi and InD, for C-RAN cluster sizes of 1 and 3, based on the throughput results taken from Figures 10-12. It is observed that for cluster 1C, 16 QAM(12) offers the highest SE for all scenarios. For cluster 3C, 64 QAM(5) offers the highest SE for scenario UMi whereas 64 QAM(8) offers the highest SE for scenario InD. For the UMa scenario, both 64 QAM(5) and 64 QAM(8) offer the highest SE. In Figure 13, we compare the throughput for three different systems, namely the proposed PT-GSFIM, the BD MU-MIMO scheme from [37] and GSM MU-MIMO from [47], where we keep constant \(N_{tx}/N_{u}=12\) and \(N_{sc}=256\), resulting in approximately 12 bpcu. We evaluate the throughput for three scenarios, namely UMa, UMi and InD. We observe that independently of the system, the throughput performance of the InD scenario is the best, as expected. This is due to its double transmitted bit rate compared to the other scenarios; the combination of \(N_{tx}/N_{u}=12\) with cluster size 3 results in a smaller number of users and has lower inter-interference. Based on the results of Figure 13, the proposed PT-GSFIM scheme clearly outperforms both GSM and conventional MU-MIMO, taking advantage of the symbols being spread over a larger number of subcarriers, exactly \(N_{f}=16\) and \(N_{df}=13\), thus exploiting the frequency diversity. Therefore, for C-RAN cluster size 3, both indoor and outdoor scenarios indicate that the total throughput achieved by our scheme is higher than that of conventional MU-MIMO and exhibits better system performance than the GSM counterpart. In Figure 14, we compare the coverage of C-RAN cluster sizes 1 and 3 for 64 QAM and 4 QAM, both with \(N_{tx}/N_{u}=8\). As expected, the coverage of modulation 4 QAM is higher than the coverage of 64 QAM, independently of the cluster size. We also observe that for 100% of transmitted power, the coverage gain of cluster size 3 (3C) compared to cluster size 1 (1C) is 1.9 for 4 QAM and increases to 3.1 for 64 QAM. Cluster size increases especially the coverage of high-order modulations such as 64 QAM modulation. \begin{table}
\begin{tabular}{c c c c c c c} \hline
**Modulation(SE)** & **UMa, 3C** & **UMi, 3C** & **InD, 3C** & **UMa, 1C** & **UMi, 1C** & **InD, 1C** \\ \hline
64 QAM(5) & 2.00 & 2.75 & 1.55 & 0.51 & 0.61 & 0.59 \\
64 QAM(8) & 2.00 & 2.29 & 1.73 & 0.65 & 0.98 & 0.88 \\
4 QAM(5) & 1.76 & 1.84 & 1.52 & 0.66 & 0.90 & 0.97 \\
4 QAM(8) & 1.36 & 1.38 & 1.16 & 0.71 & 0.80 & 1.00 \\
16 QAM(12) & 1.69 & 1.71 & 1.42 & 0.88 & 1.06 & 1.17 \\ \hline \end{tabular}
\end{table} 
Table 2: SE in bps/Hz of different scenarios and cluster sizes 1 and 3. Figure 14: Coverage vs. percentage of TRP transmitted power, for UMa scenario, with \(N_{tx}\)/\(N_{u}\) = 8, \(N_{sc}\) = 256, \(N_{rx}\) = 5, 64 QAM with 12.5 bpcu and 4 QAM with 6.5 bpcu.