## 5 Conclusions  
In this paper, we described a C-RAN aimed at 5G and beyond systems, where each BS transmits precoded space-frequency domain IM symbols. In the proposed approach, part of the information is conveyed implicitly on the indices of the active OFDM subcarriers and antennas, which also transmit normal APM signals carrying the remaining bits. The adopted PT-GSFIM scheme can benefit from the diversity effects inherent to frequency-selective channels and avoid inter-user interference at each receiver, which enables the C-RAN to accommodate flexible tradeoffs between SE, performance and complexity. A thorough system-level evaluation of the proposed PT-GSFIM-based C-RAN was performed considering standardized 5G NR scenarios with two different cluster sizes. With cluster size 3, the network is portioned into three adjacent TRP sets where each user is served by three TRPs, while the others perform inter-user interference. For RAN cluster size 1, we have the traditional cellular system. C-RAN system-level simulations considered three different three-dimensional scenarios using two different numerologies and frequency ranges: the UMa and UMi scenarios using numerology 1 at 3.5 GHz and the InD scenario using numerology 2 at 28 GHz. Throughput performance results indicate that, in spite of its higher bandwidth and higher achieved throughput, the 28 GHz band presents lower SE. The 3.5 GHz band having lower bandwidth and lower achieved throughput attains higher SE. There is an optimum number of transmitting antennas per user \(N_{tr}\)/\(N_{u}\) to achieve the highest throughput, but it depends on the cluster size of C-RAN and modulation. A higher number of \(N_{tr}\)/\(N_{u}\) with 16 QAM is optimum for traditional cellular networks with a high level of intersite interference and a lower number of users per site. A lower number of \(N_{tr}\)/\(N_{u}\) with 64 QAM is optimum for C-RAN with lower intersite interference but a higher number of users per site. Overall, the system evaluation shows that the proposed PT-GSFIM scheme can outperform both GSM and conventional MU-MIMO, exploiting its additional inherent frequency diversity. As future work, we intend to apply other alternative numerical and probabilistic methods which may be less time-intensive in order to achieve accurate system-level evaluation results.  
Conceptualization, V.V., J.P.P., N.S. and A.C.; methodology, V.V., J.P.P., N.S. and A.C.; software, V.V., J.P.P., C.G., N.S., P.S. and A.C; validation, V.V., J.P.P., C.R., C.G., N.S., P.S. and A.C.; formal analysis, V.V., J.P.P., C.R., C.G., N.S., P.S. and A.C.; investigation, V.V., J.P.P., C.R., C.G., N.S., P.S. and A.C.; resources, A.C. and N.S.; data curation, V.V., J.P.P., C.R., C.G., N.S., P.S. and A.C.; writing--original draft preparation, V.V., J.P.P., N.S. and A.C.; writing--review and editing, V.V., J.P.P., C.R., C.G., N.S., P.S. and A.C.; visualization, V.V., J.P.P., C.R., C.G., N.S., P.S. and A.C.; supervision, A.C. and N.S.; project administration, A.C. and N.S.; funding acquisition, A.C. and N.S. All authors have read and agreed to the published version of the manuscript.  
This work was supported by the Fundacao para a Ciencia e Tecnologia (FCT) under the grant 2020.05621.BD. The authors also acknowledge the funding provided by FCT/MCTES through national funds and when applicable co-funded EU funds under the project UIDB/50008/2020.  
Not applicable.  
The data presented in this study are available on request from the corresponding author. The data are not publicly available due to privacy.  
The authors declare no conflict of interest.