## 4 Conclusions  
RISs are considered a key technology for future wireless networks, in particular for B5G and 6G systems. In this article, we present an approach to increase the energy efficiency of future networks by replacing BS with RIS panels considering multi-stream multiple-input-multiple-output (MIMO)-orthogonal frequency division multiplexing (OFDM) links. We perform system-level assessments of RIS-aided B5G C-RAN scenarios operating in 3.6 GHz, mmWave (28 GHz), and sub-THz (100 GHz) bands. We have only considered scenarios wherein communication for/from different users is based on orthogonal multiple access, avoiding intra-cell interference. We have addressed smart radio environments and the task of increasing their capacity, for example, assessing the number of active IoT devices in a smart city scenario, in 3.6 GHz and 28 GHz. The minimum number of active users in the smart city environment depends  
Figure 16: Throughput vs. number of resource blocks per user for the factory scenario (LBS with BS antenna height 2 m, HBS with BS antenna height 8 m, maximum sector load, \(N_{tx}=256\), \(N_{rx}=16\) and \(N_{ris}=2304\)). on the coherence interval of the scenario because of the need for orthogonal reference pilots in the uplink. We considered 19,800 transmitting IoT devices, each one with one subcarrier per subframe of a 1 millisecond duration. The use of a C-RAN allows for decreasing the pilot pollution between sectors because of its operation mode. The introduction of RIS elements at 28 GHz in this scenario was able to increase the coverage of low-power and low-rate IoT devices located at cell borders. The coverage gain was about 7%. In the other smart scenarios evaluated, namely, the outdoor/indoor stadium with pitch scenario at 28 GHz and the indoor factory at 100 GHz, we studied the improvement in terms of coverage and throughput. The maximum number of evaluated active users in the stadium with pitch scenario was 3300, transmitting one PRB with 12 subcarriers each. We have evaluated the effect of variations in BS transmission power from 24 dBm up to 35 dBm, concluding that the performance results are almost constant. The introduction of C-RAN, in spite of its higher computing complexity, allowed for decreasing inter-cell interference between sectors. Throughput and coverage gains between 97% and 110% were observed with the introduction of RIS at cell borders of the BS on the stadium turf and in the stands. However, the throughput and coverage gains depend on the number of PRBs per user. The gains are higher when increasing the number of PRBs per user. The maximum number of active users in the indoor factory was 1650, assuming that one PRB per user was transmitted. We have evaluated different numbers of RIS elements, from 576 up to 2304, concluding that the difference in performance is small. However, for an increasing number of PRBs per user, the higher performance of RIS with 2304 elements was noticeable. Throughput and coverage gains are between 29% and 50%, depending on the number of PRBs per user. To increase throughput, the antennas of BS should be placed higher than the height of clutter density in the factory. We proposed the use of carrier aggregation of up to 16 carriers, each with 400 MHz, in order to increase the throughput in this scenario at 100 GHz. In future work, we intend to apply new semi-deterministic path loss models which are capable of accurately predicting path loss in scenarios with more obstructions and irregular layouts of factories at 300 GHz (THz band), in order to evaluate coverage and throughput performance, using system-level simulations. 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., N.S. and A.C.; validation, V.V., J.P.P., N.S. and A.C.; formal analysis, V.V., J.P.P., N.S. and A.C.; investigation, V.V., J.P.P., N.S., M.M.d.S. and A.C.; resources, A.C. and N.S. ; data curation, V.V., J.P.P., N.S. and A.C.; writing--original draft preparation, V.V., J.P.P., N.S., M.M.d.S.