## 6 Conclusions  
This work introduces an RIS-assisted mmWave M-MIMO system to improve SE, EE, and reliability. The proposed approach involves having all transmit antennas convey the same information symbol to achieve additional diversity gain. The system's performance is evaluated under 3 GPP 5G indoor and outdoor channel models for 28 GHz and 73 GHz mmWave frequencies. The evaluation considers office and street canyon channel environments. The Monte Carlo simulation results show a significant decrease in the ABER performance of the RIS-assisted mmWave M-MIMO system in the investigated indoor and outdoor channels. Interestingly, the system's ABER performance proves superior at 28 GHz compared to 73 GHz mmWave frequency, regardless of the indoor or outdoor scenario. Additionally, increasing \(N\) and \(N_{\rm T}\) significantly enhances the EE of the proposed system. In indoor environments, it is suggested to use 28 GHz for lower data rate requirements. For data-hungry applications, we recommend using 73 GHz by suitably increasing the number of reflecting components in RIS to compensate for the poor ABER performance. For outdoor environments with high data rate demands, we can use 73 GHz by appropriately selecting the number of antennas in the M-MIMO BS and the number of reflecting components in RIS.  
Finally, the ideal continuous phase RIS in the proposed system is replaced with the practically implementable discrete resolution phase shifter RIS, and its ABER and sum rate performances are observed for indoor and outdoor environments. It has been observed that doubling the value of \(P\) results in a \(\sim\)6 dB increase in SNR gain. Due to the significant vulnerability to blockages and greater path loss at mmWave frequencies, these systems exhibit lower SE values than their sub-6 GHz counterparts. Nevertheless, the ample bandwidth in the mmWave spectrum allows for higher overall achievable sum rates. As the proposed work innovatively integrates RIS, mmWave, and M-MIMO and validates the performance under 3 GPP standardized indoor and outdoor channels, it is a potential contender for physical layer technology in the upcoming landscape of B5G and 6G networks. Future work could focus on implementing an appropriate channel coding scheme to improve ABER performance. The proposed system model can be tested for blind and AP RIS configurations.  
**Author Contributions:** Conceptualization, V.V.G., V.B.K., A.B.S., and H.S.J.K.; methodology, V.V.G., V.B.K., H.K.J., A.B.S., and H.S.J.K.; software, V.V.G., V.B.K., H.K.J., A.B.S., and H.S.J.K.; validation, V.B.K., A.L.I., F.R.C.S., and S.N.S.; formal analysis, V.V.G., V.B.K., H.K.J., A.B.S., and H.S.J.K.; investigation, V.B.K., A.L.I., F.R.C.S., and S.N.S.; resources, V.V.G., H.K.J., A.B.S., and H.S.J.K.; data curation, V.V.G., H.K.J., A.B.S., and H.S.J.K.; writing--original draft preparation, V.V.G., H.K.J., and A.M.; writing--review and editing, V.V.G., H.K.J., and A.M.; visualization, V.V.G., H.K.J., and A.M.; supervision, V.B.K., A.L.I., F.R.C.S., and S.N.S.; project administration, V.B.K., A.L.I., F.R.C.S., and S.N.S. All authors have read and agreed to the published version of the manuscript.  
**Funding:** The work of [PERSON] was supported in part by the Nigerian Petroleum Technology Development Fund (PTDF) and in part by the German Academic Exchange Service (DAAD) through the Nigerian-German Postgraduate Program under grant 57473408.  
**Institutional Review Board Statement:** Not applicable.  
**Informed Consent Statement:** Not applicable.  
**Data Availability Statement:** All data are available from the corresponding author upon reasonable request.  
**Acknowledgments:** We would like to thank [PERSON] from the Department of Electronics and Electrical Engineering at IIT Guwahati for his invaluable guidance and support, which were crucial to the completion of this article.  
**Conflicts of Interest:** The authors declare no conflicts of interest.