## 3 Space Time Block Code (STBC)
### OSTBC System Model
Figure 9: BER for the 256 QAM modulation scheme versus the number of IRS reflecting elements (**a**) Alamouti STBC \(2\times 1\) deployed (**b**) OSTBC \(4\times 1\) deployed at different \(\text{E}_{\text{b}}/\text{N}_{\text{o}}\). 16 QAM, 64 QAM, and 256 QAM. Moreover, the utilization of four transmitting antennas instead of two reduces the required \(\text{E}_{\text{b}}/\text{N}_{\text{o}}\) by 6 dB. This reduction can be attributed to the diversity gain achieved through the increased number of transmitting antennas. Moreover, when deploying IRS, it outperforms the system without IRS. This is due to the constructive interference resulted by the reflections from the IRS. So, the overall system BER will decrease or the required \(\text{E}_{\text{b}}/\text{N}_{\text{o}}\) will decrease, which will let the system go more and more green in terms of technology. In addition, by increasing the number of reflecting elements, the number of transmitting antennas will decrease to avoid system complexity at the gNodeB. Moreover, the IRS is low cost in comparison with antennas, so the overall cost of the system will decrease. In the future work, the implementation of the system will be considered to compare the experimental results with the simulated results. The optimization of the IRS phase shifts will also be considered in the future work. Moreover, in the future work, OSTBC use in IRS-aided massive MIMO systems will be studied. Conceptualization, M.E.-H., M.A., H.E.-H., H.E. and B.A. ; methodology, M.E.-H. and M.A. ; software, M.E.-H. and M.A. ; validation, M.E.-H., M.A., H.E.-H., H.E. and B.A. ; formal analysis, M.E.-H. and M.A. ; resources, H.E.-H.; writing--original draft preparation, M.E.-H.; writing--review and editing, M.E.-H., M.A., H.E.-H., H.E. and B.A. ; visualization, M.E.-H.; supervision, H.E.-H., H.E., B.A. and M.A. All authors have read and agreed to the published version of the manuscript. This research received no external funding. Not applicable. Data Availability StatementData are contained within the article. The authors declare no conflict of interest. ## References

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