# Study of 5G-NR-MIMO Links in the Presence of an Interferer
## 5 Conclusions  
In this paper, we study and present the significant impact and performances of the Alamouti scheme in the presence of interferences signals. We assume that the channel is under Rayleigh slow fading, with CE modulation transmission. Understanding the effects of interference in the space-frequency domain is critical for developing a solution to cancel the interference by digital, analog, or antenna separation.  
We proved mathematically (Appendix A), and with a simulation platform, the MIMO-system based on the STC scheme performances in an interference environment decreases. When the CSI is only on the receiver side, it has destructive effects on the receiving and the decoding processes. We showed the violation of the principle of orthogonality that OSTBC produces, by creating uncorrelated channel paths and receiving every path with each antenna, independently. This is a significant issue in a multi-path scenario or urban environment because if all paths received are orthogonal to each other, a full rank in the channel matrix is obtained. Full rank leads to a maximum capacity in the channel, achieved by the sum of constant eigenvalues. The examination of the general interference, the smart jammer, and the PBN jammer showed an impact on the orthogonality region and caused the addition and duplication of received symbols. These phenomena are independent of the DOA, i.e., the transmitter, receiver, and interferer orientation, under a ULA assume.  
We showed the BER performances in relation to the SIR in the presence of interference. We showed that it is unable to reduce the BER performances for the smart jammer case unless the SIR value is above 0 dB. To overcome this, we must ensure that the CSI will be on the transmission side, and we need to make sure that the CSI will be on both sides at the same time.  
We chose to focus on the simple case with a single dominant interferer because already, in this case, we prove mathematically and with simulations performances impairments that justify identification, control and feedback. When dealing with complex problems such as multi-interference, as occurs in reality, it is necessary to simplify the problem, build models, and then deal with the complex problems.  
Our subsequent studies will aim to reach a breakthrough that enables the creation of fast, smart, and learning feedback. This feedback will be performed using an algorithm that learns the channels' characteristics between the transmitter and the interference's to the receiver, very fast and computes smart feedback between the receiver to the transmitter. This feedback must consider the trade-off between the ability to reduce and the ability to optimize energy-efficient power transmissions in the transmitter, and thus solve the classic full rank optimization problem of the interference cancellation in the receiver.  
We must research and develop models and algorithms that can design dynamic-selective channels while executing control processes, decoding SM techniques, cancellation multi-interference's signals, and can produce corrections in a unique geometric array of antennas. This solution is needed in a system with TDD techniques and an FD system, especially in HetNets sub 6 GHz, such as 5G-NR small cells under a multi-interference environment or military systems that suffer from smart jammers. In the future, we want to fix these problems, in addition to selective-fast-fading channels, and multi-interference with high-order modulation.  
Conceptualization, A.E.; methodology, A.E.; software, A.E. and Y.B.; validation, A.E.; formal analysis, A.E. and Y.B.; investigation, A.E. and Y.B.; resources, A.E.; data curation, A.E. and Y.B.; writing--original draft preparation, A.E. and Y.B.; writing--review and editing, A.E., Y.B., and Y.F.; visualization, Y.B.; supervision, Y.P.; project administration, Y.P.; All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
The authors declare no conflict of interest.