## 4 Optimization of Channel MIMO Capacity by Combining OSTBC and Polarization-Diversity with XPI
### Diversity Transmission Using OSTBC-MRRC MIMO Technique
The communication system architecture that includes a combination of OSTBC-OFDM-MRRC techniques is very widespread, and many studies have been conducted around this topic [32; 33; 34; 35; 36]. Note that the integration of architecture of the communication system described in Section 4 is identical to the architecture in this section. The architecture included OFDM-OSTBC-MRRC, and XPI mechanisms, on the transmitter side and on the receiver side, which produce polarization-diversity capability. Note that in the communication model described in Section 2, the transmission matrix \(S\) has now become a combination of OSTBC-OFDM signals as provided in [32]. In both systems, there are implementations of the optimized XPI mechanism, and both systems are under rain distortion conditions. In this simulation, we focus on the BER performance by running three different scenarios (e.g., placing all the XPI values leading to optimal, mediocre, and poor channel conditions, identical to the process we did in Sections 3 and 4) under the same rain distortion conditions. The block schemes of the simulator are similar to those described in Sections 3 and 4. In Figure 6, the BER-performance of the mmWave wireless communication system without diversity techniques under rain distortion can be seen. The three graphs represent  
Figure 5: Average Capacity Under Optimal, Mid, and Minimum Conditions with Diversity-Techniques. different XPI values by tuning the XPI mechanism. The red graph represents the optimal XPI values, the green and the blue represent the mean XPI values, and the worst XPI values, respectively, are identical to the process in Section 3. Figure 7 illustrates the BER-performance of the mmWave wireless communication system with OSTBC-MRRC and polarization diversity techniques under rain distortion. The three graphs also represent different XPI values identical to the process in Section 4. The process of comparing performance generates three essential and significant insights. The most crucial insight is that by combining diversity techniques under a scenario of rain distortion in the DP-MIMO-mmWave channels, an improvement of \(\sim\)3 dB in the optimal graph (the red graph), and an improvement of \(\sim\)2 dB, in the mid graph (the green graph), compared to a system without a combination of techniques under the same conditions can be achieved. The second insight is related to the compatibility between the results of the theoretical analysis of the three-channel conditions we analyzed and the results of the simulations. It means that in the graphical analysis of the channel performance and the visual examination  
Figure 6: BER Performance of OSTBC-OFDM-MRRC Without Diversity-Techniques. Figure 7: BER Performance of OSTBC-OFDM-MRRC With Diversity-Techniques. of the BER performance, the three graphs obtained are ranked in terms of the simulation results following the theoretical analysis we performed. The third insight is that the XPI values under certain conditions (for example, mistuning these mechanisms or setting specific values as analyzed in the theoretical process) may lead to the most destructive channel conditions and, in any case, lead to the worst performance - as reflected in the blue graph. This phenomenon appears regardless of the integration of diversity techniques in wireless communication systems or the lack of integration of these techniques. Variation of Channel Matrix Components in the Transition between \(t\) to \(t+t_{s}\) and Design of Orthogonal Conditions in the Channel Matrix with the Help of Feedback  
Up to this point, our central assumption regarding the channel's nature and statistical distribution was a Quasistatic Flat-Rayleigh-Fading. Despite the distortion in the transmitted signal polarization that occurs due to the rain phenomena, as we mentioned, it has been proven that with optimization and proper adjustment of the XPI mechanism, it is possible to achieve excellent performance in the leading parameters of the system.