**7. Conclusion**

In this chapter, we have studied and discussed the issue of hybrid precoding and combining techniques in mmWave MIMO systems for different array architectures. We presented the system models of FA, SA, and HA and solved the optimization problem of hybrid precoding and combining to maximize the spectral efficiency of each architecture. Additionally, we proposed iterative hybrid precoding and combining algorithms for all architectures. The simulation results showed that the proposed algorithms can enhance the spectral efficiency performance of mmWave MIMO systems with lower complexity and hardware requirements than traditional hybrid design methods. The findings of this chapter are expected to be of significant interest to researchers, engineers, and students working in the field of mmWave communications and MIMO systems, as they provide insights into improving the spectral efficiency and performance of wireless communication systems. Overall, this work contributes to the development of efficient and cost-effective solutions for nextgeneration wireless communication systems.
