## 8 Conclusions  
This study standardizes the definitions of frequency bands, explicitly distinguishing between microwave (300 MHz-30 GHz), millimeter-wave (30 GHz-300 GHz), and terahertz (300 GHz-10 THz) communication systems to ensure consistency in performance comparisons and practical applications. THz communication systems are indeed the next generation beyond the current wireless communication systems, as they can offer unprecedented system data rates, high spectral efficiency, and promising use cases in fields including space exploration, beyond 5G or 6G communications, and secure communication. This paper presents a detailed study of THz communication technologies' evolution, issues, and trends with reference to FSO and hybrid system implementation. The analysis of high-power THz sources shows that the development of new technologies such as quantum cascade lasers (QCLs), photonic mixers, and free-electron lasers (FELs) expands the possibilities of the THz generation even despite problems such as cooling needs and high costs of operation. THz-FSO utilization has been made more reliable through adaptive optics and advanced signal processing algorithms, showing the efficiency of using THz-FSO communication in dynamic conditions. These advances are supported by the application of machine learning to change beam orientations, construct signals, and employ methods of error dispersion to maintain proper communication when the environments are unfavorable. Integrating THz-FSO with optical and radio frequency communication systems has been proposed as a reasonable solution to the range and reliability issues of the existing THz systems. Using SDN and multi-band adaptive frameworks, these half-and-half designs allow standardized and systematic conveyance in the midst of outrageously good conditions like atmospheric babble. This review also highlights the special applications that are becoming possible for THz communication, especially in secure and military-related fields, due to its relatively small interception angle. Likewise, incorporating THz communication into 6G networks is expected to provide ultra-low latency high-capacity solutions in the next generation of applications, such as fully autonomous systems and holographic telepresence. Lastly, THz communication systems have the potential to profoundly change the practice of data transmission and provide new solutions to the continuous development of modern communication systems. However, to unlock its full potential, more interdisciplinary research activity that focuses on issues to do with energy conversion efficiency, materials, and integration into a system needs to be undertaken. With the cooperation of academia, industries, and regulators, THz technologies will open a new chapter of development in telecommunication, space communication, and secure networks, continuing the future communication paradigm into the 21 st century.  
This research was supported by the National Key Research and Development Program of China (2023 YFB2804901); National Natural Science Foundation of China (62171227, U22B2009, 62205151, 62275127); Jiangsu Provincial Key Research and Development Program (BE202079, BE2022055-2).  
The authors declare no conflict of interest.