ing applications in the space, such as satellite, and 5G and Beyond Wireless Communication Networks, First Edition. Haijian Sun, Rose Qingyang Hu, and Yi Qian. © 2024 John Wiley & Sons Ltd. Published 2024 by John Wiley & Sons Ltd. 12 Open Issues and Future Directions for 5G and Beyond Wireless Networks air communication for UAVs have different needs. Besides, their design philosophy is very much different. For example, UAV communication expects LoS scenarios (signal propa- gation is in favor condition) but power and trajectory optimization are more important design factors. Recently, the space-air-ground integrated network (SAGIN) has been pro- posed. SAGIN provides seamless coverage for larger areas, including sea, space, ground, and air. SAGIN has to consider various factors from each segment, current design focuses on pro- tocol optimization, resource allocation, performance analysis, mobility management, and inter-segment operation [112]. Furthermore, the network design and system integration in SAGIN are of great significance. 12.3 Semantic Communication Communication should be used not only for exchanging data bits, but also for semantic exchange. In fact, many scenarios involve semantic information. For example, transmitting natural languages. Current solution needs to convert language into bits via upper layer oper- ations, then simply send those bits through medium. In [150], the difference between bit and semantic transmission is shown in Figure 1 therein. Essentially, semantic communi- cation will utilize advanced ML techniques to perform semantic encoding at transmitter and correspondingly semantic decoding at the receiver end, avoiding the source encod- ing/decoding directly. The challenge is that current system will undergo significant mod- ifications, for example, the need for new metric for semantic entropy, semantic channel, and noise factor. Nevertheless, semantic communication is regarded as an important com- ponent in 5G beyond. 12.4 Data-Driven Communication System Design The exponential grow