## 8 Challenges, Future Directions, and Lessons Learned
### Future Directions  
This paper provides an in-depth look at the 5G mobile network and the technologies that enable its adoption. These technologies make 5G more trustworthy, scalable, and cost-effective. As previously described, several technological obstacles occur when deploying such technologies or offering services via a 5G mobile network. Undoubtedly, several studies have been carried out on 5G communication, especially as it relates to enabling technologies, frequency standardization, and propagation models [17; 187; 188; 189; 70; 190; 17]. However, there are still open research directions that are yet to be thoroughly explored. In this section, we identified some key areas where further research is needed. We have also considered proposals from several researchers on the integration of new technologies to facilitate the development and quick implementation of the 5G technology. * Blockchain technology for improved security: Since the development of 5G and beyond 5G networks must accommodate millions of networked devices, security and privacy are crucial considerations. Networks for 5G and beyond must address user data confidentiality and privacy concerns. To ensure the effective deployment of such networks, these difficulties must be addressed from the perspectives of service providers, network operators, and vendors in addition to those of consumers. Researchers are working on several ways to enhance security and privacy in 5G networks. Some of such recent attempts are works on the implementation of blockchain technology in 5G networks for better security and privacy [190; 191]. To further improve security and privacy in 5G and future networks, the possibility of combining blockchain technology with other technologies, such as the NFV and SDN, will be a promising area for researchers to explore. * Energy efficiency: low-complexity and low-cost optimization models are essential to meet the next-generation wireless systems' critically demanding green standards, streamlined deployment, and effective energy-saving. Because of the higher density of wireless access points, wider bandwidths, and more antennas used in 5G and beyond-5G systems compared with past generations of wireless communication networks, there are now more environmental and financial problems. As a result, EE is now a crucial prerequisite for constructing a new wireless network. Highly detailed power models, effective energy management approaches, and more advanced optimization techniques are needed to overcome this challenge. * Green 5G communication: Mobile network operators and device manufacturers have repeatedly tried to create an energy-efficient network in response to the mounting concerns over harmful carbon emissions and their negative environmental impact. Numerous cutting-edge solutions have been proposed to lower conventional energy usage and establish a 5G communication network that is sustainable [192; 193; 194; 195], as one of the primary goals of the 5G system is to develop an energy-efficient network. One of these topics that require more research by academics is the utilization of green energy. Two examples of green 5G enablers that show potential for boosting energy efficiency and lowering reliance on traditional energy sources while also promoting environmental safety are energy harvesting and smart grid integration [194]. * Multi-connectivity: Finding a suitable solution to handle a large number of users who will be using the 5G network as a result of the ongoing rise in user demand for high bandwidth is important. The prospect of combining multiple radio access technologies, such as satellite and cellular technologies, with various types of new technologies, such as femtocells and picocells, to create a 5G network communication system is an ideal topic to investigate. Multi-connectivity, which is based on heterogeneous architecture and enables the user equipment to employ component carriers from different base stations and Wi-Fi access points, is one of these unique approaches [196; 197].