## 8 Challenges, Future Directions, and Lessons Learned
### Challenges  
Despite the enormous advantages of the 5G network, several challenges must be overcome to have a seamless and effective network deployment. Radio frequency (RF) spectrum scarcity is one of the most critical challenges. The available frequency has been heavily used, making it difficult for operators to obtain new ones. A few of the several challenges are discussed as follows. 1. Interference Management: A common issue with 5G networks is handling interference among 5G devices, especially as the number of communicating devices increases [180; 181]. This is because, as the number of communicating devices and user applications increases, the interference in the network will also increase. The 5G network interference may be a result of the UE receiving interference from multiple macro-cell base stations (MBSs), other UEs, or interferences from small-cell base stations (SBSs) [182]. It is, thus, essential to find an effective interference management approach for 5G processes, such as power control, channel allocation, and load balancing. 2. Environmentally Unfriendly: Wireless communication technologies consume lots of energy, leading to very high carbon emissions, which are very harmful to the environment. The current radio access network consumes approximately 70% to 80% of the total power [183]. It is, thus, very important to develop communication systems, technologies, and hardware that will be more energy-efficient and friendly to the environment. 3. Network and UE Security and Privacy: The several encouraging features of 5G networks pose difficult challenges in the design of secure 5G networks. A large number of new communication devices, for example, may be the source of various types of attacks, such as impersonation, denial-of-service (DoS), eavesdropping, repudiation attacks, and other cyber-attacks. Another issue occurs when a large volume of data needs to be transmitted quickly and securely. 4. Economic Challenges: In terms of deployment and user incentive, a paradigm change in future mobile communications technology would have substantial economic repercussions. Because of the extraordinarily high demand for communication services in urban and rural regions, communication equipment costs are growing rapidly. Another element contributing to the high cost is the requirement for mobile operators to construct a whole new infrastructure to fulfill the tremendous demand. As a result, from the standpoint of government, regulatory bodies, and network operators, the cost of infrastructure development, maintenance, administration, and operation should be affordable. Furthermore, the cost of utilizing D2D communication should be affordable, with D2D communication devices charging no more than the cost of using a BS's services. Furthermore, revenue growth is expected to be significantly lower than traffic growth; hence, 5G networks must be built to benefit all stakeholders, including the government, network operators, and customers. 5. Health and Safety: This is another critical challenge of the 5G network because the network utilizes the millimeter wave (mmWave) in addition to the microwave utilized in the previous generations and as discussed earlier. Although the millimeter wave has a very large bandwidth, it cannot travel over a long distance because various atmospheric parameters easily attenuate it. This, then, presents the need to install the 5G antennas very close to one another and to the users in small cells, which, thus, results in the constant exposure of the users to the millimeter wave radiation [184]. This had raised concerns even before the deployment of the network, where different analyses and experiments were conducted to evaluate the radiation level and its effect on human health [185].