## 8 Challenges, Future Directions, and Lessons Learned
### Lessons Learned  
In this section, a summary of lessons learned from this survey is presented.  
1. Just as noted above, the demand for greater mobility and coverage, communications with microsecond latency, and ultra-high data speeds have soared exponentially. To keep up with the rapid advancement of ground-breaking applications, wireless service requirements, and sophisticated social needs, wireless networks are currently being updated beyond 4G systems. Even if current technologies, such as the 4G architecture, are made to offer ubiquitous wireless access and significantly reduce latency, they also have to contend with growing technological demands for better QoS, more system capacity, and faster data rates. The deployment of flexible frequency plans, small cell technology, energy-efficient transmission techniques, and other disruptive technologies in conjunction with cutting-edge security and blockchain approaches are envisaged to address these requirements in 5G and beyond technologies.
2. With the introduction of 5G technology, greater frequency allocation and adaptable spectrum management methodologies will surely be necessary to offer seamless mobile broadband support for current and future data-hungry applications. This new frequency standardization should be compatible with the more contemporary technology and services that enable 5G and other wireless communication standards. Because 5G has been designed to operate on many spectrum bands, including the licensed, unlicensed, and shared bands of the FR1 and FR 2 bands, the combination of both frequency divisions will ensure a perfect balance between providing short-range and long-range communication. Thus, it is imperative that to satisfy a range of requirements, such as those for high data rates, high reliability, the Internet of Things, and low-latency communication, these frequencies must be carefully chosen.
3. The adoption of Device-to-Device communication approaches in the current 5G network and beyond will provide better spectrum efficiency, power management, coverage, and capacity expansion, especially when radio resource reuse is implemented. Additionally, to avoid the problem of intra-cell interference, solutions such as network coding and interference avoidance multi-antenna transmission can be employed.
4. Network virtualization can reduce the load on the 5G network, allowing for more flexible service operation and simpler deployment across several sites. The NFV interface contains several virtual tools, such as machines, hypervisors, and network operations, requiring high security. To guarantee data security and the detection of malicious software in virtual functions, trusted computing can be used to make sure that only trusted software is operating on the NFV interface.
5. Blockchain technology can be used to deploy 5G and future networks. It offers many security advantages, such as full control over information when transmitted over an unsecured, unprotected network, better system performance because it does not need a centralized system to work, and network simplification.
6. Multiple antennas can be used at the transmitter and receiver ends as a successful strategy to increase spectral efficiency while lowering hardware complexity. However, addressing inter-cell interference causes the implementation to be more challenging due to the energy-intensive nature of signal processing at the BS. One of the best techniques for simplifying the MIMO network is spatial modulation (SM), which sees the antenna arrays as a spatial constellation diagram, with each antenna carrying a series of data bits.
7. When many base stations are employed, green communication can be introduced to reduce energy consumption by the base stations and, hence, lessen environmental hazards. Energy efficiency can be increased by utilizing less RF transmit power.  
Another tactic for achieving energy efficiency is utilizing a duty cycle device, which disables some base stations with low traffic.