# Machine Learning for Physical Layer in 5G and beyond Wireless Networks: A Survey
## 5 Challenges and Future Directions
On the other hand, ML will participate alone with one on one technology or manage the whole system intelligently. Besides combining the benefits of emerging technologies, there are still many crucial challenges to achieving the desired collaborative performance. Low-cost internet and the maximum digital transmission capacity of a channel are concerns for broadband transmission. Virtualization, BBU cooperation and clustering, and high fronthaul capacities are needed in C-RAN. In mmWave, the higher path loss due to higher carrier frequency and mMIMO, reciprocity error, signal-to-interference ratio (SIR), and channel coherence time requires an optimized solution. With ML, it is also necessary to choose the SDN solution; inter-operability, budget constraints, and security are primary concerns. The key challenges that need to be addressed using advanced ML-based algorithms are orchestration and integration in hybrid networks, slice isolation mechanism, security, and privacy. * 10. Backhaul 5G wireless network architecture design: It is a pivotal challenge to deploy a new backhaul network architecture design and security-aware protocols for heterogeneous ultra-dense small cell network use cases. The massive wireless network traffic caused congestion and later collapsed the backhaul network [363]. Motivations behind the backhaul wireless network architecture design are ML-based mobility within small cell networks and the optimization of the cell load distribution. Besides this, ML-based admission and congestion control algorithms are also required for quality of service and experience, mainly focused on the backhaul network. Challenges in 5G with regard to next-generation mobile networks (NGMN) are discussed and highlighted as follows [364]:  
11. Flash network traffic: The potential of large-scale events causing significant changes in network traffic patterns, either accidentally or maliciously, increases as the network capacity and the number of UEs grows. While maintaining an acceptable level of performance, the 5G system must prevent significant fluctuations in traffic utilization and be adaptable to them when they do occur. ML-based algorithms are capable of learning the environments and suggesting optimized outputs in these situations. 12. Security of radio interfaces: In existing internet generations, keys for radio interface encryption are obtained in the home core network and then sent to the visiting radio network using signaling channels such as SS7 or Diameter. When sent between network nodes, the exposed cipher key is an example of a GSM network. The connection between operators' signaling systems should be adequately secured using ML so that the radio interface session keys can be transferred via SS7 and Diameter, and such exposure is prevented. 13. User plane integrity: There was no explicit user data plane or cryptographic integrity protection until the second-generation internet; in addition, the third and fourth generation have protection, but still not for user plane data. The transport layer, application layer, or bearer layer integrity with encryption is used if data integrity is required. There is also a risk of a man-in-the-middle attack, and session hijacking is also possible. In addition, the 5G network will not add integrity protection to user plane data but at the transport or application layer with the help of ML-based algorithms. 14. Mandated security in the network: There are service-driven restrictions in the security architecture, and generally, there are measures in place to minimize the effects of these restrictions, and these measures are often not mandatory in current cellular specifications. Reducing the security dependency on the access network and on the security provided on intranet interfaces is possible, but this dependency is unlikely to be eliminated entirely. The security measures using ML must be embedded in architectural design, and otherwise, the system will not work at all--this approach is the best solution to implement mandatory security. 15.