# Machine Learning for Physical Layer in 5G and beyond Wireless Networks: A Survey
## 3 Physical Layer in 5G
### Massive MIMO and Beamforming  
MIMO refers to the fact that multiple spatially separated users are handled by the antenna array simultaneously and frequently in terms of resources. This is different from the conventional MIMO system that uses the assumed propagation abilities of the array antenna at the same time and frequency for multiple spatially scattered wireless connections. Massive MIMO requires numerous antennas at the BS to blow up the available bandwidth within a given spectrum using diversification, enabling spatial channelization. The intensive amplification of bandwidth depends on three techniques: beam steering, increased data capacity, and diversity. When the number of antennas is great, it becomes easy to select the most favorable or constructive propagation path for performing signal processing at uplink and downlink [249]. Practically, various factors draw the reader's attention to achieving high SE while having a small amount of radiated power. The 5G network provides a high quality of services by reducing radiation in contrast to 4G through massive MIMO technology, but hardware implementations have different aspects. Factors such as network design signal processing techniques, signal modeling, and EMF analysis constitute a significant obstacle to achieving high efficiency [250]. The quality of transmitted signals can be enhanced by the presence of many antennas in free spaces. The combined effect of many antennas caters to path loss, fading, and scattering issues that are different from the individual performance of single-input and single-output (SISO) and MIMO approaches. MIMO increases the linearity and precision performance of each amplifier, thus improving the robustness and prevention of system failure [251]. The jamming problem can be overcome by using a massive MIMO approach; for example, MIMO networks exploit joint path estimation and decoding to obtain channel estimation, which creates signal cancellation from intended jammers. Moreover, with MIMO, BS derives less power, which means it will receive low electromagnetic exposure [252]. Following the guidelines in [253], Figure 5 shows a massive MIMO (low complex linear processing) system for the 5G ultra-dense scenario that enhances spectral and energy efficiency, reduces power consumption, lessens fading, reduces latency, robustness, and reliability, and aids in enhanced security. Multiple-user MIMO (MU-MIMO) increases SE and overcomes the flaws of single-user MIMO. In MU-MIMO, numerous BSs interact to become an effective antenna. The primary advantage of this process is to carry out received interference and generate a quality signal from it. This process is known as a coordinated multi-point (CoMP), which is thoroughly studied by the authors in [254] to improve network performance through diversity and interference suppression at high frequencies. The CoMP provides optimized performance in dealing with blockages present in mmWave. The amalgamation of micro diversity and CoMP techniques is considered as one of the potential solutions to reduce bottlenecks. Scientists have derived a physically reliable explanation for MIMO transmission using linear processing and the collective integration of data symbols to generate a transmitted waveform to provide broadband services to multiple users, such as audio and video. Hence,by ignoring the rank constraints and through convex optimization, the capacity of MIMO can be increased. This entire process creates decoding complexity on the receiving side. The dirty paper coding (DPC) technique results in significantly improved performance in terms of understanding the capacity boundary of MU-MIMO, but there are complex problems encountered and operational issues at TX/RX [255]. Beam-forming technology is used to boost and amplify the SE of wireless media intensively. This technology deals with the concentration of wireless signals for fast and reliable communication against particular devices, rather than spreading a signal in a broadcast manner [256].