# Machine Learning for Physical Layer in 5G and beyond Wireless Networks: A Survey
## 3 Physical Layer in 5G  
The physical layer, the bottom layer of 5G, is the only layer that provides a direction to the top layers. The physical layer's primary functions include encoding/decoding, error detection, modulation/demodulation, digital and analog beam-formation, RF processing, and MIMO handling. These functions are carried out through a wireless environment surrounded by an electromagnetic field, RF, signal transmission, waves, and antenna propagation [151]. The major role of 5G is to boost the SE. Still, as soon as the bandwidth increases, the signal suffers from electromagnetic interference, pulses, and radiation, which \begin{table}
\begin{tabular}{c c c c} \hline \hline  & **4G-LTE** & **5G** & **6G** \\ \hline Use cases & MBB & eMBB,URLLC,mMTC & FeMBB, ERLLC, \\  & & & umMTC, LDHMC, ELPC \\ \hline Applications & Mobile TV/Pay, high-definition video, voice, mobile internet & Telemedicine & Autonomous vehicles, VR/AR/360\({}^{\circ}\) video, IoT, V2X, UHD videos, wearable devices, smart-city & \begin{tabular}{c} Autonomous vehicles, tactile/haptic internet, space travel, holography, Internet of Bio-Nano-Things \\ \end{tabular} \\ \hline Network characteristics & Flat and all-IP & \
\begin{tabular}{c} Slicing, cloud, \\ software, virtual \\ \end{tabular} &
\begin{tabular}{c} Virtual, software, \\ intelligent, cloud, slicing \\ \end{tabular} \\ \hline \hline \end{tabular}
\end{table} 
Table 4: 4G, 5G, and 6G characteristics, application, and use cases. Figure 2: 5G wireless communication architecture. decreases the quality of the signal. These unwanted endowments affect the signal's attributes and create unnecessary noise (i.e., additive, multiplicative, Poisson, transient, and phase noise) [152]. Additive noise is further divided into nine categories: white, additive white Gaussian, black, Gaussian, pink/flicker, Brownian, contaminated Gaussian, power-law, and Cauchy noise). Consider a simple example of a current flowing from the standard circuit to understand this concept. The circuit's elements (wire, resistor, battery) are designed to receive zero EM radiations for the current to flow without any disturbance. However, if the circuit does not follow standard regulations, unlimited reflections will be received, and the current will not flow easily. On similar lines, the circuit becomes a mono-pole antenna if the standards are not maintained in the transmission and receiving process of the wireless medium [153]. Likewise, by enhancing SE, distortion will be generated, and the signal will suffer a significant number of reflections, interrupted noises, and reflections in its path. Still, this can be overcome through massive MIMO technology, as discussed in [154]. Table 5 shows a summary of studies on physical layer functions in 5G wireless networks. Technically, electromagnetic interference is originated either from radiated or conducted sources. In the electromagnetic field and wireless communication networks, the communicating path directly links with electromagnetic propagation. Radiated interference transfers the source signal's reflection, crosstalk, and disruptive noises using inductive and capacitive coupling devices. The above description proves that at high frequency or a low wavelength, unwanted radiations and interference originate from both internal and external means in 5G architecture. According to [PERSON]'s law, these radiations may vary in electric and magnetic fields in both time and space along with alternating flux density. Contrary to this, conductive interference occurs due to impedance mismatching and the coupling of neighboring resistive elements (inductors, capacitors, and resistors), and this whole process reduces the voltage standing wave ratio [155]. Consequently, massive MIMO in the 5G architecture endures multiple Tx/Rx radiations at high SE by achieving high data rates and ensuring throughput, reliability, and bandwidth performance to tackle these limitations [156].