# Machine Learning for Physical Layer in 5G and beyond Wireless Networks: A Survey
## 3 Physical Layer in 5G
### Physical Layer Issues  
Mobile fronthaul indicates the connection of links using a centralized radio access network (C-RAN) architecture in cellular networks. It acts as a transmission medium between the remote radio unit (RRU) and baseband unit (BBU) via an optical cable. Still, it also brings research challenges, including low latency, jitter, transmission rate, and synchronization. These challenges are severe, particularly in the case of massive MIMO, where transmission faces inevitable consequences [286]. Distortion occurs due to amplitude interference, resulting in the transmission of unwanted signals between the channels, called crosstalk. The transmitted waveform produces interference or a coupling region in the neighboring signals, affecting the original signal by uncorrelated data patterns [287]. Electronic circuit elements (capacitors, inductors, and resistors) are the major sources of this coupling effect. These sources generate undesirable coupling results in the neighboring circuits. Crosstalk has two different forms:  
* Near end crosstalk (NEXT): In NEXT, the aggressor signal performs couples with the victim signal in the opposite direction. * Far end crosstalk (FEXT): In FEXT, both the victim and aggressor signal travel in the same direction [288]. At high speed, the crosstalk issue also reduces signal quality, and the entire system is affected by its associated impairments. To overcome the crosstalk challenge, it is essential to modify the impedance separation between the victim and the aggressor signal. The trace impedance has a direct relation with the crosstalk; i.e., by lowering the trace impedance in which we can decrease the crosstalk effect, as indicated in [289]. Increasing the guard band size between frame slots and the cyclic prefix (CP) can help to reduce the crosstalk noise in the communication link. Following the guidelines in [290], Figure 7 shows the frame, subframe, and subcarrier spacing standards of 5G NR and the numerology effect on a single frame. In 5G CN, an indirect relation exists between crosstalk noise and the distance between communication links, and the crosstalk factor reduces while the distance increases [291]. On the other hand, the presence of a shield among these lines reduces the undesirable coupling effects generated by crosstalk [292]. An improved version of crosstalk cancellation digital pre-distortion (CTC-DPD) is presented in [293], exploiting a novel estimating scheme and introducing a decoupling technique. This is used to compensate for crosstalk, possible delay, the non-linearity of power amplifiers (PA), and the inconsistency  
Figure 6: 5G radio resource allocation. of transmitting channels in the MIMO system. The author suggested that the design estimates the coupling coefficient and the delay at the baseband signal through shared feedback and extracts this information to improve the digital pre-distortion. Simulation graphs showed high accuracy and an improvement in crosstalk cancellation compared to previous CTC-DPC models. Before cancellation, the coupled power is simulated at \(-20\) dB, whereas the coupled power becomes \(-50\) dB after performing crosstalk cancellation. In 5G, millimeter wavelengths (mmWave 30 GHz to 300 GHz) are used to increase the wireless capacity and achieve a higher data rate speed, as frequency has an inverse relationship with wavelength, and at low wavelengths, the user may suffer attenuation, reflection, and scattering. Precise system models and architecture designs are essential for air interfaces. Fifth-generation network applications such as the tactile internet, tele-hospitals, M2M communication, and vehicle to everything (V2X) communication require an exceptionally low latency rate, which cannot be achieved easily due to the presence of this propagation challenges [294]. In [295], the authors analyzed the receiver sensitivity prerequisites in a wireless 5G NR environment related to the single-to-noise ratio (SNR) constraints at BS to achieve maximum results for throughput.