## 2 Related Work  
One key enabling solution for meeting the extreme data demand growth is the utilization of previously unused mmWave frequency spectrum. Although the mmWave spectrum offers a better way to enhance channel bandwidth and network capacity, it also causes various channel propagation issues that must be addressed prior to setting any standards for the 5G spectrum. In order to mitigate these issues, numerous studies have been performed to investigate the behavior of the propagation channel. For instance, in [41], an in-depth survey was conducted with details of various path loss models explained, based on their similarities and differences. This paper categorized more than 50 proposed models into several various groups, such as basic models, supplementary models, fading models, and ray models. Moreover, the authors in [42] presented a simple path loss model, as compared to the ITU indoor propagation model by 3 GPP that is currently in use. The authors conducted the experiments at 28 and 73 GHz frequency bands in an ultra-dense indoor office environment. Similarly, a path loss model for large-scale high-density urban scenarios at 28 and 73 GHz frequency bands has been reported in [43]. The \"3D ray-tracing\" simulation software program was utilized to design the propagation path loss models for the large-scale next-generation network [44; 45; 46]. The paper presented in [47] reported a probabilistic omnidirectional propagation path loss model using a probability distribution at the 28 and 73 GHz frequency bands. It applied CI free space reference distance (FSRD) path loss model and FI path loss model. It proposed the probabilistic weighting function that appears helpful for choosing between the best communication link for the LOS and NLOS settings. In [48], the authors investigated the channel at 73 GHz frequency band with directional antennas. Hence, an experiment was conducted in a high-density urban scenario using various combinations of a user's location for mobile and backhaul with more than 30 access points. The authors stated that omnidirectional models suffer from more path loss at higher frequency bands, thus the use of mmWave spectrum, particularly the E-band (71-76 GHz and 81-86 GHz), with the help of beam-combining and beam-steering, which could lead to a usable system for future wireless technologies through the exploitation of multipath in a high-density urban environment. In [49], the authors presented a channel measurement for RF wideband for both NLOS and LOS peer-to-peer scenarios via narrow beam antennas at 38 and 60 GHz frequency bands. The experiment was conducted using broadband sliding correlator channel sounder and steerable antenna. The outcomes were presented by characterizing the propagation path loss in dB and time delay spread for many real-world scenarios. The antenna-pointing angles relationship and root-mean-square delay spread were analyzed, and showed that the path loss and the distance between sources to destination are linearly proportional to each other. The outcomes proved that the use of a local multipoint distribution system (LMDS) spectrum seemed better in terms of user throughput, transmission time, and power optimization, particularly for the LOS scenario. Another method in [50] assessed the use of beam forming at 28 and 38 GHz mmWave bands by proposing a new path loss model that employed the industry-standard path loss models with several modifications. It was proven that the use of pointed single best directions for directional antennas could minimize the required number of the 5G BSs, as compared with the arbitrary pointing angles approach for directional antennas. The approach in [51] proposed a height-dependent path loss model for NLOS urban micro outdoor scenarios. The authors utilized omnidirectional antennas and a high-power gain amplifier, which were later compared with the outcomes of 3 GPP path loss models at 3.5 GHz. They also investigated the impact of shadowing and penetration losses caused by the human body and herbage environment.