## 2 Related Work
The large-scale path loss model was analyzed in [52] by using co and cross antenna polarizations for directional Tx and Rx antennas, particularly at the 32 GHz frequency band. The study showed that in a comparison of the FI model, the use of CI free space path loss model displayed some limitations in the NLOS environment. As for the propagation channel analysis, the urban measurement campaign was conducted with a synchronous channel sounder system at the 28 GHz frequency channel [53]. This paper investigated the omnidirectional spatial channel model for the NLOS path based on wideband propagation measurements. The authors in [54] assessed the indoor channel using a 28 GHz multibeam MIMO prototype. The authors performed multibeam forming using continuous aperture-phased MIMO (CAP-MIMO) that utilized a lens antenna array. They claimed to use a first-time simultaneous multibeam channel measurement of about 4\({}^{\circ}\) beam width. They also stated that by using a CAP-MIMO transceiver in place of the existing conventional sounder it not only enabled spatial resolution, but also allowed them to practice using the simultaneous multibeam, which enhanced network performance. In the study reported in [55], the investigation achieved the angularly resolved path loss measurement in an urban microcell scenario at the 28 GHz frequency band. The authors stated that most of the signal energy was lost in the NLOS path, as compared with the LOS path. Nevertheless, only a few of the small refraction and reflection objects gave better signal quality, as received for the LOS path. The two similar approaches in [56; 57] compared the propagation characteristics of 11, 16, 28, and 38 GHz by utilizing space-alternating generalized expectation (SAGE) maximization to process the measurement data. This study analyzed some new massive MIMO propagation properties, such as spherical wavefront property, cluster birth-death property, and non-stationary over the array, by investigating the variations of channel parameters. Their work showed that these massive MIMO propagation characteristics could be used for large antenna array systems for mmWave channel modelling. Another study in [58] examined the frequency bands at 60 GHz and 73 GHz using the Stanford University Interim (SUI) and standard theoretical free space (FS) path loss models, including their potential ability to deliver performance. The results proved, explicitly, that accounting for the number of beams (or discrete angles) combined at the Rx could result in receiving a strong signal power. An approach in [59] studied the urban microcell wideband measurement at 28 and 38 GHz by using a channel sounder equipped with an omnidirectional and steerable directional antenna. This study characterized various small- and large-scale parameters, such as angular speed, shadow fading, delay spread path loss, and clustering, in order to determine the potential of mmWave frequencies. Their results predicted that the PLEs value of 2.0 for the LOS environment, while, for the NLOS it hit 3.0. They concluded that the LOS PLEs of the UMi environment at 28 and 38 GHz were similar in value to each other and were quite close to the theoretical free space value of 2. Moreover, the large- and small-scale parameters at 28 GHz were similar as achieved at 38 GHz. In [60], the dual-directional path loss model that incorporated both the LOS and NLOS transmissions was proposed. The authors analyzed the results in terms of average data rates achievable and coverage probability. Furthermore, in [61], the access links and backhaul links for D2D communication have been examined, along with their impact due to outdoor path loss models in the presence of solar radio emissions at a frequency band of 60 GHz. The research highlighted that the solar radio emission could be useful in minimizing carrier-to-noise ratio (CNR) and in increasing the corresponding PLE values for path loss channel models with large-scale propagation.