Table 2.

Evaluation of the relevant works on path loss models for millimeter wave propagation.

Ref	Frequency (GHz)	Environment	Scenario	Methodologies	Model	Important Reasons
[36]	60	Indoor	NLOS and LOS	Efforts were made to properly scrutinize the workability and the performance of the SBR/IM method on the basis of accuracy, reliability and the ease of use. The simulation of the important components of the channel propagation such as RMS delay spread and the path loss was carried out.	SBR/IM method	It was inferred that the RMS delay spread has a very low value and a rise in the coherent bandwidth at a frequency of 60 GHz, but at 2.4 GHz, there is a tendency for anti-interference.
[38]	2–73	Urban microcell	LOS and NLOS	The analysis of millimetre-wave channel characteristics in urban microcell environment based on the SBR method adopts a vertically polarized antenna which is omnidirectional in nature in the transmitter and the receiver section.	SBR technique	The results of this method indicate that some of the features of the millimetre wave channel in UMi is a good background for propagation but only in the outdoor scenarios.
[39]	28 and 73	Indoor	LOS and NLOS	An experiment on ultra-wide band propagation with the statistics of the large scale path loss for present as well as future use was conducted by adopting directional horn antennas.	CI and FI	It was observed during analysis that simple CI and FI models can be used to model large scale path loss (with distance and frequency) in millimetre wave indoor wireless channels with the correctness intact while using one or two functions that have a connection with the transmitted power.
[42]	2–73.5	Indoor	LOS and NLOS	Data were obtained from about 20 measurement campaigns for frequency bands 2 GHz and 73.5 GHz over a path of distances starting at 5 m and stopping at 1429 m.	CI and ABG	Observation of the analysis of the result shows that the simulation accuracy of the CI model is better than the ABG model even though it is a three parameter model. The former offers more stable and acceptable performance in all the frequencies and the range of distance considered in the course of the experiment which is not so in the latter.
[43]	2–73	Urban microcell, shopping malls and indoor office	LOS and NLOS	The analogy of three most common models of path loss that is CI, CIF and ABG models was done in the range of data sets of distance ranging from 4 m to 1238 m and frequency of 2–73 GHz.	CI, CIF and ABG	It was concluded that for outdoor use, the CI model is more preferable but in the case of indoor modelling, the CIF is the better model.
[45]	0.5–100		LOS and NLOS	A study of rural macrocell path loss models for millimetre wave wireless communications gives a comprehensive understanding of the present 3GPP, RMa LOS as well as NLOS scenarios of models of path loss in range of frequencies of 0.5 GHz–100 GHz. The use of directional antennas was adopted for real-time measurement campaign in a rural area	CI and CIH	The observation queries continued application of the present 3GPP RMa path loss models for frequencies above 6 GHz. The result of the measured data validates the accuracy, reliability and frequency dependence of the CIH model even beyond the first meter distance of propagation.
[46]	6.5, 10.5, 15, 19, 28 and 38	Indoor	LOS and NLOS	Characteristics analysis of millimetre wave channels in the frequency bands of 6.5 GHz, 10.5 GHz, 15 GHz, 19 GHz, 28 GHz and 38 GHz in an indoor scenario was done with measurement campaign taken across 4000 power delay profiles using horn directional antenna in the receiver and an omnidirectional antenna in the transmitting section.	Path loss models, known as frequency attenuation model ABGX, CIX as well as ABG with XPD were proposed.	The results of this model show its simplicity, less path loss exponents, better RMS delay spread and good dispersion factor values.
[47]	19	Outdoor	LOS and NLOS	Examination of the features of millimetre wave 5G channels in order to find out the components of path loss together with dispersion of time for an outdoor scenario was carried out.	Free space path loss model	This experiment deduced that there is a very great drop in the mean values of the delay spread in both LOS and NLOS situations when adopting the use of horn-horn as well as Horn-Omni antenna applications. It indicates that the influence of the delay spread is minimal to the directional horn antenna at the transmitting side.
[48]	40	Indoor	LOS and NLOS	Measurements were conducted in order to access the functionality of 5G wireless propagation and the path loss exponent.	CI and FI Models	Results of this experiment show that in both the LOS as well as NLOS the FI together with CI models have similar values for the PLE and slope line. This is a very strong indication that the FI together with the CI model have the best performance for indoor use at the frequency band of 40 GHz in a 5G propagation system.
[49]	10, 20, and 26	Indoor and Outdoor	LOS and NLOS	The measurement of the diffraction as well as the prediction models of the signal strengths in environments where there are corners, irregular objects, and pillars in both indoor and outdoor scenarios were conducted. The continuous wave channel sounder with similar antenna pairs that is a directional horn type at the receiving as well as at the transmitter section were used.	The KED was the reference model.	Observation shows that the KED fits accurately with the indoor scenarios as well as with the rounded corners in the outdoor situations. However, there is a notice of the increase in diffraction as a result of rise in frequency in the outdoor scenarios, but less in indoor environments.
[52]	2.4	Indoor	LOS and NLOS	A laboratory measurement for indoor scenario was conducted for LOS and NLOS propagation at a frequency of 2.4 GHz for proposing a path loss with the spatial diversity.	FI and log-distance models	Results show that the average path loss value in four coherent signals for a LOS scenario brings about a path loss reduction of 89.4% in comparison to the technique of using NCC scenario which only reduces the path loss by 55.98%.
[53]	27.5	Indoor	NLOS and LOS	The measurement took place in an environment that is totally blocked for a deep fading. The model of dual slope, which takes into account this deep fading, was adopted. The performance of the model was compared with the single slope path loss model which is the most common in both LOS and NLOS scenarios using omni directional antennas.	Dual slope model	Results show that the proposed dual slope model has a very good fitting with the values of the measurement campaigns but with a little shadow factors.
[56]	28–73	Indoor	NLOS and LOS	Capacity analysis of non-orthogonal multiple access with mm wave massive MIMO systems	Uniform random single-path (UR-SP) model	Observations show that there is an improvement in the capacity when compared with the present LTE systems. Other results indicate the improvement and compromise between energy usage and the capacity advancement which can also be an advantage in the mm wave propagation.