Table 1.

Comparison of general works on millimeter wave propagation.

Ref	Frequency (GHz)	Scenario	Environment	Area of Focus/Methodology	Important results
[1]	0.5–100	LOS and NLOS	Indoor and Outdoor	Rappaport et al. did a thorough investigation of the fundamental propagation modelling techniques for the 5G networks in millimetre wave wireless communication systems.	The data gathered on shadowing and path loss will help with the improvement expected knowing fully well that millimetre wave is still in the research era especially in the use for 5G propagation.
[18]	14, 18 and 22	LOS and NLOS	Indoor	The measurements for two different path loss models at frequencies 14 GHz, 18 GHz and 22 GHz in an indoor situation for LOS and NLOS communication scenarios.	Analysis in the LOS indicates that CI and FI models are similar in execution at all frequencies used. In the CI model, a notable increment of the PLE was observed with the increase in frequency.
[25]	60	NLOS and LOS	Indoor	Analysis for the deployment of wireless high-speed local and personal area networks (WLANs/WPANs)	The millimetre wave propagation was observed to be characterised with propagation loss due to obstructions like walls, furniture and human blockage, to the level of between 25-30 dB.
[26]	23, 25, 28, and 38	LOS	Outdoor	Dynamic rain-aware link adaptation scheme to allow the system to fit the modulation and coding scheme for rain intensity levels	Result shows that the improbability lies between the theoretical and practical signal induced with rain in the range 1.5–4.5 dB, which was attenuation due to unstable conditions of the weather.
[27]	55–65	NLOS	Outdoor	On-chip wireless channel characteristics in conjunction with antenna implementation with near field and multipath propagation effects	It shows that the near field/transition region is where the propagation in the on-chip wireless channel is located thus making the channel complicated. It was also observed on antennas that the directional antennas are less affected by channel time dispersion, however, there are higher losses. The characteristics are reversed for omnidirectional antennas.
[28, 29, 30]	28 and 73	NLOS and LOS	Indoor	Analysis of millimetre wave propagation at different frequency bands	This implies that a rise in the path loss brings about a corresponding increase in the separation distance as a result of issues associated with the directivity of the antenna. Also, the scheme has been able to surmount the problem of bandwidth in electronic devices and opens the room for the development of low-cost infrastructure demand for the broadband mobile devices.
[32]	73	LOS and NLOS	Outdoor	Proposed a new technique known as Q learn-based scheme which encompasses edge computing function in an adjustable power and angle, which uses sub-6 GHz user equipment (UE)	The result from this investigation indicates that the user equipment with this scheme was able to achieve high energy efficiency and thus creating a room for a very robust and stable capacity in transmission.
[33]	60	NLOS and LOS	Outdoor and Indoor	Made us to realise that there are unallocated spectrum in the millimetre wave bands which is not making the full opportunity of the use of large antenna arrays for high speed data rates to be achievable.	It was discovered that millimetre wave communication has played a vital role in the deployment of 5G and it is expected that the fundamental improvement in radio and network will be made to help in the deployment of the upcoming 6G.
[34]	6 and 70	LOS and NLOS	Indoor	Fuschini et al [32] studied the narrowband and wideband features of in-room 70 GHz wireless channel using Ray Tracing (RT) simulations and making the measurement to be directional.	Observations show that reflection is the most pronounced mode of propagation, scattering is still present and it appears more than when the frequencies are below 6 GHz.
[35]	60	LOS	Indoor	The use of vector network analyser (VNA) was adopted to verify the response of three different types of antennas to the parameters of power loss.	The investigation shows that large aperture antennas has a significant guided wave effect than those with the narrow apertures, thus making the latter to have a more accurate path loss model. The measurements also indicate that placing an omnidirectional antenna in the access point (AP) gives an improved radiation than at a corner in the meeting room, since the shadowing effects caused by human obstruction are reduced.
[36]	2.4 and 60	LOS	Indoor	Wang et al. validated the eligibility of the SBR modelling method. The SBR method makes use of some propagation parameters such as path loss, RMS delay speed etc.	The results show that strong attenuation leads to smaller coverage area for millimetre waves than lower frequencies.
[37]	60	LOS and NLOS	Indoor	Verification of multipath effects in an indoor environment	This investigation shows that the multipath effect is more pronounced in indoor NLOS surroundings because the reflection effect on the received power is more as compared to the diffraction effect.
[38]	73	LOS and NLOS	Indoor	The work uses simulated surroundings at a frequency of 73 GHz in LOS as well as NLOS situations using SBR method.	The measured results are put in comparison with the simulated results of path loss to be able to authenticate the correctness of the SBR method. The profiles of the power angle as well as the power delay were critically examined.