Choose a Propagation Model

Introduction

Propagation models allow you to predict the propagation and attenuation of radio signals as the signals travel through the environment. You can simulate different models by using thepropagationModelfunction. Additionally, you can determine the range and path loss of radio signals in these simulated models by using therangeandpathlossfunctions.

The following sections describe various propagation and ray tracing models. The tables in each section list the models that are supported by thepropagationModelfunction and compare, for each model, the supported frequency ranges, model combinations, and limitations.

Atmospheric

Atmospheric propagation models predict path loss between sites as a function of distance. These models assume line-of-sight (LOS) conditions and disregard the curvature of the Earth, terrain, and other obstacles.

Model	Description	Frequency	Combinations	Limitations
freespace (`FreeSpace`)	Ideal propagation model with clear line of sight between transmitter and receiver	No enforced range	Can be combined with rain, fog, and gas	Assumes line of sight
rain (`Rain`)	Propagation of a radio wave signal and its path loss in rain. For more information, see[3]。	1 GHz to 1000 GHz	Can be combined with any other propagation model	Assumes line of sight
gas (`Gas`)	Propagation of radio wave signal and its path loss due to oxygen and water vapor. For more information, see[5]。	1GHz to 1000 GHz	Can be combined with any other propagation model	Assumes line of sight
fog (`Fog`)	Propagation of the radio wave signal and its path loss in cloud and fog. For more information, see[2]。	10GHz to 1000 GHz	Can be combined with any other propagation model	Assumes line of sight

Empirical

Like atmospheric propagation models, empirical models predict path loss as a function of distance. Unlike atmospheric models, the close-in empirical model supports non-line-of-sight (NLOS) conditions.

Model	Description	Frequency	Combinations	Limitations
close-in (`CloseIn`)	Propagation of signals in urban macro cell scenarios. For more information, see[1]。	No enforced range	Can be combined with rain, fog, and gas	—

Terrain

Terrain propagation models assume that propagation occurs between two points over a slice of terrain. Use these models to calculate the point-to-point path loss between sites over irregular terrain, including buildings.

Terrain models calculate path loss from free-space loss, terrain and obstacle diffraction, ground reflection, atmospheric refraction, and tropospheric scatter. They provide path loss estimates by combining physics with empirical data.

Model	Description	Frequency	Combinations	Limitations
longley-rice (`LongleyRice`)	Also known as Irregular Terrain Model (ITM). For more information, see[4]。	20 MHz to 20 GHz	Can be combined with rain, fog, and gas	Antenna height minimum is 0.5 m and maximum is 3000 m
tirem (`TIREM`)	Terrain Integrated Rough Earth Model™	1 MHz to 1000 GHz	Can be combined with rain, fog, and gas	Requires access to external TIREM library Antenna height maximum is 30000 m

Ray Tracing

Ray tracing models, represented byRayTracingobjects, compute propagation paths using 3-D environment geometry[7][8]。他们确定的路径损耗和相移each ray using electromagnetic analysis, including tracing the horizontal and vertical polarizations of a signal through the propagation path. The path loss includes free-space loss and reflection losses. For each reflection, the model calculates losses on the horizontal and vertical polarizations by using the Fresnel equation, the incident angle, and the relative permittivity and conductivity of the surface material[5][6]at the specified frequency.

While the other supported models compute single propagation paths, ray tracing models compute multiple propagation paths.

These models support both 3-D outdoor and indoor environments.

Ray Tracing Method	Description	Frequency	Combinations	Limitations
shooting and bouncing rays (SBR)	Supports calculation of approximate propagation paths for up to ten path reflections. The locations of receiver sites calculated by the SBR method are not exact. The accuracy of the calculated propagation paths decreases as the length of the paths increases. Computational complexity increases linearly with the number of reflections. As a result, the SBR method is generally faster than the image method.	100 MHz to 100 GHz	Can be combined with rain, fog, and gas	Does not include effects from diffraction, refraction, and scattering
image	Supports up to two path reflections and calculates exact propagation paths. Computational complexity increases exponentially with the number of reflections.	100 MHz to 100 GHz	Can be combined with rain, fog, and gas	Does not include effects from diffraction, refraction, and scattering

SBR Method

This figure illustrates the SBR method for calculating propagation paths from a transmitter,Tx, to a receiver,Rx。

$Ray tracing reflection and diffraction using the SBR method$

The SBR method launches many rays from a geodesic sphere centered atTx。The geodesic sphere enables the model to launch rays that are approximately uniformly spaced.

Then, the method traces every ray fromTxand can model different types of interactions between the rays and surrounding objects, such as reflections, diffractions, refractions, and scattering. Note that the implementation considers only reflections.

When a ray hits a flat surface, shown asR, the ray reflects based on the law of reflection.
When a ray hits an edge, shown asD, the ray spawns many diffracted rays based on the law of diffraction[9][10]。Each diffracted ray has the same angle with the diffracting edge as the incident ray. The diffraction point then becomes a new launching point and the SBR method traces the diffracted rays in the same way as the rays launched fromTx。A continuum of diffracted rays forms a cone around the diffracting edge, which is commonly known as aKeller cone[10]。能源部的当前实现SBR方法s not consider edge diffractions.

For each launched ray, the method surroundsRxwith a sphere, called a reception sphere, with a radius that is proportional to the angular separation of the launched rays and the distance the ray travels. If the ray intersects the sphere, then the model considers the ray a valid path fromTxtoRx。

Image Method

This figure illustrates the image method for calculating the propagation path of a single reflection ray for the same transmitter and receiver as the SBR method. The image method locates the image ofTxwith respect to a planar reflection surface,Tx'。Then, the method connectsTx'andRxwith a line segment. If the line segment intersects the planar reflection surface, shown asRin the figure, then a valid path fromTxtoRxexists. The method determines paths with multiple reflections by recursively extending these steps.

Ray tracing using the image method

References

[1]Sun, Shu, Theodore S. Rappaport, Timothy A. Thomas, Amitava Ghosh, Huan C. Nguyen, Istvan Z. Kovacs, Ignacio Rodriguez, Ozge Koymen, and Andrzej Partyka. “Investigation of Prediction Accuracy, Sensitivity, and Parameter Stability of Large-Scale Propagation Path Loss Models for 5G Wireless Communications.”IEEE Transactions on Vehicular Technology65, no. 5 (May 2016): 2843–60. https://doi.org/10.1109/TVT.2016.2543139.

[2]International Telecommunications Union Radiocommunication Sector.Attenuation due to clouds and fog。Recommendation P.840-6. ITU-R, approved September 30, 2013. https://www.itu.int/rec/R-REC-P.840-6-201309-S/en.

[3]International Telecommunications Union Radiocommunication Sector.Specific attenuation model for rain for use in prediction methods。Recommendation P.838-3. ITU-R, approved March 8, 2005. https://www.itu.int/rec/R-REC-P.838-3-200503-I/en.

[4]Hufford, George A., Anita G. Longley, and William A.Kissick.A Guide to the Use of the ITS Irregular Terrain Model in the Area Prediction Mode。NTIA Report 82-100. National Telecommunications and Information Administration, April 1, 1982.

[5]International Telecommunications Union Radiocommunication Sector.Effects of building materials and structures on radiowave propagation above about 100MHz.Recommendation P.2040-1. ITU-R, approved July 29, 2015. https://www.itu.int/rec/R-REC-P.2040-1-201507-I/en.

[6]International Telecommunications Union Radiocommunication Sector.Electrical characteristics of the surface of the Earth。Recommendation P.527-5. ITU-R, approved August 14, 2019. https://www.itu.int/rec/R-REC-P.527-5-201908-I/en.

[7]Yun, Zhengqing, and Magdy F. Iskander. “Ray Tracing for Radio Propagation Modeling: Principles and Applications.”IEEE Access3 (2015): 1089–1100. https://doi.org/10.1109/ACCESS.2015.2453991.

[8]Schaubach, K.R., N.J. Davis, and T.S. Rappaport. “A Ray Tracing Method for Predicting Path Loss and Delay Spread in Microcellular Environments.” In[1992 Proceedings] Vehicular Technology Society 42nd VTS Conference - Frontiers of Technology, 932–35. Denver, CO, USA: IEEE, 1992. https://doi.org/10.1109/VETEC.1992.245274.

[9]International Telecommunications Union Radiocommunication Sector.Propagation by diffraction。Recommendation P.526-15. ITU-R, approved October 21, 2019. https://www.itu.int/rec/R-REC-P.526-15-201910-I/en.

[10]Keller, Joseph B. “Geometrical Theory of Diffraction.”Journal of the Optical Society of America52, no. 2 (February 1, 1962): 116. https://doi.org/10.1364/JOSA.52.000116.