[[File:manuals/emagware/emcube/modules/propagation/wireless-propagation-primer/the-need-for-wireless-propagation-modeling/urban.png]]
== A Wireless Propagation Primer ==
=== Free Space Propagation Channel ===
In a free-space line-of-sight (LOS) communication system, the signal propagates directly from the transmitter to the receiver without encountering any obstacles (scatterers). Electromagnetic waves propagate in the form of spherical waves with a functional dependence of e<sup>j(Ï?</sup><sup>t-k<sub>0</sub>R)</sup>/R, where R is the distance between the transmitter and receiver, Ï ? = 2Ïf2pf, f is the signal frequency, k<sub>0</sub> = Ï?/c = 2Ï2p/λ?<sub>0</sub>, c is the speed of light, and λ?<sub>0</sub> is the free-space wavelength at the operational frequency. By the time the signal arrives at the location of the receiver, it undergoes two changes. It is attenuated and its power drops by a factor of 1/R<sup>2</sup>, and additionally, it experiences a phase shift of 2ÏR2pR/λ?<sub>0</sub>, which is equivalent to a time delay of R/c. The signal attenuation from the transmitter to the receiver is usually quantified by '''Path Loss''' defined as the ratio of the received signal power (P<sub>R</sub>) to the transmitted signal power (P<sub>T</sub>). Assuming isotropic transmitting and receiving radiators (i.e. radiating uniformly in all directions), the Path Loss in a free-space line-of-sight communication system is given by Friisâ formula:
[[File:manuals/emagware/emcube/modules/propagation/wireless-propagation-primer/free-space-propagation-channel/friis1.png]]
[[File:manuals/emagware/emcube/modules/propagation/wireless-propagation-primer/free-space-propagation-channel/los.png]]<br /> Figure 1: A Line-of-Sight (LOS) Propagation Scenario.
=== Multipath Propagation Channel ===
Free-space line-of-sight communications is an ideal scenario that is typically used to model aerial or space applications. In ground-based systems, the presence of the ground as a very large reflecting surface affects the signal propagation to a large extent. Along the path from a transmitter to a receiver, the signal may also encounter many obstacles and scatterers such as buildings, vegetation, etc. In an urban canyon environment with many buildings of different heights and other scatterers, a line of sight between the transmitter and receiver can hardly be established. In such cases, the propagating signals bounce back and forth among the building surfaces. It is these reflected or diffracted signals that are often received and detected by the receiver. Such environments are referred to as âmultipathâ. The group of rays arriving at a specific receiver location experience different attenuations and different time delays. This gives rise to constructive and destructive interference patterns that cause fast fading. As a receiver moves locally, the receiver power level fluctuates sizably due to these fading effects.
Figure 1: A multipath propagation scene showing all the rays arriving at a particular receiver.
=== The SBR Method ===
EM.Cube's Propagation Module provides an asymptotic ray tracing simulation engine that is based on a technique known as Shooting-and-Bouncing-Rays (SBR). In this technique, propagating spherical waves are modeled as ray tubes or beams that emanate from a source, travel in space, bounce from obstacles and are collected by the receiver. As rays propagate away from their source (transmitter), they begin to spread (or diverge) over distance. In other words, the cross section or footprint of a ray tube expands as a function of the distance from the source. EM.Cube uses an accurate equi-angular ray generation scheme to that produces almost identical ray tubes in all directions to satisfy energy and power conservation requirements.
A new reflected ray is generated at the specular point, which starts traveling and bouncing around in the scene. If the obstructing surface is penetrable, a second transmitted ray is generated and added to the scene. If the ray hits the edge of an obstacle, it is diffracted from that edge. This leads to the creation of a cone of new rays, which greatly complicate the computational problem. The Uniform Theory of Diffraction (UTD) is used to calculate the wedge diffraction coefficients at the edges of scattering blocks. Note that reflection, transmission and diffraction coefficients are all dependent on the polarization of the incident plane wave.
A receiver may receive a large number of rays: direct line-of-sight rays from the transmitter, rays reflected or diffracted off the ground or terrain, rays reflected or diffracted from buildings or rays transmitted through buildings. Each received ray is characterized by its power, delay and angles of arrival, which are the spherical coordinate angles θ ? and Ï f of the incoming ray. The actual signal received and detected by the receiver is the superposition of all these rays with different power levels and different time delays. Most of the time, you will be interested in the coverage map of an area, which shows how much power is received by a grid of receivers spread over the area from a given fixed transmitter.
=== Ray Reflection & Transmission ===
The incident, reflected and transmitted rays are each characterized by a triplet of unit vectors:
[[File:manuals/emagware/emcube/modules/propagation/wireless-propagation-primer/ray-reflection/frml6.png]]
=== Penetration Through Thin Walls Or Surfaces ===
In "Thin Wall Approximation", we assume that an incident ray gives rise to two rays, one is reflected at the specular point, and the other is transmitted almost in the same direction as the incident ray. The reflected ray is assumed to originate from a virtual image source point. Similar to the case of reflection and transmission at the interface between two dielectric media, here too we have three triplets of unit vectors, which all form orthonormal basis systems.
[[File:manuals/emagware/emcube/modules/propagation/wireless-propagation-primer/transmission-through-thin-walls/frml21.png]]
=== Wedge Diffraction From Edges ===
For the purpose of calculation of diffraction from building edges, we define a "Wedge" as having two faces, the 0-face and the ''n''-face. The wedge angle is α a = (2-''n'')Ïp, where the parameter ''n'' is required for the calculation of diffraction coefficients. All the diffracted rays lie on a cone with its vertex at the diffraction point and a wedge angle equal to the angle of incidence in the opposite direction. A diffracted ray is assumed to originate from a virtual image source point. Three triplets of unit vectors are defined as follows:
* [[File:manuals/emagware/emcube/modules/propagation/wireless-propagation-primer/wedge-diffraction/frml19_tn.png]] representing the unit vector normal to the edge and lying in the plane of the 0-face, the unit vector normal to the 0-face, and the unit vector along the edge, respectively.
where ''N<sup>±</sup>'' are the integers which most closely satisfy the equations 2''n''π''N<sup>±</sup>'' - ν = ±π.
=== SBR As An Asymptotic EM Solver ===
EM.Cube's SBR simulation engine can be used as a versatile and powerful asymptotic electromagnetic (EM) solver. If you compare EM.Cube's Propagation Module with its other computational modules, you will notice a lot of similarities. While other modules group objects primarily by their material properties, Propagation Module categorizes the types of obstructing surfaces. Besides sharing the same ray-surface interaction mechanisms, all the objects belonging to a surface group also share the same material properties. Propagation Module offers similar source types and similar observable types as the other computational modules. For instance, the Hertzian dipole sources used in a SBR simulation are identical to those offered in PO, MoM3D and Planar modules. The plane wave sources are identical across all computational modules. Propagation Module's sensor field planes, far field observables (either radiation patterns or RCS) and Huygens surfaces are all fully compatible with EM.Cube's other computational modules.
As an asymptotic EM solver, the SBR engine can be used to model large-scale electromagnetic radiation and scattering problems. An example of this kind is radiation of simple or complex antennas in the presence of large scattering platforms. You have to keep in mind that by using an asymptotic technique in place of a full-wave method, you trade computational speed and lower memory requirements for modeling accuracy. In particular, the SBR method cannot take into account the electromagnetic coupling effects among nearby radiators or scatterers. However, when your scene spans thousands of wavelengths, an SBR simulation might often prove to be your sole practical solution.
=== Novelties Of EM.Cube's SBR Solver ===
EM.Cube's new SBR simulation engine utilizes an intelligent ray tracing algorithm based on the concept of k-dimensional trees. A k-d tree is a space-partitioning data structure for organizing points in a k-dimensional space. k-d trees are particularly useful for searches that involve multidimensional search keys such as range searches and nearest neighbor searches. In a typical large radio propagation scene, there might be a large number of rays emanating from the transmitter that may never hit any obstacles. For example, upward-looking rays in an urban propagation scene quickly exit the computational domain. Rays that hit obstacles on their path, on the other hand, generate new reflected and transmitted rays. The k-d tree algorithm traces all these rays systematically in a very fast and efficient manner. Another major advantage of k-d trees is the fast processing of multi-transmitters scenes. Unlike the previous versions of the SBR solver which could handle one transmitter at a time and would superpose all the resulting rays at the end of the simulation, the new SBR shoots rays from all the transmitters at the same time.
EM.Cube's new SBR simulation engine performs fully polarimetric and coherent SBR simulations with arbitrary transmitter antenna patterns. The new engine solves directly for the vectorial field components at the receiver locations or field observation points. This is far more rigorous than the previous versions of the SBR solver which primarily utilized ray power calculations based on the two vertical and horizontal polarizations. In other words, EM.Cube's new SBR engine is a truly asymptotic "field" solver. As a result, you can visualize the magnitude and phase of all six electric and magnetic field components at any point in the computational domain. For power calculations at the receiver location, an isotropic, polarization-matched, receiving antenna is assumed.
In most scenes, the buildings and the ground or terrain can be assumed to be made of homogeneous materials. These are represented by their electrical properties such as permittivity ε e and electric conductivity Ïs. More complex scenes may involve a multilayer ground or multilayer building walls. In such cases, one can no longer use the simple reflection or transmission coefficient formulas for homogeneous medium interfaces. EM.Cube calculates the reflection and transmission coefficients of multilayer structures as functions of incident angle, frequency and polarization and uses them at the respective specular points.
=== Limitations of EM.Cube's SBR Solver ===
It is very important to keep in mind that SBR is an asymptotic electromagnetic analysis technique that is based on Geometrical Optics (GO) and the Uniform Theory of Diffraction (UTD). It is not a "full-wave" technique, and it does not solve Maxwell's equations directly or numerically. SBR makes a number of assumptions, chief among them, a very high operational frequency such that the length scales involved are much larger than the operating wavelength. Under this assumed regime, electromagnetic waves start to behave like optical rays. Virtually all the calculations in SBR are based on far field approximations.
In order to maintain a high computational speed for urban propagation problems, EM.Cube's SBR solver ignores double diffractions. Recall that diffractions from edges give rise to a large number of new secondary rays. The power of diffracted rays drops much faster than reflected rays. EM.Cube ignores diffracted rays that are not detected by any receiver. In other words, an edge-diffracted ray does not diffract again from another edge. However, reflected and penetrated rays do get diffracted from edges just as rays emanated directly from the sources do.
== Anatomy Of A Propagation Scene ==
An EM.Cube propagation scene typically consists of several elements. At a minimum, you need a transmitter (Tx) at some location to launch rays into the scene and a receiver (Rx) at another location to receive and collect the incoming rays. A transmitter and a receiver together make the simplest propagation scene, representing a free-space line-of-sight (LOS) channel. A transmitter is one of EM.Cube's several source types, while a receiver is one of EM.Cube's several observable types. A simpler source type is a Hertzian dipole. A simpler observable is a field sensor that is used to compute the electric and magnetic fields on a specified plane.
Figure 1: The Navigation Tree of EM.Cube's Propagation Module.
=== The Various Types Of Surfaces & Blocks ===
In a SBR simulation, the propagating rays hit the surface of building structures, walls, terrain (or global ground) and bounce back into the scene (reflection). Some rays penetrate thin walls or other penetrable surfaces and continue their path on the other side of the surface (transmission). The field intensity, phase and power of the reflected and transmitted rays depend on the material properties of the obstructing surface. The specular surface can be modeled as a simple homogeneous dielectric half-space or as a multilayer structure. In that respect, the buildings, walls, terrain or even the global ground all behave in a similar way:
# '''Terrain Surfaces:''' These blocks are used to provide one or more impenetrable, ground surfaces for the propagation scene. Rays simply bounce off terrain objects. The global ground acts as a flat super-terrain that covers the bottom of the entire computational domain.
EM.Cube's Propagation Module allows you to define block groups of each of the above three types. Each block group has the same color or texture and its members share the same material properties: permittivity εe<sub>r</sub> and conductivity Ïs. Also, all the penetrable surfaces belonging to the same block group have the same wall thickness. You can define many different block groups with certain properties and underneath each introduce many member objects with different geometrical shapes and dimensions. The table below summarizes the characteristics of each block type:
{| class="wikitable"
|}
=== Impenetrable Surfaces For Outdoor Scenes ===
In outdoor propagation scenes such as "Urban Canyons", you are primarily interested in the wireless coverage in the areas among buildings. You can assume that rays bounce off the exterior walls of these buildings but do not penetrate them. In other words, you ignore the transmitted rays and assume that they are either absorbed or diffused inside the buildings. This is not an unrealistic assumption. EM.Cube offers "Impenetrable Blocks" to model buildings in outdoor propagation scenes. A penetrable block has a color or texture property as well as material properties: permittivity (εe<sub>r</sub>) and conductivity (Ïs). By default, a brick building is assumed with εe<sub>r</sub> = 4.4 and Ï s = 0.001S/m. Impinging rays are reflected from the facets of impenetrable buildings or diffracted from their edges.
To define a new impenetrable block group, follow these steps:
# Right click on either the '''Impenetrable Surfaces''' item of the Navigation Tree and select '''Insert New Block...''' A dialog for setting up the block properties opens up offering a preloaded material type (Brick) with predefined color and texture.
# Specify a name for the block group and select a color or texture.
# The electromagnetic model that determines ray-block interaction is selected under '''Specular Interface Type'''. Two options are available: '''Standard Material''' or '''User Defined Model'''. The former is the default choice and requires material properties, '''Permittivity''' (εe<sub>r</sub>) and '''Electric Conductivity''' (Ïs), which are set to "Brick" by default. No magnetic properties are allowed for blocks.
# Click the '''OK''' button of the dialog to accept the changes and close it.
Figure 2: Propagation Module's "Edit Layer" dialog corresponding to impenetrable surfaces.
=== Penetrable Surfaces For Indoor Scenes ===
A typical indoor propagation scene usually involves an arrangement of walls that represent the interior of a building. The transmitters and receivers are then placed in the spaces among such walls. From the point of view of EM.Cube's SBR simulator, walls act like thin penetrable surfaces. EM.Cube uses the "Thin Wall Approximation" to model penetrable surfaces. It assumes that rays simply penetrate a wall and exit at the same specular point on the opposite side of the wall. In other words, rays are not displaced by the walls, nor do they get trapped inside the walls (no internal reflection). This is equivalent to assuming a zero thickness for penetrable surfaces for the purpose of geometrical ray tracing, while the finite thickness of the "thin" surface is used for electromagnetic calculation of transmission coefficient. EM.Cube offers "Penetrable Surface Blocks" for the construction of rooms in indoor propagation scenes as well as modeling of hollow buildings and other structures. You can define many penetrable surface groups with arbitrary thicknesses and material properties (color, texture, permittivity and electric conductivity).
# Specify a name for the surface group and select a color or texture.
# The properties of a penetrable surface are identical to those of an impenetrable surface, plus an additional thickness property.
# By default, a brick wall with a thickness of 0.5 units is assumed. You can change the '''Thickness''' of the penetrable surface as well as its '''Permittivity''' εe<sub>r</sub> and '''Electric Conductivity''' Ïs.
# Click the '''OK''' button of the dialog to accept the changes and close it.
You can construct several thin walls and arrange them as rooms. A regular room can be built by placing four vertical wall objects together with an optional horizontal wall at the top for the ceiling. Alternatively, you may use EM.Cube's hollow box objects or boxes with one or two capped end(s). '''Keep in mind that all the penetrable surfaces belonging to a group have the same wall thickness, which is initially set to 0.5 project units by default. Also, note that solid CAD objects belonging to a penetrable surface group are treated as air-filled hollow structures.''' The thickness of penetrable surfaces is implied and not visualized when displaying objects in the project workspace.
=== Computational Domain & Global Ground ===
The SBR simulation engine requires a finite computational domain. All the stray rays that hit the boundaries of this finite domain are terminated during the simulation process. Such rays exit the computational domain and travel to the infinity, with no chance of ever reaching any receiver in the scene. When you define a propagation scene with various elements like buildings, walls, terrain, etc., a dynamic domain is automatically established and displayed as a wireframe box with green lines that surrounds the entire scene. Every time you create a new object, the domain is automatically adjusted and extended to enclose all the objects in the scene. You can change the size and color of the domain box through the Ray Domain Settings Dialog, which can be accessed in one of the following three ways:
Figure 1: Propagation Module's Domain Settings dialog.
Most outdoor and indoor propagation scenes include a flat ground at their bottom, which bounces incident rays back into the scene. EM.Cube's Propagation Module provides a global flat ground at z = 0. The global ground indeed acts as an impenetrable surface that blocks the entire computational domain from the z = 0 plane downward. It is displayed as a translucent green plane at z = 0 extending downward. The color of the ground plane is always the same as the color of the ray domain. The global ground is assumed to be made of a homogeneous dielectric material with a specified permittivity εe<sub>r</sub> and electric conductivity Ïs. By default, a rocky ground is assumed with εe<sub>r</sub> = 5 and Ï s = 0.005 S/m. You can remove the global ground, in which case, you will have a free space scene. To disable the global ground, open up the Global Ground Settings Dialog, which can be accessed by right clicking on the '''Global Ground''' item in the Navigation Tree and selecting '''Global Ground Settings... '''Remove the check mark from the box labeled '''"Include Half-Space Ground (z<0)"''' to disable the global ground. This will also remove the green translucent plane from the bottom of your scene. You can also change the material properties of the global ground and set new values for the permittivity and electric conductivity of the impenetrable, half-space, dielectric medium. '''Do not forget to disable the global ground if you want to model a free space propagation scene.'''
[[File:PROP4.png]]
Figure 2: Propagation Module's Global Ground Settings dialog.
=== Terrain Surfaces vs. Global Ground ===
A terrain surface acts as a custom, unlevel or irregular ground for your propagation scene. EM.Cube's default global ground blocks the z < 0 half-space everywhere in the computational domain. You can simply turn off the global ground and create one or more terrain objects and place them arbitrarily in the scene. You can also import an external terrain model or file. A terrain represents an impenetrable surface with a more complex surface profile. You can have one or more terrain objects of finite extents and place them on or above the global ground.
* Right click on the '''Terrain''' item in the Navigation Tree and select '''Insert New Terrain...''' A dialog for setting up the terrain properties opens up offering a of preloaded material type (Rock) with predefined green color and no texture.
* Specify a name for the terrain group and select a color or texture.
* Similar to other blocks, you have to specify the material properties, Permittivity (εe<sub>r</sub>) and Electric Conductivity (Ïs), of the terrain group. Rock with εe<sub>r</sub> = 5 and Ï s = 0.005S/m is the default material choice for a new terrain.
* Click the '''OK''' button of the dialog to accept the changes and close it.
# Import an external terrain file of "'''.DEM'''" type.
=== Using Terrain Generator ===
EM.Cube provides a convenient and powerful Terrain Generator for creating a variety of terrain surface objects. EM.Cube's Terrain Generator looks very similar to CubeCAD's Surface Generator. However, whereas the Surface Generator creates a generic or polymesh surface object, Terrain Generator always creates another special type of object known as a '''Tessellated Object'''. A terrain object is much simpler than EM.Cube's polymesh objects and is usually made up of triangular or quadrilateral facets. As such, terrain objects have limited editing capabilities. For example, you can cut, copy, paste, translate or rotate terrain objects. But operations like scaling, mirroring, grouping (composite), arraying, exploding, linking or Boolean operations do not work on terrain objects.
Figure 1: Propagation Module's Terrain Generator dialog.
Some surface types have an additional shape factor called '''Alpha''' that is identical to the alpha parameter in the surface generator. For example, a Gaussian Hump is defined as exp(-r<sup>2</sup>/(2α2a<sup>2</sup>)), where r is the polar radius. For a Super-quadratic Hump, the input parameter α a defines the degree of the super-quadratic surface. α a = 2 corresponds to an ellipsoid. Larger values of α a get close to a rectangular base with rounded corners. An undulated sinusoidal surface is defined by cos(Ïαxpax/D<sub>x</sub>)*cos(Ïαypay/D<sub>y</sub>), and an undulated sinc is defined by D<sub>x</sub>*D<sub>y</sub>*sin(Ïαxpax/D<sub>x</sub>)*sin(Ïαypay/D<sub>x</sub>)/(2Ïxy2pxy), where D<sub>x</sub> and D<sub>y</sub> are the X and Y dimensions, respectively. Terrain Generator creates a unit cell based on the specified surface type. From the same dialog, you can also produce an array arrangement of such unit cells. Simply enter any number of elements along the X and Y directions in the boxes labeled '''Array'''.
[[File:PROP19.png]]
Figure 3: Two noisy custom terrain surfaces both defined as z = (x.y)/20: (Left) RMS noise amplitude = 0.2, (right) RMS noise amplitude = 0.5.
=== Generating Grid-Based Terrain ===
Every time you create a new terrain object using Terrain Generator, an ASCII data file named "GeneratedTerrain" with a "'''.TRN'''" file extension is created and placed in your project folder. This is EM.Cube's simple native terrain file format that basically lists all the (x, y, z) coordinates of the generated surface points on a horizontal, rectangular XY grid. Terrain Generator simply takes your custom function definition or one of the selected catalog surface types and generates the digital elevation data on the specified grid.
A grid-based terrain object.
=== Importing & Exporting Terrain Models ===
You can import two types of terrain in EM.Cube's Propagation Module. The first type is "'''.TRN"''' terrain file, which is EM.Cube's native terrain format. It is a basic digital elevation map with a very simple ASCII data file format. The resolution of the terrain map in the X and Y directions is specified in meters as STEPS. The (x, y, z) coordinates of the terrain points are then listed one point per line. The other type of terrain format supported by EM.Cube is the standard '''7.5min DEM''' file format with a '''.DEM''' file extension.
Figure 1: An imported external terrain model.
=== Multilayer Surface Models ===
Most of the time, your outdoor propagation scene consists of simple buildings made of single-layer walls with standard material properties (εe<sub>r</sub> and Ïs). In the case of a single-layer impenetrable surface, the specular interface is an infinite dielectric half-space, which reflects the impinging rays. Single-layer penetrable surfaces, on the other hand, involve finite-thickness dielectric walls, which both reflect and transmit the incident rays. Similarly, most of your indoor propagation scenes involve simple single-layer penetrable walls with the specified material properties εe<sub>r</sub> and Ïs. A thin wall acts like a finite-thickness dielectric slab that both reflects and transmits incident rays. In the case of the global ground or terrain objects, only ray reflection off the ground surface is considered.
In EM.Cube's Propagation Module, you can define multilayer surfaces with both reflection and transmission properties. You can define multilayer impenetrable buildings, multilayer penetrable walls, and multilayer terrain, with an arbitrary number of layers having different material compositions. You define a multilayer surface in the property dialog of a block, whether impenetrable, penetrable or terrain. In the section entitled '''Surface Type''', two options are available: '''Standard Material''' or '''User Defined Model'''. For simple multilayer walls, select the '''Standard Material''' option. You can add new layers with arbitrary thickness and material parameters to the existing layers. To insert a new layer, deselect any items in the layer list, and click the '''Add/Edit''' button to open the "Add Layer" Dialog. Here you can enter a name for the new layer and values for its '''Thickness''', εe<sub>r</sub> and Ïs. You may also delete any layer by selecting and highlighting it and clicking the '''Delete''' button. You can move layers up or down using the '''Move Up''' and '''Move Down''' buttons and change the layer hierarchy.
You can also search EM.Cube's material database by clicking the '''Material''' button of "Add Layer" or "Edit Layer" dialogs. This opens the '''Materials''' Dialog. Inside the material list select and highlight any row and click the '''OK''' button. The selected material will fill out all the fields in the "Add Layer" or "Edit Layer" dialogs. Inside the Materials Dialog, you can type the few first letters of any material, and it will take you to the corresponding row of the list.
Figure 2: EM.Cube's material list.
=== Transferring Objects From Or To Other Modules ===
When you start a new project in EM.Cube's Propagation Module and draw a solid object like a box in the project workspace without having defined any surface groups, it is assumed to be of the impenetrable surface type. A default impenetrable surface group called Block_1 is automatically added to the Navigation Tree, which holds your newly drawn object. The default group has the material properties of "Brick" (εe<sub>r</sub> = 4.4 and Ï s = 0.001 S/m.) with a dark brown color. You can continue drawing new objects in the project workspace and adding them under this block node. Or you can define a new surface type with different properties. By default, the last surface group that was defined is '''Active'''. The current active surface group is always listed in bold letters in the Navigation Tree. When you draw a new object, it is always inserted under the current active surface group. Any surface group can be activated by right clicking its name in the Navigation Tree and selecting the '''Activate''' item of the contextual menu.
You can move any object from its current surface group into any other available surface group. First select the object, then right click on its surface and select '''MoveTo > Propagation >'''. A submenu appears which lists all the available surface groups where you can transfer the selected object. You can also move objects among surface groups by selecting their names in the Navigation Tree and using the contextual menu. In a similar way, you can transfer objects from Propagation Module to EM.Cube's other modules or vice versa. '''Keep in mind that all the external model files such as STEP, IGES, STL, etc. are first imported to EM.Cube's CubeCAD, from which you can transfer them to other modules.''' First select the object, then right click and select '''MoveTo >'''. In the submenu you will see a list of all the EM.Cube modules that have at least one available group where you can transfer your selected object. You can select multiple objects for transfer. When using the keyboard's '''Shift Key''' or '''Ctrl Key''' for multiple selection, make sure that those keys are held down, when you right click to access the contextual menu.
The simplest SBR simulation can be performed using a short dipole source with a specified field sensor plane. In this way, EM.Cube computes the electric and magnetic fields radiated by your dipole source in the presence of your multipath propagation environment. A "classic" urban propagation scene can be set up using a "Transmitter" source and an array of "Receiver" observables. A transmitter is a point radiator with a user defined radiation pattern. A receiver is a polarization-matched isotropic point radiator that collects the received rays at its aperture. Using receivers, you can calculate the received power coverage map of your propagation scene. You can also calculate your channel's path loss between the transmitter and all the receivers. <br />
=== Hertzian Dipole Sources ===
Earlier versions of EM.Cube's Propagation Module used to offer an isotropic radiator with vertical or horizontal polarization as the simplest transmitter type. This release of EM.Cube has abandoned isotropic radiator transmitters because they do not exist physically in a real world. Instead, the default transmitter radiator type is now a Hertzian dipole. Note that before defining a transmitter, first you have to define a base set to establish the location of the transmitter. Most simulation scenes involve only a single transmitter. Your base set can be made up of a single point for this purpose.
Figure 1: Propagation Module's Transmitter dialog with a short dipole radiator selected.
=== Defining Base Point Sets ===
In order to tie up transmitters and receivers with CAD objects in the project workspace, EM.Cube uses point objects to define transmitters and receivers. These point objects represent the base of the location of transmitters and receivers in the computational domain. Hence, they are grouped together as "Base Sets". You can easily interchange the role of transmitters and receivers in a scene by switching their associated bases. The usefulness of concept of base sets will become apparent later when you place transmitters or receivers on an irregular terrain and adjust their elevation.
Once a base set node has been added to the Navigation Tree, it becomes the active node for new object drawing. Under base sets, you can only draw point objects. All other object creation tools are disabled. A point is initially drawn on the XY plane. Make sure to change the Z-coordinate of your radiator, otherwise, it will fall on the global ground at z = 0. You can also create arrays of base points under the same base set. This is particularly useful for setting up receiver grids to compute coverage maps. Simply select a point object and click the '''Array Tool''' of '''Tools Toolbar''' or use the keyboard shortcut "A". Enter values for the X, Y or Z spacing as well as the number of elements along these three directions in the Array Dialog. In most propagation scenes you are interested in 2D horizontal arrays along a fixed Z coordinate (parallel to the XY plane).
=== Defining Transmitter Sets ===
A short dipole is the closest thing to an omni-directional radiator. The direction or orientation of the short dipole determines its polarization. In many applications, you may rather want to use a directional antenna for your transmitter. You can model a radiating structure using EM.Cube's FDTD, Planar, MoM3D or PO modules and generate a 3D radiation pattern data file for it. These data are stored in a specially formatted file with a "'''.RAD'''" extension, which contains columns of spherical Ï f and θ ? angles as well as the real and imaginary parts of the complex-valued far field components '''E<sub>θ?</sub>''' and '''E<sub>Ïf</sub>'''. The θ?- and Ïf-components of the far-zone electric field determine the polarization of the transmitting radiator.
To define a directional transmitter radiator, you need to select the "User Defined" option in the "Radiator" section of the Transmitter Dialog. You can do this either at the time of creating a transmitter set, or afterwards by opening the property dialog of the transmitter set. In the "Custom Pattern Parameters", click the '''Import Pattern''' button to set the path for the radiation data file. This opens up the standard Windows Open dialog, with the default file type or extension set to ".RAD". Browse your folders to find the right data file. A radiation pattern file usually contains the value of "Total Radiated Power" in its file header. This is used by default for power calculations in the SBR simulation. However, you can check the box labeled "'''Custom Power'''" and enter a value for the transmitter power in Watts. EM.Cube can also rotate the imported radiation pattern arbitrarily. In this case, you need to specify the '''Rotation''' angles in degrees about the X-, Y- and Z-axes. Note that these rotations are performed sequentially and in order: first a rotation about the X-axis, then a rotation about the Y-axis, and finally a rotation about the Z-axis.
Figure 1: Propagation Module's Transmitter dialog with a user defined radiator selected.
=== Multiple Transmitters vs. Antenna Arrays ===
EM.Cube's SBR simulations are fully coherent and 3D-polarimetric. This means that the phase and polarization of all the rays are maintained and processed during their bounces in the scene. Your propagation scene can have more than one transmitter. During an SBR simulation, all the rays emanating from all the transmitters are traced in the propagation scene. All the received rays at a given receiver location are summed coherently and vectorially. This is based on the principle of linear superposition. All the transmitters belonging to the same transmitter set have the same radiation properties. They are either parallel short dipole radiators with the same current amplitudes and phases, or parallel user defined radiators with identical radiation patterns. As these transmitters are placed at different spatial locations, they effectively form an antenna array with identical elements. The array factor is simply determined by the coordinates of the base points. If you want to have different amplitude or phases, then you need to define different transmitter sets.
If that radiators are indeed the elements of an actual antenna array with a half wavelength spacing or so, we recommend that you import the radiation pattern of the array structure instead and replace the whole multi-radiator system with a single point transmitting radiator in your propagation scene. This case is usually encountered in MIMO systems, and using an equivalent point transmitter is an acceptable approximation because the total size of the array aperture is usually much smaller than the dimensions of your propagation scene and its representative length scales. In that case, you need to position the equivalent point radiator at the radiation center of the antenna array. This depends on the physical structure of the antenna array. However, keep in mind that any reasonable guess may still provide a good approximation without any significant error in the received ray data.
=== Defining Receiver Sets ===
Receivers act as observables in a propagation scene. The objective of a SBR simulation is to calculate the far-zone electric fields and the total received power at the location of a receiver. In that sense, receivers indeed act as field observation points. You need to define at least one receiver in the scene before you can run a SBR simulation. You define the receivers of your scene by associating them with the base sets you have already defined in the project workspace. Unlike transmitters that usually one or few, a typical propagation scene may involve a large number of receivers. To generate a wireless coverage map, you need to define an array of points as your base set.
Figure 1: Propagation Module's Receiver dialog.
=== Defining Field Sensors ===
As an asymptotic electromagnetic field solver, the SBR simulation engine can compute the electric and magnetic field distributions in a specified plane. In order to view these field distributions, you must first define field sensor observables before running the SBR simulation. To do that, right click on the '''Field Sensors''' item in the '''Observables''' section of the Navigation Tree and select '''Insert New Observable...'''. The Field Sensor Dialog opens up. At the top of the dialog and in the section titled '''Sensor Plane Location''', first you need to set the plane of field calculation. In the dropdown box labeled '''Direction''', you have three options X, Y, and Z, representing the"normals" to the XY, YZ and ZX planes, respectively. The default direction is Z, i.e. XY plane parallel to the substrate layers. In the three boxes labeled '''Coordinates''', you set the coordinates of the center of the plane. Then, you specify the '''Size''' of the plane in project units, and finally set the '''Number of Samples''' along the two sides of the sensor plane. The larger the number of samples, the smoother the near field map will appear.
[[File:PMOM88.png]]
=== Computing Radiation Patterns In SBR ===
Coming Soon...
== Scene Discretization & Adjustment ==
=== The Need For Discretization Of Propagation Scene ===
In a typical SBR simulation, a ray is traced from the location of the source until it hits a scatterer. The SBR method assumes that the ray hits either a flat facet of the scatterer or one of its edges. In the case of hitting a flat facet, the specular point is used to launch new reflected and transmitted rays. The surface of the facet is treated as an infinite dielectric medium interface, at which the reflection and transmission coefficients are calculated. In the case of hitting an edge, new diffracted rays are generated in the scene. However, only those who reach a nearby receiver in their line of sight are ever taken into account. In other words, diffractions are treated locally.
You can build a variety of surface and solid objects using EM.Cube's native "Curve" CAD objects like lines, polylines, circles, etc. You can use tools like Extrude, Loft, Strip-Sweep, Pipe-Sweep, etc. to transform curves into surface or solid objects. '''However, keep in mind that all the "Curve" CAD objects are ignored by the SBR mesh generator and are therefore not sent to the simulation engine.'''
=== Viewing SBR Mesh ===
You can view and examine the discretized version of your scene objects as they are sent to the SBR simulation engine. To view the mesh, click the '''Mesh''' [[File:manuals/emagware/emcube/modules/propagation/hybrid-simluations/illuminating-periodic-walls-using-sbr/mesh_tool.png]] button of the Simulate Toolbar or select '''Simulate > Discretization > Show Mesh''', or use the keyboard shortcut '''Ctrl+M'''. A triangular surface mesh of your physical structure appears in the project workspace. In this case, EM.Cube enters it mesh view mode. You can perform view operations like rotate view, pan, zoom, etc. But you cannot select objects, or move them or edit their properties. To get out of the Mesh View and return to EM.Cube's Normal View, press the '''Esc Key''' of the keyboard, or click the Mesh button of the Simulate Toolbar once again, or go to the Simulate Menu and deselect the '''Discretization >''' '''Show Mesh''' item.
Figure 1: Propagation Module's Mesh Settings dialog.
=== Special Discretized Object Types ===
In EM.Cube, terrain objects are represented by and saved as special "Tessellated" objects with quadrilateral cells. This is true of terrain objects that you create yourself using EM.Cube's Terrain Generator as well as all the terrain objects that you import from external files to your project. The center of each cell represents the terrain elevation at that point. Tessellated objects are considered as discretized objects by EM.Cube and they are not meshed one more time by the SBR mesh generator. Each quadrilateral cell is divided into two triangular cells before being passed to the SBR simulation engine. Therefore, when using EM.Cube's Terrain Generator to create a new terrain object, you have to pay special attention to the resolution of the terrain object as it determines the total number of terrain facets sent to the simulation engine. A high resolution terrain, although looking better and more realistic, may easily lead to an enormous computational problem.
You can use EM.Cube's "Polymesh" tool to discretize solid and surface CAD objects. You can manually control the mesh characteristics of polymesh objects including inserting new nodes on faces and edges or deleting existing nodes. In addition, EM.Cube's Solid Generator and Surface Generator tools create ploymesh solids and surfaces, respectively. Like tessellated object, polymesh objects are also considered as discretized objects by EM.Cube and they are not meshed again by the SBR mesh generator.
=== SBR Mesh Rules & Considerations ===
Coming Soon...
=== Adjusting Block Elevation On Terrain ===
In EM.Cube, buildings and all other CAD objects are initially created on the XY plane by default. In other words, the Z-coordinate of the local coordinate system (LCS) of all blocks is set to zero until you change them. As long as you use the global ground, all is fine as your buildings are seated on the ground. When your propagation scene has an irregular terrain, you want to place your buildings on the terrain and not buried under it. Buildings in EM.Cube are not adjusted to the terrain elevation automatically. You need to instruct EM.Cube to do so.
A Scene with Buildings and Terrain Before and After Adjusting Elevation
=== Transmitters & Receivers Above An Irregular Terrain ===
In EM.Cube, all the transmitters and receivers are tied up with point objects in the project workspace. These point objects are grouped and organized in base sets. When you move the point objects or change their coordinates, all of their associated transmitters or receivers immediately follow them to the new location. For example, you usually define a grid of receivers using a base set that is made up of a uniformly spaced array of points and spread them in your scene. All of these receivers have the same height because their associated base points all have the same Z-coordinate. When your receivers are located above a flat terrain like the global ground, their Z-coordinates are equal to their height above the ground, as the terrain elevation is fixed and equal to zero everywhere. The same is true for transmitters, too.
Figure 1: Transmitters and receivers adjusted above an uneven terrain and their associated base sets.
== Running A SBR Simulation ==
EM.Cube's Propagation Module offers three types of ray tracing simulations:
Figure 1: Propagation Module's Simulation Run dialog.
=== SBR Simulation Parameters ===
There are a number of SBR simulation settings that can be accessed and changed from the SBR Settings Dialog. To open this dialog, click the button labeled '''Settings''' on the right side of the '''Select Engine''' dropdown list in the Run Dialog. EM.Cube's SBR simulation engine allows you to separate the physical effects that are calculated during a ray tracing process. You can selectively enable or disable '''Ray Reflection''', '''Ray Transmission''' and '''Ray Diffraction'''. By default, all three effects are checked and included in the computations. Separating these effects sometimes help you better analyze your propagation scene and understand the impact of various blocks in the scene.
Figure 1: Propagation Module's SBR Engine Settings dialog.
=== The Coverage Map ===
If the associated radiator set is isotropic, so will be the transmitter set. By default, an isotropic transmitter has vertical polarization. You can use the '''Polarization''' radio button to select one of the two options: '''Vertical''' or '''Horizontal'''. If the associated radiator set consists of '''Short Dipole''' or '''User Defined''' radiators, it is indicated in the transmitter property dialog. In the case of a short dipole radiator, you can set a value for the dipole current in Amperes. The radiation resistance of a short dipole of length ''dl'' is given by:
Output Plot Settings
=== The Ray Data ===
At the end of a SBR simulation, each receiver receives a number of rays. Some receivers may not receive any rays at all. You can visualize all the rays received by a certain receiver from the active transmitter of the scene. To do this, right click the '''Receivers''' item of the Navigation Tree. From the context menu select '''Show Received Rays'''. All the rays received by the currently selected receiver of the scene are displayed in the scene. The rays are identified by labels, are ordered by their power and have different colors for better visualization. You can display the rays for only one receiver at a time. The receiver set property dialog has a list of all the individual receivers belonging to that set. To display the rays received by another receiver, you have to change the '''Selected Receiver''' in the receiver set's property dialog. If you keep the mouse focus on this dropdown list and roll your mouse scroll wheel, you can scan the selected receivers and move the rays from one receiver to the next in the list. To remove the visualized rays from the scene, right click the Receivers item of the Navigation Tree again and from the context menu select '''Hide Received Rays'''.
Analyzing a selected ray from the ray data dialog.
=== Plotting Other Simulation Results ===
Besides visualizing the coverage map and received rays in the EM.CUBE's Propagation Module, you can also plot the '''Path Loss''' of all the receivers belonging to a receiver set as well as the '''Power Delay Profile''' of individual receivers. To plot these data, go the '''Observables''' section of the Navigation Tree and right click on the '''Receivers''' item. From the context menu, select '''Plot Path Loss''' or '''Plot Power Delay Profile''', respectively. The path loss data between the active transmitter and all the receivers belonging to a receiver set are plotted on a Cartesian graph. The horizontal axis of this graph represents the index of the receiver. Power Delay Profile is a bar chart that plots the power of individual rays received by the currently selected receiver versus their time delay. If there is a line of sight (LOS) between a transmitter and receiver, the LOS ray will have the smallest delay and therefore will appear first in the bar chart. Sometimes you may have several rays arriving at a receiver at the same time, i.e. all with the same delay, but with different power level. These will appear as stacked bars in the chart.
You can also plot the path loss and power delay profile graphs and many others from EM.CUBE's data manager. You can open data manager by clicking the '''Data Manager''' [[File:manuals/emagware/emcube/modules/propagation/running-a-sbr-simulation/plotting-other-simulation-results/data_manager_icon.png]] button of the '''Compute Toolbar''' or by selecting '''Compute [[File:manuals/emagware/emcube/modules/propagation/hybrid-simluations/illuminating-periodic-walls-using-sbr/larrow_tn.png]]Data Manager''' from the menu bar or by right clicking on the '''Data Manager''' item of the Navigation Tree and selecting Open Data Manager... from the contextual menu or by using the keyboard shortcut '''Ctrl+D'''. In the Data manager Dialog, you will see a list of all the data files available for plotting. These include the theta and phi angles of arrival and departure of the selected receiver. You can select any data file by clicking and highlighting its '''ID''' in the table and then clicking the '''Plot''' button.
=== Output Data Files ===
At the end of an SBR simulation, the results are written into a main output data file with the reserved name of SBR_Results.RTOUT. This file has the following format:
A typical SBR output data file.
=== Running A Frequency Sweep With SBR ===
By default, you run a single-frequency simulation in EM.CUBE's Propagation Module. You set the operational frequency of a SBR simulation in the project's '''Frequency Dialog''', which can be accessed in a number of ways:
Animation controls dialog in the project workspace.
=== Running a Parametric Sweep with SBR ===
In EM.CUBE, all the CAD object properties as well as certain source, material and mesh parameters can be assigned as variables. Variables are defined to control and vary the values of such parameters either for editing purposes or to run parametric sweep or optimization. Variable are defined using the '''Variables Dialog''', which can be accessed in the three ways:
The coverage map of the scene at the end of a parametric sweep where the sweep variable is the transmitter height.
=== Statistical Analysis of Propagation Scene ===
EM.CUBE's coverage maps display the received power at the location of all the receivers. The receivers together from a set/ensemble, which might be uniformly spaced or distributed across the propagation scene or may consist of randomly scattered radiators. Every coverage map shows the '''Mean''' and '''Standard Deviation''' of the received power for all the receivers involved. These information are displayed at the bottom of the coverage map's legend box and are expressed in dB.