Wireless carriers have begun to explore new ways to maximize the spectral efficiency of their networks and improve their return on investment. Research efforts investigating methods of improving wireless systems performance are currently being conducted worldwide. In the design of wireless networks for companies and institutions, there is no room for chance. Failure to even potentially the smallest factor can cause errors that will make our project become useless. To organize the design process, the concept of splitting the process into three phases is introduced [ 1 ]: initial phase —collects information about the requirements and expectations of the client; design phase —identifying the best access point, AP, location by application of simulation based on the EM wave propagation models; and measurement phase —implementation of the project and the introduction of any amendments arising from the difference between the results of measurements the real behavior of the network , and simulation.
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In the circumstances in which radio waves are many phenomena, the designation of a useful signal level is extremely complex and requires the introduction of appropriate propagation models. Such model is a collection of mathematical expressions, graphs, and algorithms used to produce propagation characteristics of EM waves in chosen environment. There are also models which are a combination of both of these types. Presented models allow streamlining the design process for wireless networks. On the market, there are a lot of solutions, commercial computer codes, based on these concepts.
Wireless network design requires that you specify the size and shape of the areas covered by the access points. To this end, the link budget is performed:. The most difficult to determine the part of the link budget is the attenuation loss L of the propagation route. Typical wireless systems environment is located inside the buildings filled with walls, furniture, peoples, and other objects.
In such conditions, the mechanism of propagation of the EM waves is very complex. A number of EM waves distributed inside buildings belong to different physical phenomena [ 2 ]: Diffraction : when signal encounters on the road, an impermeable barrier, whose dimensions are larger than the wavelength. At the edges of the obstacles is the deflection of the wave causing the attenuation, dispersion, and a change in the direction of EM wave propagation,.
Dispersion : when on the road the wave contains obstacles, whose dimensions are comparable to the wavelength. In this case, the radio waves are directed in more directions, which is difficult to predict and model,. Reflection : when radio wave on the way encounters an obstacle, whose dimensions are much larger than the wavelength of the incident EM wave, they reflect itself from the obstacle.
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In cases when at the receiving end, there are many of reflected waves signal can be very unstable,. Penetration over obstacles : when radio wave encounters an obstacle, which is to some extent transparent for radio waves, allowing the reception of radio signals inside buildings waves penetrate through walls and ceilings ,. Absorption : caused by the appearance of the plants on the propagation route, peoples with high absorption capacity. Radio waves are absorbed also by other obstacles, such as walls, furniture, and painting of the walls, curtains on the windows, etc.
Running along the tunnels and corridors : guided wave phenomenon can be dealt with as a special mechanism to describe the propagation in tunnels or corridors, arising as a result of multiple reflections and interference of the EM waves along the route. The designation of the attenuation of a route in such conditions is extremely difficult. Having regard to these mechanisms of propagation, the propagation models have been developed. The propagation model is an algorithm to analyze the propagation of radio waves in the environment taking into account the mechanisms described above.
The algorithm described it in the proper order by means of the specified mathematical expressions, charts, and tables of some coefficients, and they are most frequently served in the form of recommendations of the ITU, IEEE and others worldwide standardizing institutions.
The propagation models permit to determine the average value of the propagation loss in a proper place. For the complete modeling of the propagation environment, the statistics of the received signal should be provided, which lets you include slowly variable and quick-exchange of signal dropouts. The slowly variable dropouts are understood as fluctuations in the average value of a signal over a distance of several wavelengths.
The quick-exchange dropouts shall be understood as fluctuations in the average value of the signal caused by changes in the propagation environment, for example, the movement of people in the building. In order to facilitate the description of the Rice distribution, the k parameter is used [ 8 ]:.
In the case of a large movement of people in buildings, it is advisable to use the Rayleigh distribution [ 8 ]:. Empirical models are based on measurements and observations made under different conditions. Their accuracy depends not only on the results of measurements but also from the similarity of the present environment and the environment in which the modeling measurements were conducted.
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The propagation environments inside buildings are strongly differentiated. The use of empirical models in such conditions can lead to less accurate results relative to other promotional environments. Therefore, for indoor environments, more accurate models are considered to be deterministic models.
Despite this, the easy implementation and fast calculation time that empirical models are not lost in popularity [ 8 ]. The one-slope model is the simplest model used in the indoor environment. It does not take into account the details of the structure of the building but only the distance and n parameter describing an environment [ 8 ]:. The parameters of the L0 and n are empirical parameters assigned to the specified environment.
By appropriate selection of their values, we can adjust the model to a specific type of building, for which we design a wireless network. Examples of model values for different propagation environments are shown in Table 1. Building 1 represents buildings introducing the large attenuation with high density of users, furniture, walls, and other obstacles. Building 2 represents the typical office buildings of medium attenuation. Building 3 represents the big empty spaces of exhibition halls, warehouses, and large offices with a small amount of furniture and other obstacles of the generally small damping.
Parameter n in the case of propagation in the corridor is only 1, 2—what is apparent from the account of the phenomenon of driving in the tunnel waveguide , which amplifies the signal. The most popular models of empirical studies to take into account the effect of the walls and ceilings are multi-wall models.
Linear multi-wall model also called the Motley-Keenan model specifies the attenuation on the direct route between transmitter and receiver, taking into account the attenuation by walls and ceilings of the building [ 8 ]:. Conditions of the propagation inside the building more accurately reflects, so called, nonlinear model of multi-walls, which was developed within the framework of the European project COST and approved by the ITU as recommended for third generation of cellular mobile systems projects.
Attenuation of radio link referred to the model is given by following equations [ 4 ]:. The M-W models are used in computer programs commercial and academic. Figure 2 shows a comparison of the simulation results obtained in the building type 1 by applying the model of one-slope b and nonlinear model of multi-walls c.
The projection of floors and location of the access point a , the distribution of the signal for one-slope model b , and for nonlinear model of multi-walls c . The M-W model in indoor environments takes into account the impact of the walls on the suppression of radio wave but only on the direct of transmission transmitter to receiver route. As mentioned earlier, the specificity of phenomena in indoor environments causes if a direct signal is often not the strongest one.
In that cases in the received signal the reflected as well as the diffraction rays should also be considered [ 4 ]. In the dominant path model D-P , the losses shall be determined for several wave propagation routes. Analysis of the results of measurements also showed that the adjacent rays decompose the same phenomena and are almost identical.
This fact was used in the D-P model, which is looking for the routes with the smallest attenuations. In Figure 3 , the concept of the D-P model is shown. The losses on selected routes shall be, according to [ 5 ]:. The D-P model allows you to take into account the situation, when the dominant radius is not the dominant one, it means the attenuation of EM waves is not lowest one.
So, on the beginning the attenuations for some few routes are determined, for which are expected to be the smallest. On further analysis, only those routes have been taken into account. It is important that the exact designation of the losses is associated with some encountered phenomena reflection and diffraction and strengthening related to the phenomenon of the driving waves in the corridors the waveguide effect. The designation of a wave driving factor W and suppression caused by diffraction and reflection can greatly vary depending on the type of building.
Determination of the coefficient W starts the analysis of three parameters for each of the walls and at the point of x. The factor wi x that represents the interim impact of the ith wall on the conduct of the wave at the point x , can be appointed from [ 5 ]:. The factor w x that represents the partial effects of all N walls in section x specifies the pattern [ 5 ]:.
The factor is normalized relative to the w0 —maximum value of the conducted wave. Resultant factor wave driving is the superposition of all partial coefficients in the way of propagation [ 5 ]:. The value of the coefficient W can range between a value of 0 the conducting of EM wave phenomenon does not exist and 1 the ideal case of the conducting EM wave phenomenon. This parameter is determined as a factor W of the wave driving.
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Besides, you should also know the suppression resulting from the transition of the wave by an obstacle and the distance between the transmitter and the receiver. Figure 4 shows a comparison of the simulation results obtained from: a only the direct radius nonlinear of M-W model, b the strongest ray the dominant path D-P model , and c the ray tracing, 3D RT, approach c.
Simulation results obtained using: only the direct radius nonlinear of M-W model a , of the strongest ray the dominant path D-P model b , and ray tracing, 3D RT, approach c. In this example, the direct radius is weaker than the strongest radius of about 5. This result proves the superiority of the D-P model over the M-W model. Unfortunately, the very complex initial calculations make D-P model difficult to implement. Deterministic models are based on physics laws and allow for precise modeling of the propagation of electromagnetic waves.
These models take into account the phenomena such as reflection, diffraction, absorption, and wave running, which are essential in the conditions inside buildings as well as in the outdoor area of dense building centers. Unlike the empirical models, the deterministic models are not based on measurements and thus it accuracy does not depend on the similarity of the standard environment and concerned one.
To accurately model the phenomenon of propagation of electromagnetic waves, deterministic models require accurate rendering of environment propagation periods. In addition to the level of the model of the environment and on the quality of results, calculation has the effect of repeat accuracy phenomena, which are subject to the electromagnetic waves. The EM waves propagation environments inside buildings are strongly differentiated. Therefore, for these environments, deterministic models are considered to be more accurate in comparison with models of empirical studies.
In order to wave propagation modeling, two concepts were developed. The method requires a transition from continuous space and time distribution of electromagnetic field to a discrete spatially grid that contains in its nodes of the field values in a certain moment of time. The transition from derivatives for odds ratios differential allows you to create an algorithm to calculate the distribution of the field in the next time step of the fields at the time of the previous one.
The distribution of the structure given by the material constants is specified in each of the nodes of the grid. This algorithm has been provided by Kane Yee. In accordance with the algorithm EM field value in the node of the grid depends only on the values of the EM field at this point in the previous time step, as well as of the values of other EM fields in the adjacent nodes of the grid, and of the known features of magnetic and electrical sources. For this reason, it is rarely used in computer programs that support design of the wireless networks.
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The FDTD method is limited in suburban environments, in which the structures are situated close to antennas with complex material properties [ 7 , 23 , 24 , 25 , 26 , 27 ]. Deterministic models are based primarily on methods of geometrical optics ray optical method RO , or geometrical optics GO , which are based on the assumption of rectilinear propagation of electromagnetic waves. Each of the rays carries the part of the radiated power. If it encounters obstacles, the wave is: reflected, refracted, diffracted, or dispersed. At the beginning, all possible routes between the transmitter and the receiver are determined.
In several recent years, the largest popularity won two techniques to determine the possible paths of propagation: method of images image method, IM and method of the rays shooting ray lunching, RL. These methods allow you to recreate the three-dimensional spread of the waves by taking into account of losses on the transmitter-receiver road. The accuracy of the two methods depends largely on the number of phenomena that are included in the models. The more the phenomena will be included, we can get more accurate results. The image method IM lets you designate all possible routes of the signal propagation between the transmitter and the receiver the center of the pixel with regard to the phenomena of reflection and transmission.
To find a ray route, the mirrors of transmitter relative to the illuminated surrounding surfaces are created. The intersection of the straight line connecting the virtual source and receiving point of the illuminated plane designates the place of reflection of the rays Figure 5a. Routing of the reflected rays in the method of images IM a , and routing of the reflected rays in the rays launching R-L method . In the case of multiple reflections, the following mirrors of virtual source are generated and the algorithm is repeated. The analysis can be carried out in three dimensions, or separately in the vertical and horizontal planes.
Method of images allows you to designate the exact routes of the reflected rays. Method of images can be used to find the routes of diffracted rays [ 8 ]. In the RL method, the created rays are shooting directly from the source. The azimuth elevation angle of rays increases gradually at constant values Figure 5b. The route of each of the ray is tracked independently of the other. For each of ray is allocated a part of the sphere forming the wave front Figure 6b.
In the simplest solution, an excerpt of the sphere is approximated by a circle. Then, the circle creates the so-called received sphere Figure 6a , in which the ray for 2D model is expressed by 17 , and for the 3D model by 18 [ 8 ]:. This method introduces errors of interpretation arising from the limited geometrical possibility of approximation of a sphere by concentric circles.
In order to increase the accuracy, portions of a sphere are approximated by the rectangles Figure 6. The intensity of the electric field at the receiving end is the sum of the intensities of all rays, whose distances from a receiving point are less than R S. The direction of the rays, penetrating the obstacle shall be determined with the law of Snell. The RL method can be used to find the routes of diffracted rays. The source of diffracted rays may be found when the rectangle or circle associated with the ray crosses the diffraction edge [ 8 ].
The RL method has a disadvantage in relation to the IM method because it may neglect the very narrow obstacles lying on the extension of transmitter, which can be placed between two rays. To ensure satisfactory resolution of the simulation, the technique of rays splitting RS was introduced, which allows you to split the rays when the radius of the receiving sphere or side of the rectangle reaches its maximum size Figure 6.
The RL method is easier to implement with respect to the IM method. It is characterized by the weaker resolution and longer time of computation. In several recent years, research to refine the method of RT has been conducted. Thus the smart method of ray tracing, that is, intelligent ray tracing IRT Figure 8b has been developed. It is based on simple assumptions [ 10 , 14 , 15 ]: 1 only a few rays takes an essential part of the energy of the electromagnetic field, 2 visibility of faces and edges do not depend on the position of the of transmitter antenna, 3 often the bordering receiving points pixels are reachable by the rays with a very similar the properties.
In accordance with the objectives the calculation were optimized. Pretreatment process of database processing with a collection of information about the obstacles encountered in the model shall be carried out only once. The idea is that each of these obstacles is divided into small pieces called tiles and on the borders in the form of episodes. Mutual relations of visibility between objects are calculated once and stored in the database. The tiles and the edges are represented only by their center points.
The idea of the relationship of visibility center points of the edges or tiles is that if the two center points are in the zone, direct visibility defines the rays from the center of the first tile to the corners of the next one Figure 7a. The division of the rays in the RT method  a and the geometry of the inteligent method of ray tracing IRT . These rays and the throw of their angles are data indicating the relationship prevailing between the two center points.
Such relationships are created also between edges, as well as in the case of the edge of the tiles. Important are the angles that define the angular distance of possible diffraction and reflection. During the process of routing, the propagation links the information about the relationship of visibility are readily available and you do not need to set dependencies between the obstacles for each ray, which greatly increase the computation speeds [ 10 ].
The introduction of the 3D model is associated with the growth of databases and considerable complexity of algorithms. This causes prolongation of the calculation time. In this solution, two independent analyses are carried out: in vertical and horizontal planes [ 8 ]. In order to further reduce the calculation time proposed the division of the algorithms in the class due to the number of the relevant phenomena for each of the waves Table 2.
Further increasing the maximum number of, at issue, propagation phenomena takes longer simulation time. In order to reduce calculation time the receiving coverage surface is divided into smaller portions, called the pixels. Each pixel is represented by its center point. The route of the rays is determined only between source and any center point. Increasing the size of the pixels will reduce the resolution of the simulation and the calculation time. After you specify all the routes between the transmitter and receiver, the components of the electrical field strength Ei at the receiving end originating in from each of the rays are determined [ 9 ]:.
The intensity of the electric field from the all rays arriving at the receiver is calculated according to the formula [ 8 ]:. Power PR delivered by the receiving antenna to receiver depends on the effective area or the effective length ASK of the receiving antenna and power density S [ 8 ]:. The coefficients in the expression 21 are defined by the modeling techniques of propagation phenomena used in the RT method. In the methods of geometrical optics, GO, reflection phenomenon of reflection of electromagnetic waves describes the Fresnel coefficients, expressing ratios of electromagnetic fields strengths of the reflected R and incident I waves.
The two Fresnel coefficients are different for the EM vectors intensities, namely the parallel and perpendicular components to the plane of incidence. The degree of reflection and transmission of both Fresnel vectors are varying quite different. The plane of incidence is defined as the plane determined by the wave vector of the incident and the normal to the boundary of separation. Figure 8 shows the mechanism of the reflection phenomenon [ 8 ].
In real terms, the walls are constructed of several layers. On each border of the wall layers the wave is splitting on two components, the reflected wave from the surface of layer and transmitted wave to the layer. The result is that the actual coefficients of transmission and reflection depend on the angle of incidence [ 9 ].
The incident wave is dissipated as a result of surface roughness. To determine the surface roughness, the criterion of Rayleigh is used [ 12 ]:. For small angles, the height H of Rayleigh and minimum distance between inequalities S determine the following equations [ 12 ]:. According to the Rayleigh criterion, if the height surface roughness is greater than H and the distance between them is greater than S , then consider the surface that causes the dispersion of incident wave [ 12 ].
The phenomenon of EM wave scattering can be taken into account by reduction of the reflection coefficients transmission coefficients are not changed. These coefficients can be multiplied by the value of slightly less than unity, where the exact value depends exponentially on the level of surface roughness calculated in accordance with the theory of Rayleigh:. Studies have proven that the absorption of the wave by different types of objects first of all living beings is very difficult to model.
The ray tracing method RT does not take diffraction into account. In order to model this phenomenon, the deterministic models based on ray tracing method are enhanced with correct techniques. There are several methods for modeling a phenomenon of diffraction: PAW perfectly absorbing wedge , GTD geometrical theory of diffraction and UTD uniform theory of diffraction. One of the biggest problems of modeling of diffraction is the precise definition of diffraction edges of the buildings and other objects. The most commonly used method is the UTD, which takes into account the wave polarization and the material properties of the diffraction edge.
The EM wave coming on the diffraction edge is scattered on a cone whose vertex is at the diffraction point. In Figure 10a , the example of a diffraction cone is shown [ 8 ]. Diffraction cone in the UTD method a and an example of the radius diffracted on the edge b . Figure 10b shows the transmission of the ray, which includes the phenomenon of diffraction by means of the UTD method.
The intensity of the electric field at the point of reception can be designated from [ 13 ]:. Figure 4c shows the results obtained by RT 3D method. Method of geometrical optics, GO, is used to model the distribution of electromagnetic fields inside the buildings. Also, in wide band of frequencies around 60 GHz, the radio waves are strongly attenuated by molecular oxygen in the atmosphere.
In , Heinrich Hertz became the first to demonstrate the existence of such waves by building an apparatus that produced and detected microwaves in the ultra high frequency region. Later work by others led to the invention of wireless communications, based on microwaves. In a US-French consortium demonstrated an experimental microwave relay link across the English Channel using 10 foot 3m dishes, one of the earliest microwave communication systems. Telephony, telegraph and facsimile data was transmitted over the 1. However it could not compete with cheap undersea cable rates, and a planned commercial system was never built.
A microwave link is a communications system that uses a beam of radio waves in the microwave frequency range to transmit video, audio, or data between two locations, which can be from just a few feet or meters to several miles or kilometers apart. Operating Distances for microwave links are determined by antenna size gain , frequency band, and link capacity. Microwave links are commonly used by television broadcasters to transmit programmes across a country, for instance, or from an outside broadcast back to a studio.
Mobile units can be camera mounted, allowing cameras the freedom to move around without trailing cables. These are often seen on the touchlines of sports fields on Steadicam systems. Microwave signals are often divided into three categories: ultra high frequency UHF 0. In addition, microwave frequency bands are designated by specific letters. The designations by the Radio Society of Great Britain are given below. For other definitions, see Letter Designations of Microwave Bands. Lower Microwave frequencies are used for longer links, and regions with higher rain fade.
Conversely, Higher frequencies are used for shorter links and regions with lower rain fade. Rain fade refers primarily to the absorption of a microwave radio frequency RF signal by atmospheric rain, snow or ice, and losses which are especially prevalent at frequencies above 11 GHz. It also refers to the degradation of a signal caused by the electromagnetic interference of the leading edge of a storm front.
Rain fade can be caused by precipitation at the uplink or downlink location. However, it does not need to be raining at a location for it to be affected by rain fade, as the signal may pass through precipitation many miles away, especially if the satellite dish has a low look angle. From 5 to 20 percent of rain fade or satellite signal attenuation may also be caused by rain, snow or ice on the uplink or downlink antenna reflector, radome or feed horn. Possible ways to overcome the effects of rain fade are site diversity, uplink power control, variable rate encoding, receiving antennas larger i.
In terrestrial microwave links, a diversity scheme refers to a method for improving the reliability of a message signal by using two or more communication channels with different characteristics. Diversity plays an important role in combatting fading and co-channel interference and avoiding error bursts.