Method for predictive estimation of radio cell coverage

0001The present invention relates to the definition phase of the cellular telephone networks to be implemented on a territory and in particular to a method for estimating radio coverage of the territory, with a view to determining operating parameters and optimal positions of the base stations or network repeaters, ie the boundaries of the corresponding radio cells.

0002It will be recalled that a cellular radiotelephone network consists of a plurality of base earth stations which are interconnected through the wired telephone network and to which the mobile terminals can access when they are in the radio cell of the station.

0003The radio propagation in a cell must satisfy the two essential requirements, which are the emission of a not excessive power by the base station and the reception by the terminals of radio signals of sufficient power.

0004Indeed, the scope of each base station must, first of all, be sufficient for the cell concerned to overflow on the neighbors, to avoid any risk of communication failure when a mobile terminal changes cell. This requires increasing the transmit power of stations beyond what is strictly necessary.

0005Then, as the radio links have a substantially linear path at ground level, it is necessary to cover the station's radio shadow zones, due to the natural reliefs or buildings. A shaded area is an area in which the radio propagation attenuation between a mobile terminal and the station exceeds the limit of the sensitivity specification of the radio circuits, so that the reception level is insufficient to correctly detect transmitted bit packets representing speech or data to be exchanged. However, it is impossible to increase the emission levels.

0006Indeed, on the base station side, any increase would increase the size of the cell, causing excessive interference between neighboring cells. Side mobile devices, their maximum power is limited by the requirements of personal safety and battery life.

0007It is also necessary to avoid unnecessarily multiplying the number of base stations or repeaters for the micro-cells represented by the shadows, in order to limit costs and interference.

0008In a conventional manner, for a cell, a prediction calculation of the attenuations is made at a plurality of points of the cell, using a vector database provided by the National Geographical Institute (IGN), among others, and representing the map of the geographical area considered with the buildings and other reliefs of the sursol it contains. Different code words define the type of sursol, such as forest, water, pavilions, specified in Lambert coordinates and in height above the local altitude of the ground relative to the sea.

0009To estimate the predicted attenuation at any point of the cell, a propagation simulation of radio signals is carried out. The propagation is modeled by fictitiously launching, by calculation in a computer of a radio coverage prediction production line, an electromagnetic ray from the base station, in a determined direction, and one calculates its conditions of propagation in the angle elementary solid that he occupies. Apart from the propagation in free space when the station is in direct view of a fictitious radio terminal, known linear attenuation propagation, the path of the ray encounters obstacles which attenuate or deviate it further, in particular in the microcells. whose stations are often at an altitude lower than the roofs of buildings.

0010Thus, in a street, the ray can be deflected by reflection or refraction. The angular opening of its solid angle can even be increased.

0011These calculations are repeated for a plurality of elementary solid angles distributed in a global solid viewing angle of the entire cell, such as a globally horizontal ring, so as to sample the various conditions of propagation of the space of the cell. , see for example <patcit id="pcit0001" dnum="WO9744977A"><text>WO-A-97/44977</text></patcit>.

0012At each point of the trajectory of each ray, the calculator consults the vector database to determine if there is an obstacle. The corresponding vector calculations require a considerable computing power and extend over a working day, in practice they must be launched in the evening, when using conventional computers.

0013The present invention aims to reduce the computing power required to calculate the radio coverage of such cells, whether they are large cells, or macrocells, or microcells.

0014To this end, the invention relates to a method for the provisional estimation of the radio coverage of a cell, of a cellular radio network, by a radio station for managing the traffic of the cell, by means of a base of relief map data specifying positions and types of reliefs of the cell, in which method, a calculation system in an exploitation phase,<ul id="ul0001" list-style="dash" compact="compact"><li>fictively launches in the cell a sampling radius representative of the radio propagation conditions, according to an initial trajectory section from a home position and according to a determined direction and propagation conditions,</li><li>by reading the database, the calculation system compares the trajectory section with the relief map data to identify the position and the nature of any current point of impact of the trajectory section with a relief,</li><li>according to the nature specified in the database of the relief considered, the calculation system determines new propagation conditions on a downstream section of trajectory beyond the point of impact,</li><li>the calculation system repeats, if necessary, the two preceding steps a given number of times for other impacts of downstream sections of trajectory,</li><li>according to the propagation conditions over the entire trajectory, the calculation system determines a cumulative attenuation for any selected point thereof, and</li><li>the calculation system repeats all the steps above a plurality of times for a plurality of initial directions in order to sample the entire cell and thus to determine an attenuation map of the selected points, characterized in that, in a phase initial,</li><li>the system generates the database in the form of a data matrix, with direct access, of local and autonomous specification of the positions and natures of the respective reliefs of a plurality of predetermined meshes of a corresponding mesh of the map, and ,</li><li>the system stores geographic orientation data of the reliefs in the matrix database in the exploitation phase, the calculation system</li><li>compares the position of the current point to the mesh to identify an impact mesh,</li><li>calculates the conditions of propagation after impact from local and autonomous terrain specification data of the impact mesh, and</li><li>calculates a direction of a downstream section reflected radius from the orientation data.</li></ul>

0015The propagation data having thus been fragmented into a plurality of autonomous data, and therefore of reduced volume, it is then easy to access it quickly to read them in a usable form.

0016Thus, the calculation of the propagation conditions downstream of the point of impact is made from local data and limited volume, direct access and not sequential as in the case of a vector base, data that define, for the point of impact, the conditions of radioelectric anisotropy determining, by attenuation, reflection or diffraction with possible diffusion, a possible angular deflection and a new attenuation of propagation.

0017In the absence of obstacle terrain, the free space propagation conditions are well known. In particular in this case, a reading of the data of a mesh traversed immediately shows that there is no obstacle and therefore we go to the next mesh, without requiring the very heavy calculations, of the prior art, vector reconstruction of local data from a global database

0018It will be noted that the term relief here considered globally designates any obstacle to propagation, including the ground, even horizontal, which is capable of absorbing or reflecting at least partially the rays.

0019Advantageously, the system stores, in the initial phase, geographic orientation data of the reliefs in the matrix database and, in operation, calculates a direction of a reflected radius of the downstream section from the orientation data.

0020Preferably, in this case, the storage is limited to azimuth data and, in operation, the direction of the downstream section reflected ray is calculated by considering that the reflection reliefs are vertical.

0021The calculations are thus limited.

0022In particular, it is preferable to include, in the matrix database, data specifying a nature of a sursol and, in operation, the system calculates the conditions of propagation on the downstream section according to said nature of sursol.

0023The accuracy of the propagation calculations is thus better.

0024In order to take into account the details of the reliefs, in the initial phase, data relating to relief edges can be integrated into the matrix database and, in operation, the calculation system calculates a direction of downstream section of radius refracted from said edge data.

0025More preferably, in the initial phase, relief data of reliefs are stored in the matrix database and, in operation, the system calculates a propagation attenuation on the downstream section from said attenuation data.

0026In such a case, the attenuation data may relate to the relief reflection and is used to calculate the attenuation on the downstream section, and / or the attenuation data relates to the propagation through the reliefs and is used to calculate the attenuation of the rays propagated there.

0027Advantageously, the propagation attenuation data through the reliefs further include propagation medium transition data specifying relief penetration attenuations, which are used to determine a local penetration attenuation.

0028For an improved quality modeling of the propagation conditions, and having an algorithm for calculating the angular dispersion of the radius after impact, the system calculates a plurality of directions of downstream sections, with specific attenuations, forming a solid angle of diffusion of the radius beyond the impact.

0029It is furthermore possible to count the successive impacts on the trajectory and, in the case of a second impact, the system considers, for calculating said propagation conditions beyond, that the ray has been polarized during the first impact.

0030In order to limit the required calculations, the system counts the successive impacts on the trajectory and compares the total to a high threshold value to stop performing the steps of the process when the threshold is reached, or, at each impact, the system determines the cumulative propagation attenuation and compares it to a maximum attenuation threshold value to stop performing the process steps when the threshold is reached.

0031To make the most of the results, the attenuation map is stored in three dimensions.

0032It is thus possible to estimate a predicted quality level of the links according to the floors of the buildings.

0033In particular, in order to develop the database, the system represents the map by a bundle of vertically extending pixel chains and cuts the pixel chains to form stacked elementary volumes of volume meshes each having particular data.

0034Preferably, the system takes, as the original position, a position provided for the station, but, however, can provide to take, as the original position, any position in the cell and the ray launch direction is chosen , according to the positions and natures of the near reliefs, so that the ray passes close to the station.

0035Having calculated, according to the method of the invention, the conditions of propagation in a microcell in contact with a cell, therefore larger and still called macrocell, the system can then calculate propagation conditions in the cell and then performs a calculation of smoothing the results of the two calculations relating to a boundary zone between cell and microcell.

0036The method thus makes it possible to better integrate the two types of cells functionally.

0037The invention will be better understood with the aid of the following description of a preferred embodiment of the invention, with reference to the appended drawing, in which:<ul id="ul0002" list-style="dash" compact="compact"><li>the <figref idref="f0001">figure 1</figref> represents a relief geographical map forming a database, on which has been reported the position of a cell of a cellular radio network being defined,</li><li>the <figref idref="f0002">figure 2</figref> represents a radio attenuation curve in the cell, on a trajectory with obstacles and whose base station constitutes one of the extremities,</li><li>the <figref idref="f0003">figure 3</figref> is a plan view of the outline of a building on a portion of the map,</li><li>the <figref idref="f0004">figure 4</figref> is counterpart of the <figref idref="f0003">figure 3</figref> and illustrates a reflection and a refraction on a building, and</li><li>the <figref idref="f0005">figure 5</figref> is a vertical sectional view illustrating the propagation between buildings of a macrocell.</li></ul>

0038The <figref idref="f0001">figure 1</figref> represents the position of a cell 1, a cellular radiotelephone network provided for terminals such as that referenced 21, reported on a portion of the geographic map 31 also having the expected position of a base radio station 11 management of the cell 1. The card 31 is used to determine predictive contours of a plurality of cells constituting the radio network to be implanted, by adjusting their number, size and position in order to optimize the volume of equipment while ensuring the desired radio coverage with a quality of service. specified.

0039The reference 31 designates here the presentation of the elements of the ground, ground and sursol, of the zone considered. The corresponding geographical data making it possible to generate this presentation are stored in a database of relief 32, 33 of a computer 30. This database includes a memory block 32 specifying the shape and relief of the terrain, ground and sursol, associated with a memory block 33 of soil morphology and sursol specifying the natures or radio propagation characteristics of the various occupations of the ground, in a determined frequency range corresponding to the frequencies used by the base stations, these data of morphology of the memory block 33 present the various points of the ground considered with the reliefs or forms of the geographical block 32. It will be noted that the term "relief" must be taken in a broad sense to designate any obstacle likely to be reached by a radius, direct or previously deviated, of the station 11. It is therefore basically the ground and the ground. In addition to buildings, hills and equivalents, it can also be flanks of valleys or plains or water bodies.

0040As explained below, the data in block 33 makes it possible to determine, directly or through a correspondence table between nature and radio propagation characteristics data, the disturbance to an incident radio beam, in order to determine the direction and the attenuation or amplitude of a corresponding downstream radius. The card 31 itself therefore has only a didactic purpose here, since the data defining it are contained in the blocks 32, 33 that a calculator uses.

0041Alternatively, the base station 11 could be replaced by a station of the same function, but of reduced scope, to define a microcell. As indicated at the beginning, the microcells are located in areas of high relief or in the city, to cover the radio shadow areas of conventional cells.

0042It is assumed here that the trajectory of a radio beam, or narrow beam, connecting the station 11 to the mobile 21 in the cell 1, encounters obstacles 41 and 42, respectively a building and the trees of a forest. For the sake of simplicity, it is assumed here that the obstacles 41, 42 of the<figref idref="f0001">figure 1</figref> do not deviate the trajectory, which remains rectilinear, without reflection or refraction by them.

0043The <figref idref="f0002">figure 2</figref> represents, in decibels (dB), the level S of the radio signal as a function of the distance X traveled on the trajectory, plotted on the abscissa. Starting from the emission level, the decay of the level with the propagation distance thus represents the attenuation. This attenuation is the sum of the attenuations of various segments or sections of the trajectory, each section corresponding to a specific propagation medium.

0044There are here five sections, referenced in order 51 to 55, respectively corresponding for the first 51 to the attenuation due to the air path from the station 11 to a building 41, for the second 52 to the attenuation due to the crossing of building 41, for the third 53 at the attenuation due to the air path that follows to the edge of a forest 42, for the fourth 54 at the attenuation due to the crossing of the forest 42 and for the fifth at the attenuation due to the air path to the mobile terminal 21.

0045The linear attenuation of radio propagation in free space in the air, represented by the slope of the level on the <figref idref="f0002">figure 2</figref>, is a well-known physical constant for a determined carrier frequency, and the computer 30 can therefore, from the propagation distances found in the database representative of the card 31, calculate the three corresponding attenuations. On the other hand, the obstacles 41, 42 correspond to increased attenuations. In addition, as mentioned, certain types of obstacles can also deviate the trajectory, as illustrated by the<figref idref="f0004">figure 4</figref>. However, to verify the predicted radio coverage, it is necessary to be able to estimate by calculation the field level at various points distributed throughout the cell 1, for the links between the base station 11 and the mobile terminal 21.

0046The predicted link budget thus calculated, or the sum of the attenuations associated with the path sections 51 to 55, must not exceed the difference between a maximum transmission level Nm of the station 11 and a predetermined threshold Sm of the sensitivity of the terminal 21. It is the same for the direction said amount of communication from the terminal 21 to the station 11, as regards a maximum emission level of the terminal 21, here 2 watts, and a sensitivity level of the station 11. These sensitivity levels take into account a code for detecting propagation errors, and for correcting a limited number of erroneous bits, in the packets of bits exchanged through time slots of a radio frame.

0047For the sake of efficiency, computer mitigation calculations need only take a limited time, much less than a day's work.

0048To do this, the card 31 is in the form of a digital map in relief representing the cell 1 and the database 32, 33 specifies the positions of the reliefs as 41 and 42 and others, and their natures determining the conditions radio propagation, such as high buildings, forest, suburban area, lake and others. In an initial phase, before the operating phase for calculating the attenuations, a predetermined mesh of the card 31 is made and, at each mesh 34 of the geographical matrix thus determined, the numerical data of the reliefs that it comprises is associated, to thus have a matrix database, or mosaic, of specification of the reliefs, usable in the phase of exploitation.

0049In practice, a human operator or the computer 30 defines the mesh and stores, in the blocks 32 and 33 respectively, the relief shape data of each mesh 34 and the nature data of the reliefs. As mentioned above, these data identify the reliefs, such as forest, buildings, and a general correspondence table makes it possible to determine radio characteristic values, that is to say, parameters determining the propagation conditions of the radii. reaching the relief considered. As a variant, the values ​​of the propagation parameters are directly stored in the block 33, without the need to store the types of the reliefs.

0050In the operation phase, carried out in practice by the computer 30, for the provisional estimation of the radio coverage of the cell 1 by the radio management base station 11 of the cell 1, using the base of data 32, 33 embossed map specifying the positions and types of reliefs as 41 and 42 of the cell 1.

0051The computer fictitiously launches in the cell a sampling radius representative of the conditions of radio propagation, according to an initial trajectory section for example 11, 41 from a home position and according to a determined direction and propagation conditions, it is in practice through the air.

0052By reading the database, the computer compares the trajectory section 11, 41 to the relief map data, specifically those of the block 32, to identify the position and the nature of any current point of impact of the trajectory section. with a relief 41 in the example.

0053According to the specified nature of the relief considered 41, the computer determines new propagation conditions on a downstream portion of trajectory 41, 42 (section 52) beyond the point of impact at 41.

0054The computer reiterates, if necessary, the two preceding steps a given number of times for other impacts of downstream portions 53, 54 of trajectory.

0055According to the propagation conditions over the entire trajectory 11 to 21, the calculator determines a cumulative attenuation for any selected point thereof, and repeats all of the above steps a plurality of times for a plurality of initial directions in order to sample the entire cell and thus determine an attenuation map of the selected points.

0056In addition, having previously prepared, in the initial phase, the database 32, 33 in the form of a matrix, or mosaic, of local and autonomous specification data of the plurality of predetermined meshes 34 of the corresponding mesh of the map 31, this allows, in the operation phase,<ul id="ul0003" list-style="dash" compact="compact"><li>comparing the position of the current point to the mesh to identify the impact mesh 34, and</li><li>to calculate the propagation conditions after impact (section 41-42 for example) from the local and autonomous terrain specification data of the impact mesh 34.</li></ul>

0057Thus, in a general manner, the database 32, 33 specifies, in each cell, in particular in the memory 33, the local radio anisotropy, that is to say the distortions introduced into the beam, such as deflection, attenuation polarization diffraction and others. It is in a way defined, for each mesh 34, an ellipsoid of anisotropy determining the conditions of propagation in space, ellipsoid related to three eigenvectors, for example three orthogonal unit vectors of abscissa, ordinate, such as parallel, local meridian and vertical.

0058The above ellipsoid is in fact multiple, since it specifies the values ​​of several propagation variables, making it possible to calculate, for example, the direction of emergence of an incident ray as a function of its angle of arrival, on the card 31. , or the corresponding attenuation crossing the mesh 34, depending on the two directions, incidence and emergence. It is thus a matrix of transformation of the conditions of propagation of the various meshes, whose data are stored by independent zones in the block 33.

0059Reading, in block 33, the appropriate zone associated with the impact mesh 34 thus makes it possible to quickly determine the propagation conditions of the mesh exit from the input trajectory section for example 11, 41. In particular, one bit per cell 34 can specify whether the mesh 34 considered contains an obstacle or not. By direct reading thereof, possibly stored with its counterparts in a limited size register and quick access, a lack of obstacle indication is immediately detected and the computer 30 immediately goes to the next mesh examination without calculating new propagation conditions. Thus, apart from reading the bit indicating the presence of an obstacle, the computer 30 does not consult the data block 33 specifying the nature of the radio obstacles. A calculation of attenuation of current point, mesh mesh, on the trajectory is not necessary. The cumulative attenuation can in fact be calculated only in the case of an obstacle, by calculating the distance between the two end meshes 34 of the section considered. Experience has shown that calculations for a cell 500 meters in average radius require about 1 minute of average power calculator.

0060The computer 30 having fixed or calculated the angle of the incident ray, in the space of the card 31, can thus directly read, in the block 33, all the corresponding values ​​of the propagation parameters of the outgoing radius of the mesh 34, if the ray can actually come out.

0061Note that, the card 31 being in relief, the data mesh is preferably, as in this example, performed in three independent dimensions, such as those mentioned above. In other words, it is possible to define a bidirectional mesh of so-called horizontal meshes 34, with each horizontal mesh 34 corresponding to a vertically extending volume cut at various altitudes, possibly specific to each horizontal mesh 34, by planes or other surfaces, so as to determining elementary volumes each having particular propagation data stored in an area of ​​block 33. In most cases, only two elementary volumes per horizontal mesh 34 are sufficient, the lower one containing for example the entire building, and the upper one corresponding to the free space. On the other hand, in the case of reliefs with overhang, such as arched buildings or bridges, a third elementary volume, free propagation, is to be provided under an elementary volume containing the obstacle in question.

0062In other words, the map 31 is represented by a vertically extending group of strings of pixels, for each cell 34, and the pixel chains are cut to form stacked elementary volumes of volume meshes each having particular data.

0063The specification data characteristics or nature of the reliefs of each mesh 34, integrated during the initial phase, may correspond to one or more of the following data.

0064The specification data of the reliefs of the memory 32 may consist of geographical orientation data of the reliefs, indicating, for example, a plane of radio reflection. Knowing the incident path portion or segment, the direction of the reflected downstream trajectory portion, which is symmetrical with respect to the normal to the reflection plane at the point of impact, is calculated. In the air, in the absence of an obstacle in the first Fresnel ellipsoid (direct propagation), attenuation in the near field is approximately 20 dB / km over the first 500 meters of trajectory; beyond this, the value is 30 dB / km.

0065The <figref idref="f0003">figure 3</figref> thus illustrates the contents of a matrix data table of block 33, which content is schematically presented in the graphic form of a portion of the card 31, to be explained more clearly.

0066The <figref idref="f0003">figure 3</figref> represents some horizontal meshes 34 on which the surface occupied by a building has been reported. The building in question has four rectilinear walls, referenced 61 to 64, and has a trapezoidal shape with four corners of walls or vertical ridges 65 to 68.

0067In block 33, the four meshes having one of the wall edges include data specifying this characteristic. It may further be provided to specify the value of the edge angle and even the orientation of its sides. The other meshes 34 traversed by one of the walls 61 to 64 include data specifying this feature. In practice here, these data specify the orientation of the wall considered, that is to say, its azimuthal direction. It can be expected to also specify the inclination of the relief when it comes to other reliefs, for example natural. In such a case, these orientation data of the plane of the relief can be defined by the normal to the relief, expressed for example according to the three-dimensional coordinates mentioned above.

0068However, in order to limit the size of the block 33, it is possible to limit the orientation data of the relief to the azimuth data, and, in operation, the direction of the reflected radius of the downstream section is calculated by considering that the reflection reliefs radius are vertical, which is the general case in the city.

0069It is also possible to integrate, in the block 33 of the matrix database, data specifying a nature of a sursol, such as forest, suburban zone and others, and, in operation, the system 30 calculates the conditions of propagation on the downstream section. according to the nature of the sursol as specified.

0070In the initial phase, if the relief edge data are integrated in the block 33 of the matrix database, this allows, in operation, to calculate a downstream section direction of the refracted ray from the data of edges. As indicated, the refraction is usually caused by vertical corners or edges of buildings. However, roof ridge edges and ridges can likewise be specified in block 33 to likewise determine a direction of refraction of the incident ray, thus deflected downward for example. The above deflections increase the size of the coverage area of ​​cell 1 since they direct the refracted ray towards a volume which, in linear propagation, would be a shadow zone.

0071In the initial phase, the system integrates relief attenuation data into the memory in the matrix database 33 and, in operation, the system determines an attenuation of the radius from the attenuation data above.

0072The attenuation data may relate to the reflection on the reliefs 60 and they are then used to calculate the attenuation of reflected rays, for example about 7 dB, which value depends on the morphological nature of the facade, such as glass, brick or other.

0073In addition or instead, the attenuation data may relate to the propagation through the reliefs 41, 42, 60 and they are then used to calculate the attenuation of the rays propagating thereon, as illustrated in FIG. <figref idref="f0002">figure 2</figref>.

0074In particular, the propagation attenuation data through the reliefs 41, 42 may further comprise propagation medium transition data, specifying relief penetration attenuations, or propagation medium changes, which are used. to determine a local attenuation of penetration, air / building for example.

0075The <figref idref="f0004">figure 4</figref> is counterpart of the <figref idref="f0003">figure 3</figref> but the building 70 it represents has, in top view, a simplified form, triangular in this example, for the simplicity of the presentation. An incident ray 81 reaches a point of a facade 71 of the building 70, the point of impact which is located in a mesh 34 crossed by the facade 71.

0076According to the corresponding data of the mesh of block 33, indicating the azimuthal direction of the facade 71 and indicating that the mesh 34 considered is (entirely) traversed by the facade 71, the computer 30 determines that the incident ray 81 is reflects according to a radius 82 whose direction of emergence it calculates, a direction which forms, with the local normal 72 at the facade 71, an angle equal to and opposite to that of the incident ray 81.

0077As shown, the reflected ray 82 in fact defines the main direction of a lobe 84 of surrounding rays 83 which surround it, thus forming, in space, a solid angle. Indeed, the ray 81 excites the area it reaches and it thus generates a secondary source that diffuses the electromagnetic radiation in a more diffuse pattern, that is to say more isotropic, than a conventional primary source.

0078In order to better model the propagation, in this example the system determines, by an angular dispersion calculation algorithm, the plurality of directions of the rays 82 and 83, with specific attenuations, forming the solid angle of diffusion of the incident ray 81 at the beyond the point of impact. This may concern rays reflected by inhomogeneous surfaces, such as facades of buildings with windows and balconies, and diffracted rays. To illustrate these, an incident ray 91 still reaches the facade 71 but in a vertical edge region 73. The data of the block 33 of the mesh 34 considered then specify the presence of the edge, indicating a main diffraction direction. for an emergent radius 92 and a solid angle 94 of associated diffracted rays 93. The above explanation is also valid for vertically inclined edges. In particular, we can also consider the<figref idref="f0004">figure 4</figref> as a vertical section of a relief, through superposed horizontal rows of meshes, illustrating the fact that the diffracted ray can "dive" towards the ground, in a volume that one could a priori suppose to be a zone of radio shadow.

0079Note that the block 33 can contain both orientation data for calculating a partial reflection (82) and calculation data of a refracted ray 92, if it is considered that the incident ray 81, 91 has a section of the order of magnitude of a mesh 34, which is only partially affected by the presence of the edge 73.

0080With reference to the <figref idref="f0005">figure 5</figref>, the diffraction attenuation on a roof Ltàm (mobile roof), can be calculated by the following formula:<maths id="math0001"><math display="block"><mi mathvariant="normal">LTAM</mi><mo mathvariant="normal">=</mo><mo mathvariant="normal">-</mo><mn mathvariant="normal">16</mn><mo mathvariant="normal">,</mo><mn mathvariant="normal">9</mn><mo mathvariant="normal">-</mo><mn mathvariant="normal">10</mn><mspace width="1em" /><mi>log</mi><mfenced><mi mathvariant="normal">W</mi></mfenced><mo mathvariant="normal">+</mo><mn mathvariant="normal">10</mn><mspace width="1em" /><mi>log</mi><mfenced><mi mathvariant="normal">f</mi></mfenced><mo mathvariant="normal">+</mo><mn mathvariant="normal">20</mn><mspace width="1em" /><mi>log</mi><mo>⁢</mo><mfenced><mi>hb</mi><mo mathvariant="normal">-</mo><mi>hm</mi></mfenced></math><img file="EP1283643B1_D0001.tif" /></maths> or taken = 0, if the above calculation gives Lt &lt;0 with<ul id="ul0004" list-style="none" compact="compact"><li>W: width of a reception street seen from the antenna of station 11,</li><li>F: frequency in MHz</li><li>Hb: roof height diffracting towards the mobile 21</li><li>Hm: antenna height of mobile 21</li></ul>

0081In order to further improve the accuracy of the attenuation estimates, in this example the successive impacts on the trajectory (51 to 55, 81, 82, 91, 92) and, in the case of a second impact, are considered, to determine the propagation conditions beyond this impact, that the ray was polarized during the first impact. Indeed, a ray that has been reflected or refracted by a building facade undergoes a polarization, at least partial, substantially vertical. As a result, in the absence of the horizontal polarization components, eliminated during the first impact and which accounted for a large part of the total attenuation, the attenuation has a reduced value during the following impacts. The ray has thus somehow been adapted to obstacles. Reflection attenuation thus passes, on average, 7 dB for the first reflection to 3 dB for the following.

0082In order to obtain radio coverage data for the various floors of the buildings and also to treat the overhanging forms mentioned above, in this example the map or ground base of the attenuations is established in three dimensions.

0083To avoid unnecessarily prolonging the calculation time, the system counts the successive impacts of the radius and the total is compared to a high threshold value to stop performing the process steps when the threshold is reached.

0084For the same purpose, in addition or instead, at each impact, the system determines the cumulative attenuation and compares it to a maximum attenuation threshold value to stop performing the process steps when the threshold is reached.

0085It will be noted that it is possible, as a variant of this particular example, to apply the inverse return principle of the radius, that is to say to transmit it from any position of the terminal 21 towards the station 11, by moving as previously the position of the terminal 21 in the whole cell 1.

0086However, in such a case, there is some uncertainty as to the success of each test, since a ray emitted towards the station 11 may be deflected and, conversely, a ray emitted in another direction will be deflected by a relief and will reach it. It will therefore be necessary to provide an increased number of ray launches, for example in a large solid angle containing the station 11, taking into account the foreseeable deviations, for example a roof of a refractive building, close to the terminal 21.

0087Thus, it is possible to take, as the original position, a position provided for the station 11, or we take, as the original position, any position in the cell 1 and the launching direction of the spoke is chosen, after the positions and natures of the close reliefs, so that it passes close to the station 11. Thus, a sufficient proportion of calculations is exploitable.

0088The method can in particular be used for microcells in contact with, if not included in, (macro) cells. Given the high antenna height of the latter, the propagation is less disturbed and calculations can be performed by a conventional propagation method. Having also calculated, according to the present method, the conditions of propagation in a microcell in contact with a (macro) cell, propagation conditions are calculated in the cell and a smoothing calculation of the results of the two calculations relating to a border zone between cell and microcell

0089The map 31 here has a grid step of about 5 meters along parallels and meridians. In particular, it can be obtained by linear interpolation from an altimetric map of the smaller scale IGN, with meshes essentially in the form of large squares of 50 meters in length, having a vector database defining, in addition to the altimetry of the terrain, the positions of the reliefs and their natures. The calculating system knits it at a pace of 5 meters, cutting each large square into one hundred small squares of 5 meters on each side. The area delimited by each small square thus determines a corresponding subset of the data defining the positions and natures of the reliefs.

0090Then, the computing system performs a smoothing, or low-pass spatial filtering, by an interpolation calculation taking into account the above altimetry data of the large squares adjacent to that considered, this in line, in column and in diagonal. The computer 30 thus modulates, for example, the altitude data of the average ground of the large square considered in order to determine a most probable local value for each small square, to thereby obtain a sub-block of altimetry data matrix, constituting the one of the plurality of areas of the block 32.

0091Other data of global order, for example of specification of the sursol, such as forest, suburban area, empty terrain or other, can be calculated in this way. The more punctual order data, specifying, for example, a building facade orientation for the block 33, are instead preferably based on field surveys, for example aerial photos. The sub-block of supersoft data data is preferably, as here, supplemented by a matrix sub-block of data of height of the sursol, obtained by difference between the altitude of the latter and the altitude of the ground, with respect to The data of blocks 32, 33 are thus more accurate and up-to-date.