METHOD AND APPARATUS FOR DETERMINING RADIO WAVE PROPAGATION ATTENUATION

Abstract: The invention relates to a method and an apparatus for determining radio wave propagation attenuation in the desired area in a radio system. An at least multi-dimensional vector map delineating the environment of a base station is used in determining the coverage area of the system base station. Propagation attenuation at some points of the desired area is determined by a ray tracing method. At other points of the desired area, the propagation attenuation is calculated by an empirical model, the parameters of said model being initialized by the results obtained by the ray tracing method. By the method, propagation attenuation can be calculated much more rapidly than before.

Description: METHOD AND APPARATUS FOR DETERMINING RADIO WAVE PROPAGATION ATTENUATION FIELD OF THE INVENTION The invention relates to a method for determining radio wave propagation attenuation in the desired area in a radio system, said method using an at least two-dimensional vector map delineating the environment of a base station in determining the coverage area of the system base station, and said method determining the strength of the transmission of a transmitter at different points of the desired area. DESCRIPTION OF THE PRIOR ART When a radio system is being constructed, an effort is made at achieving the desired coverage area as cost-effectively as possible. When considering the locations of the base stations of the system, the required traffic capacity and the achieved coverage area are taken into account. The aim is to locate the base stations in such a manner as to ensure an extensive coverage area and an advantageous location of the base station as far as radio wave propagation is concerned. Different methods and instruments have been developed for radio network planning. Digital maps providing modeled information about the ter- rain and buildings in the desired area are generally used instruments in radio network planning. By means of a vector map a computer can be used to calculate coverage areas and parameters concerning network operation for different base station locations. However, to determine indoor attenuation particularly is difficult. A generally applied method is to determine indoor coverage area by estimating the general field strength outside a building and subtracting from this the general indoor attenuation in the building. Results obtained by this method are, however, highly inaccurate for small cells in which the field strength can vary widely even indoors, and to use a general attenuation factor is insufficient. Moreover, the method is unsuitable for determining the coverage areas of indoor base stations. Another known method is to perform measurements in different buildings and locate a base station in such a manner as to achieve the best audibility. The drawback in this is, however, that the measurements are ex- pensive and time consuming. Moreover, in practice the entire desired area cannot be measured. A third method is to calculate coverage areas by so-called ray tracing method. This is, however, impractical in determining the indoor coverage areas of buildings, since there are typically many partitions inside a building, and to ensure accurate predictive calculations the rays propagating through and being reflected from the walls must be taken into account. Consequently, in practice the calculation time becomes too long using present-day computers as far as network planning is concerned. BRIEF DESCRIPTION OF THE INVENTION The object of the present invention is thus to implement a method by which the disadvantages of known methods can be avoided. This is achieved by the method described in the preamble, said method being characterized in that the propagation attenuation at some points in the desired area is determined by a ray tracing method, and that the propagation attenuation in the desired area is calculated by means of an empirical model, the parameters of said model being initialized by the results obtained by the ray tracing method. The invention further relates to an apparatus for determining radio wave propagation attenuation in a radio system in the desired area, said system comprising at least one base station, and said apparatus comprising means for maintaining an at least two-dimensional vector map delineating the environment of the desired base station, and means for determining the transmission strength of a base station transmitter at different points of the environment. The apparatus of the invention is characterized in that it further comprises means for estimating propagation attenuation at some points of the desired area delineated by the vector map by a ray tracing method, and means for calculating propagation attenuation in the desired area by means of an empirical model, and means for initializing the parameters of the model by the results obtained by the ray tracing method. The method of the invention has several advantages. In the solution of the invention the calculation time is short, a fraction of the time taken if the entire calculation were performed by a ray tracing method. Since the empirical model is initialized by accurate values, the results obtained are, however, suf- ficiently accurate. The solution of the invention is particularly suitable for estimating indoor propagation attenuation. BRIEF DESCRIPTION OF THE DRAWINGS The invention is described in greater detail in the following with ref- erence to the examples in accordance with the accompanying drawings, in which Figure 1 illustrates a radio system, to whose planning the method of the invention can be applied, Figure 2 illustrates a vector map of a coverage area of a cellular ra- dio system, Figure 3 illustrates an example of a final propagation attenuation curve and Figure 4 is a block diagram illustrating the structure of the apparatus of the invention. DETAILED DESCRIPTION OF THE INVENTION The method of the invention can thus be preferably applied in radio system planning. It is particularly suitable for planning radio systems which are implemented by micro cell and pico cell techniques in an urban environment. Some typical radio systems are illustrated in Figure 1. Two systems are shown in the figure, a typical cellular radio network 100 and an indoor network 104 of a building 102. In the cellular radio network 100 a base station 106 is located outdoors, and it comprises a number of subscriber terminals 108 to 110, some 108 of which can be located outdoors and some 110 inside buildings 112. The subscriber terminals have a bi-directional connection to the base station. An indoor pico cell network 104 of the building 102 comprises an indoor base station 114 and a number of indoor terminals 116 to 118. Let us take a closer look at the method of the invention by means of an example. Figure 2 illustrates a vector map of a typical office environment in which an indoor cell is to be located for a radio system. The vector map com- prises the structural parts of the area which affect radio wave propagation. The aim is to locate the base station in the area so as to achieve the best possible audibility in different parts of the building. In practice some probable locations where the base station could be located are selected, and a calculation is performed by these defaults. The best alternative is finally selected to be the final location of the base station. A similar calculation is thus performed in each presumed location of the base station. An initial calculation is thus determined by a ray tracing method at some points of the desired area. Ray tracing can be implemented in two manners: by means of multiple mirror images or by a ray launching method. The method is described in closer detail in T. Huschka: Ray Tracing Models for Indoor Environments and Their Computational Complexity, PIMRC '94. AAB5.1 , 1994. Several different empirical models have been developed for determining propagation attenuation. Thus far the problem has been the difficulty of setting their parameters. A possible propagation model is disclosed in the following formula which assumes that the powers are given on a dB scale: P r = P t - C 0 - 10 * C ή *log (d) - C 2 d where P r and P ( are transmitted and received power and d is the distance between the transmitter and the receiver. C 0 is a scaling factor depending on radio frequency and antenna properties. C and C 2 are an attenuation slope and an additional obstacle loss. Parameters C, can thus be initialized by information obtained by the ray tracing method. Another possible empirical model is disclosed in J.M. Keenan, A.J. Motley: Radio Coverage in Buildings, British Telecom Technology Journal, Vol. 8., No 1 , January 1990, and is as follows: P r = P t - C 0 - 10'C Iog (d) - n,*D, -n 2 *D 2 where P t is transmission power, C 0 a constant term depending on radio frequency and antenna properties, C 1 an attenuation slope, d the distance between the transmitter and the receiver, n, the number of obstacles on the line of sight and belonging to class i, and D, an attenuation constant of obstacles belonging to class i. In buildings these obstacles usually are walls made from different materials, the attenuation of said walls depending on the building material used and which can be suitably classified. In the solution of the invention, the attenuation constants C, and D, can be preferably and accurately determined by means of a ray tracing method. An example of a final propagation attenuation curve is illustrated in Figure 3. The measurement point is shown on the horizontal axis and the field strength on the vertical axis. The measurement points result from presuming that a terminal inside the building in Figure 2 takes the route marked by the dotted line. The result (the upper curve) obtained by the method of the invention and the result (the lower curve) obtained by a measurement in practice are documented in the figure for the sake of comparison. The estimated curve has been raised by 15 dB from the actual result for the sake of clarity. The advantage of the solution of the invention is particularly its great rapidity. An initial ray tracing calculation in a building in accordance with Figure 2 takes nine minutes performed by an effective work station, while a final propagation attenuation estimate was calculated in one second utilizing the latter initialized empirical model. If an area comprises, for instance, a hundred buildings with 25 floors in each, and information about each building is needed, the calculation by mere ray tracing method would take 15 days, while the solution of the invention would take less than one hour. The solution of the invention is suited to outdoor and indoor calculation. If necessary, a different empirical model can be used in different parts of the desired area, depending on the type of the areas. The calculation can, of course, be performed three-dimensionally, depending on the dimension of the vector map used. Lets us next have a closer look at the structure of an apparatus implementing the method of the invention by means of a block diagram shown in Figure 4. The apparatus comprises means 400 for maintaining an at least two- dimensional vector map delineating the environment of the desired base station. Means 400 are typically implemented by means of memory circuits. The apparatus further comprises processor means 402 for determining the transmission strength of a base station transmitter at different points of the environment. In the apparatus of the invention the processor means 402 determine propagation attenuation at some points of the desired area by a ray tracing method. The processor means 402 further determine propagation attenuation in the same area by means of an empirical model, the parameters of said model being initialized by the results obtained by the ray tracing method. In the apparatus of the invention the values calculated by a ray tracing method can temporarily be stored in the memory means 404 of the apparatus, where the values can be read when the initial values of the empirical model are set. The means 402 can preferably be implemented by means of microprocessor or corresponding detached logic circuit, whereby the procedures of the invention can preferably be implemented by software. The particular area being calculated can be introduced as an input 403 to the processor devices. Even though the invention has been explained above with reference to the example in accordance with the accompanying drawings, it is obvious that the invention is not restricted to it but can be modified in many ways within the scope of the inventive idea disclosed in the appended claims.

Claims: CLAIMS 1. A method for determining radio wave propagation attenuation in the desired area in a radio system, said method using an at least two-dimensional vector map deline- ating the environment of a base station in determining the coverage area of the system base station, and said method determining the strength of the transmission of a transmitter at different points of the desired area, characterized in that propagation attenuation at some points in the desired area is de- termined by a ray tracing method, and that propagation attenuation in the desired area is calculated by means of an empirical model, the parameters of said model being initialized by the results obtained by means of the ray tracing method. 2. A method as claimed in claim 1, characterized in that a different empirical model is used in different parts of the desired area. 3. A method as claimed in claim 1, characterized in that the empirical model is of the form P r =P t -C 0 - W^ log (d) - n 1 *D 1 -n 2 * D 2 where P, is transmission power, C 0 a constant term depending on radio frequency and antenna properties, C 1 an attenuation slope, d the distance between the transmitter and the receiver, n, the number of obstacles on the line of sight and belonging to class i and D, an attenuation constant of obstacles belonging to class i, and one or more of the attenuation constants C, and D, are determined by a ray tracing method. 4. A method as claimed in claim 1, characterized in that the method is applied to the calculation of the indoor attenuations of buildings. 5. An apparatus for determining radio wave propagation attenuation in a radio system in a desired area, said system comprising at least one base station, and said apparatus comprising means (400) for maintaining an at least two-dimensional vector map delineating the environment of the desired base station, and means (402) for determining the transmission strength of a base station transmitter at different points of the environment, characterized in that it further comprises means (402) for estimating propagation attenuation at some points of the desired area delineated by a vector map by a ray tracing method, and means (402) for calculating propagation attenuation in the desired area by means of an empirical model, and means (402, 404) for initializing the parameters of the model by the results obtained by the ray tracing method.

Inventors: LEPPAENEN RISTO, SIPILAE KARI, LEPPAENEN, RISTO, SIPILAE, KARI
Applicants: LEPPAENEN RISTO, NOKIA TELECOMMUNICATIONS OY, SIPILAE KARI, LEPPAENEN, RISTO, SIPILAE, KARI
Application year: 1998
Main classification: H04B7
IPC codes: H04B7, H04B17, H04W16