Building Penetration Loss Models

The building penetration loss is defined as the power loss that an electromagnetic wave undergoes as it propagates from outside a building towards one or several places inside this building. This parameter is determined from the comparison between the external field and the field present in different parts of the building where the receiver is located. Penetration loss models, being integrated into the coverage prediction tools, must take into account the environment around the buildings which is under consideration.

Diffracted Building
Fig. 7.15. Diffracted and reflected ray paths between an emitter E and a receiver R

The electromagnetic field inside buildings is subject to the influence of different parameters, including the position of the building with respect to the emitter and to other buildings or the architectural characteristics of the building, for instance the materials, the interior layout or the size of the windows. Values obtained for a given building should not, therefore, be generalised to any kind of building when performing coverage predictions. While a database integrating a large enough number of characteristics would help improving the precision of building penetration loss models, the development on a large scale of such a database remains difficult (Toledo 1998).

The value of the building penetration loss is influenced by a number of different physical parameters, whose effects intermingle most of the time. Among these different parameters the following are traditionally distinguished:

- the near environment : a distinction is drawn between districts with high towers more or less separated from each other and more traditional districts with buildings of average height,

- the reception depth in buildings: the amplitude of the field decreases as the mobile moves from the front of the building towards a room located inside it, while the influence of the inhomogeneities decreases as the penetration depth inside a building increases. Waves penetrate more easily through window panes than through brick walls. Accordingly, the paths followed by radio waves will be more or less attenuated, and might even be occulted. The building penetration loss is generally 6 dB lower for glazed walls compared with non glazed walls (Rappaport 1994). Although the level of attenuation is higher in the back of buildings, it is also much more homogeneous there.

- the incidence angle, which determines the reflection and transmission coefficients at a surface,

- the reception height, more commonly described as 'floor effect'. This parameter induces effects in the form of a reduction of the building penetration loss or of a relative power gain, as compared with the lower floor. The calculation starts therefore from the consideration of the building penetration loss at the ground floor, as determined from a comparison with the external field. In small cell, power gains are typically of the order of approximately 2 to 3 dB per floor at the 900 and 1800 MHz frequencies. However the great diversity of situations results in a scattering of these gains per floor; values ranging from 4 to 7 dB have already been measured in practice (Gahleitner 1994). The lower floors are illuminated by rays reflected and diffracted at the roofs and in the street, whereas the upper floors are generally under stronger illumination, and even sometimes under direct line-of-sight illumination. As a consequence, the values of the building penetration loss vary between the lower floors and the upper floors (Rappaport 1994; Walker 1983).

- the distance between the transmitter and the receiver, when the building where the mobile is located is in line-of-sight of the transmitting antenna. The building penetration loss in this case depends on distance as predicted by the freespace propagation law.

- the height of the emitting antenna,

- the frequency,

- the nature of the materials. The attenuation that electromagnetic waves undergo varies from 4 dB in the case of wood to 10 dB in the case of concrete walls (COST231 1999).

Several different measurement techniques have been developed in order to characterise the building penetration loss associated with the materials of the buildings. In particular, the method based on the use of two reverberation rooms can be mentioned here (Foulonneau 1996).

The most classical models draw on the Motley-Keenan model (Motley 1988) used for the study of propagation inside buildings. The parameters considered in these models for the determination of the building penetration loss include:

- the distance between the emitter and the external wall of the building where the receiver is located,

- the distance between the external wall and the receiver,

- the number of internal walls present along the emitter-receiver profile,

- the floor effect,

- the attenuation due to the external walls of the building,

- the attenuation due to the internal walls of the building.

The loss path L is expressed as the sum of the free-space loss Lo of the losses due to the obstacles present along the direct line-of-sight path (tiles, walls, doors, windows) and of a constant Lc (Motley 1988). Databases can be used for differentiating between the different obstacles to which specific attenuation values are attached. This model is the most commonly used.

j=l where Nj is the number of walls of type j present along the line-of-sight path, Lj is the losses due to walls of type j, N is the number of types of walls, Nf is the number of tiles present along the line-of-sight path and Lf is the losses due to tiles.

Typical values of transmission losses for different types of materials used for the external walls in the 1-2 GHz frequency band are summarised in Table 7.2 (COST 231 1999).

Table 7.2. Transmission losses for external walls made of different types of materials in the 1-2 GHz frequency band

Materials Losses [dB]

Porous concrete 6.5

Reinforced glass 8

Concrete (3q centimetres) 9.5

Thick concrete wall (25 centimetres) with 11 large glazed panes

Thick concrete wall (25 centimetres) with- 13 out glazed panes

Thick wall (> 20 centimetres) 15

Tile 23

Radio Wave Diffraction Diagrams

Fig. 7.16. Propagation paths considered in building penetration loss models

Points used for calculating the field strength outdoors

Fig. 7.16. Propagation paths considered in building penetration loss models

The main disadvantage of these models lies in the empirical evaluation of the parameters. These parameters, for instance the value of the wall attenuation, may indeed present significant fluctuations from one building to another, which results in a reduction of the degree of precision of the prediction of the field strength. Furthermore, the internal architecture of buildings is not at the present time integrated by any database. Even though this solution could be considered for certain specific buildings, it is doubtful that it could ever be implemented on a large scale.

Building penetration loss models can be enhanced through the use of microprofiles joint the indoor mobile to external reference points, as represented in Fig. 16. The information relating to the external environment and to the interior of buildings and extracted from the microprofiles allows determining the total attenuation, which consists of two terms: a building penetration loss and an outdoor path loss between the base station and the building (COST 231 1999).

The propagation of radio waves in buildings depends primarily on the nature of the environment. The propagation environment can be characterised as either dense (office type buildings), open (office type buildings, offices with large capacity), broad (buildings with very large rooms, for instance warehouses, airports or railway stations) and corridor (in the case where the emitter and the receiver are located in the same corridor). Indoor propagation is also multipath: the predominant propagation mechanisms are reflection, transmission, diffraction and scattering (Hashemi 1993b; Valenzuela 1997).

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