7.12. Experimental studies of the radiation levels of antennas BTS. cellular communication standard GSM.

The attenuation of radio signals during the propagation of radio waves along real paths is due to the complex influence of the environment.

Single-, double- and multipath models of electromagnetic wave propagation are suitable for special cases where the propagation paths can be accurately described. Therefore, to assess the real distribution of the components of the electromagnetic field within a cell, it is necessary to conduct experimental studies of the functions P (g, /, A, h2), P (g, /, hi, h2) and E (g, /, hi, h2), that is, changes in power flux density, power and electric field strength at the point of reception from the distance g, the operating frequency /, the elevation of the BTS-hi and MS-h2 antennas above the ground or the average ground level. Since these functions vary depending on the objects in the path of propagation of radio waves, we introduce the concept of the following types of surfaces:

- open areas: areas with very few obstacles, such as trees or sparse structures (for example, farmers land or open fields),

- suburban areas: areas with one-story houses, small buildings and trees, often located near mobile stations,

- urban areas: areas densely built up with high-rise buildings and high-rise buildings.

This classification did not include transition zones in which propagation conditions should be specifically studied.

We introduce the concept of a quasi-smooth surface, by which we mean a piece of land with an average height of irregularities not exceeding a certain size (for example, 20 m).

The introduction of this concept allows us to consider a specific terrain as “flat” or “uneven” in accordance with the Rayleigh criterion.

where o is the standard deviation of the surface from the average height of the surface, X is the working wavelength, 0 is the angle of incidence of the electromagnetic wave in radians relative to the horizontal.

As experiments show:

- if the value of kr <0.1, then the surface can be considered smooth; For this surface, one can use classical models for estimating losses during radio wave propagation;

- if kg> 10, the surface should be considered uneven, in which case the amplitude of the reflected wave is small, and the received signal power is inversely proportional to the fourth power of the distance between the BTS and MS antennas - P ~ (1 / g4).

If we estimate the value of kg for the mobile communication system of the GSM standard, then with / = 900 MHz: with g = 2 m - kg = 76 0, with g = 20 m - kg = 760 0, with g = 30 m - k = 1140 0, that is, all surfaces for the service area when b2 ~ 2m are non-smooth.

In ch. 3 experimental studies of the levels of the received signal emitted by the BTS antennas in the conditions of the city, suburb and forests are considered. It should be noted that for estimated calculations of the electric field strength in urban conditions, when pico or micro cells are to be organized, semi-empirical expressions obtained by various foreign researchers in [7.21] can be used.

Consider for example the results of experimental studies of the levels of power flux density in the center of Riga. To estimate the integral median power flux-density levels in the frequency range from 0.3 to 1000 MHz under the conditions of the city of Riga, a NARDA device was used, having a measuring range in P from 0.01 to 20 μW / cm2.

The values ​​of P (d) were determined at various points in the city of Riga, and thus obtained:

1) in the area of ​​the television tower (Zakusala) the range of changes in the P (g) levels ranged from 0.6 μW / cm2 to (0.38 ... 0.48) μW / cm2.

2) in the area of ​​the Academy of Sciences (high-rise building) and the Town Hall Square, the value of P (g) was 0.28 ... 0.38 μW / cm2.

Thus, experiments have shown that the levels of the electromagnetic field in the frequency range of the GSM, NMT mobile communications within the central part of Riga do not exceed a safe background value of 1 µW / cm2, which indicates normal living conditions for city residents (in terms of electromagnetic safety).