The GSM system is a digital system of cellular communications developed on the basis of European standards and distinguished among its analog predecessors both for good technical and operational characteristics and high environmental indices. This is the result of a number of advanced technical solutions used in the system, which significantly reduces the output per voice channel. Those solutions are: time division of channels, control of transmitter power with respect to signal level on receiver input, transmitter’s "silence" in the pauses during conversation, original method of efficient coding of the audio signal.

The frame of time-division multiplex used in GSM system consists of 8 time intervals, out of which only one telephone signal is transmitted per time interval. That is why the output of the mobile handset transmitter is reduced by 8 times, while the base stations can, at the same time, derive 8 times as many channels under the same output. If we take into consideration that each party of a telephone conversation speaks during one half of the total period of time, we can say that the output of a mobile handset transmitter is additionally reduced twice with respect to its total output. The power of the electromagnetic field established by base stations is also reduced twice. As a result, the real transmission power of a GSM mobile handset with a 2-watt transmitter is 16 times less than the maximum power and makes 125 milliwatts. The power of the electromagnetic field set up by the base stations is almost doubly reduced for the same reason. Additional reduction of the average power of the transmitters of both the mobile handset and the base station is conditional on the system of automatic power control according to the distance between the subscriber and the base station. Precise quantitative estimation of such power is impossible as it depends on the geometric distribution of subscribers among base stations. We can only mention here that the denser the base station network, the bigger is the "win" in power.  One more thing is that the range of the dynamic regulation of the transmitter power of a GSM base station is 30 dB, and its power can correspondingly vibrate between 20 watts and 0.02 watts.

One third of Magti GSM network base stations are located in Tbilisi, and the rest in various regions of Georgia. The most significant thing from an environmental point of view is the location of base stations and their configuration, in particular the number of working sectors, types and orientation of transmitting antennas. The configuration of Magti GSM network base stations is selected based on the desired coverage area. One part of the base stations operate in the 900 MHz frequency range, and the other part – 900/1800 MHz. Base station antennas are located at least 10 or 15 meters higher than the roofs of adjacent buildings. This is done to secure the stability of mobile communication and to achieve high environmental security. The nearest buildings are on a vertical plane, outside the range of the effect of the main lobe of antenna directivity, and the power of the electromagnetic wave towards their direction is at least 14 or 15 dB less than in the main direction.

We use three main methods of erecting antennas:

1. At the perimeter of the building roof, when we
manage to find a building much higher than
the ones nearby.

2. On a supplementary tower mounted on a building roof, when we fail to choose a building as high as needed.

3. On a separately erected tower, mainly in outlying districts, far from populated areas.

We have carried out a theoretical estimation and the instrumental measurement of electromagnetic fields set up by those three types of stations. The analysis of the results show that the density of the energy flux on the buildings around GSM base stations does not exceed 0.5 mkW/cm_, which is at least 20 times less than the admissible peak value (such value makes 10 mkW/cm_ according to  sanitary norms acting in Georgia).

The electromagnetic field is weaker still in the buildings with antennas mounted on the top of the roofs. The reason is that antennas do not establish an electromagnetic field vertically down, and the electromagnetic field reached there is mainly the one reflected from surrounding objects.

The cumulative power of electromagnetic fields in base stations with both 900 MHz and 1800 MHz transmitters  (the power of the latter makes 8 watts instead of 20) is significantly less than the norm, in particular, the density of the energy flux on surrounding objects reaches 0.75 mkW/cm_, which is at least 13 times less than the normal value. Even when 4 pairs of these transmitters are located in a base station, the density of energy flux reaches 3 mkW/cm_, which is quite admissible, and what is more, even the hygienic condition of a building with two base stations of two different operators will correspond to the norms.

Environmental security requires the estimation of the influence of separate base stations as well as their combined influence. However, there is nothing alarming here. The power of electromagnetic fields set up by each transmitter is attenuated according to square law, and its share in the cumulative field established on the surrounding objects of neighboring stations is very weak. This is assisted by the fact that, according to cellular technology, the antennas of neighboring stations never face each other. They are directed at a certain angle towards each other in order to achieve full coverage. Drawing 1 illustrates the range card of three-sector base station antennas in an idealized cellular structure.

In conclusion, we would like to mention that under the order and on the initiative of MagtiCom, the Magti GSM network project was sent to the Ministry of Environment and Natural Resources Protection of Georgia for environmental impact assessment which attested to the harmlessness of the Magti GSM network. According to the findings of the State Commission, "Magti GSM cellular communications network fully complies with the norms active in Georgia with its parameters of electromagnetic fields".
 

Givi Murjikneli
Professor,
Technical Manager
MAGTICOM