high quality and safe service

Giorgi Tabuashvili
Manager of Radio Frequencies of MagtiCom

    Mobile communication, the highly convenient and often indispensable means of communication, has undergone rapid development in the past 20-30 years. Cellular systems have been widely utilized, in particular GSM (Global System for Mobile Communication) based on the greatest scientific and technical achievements of the last 20 years of the past century. The principal operational environment of such systems is the limited range of radio frequencies, where the coinciding working frequencies are used by base stations separated by space and by mobile handsets located in the area regulated by those stations. That is why the proper distribution of base stations and power regulation requires a scrupulous and precise network design.

    The principle of cell design implies the existence of low-power base stations. Considering the 2 or 3 db loss of energy in the feeders connecting the transmitters to the antennae, the maximum power input into the antennae is 10 or 12 watts for the 900 MHz band, and 8 or 10 watts for the 1800 MHz one. However, as was mentioned, those are the maximum figures. The GSM standards, to ensure the flexibility of the system, allow regulation of the power of base stations within a wide range (between 0.00062 and 12 watts). One of the elements of optimizing base stations is reducing the power to such a point that would allow an efficient re-distribution of traffic among the neighboring cells and the reduction of harmful effect on the cells operating on coinciding frequencies.
   In our age of electrification when both indoors and outdoors we are surrounded by numerous sources of electromagnetic fields of different power, the given above figures seem negligent: 200 volt wiring, high-voltage transmitting lines, TV-sets, microwave ovens, etc. (i.e. anything, where the voltage with certain frequency changes its value) belong to such sources. It is known that a light beam represents an electromagnetic wave. 90 or 95% of the power consumed by an ordinary bulb is transformed into heat, while the remaining 5 or 10% is spread as an electromagnetic field or light. In other words, if the cumulative consumed power of a chandelier is 300 watts, then the power falling on a visible electromagnetic field accordingly accounts for some 15 or 30 watts.
   A GSM mobile handset is a sophisticated instrument utilizing the latest scientific and technical achievements. According to their purpose, mobile phones can be of different types, the majority being pocket and portable ones. Their maximum power capacity is 2 watts for the 900 MHz and 0.8 watts for the 1800 MHz band [33 and 29 decibel milli watts (dbmw) accordingly]. If the signal level on the input of a handset receiver is higher than required, the handset, following GSM standards, does not use the maximum transmission power, decreasing it to the minimum value of 0.08 watts (19 dbm), which, on the one hand, prolongs the operation time of the handset accumulator and, on the other, improves the general background of interference (the effect of one transceiver on another).
   This is easy to see if one takes the same fully charged handset and speaks first from a properly covered area (where the antenna index is maximum, exceeding -65 dbm) and then from a poorly covered area (where the antenna index does not exceed 3 bars, or is less than -85 dbm). In more simple terms, the power of -60 dbm, i.e. one billionth of 1 watt corresponds to a well covered area that is on average a good radio signal, while the power of -90 dbm or - 10-12 watt corresponds to a poorly covered area, i.e. a weak radio signal. In the first case the total talk time will be more than in the second case. In this respect, when operating small powers, it is desirable to have a large number of base stations in densely populated areas, especially as the power of a radio signal in free space decreases inversely proportional to the square of distance, while in practice such a decrease is at best inversely proportional to the cube of the distance. The dependence of signal power on distance is determined by a number of factors, including the applied frequency band and the urbanization level (Draw. 1).

   It is apparent from the drawing that the weakening of a radio signal is stronger in the higher DSC 1800 MHz frequency range than in the GSM 900 MHz range. The level of radio signal is 40 db weaker (10,000 times less) 3 kilometers away from a base station than 0.2 kilometers away. It is more convenient to use decibels in calculating a radio-line budget, because the values of amplification and attenuation on the different stages of transmission path are simply added with corresponding signs. Like a financial budget, a radio-line budget determines the required level of communication to fulfill the stated task. When analysing such a budget we must take into account both the decibel-increasing (transmitter power, the coefficient of antenna amplification) and the decibel-decreasing factors (e.g. signal attenuation). To achieve the guaranteed quality of communication the input of the receiver must have a certain level of signal in db plus some reserve against attenuation. As the wavelength is 30 cm for the GSM 900 MHz and 15 cm for the DCS 1800 MHz, the environment has a great influence over the transmission of a signal. Any object, be it a hill, a building, a piece of furniture or a car, decreases the signal level, changing its direction or qualities. The results of such an influence may be divided into the following categories:

   Attenuation occurs when an electromagnetic field suffers energy loss as it encounters some obstacle in its path. As a rule, attenuation is quite significant for GSM frequency range and depends on the substance and size of the obstacle that causes it.

   Reflection occurs when a radio wave falls on a comparatively flat and conductive surface. The wave is reflected at the falling angle. This is the reason why seawater is a better reflector than, say, sand.

   When a radio wave falls on an uneven, rough surface, it is reflected in many directions, resulting in the scattering of energy.

   Diffraction is a deviation in the direction of a wave at the edge of an obstacle in its path. It takes place when a radio signal meets some sharp object or an angle of a house or wall on its way. The level of diffraction increases with the increase of frequency.

   Cases of an intensive attenuation are demonstrated in the shadow zones - the comparatively weak places in the coverage area. In such places the indicator of a signal level on a mobile handset shows fewer bars. Research carried out in different cities showed that shadow zones result in a 2 db attenuation of a signal even in the short distances (i.e. the signal is weakened 100 times). The reason for attenuation may be even the plant life of a given place: in New Jersey, USA, an experiment conducted on 836 MHz frequency band showed that the signal level is 10 db less in summer than in winter, when the trees are bare. The main factor, necessary to consider while designing a cellular infrastructure, is the multi-pathing propagation of a signal as a result of reflection. It is a good thing on one hand, because the radio waves penetrate buildings, underground garages, tunnels, etc, but on the other hand it causes such problems as the increase of signal delay, and others. These problems are caused as a result of the reception of various-phase signals. This phenomenon has already been investigated by British physicist John William Rayleigh (1842-1919). In cases when the phases are shifted by 180º, the cumulative signal may even fall to zero. In real space the radio signal passes a complicated path from a base station to a mobile terminal and back, thus undergoing complex changes. There are different methods for modeling this path. Actually, the modeling of radio signal propagation is an algorithm of changes of electromagnetic field in the space taking into account the configuration of base stations and mobile terminals under the different conditions of urbanization and frequencies. The Japanese engineer Okumura explored these questions back in the 1960s. His method was based on experiments, describing the signal propagation for frequency bands between 150 and 2000 MHz and distances between 1 and 100 kilometers under different conditions. Later his countryman Hata expanded the Okumura method making it practically applicable. Today many of the world's largest operators, including Magti  GSM, are using this method among others, for instruments of planning and analysis, as well as in models for projecting signal propagation.
   We have tried to give our readers a simple description of the technical complexities faced by radio engineers or network designers in companies operating GSM systems. One of them is Magti GSM - a company that always puts the interests and safety of its customers first.