INTRODUCTION
In the master's work analysis of the main features of distribution of radio-waves in system of a mobile communication of standard GSM and the models, used for calculation of radio of lines is offered. Modelling of distribution of a signal with application of various models and matching of the received data.
URGENCY
The system of cellular communication GSM900/1800 is the most widespread in the world and universal in usage, having perspectives of maintenance and development the next 5-10 years. Therefore the choice of system GSM900/1800 for research in the given operation is actual in sphere of telecommunications.
SETTING OF TASKS
In operation it is necessary to carry out the analysis of the existing problems arising at signaling in network GSM.
To offer the main ways of struggle against distortions of signals.
To consider criteria and degrees of quality of services of network GSM.
To spend modeling of segments of a network of cellular communication and to analyze existing models on efficiency parameter.
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Influence of external factors on distribution of signals in a mobile communication network
At distribution of radio-waves to the free space peak value of an electric field strength of a signal of Etsv on distance r from the transferring aerial of the base server is defined under the formula [4]:
where
P - radiated power of the transferring aerial;
Dпрд – coefficient of the routed operation of the transferring aerial;
F- the characteristic of a trend of the aerial in horizontal and vertical planes.
The formula shows that the strength of the field of a signal in a reception place decreases proportionally to distance r owing to "transmission losses" in the free space.
At distribution of radio-waves to troposphere of loss of strength of the field will be defined by a tangent of angle of losses:
Where
sigma, epsilon – accordingly specific conductivity and dielectric permeability of troposphere, and lyambda - a wavelength.
Dispersing properties of a ground layer of troposphere are defined by its gas structure, temperature, pressure and humidity.
Figure 1 - Influence of heterogeneity of the environment on distribution of radio-waves
"The normal troposphere" with parameters Т=288 grad, р=0,1013 kPa and humidity of 60 % for VHF range in which radio-waves extend practically lost-free. However, the real troposphere is not homogeneous for the structure. In a ground layer of troposphere there are water steams (a fog, a rain) or the weighed particles (a smoke, a dust). It causes reduction of strength of the field through thermal losses on movement of molecules of gas. The absorption value is estimated by coefficient of losses in heterogeneity allowed band epsilon:
Where
epsilon – coefficient of easing of strength of the field of a signal in heterogeneity allowed band;
alfa – a constant of fading of strength of the field;
r - length of allowed band of heterogeneity.
With increase in length of allowed band of heterogeneity rзн absorption of energy of a radio signal increases. Losses for spectra UHF (on which the mobile communication works) are especially notable. Influence of troposphere on distribution of radio-waves appears not the core. More important influence the spreading surface renders, radio communication lines. The earth ground includes dry ground and water solutions of salts which define the important discrepancies of dispersing parameters: epsilon – dielectric permeability and sigma – specific conductivity of the earth. Thereof on a radio communication line there can be effects of reflection and absorption of radio-waves. Conditions of distribution of radio-waves are defined by a tangent of angle of losses in the earth.
At low lifted aerials of base servers (to 20м) absorption of energy of an electromagnetic field in the earth increases. The coefficient of thermal losses in the earth epsilonз is defined by coefficient of Van der Poll[4]:
where
At the semi raised aerials of loss in the earth are estimated by Vvedenskiy coefficient:
where
Taking into account influence of an inhomogeneous medium of distribution of radio-waves and a spreading surface, expression of peak value of strength of the field of a signal in a place of reception becomes:
Because of influence of coefficients epsilonз, epsilonзн the strength of the field of signals in a place of reception of radio-waves essentially decreases.
Real lines of a radio communication of transportable systems have sites of a spreading surface with different values epsilonз, sigmaз. However, owing to rather small distances between БС and MSEC of value of parameters can be taken by the averaged. Are usually taken value ?з, ?з such which respond parameters of "a damp ground" sigmaз = (0.1...0.011), epsilonз = (15... 30).
Thus, the spreading surface on lines of a transportable radio communication for range UHF is the semiconductor.
The reasons of losses of strength of the field of radio signals in a reception place is also the lay of land. As aerials of base servers are in immediate proximity from the earth on road radio communications appear large-scale objects which shield receiving aerials from the transferring. The more interestedness districts, the it renders a greater influence on conditions of direct visibility of servers. Easing of the field of a signal thus depends not only on the value of a gleam of a line of a radio communication, but also from distance to the object which shields (fig. 2,). Parameters of a line of radio communication H0, ra, rb, and also a wavelength lyambda define value of the generalized parameter of losses d:
The generalized parameter d defines the sizes of that part of space between БС and the MSEC in which the main body of energy of an electromagnetic field [4] extends. If the value of the screen does not exceed radius R of the first allowed band Frenelja (fig. 2,) the strength of the field of a signal in a reception place will respond practically to strength of the field at an open line. If the screen value is more radius of the first allowed band Frenelja despite formally closed line, easing of strength of the field of a signal will be defined depending on diffraction parameter d.
Figure 2 - MS Shielding on a radio communication line
Statistical researches of lines of a transportable radio communication in VHF ranges show that separate grounds can be objects which mirror, and also separate objects for which the condition tg (sigma)> 1 is satisfied. Objects which mirror, will play a role of secondary (passive) radiators (fig. 3).
The mirrored rays thus will have a course miscellany as distances from secondary radiators to receiving radio station will be different. Thus, on a radio receiver input even at shielding of a regular ray there will be a radio signal derivate by path interference of addition of mirrored signals. As in the course of functioning of system of MS permanently move, the quantity of objects which mirror, with different efficiency of reflection and a difference of a course of rays varies also. Thereof, the mirrored signal on an input of the receiver of MS will fluctuate permanently to (fluctuate).
Figure 3 – Reflection of radio-waves on a radio communication line
Conclusions
At cover zone definition all features of a cross-country terrain for the purpose of a maximum exception of shadow sites of possible lines of a radio communication, fading in deposits, reflection, an interference, influence of discontinuities should be considered. In a cover zone a radio communication it should be provided practically for any point of a finding of MS. It is reached not only co-ordinate allocation BS, but also a choice of heights of a raising of their aerials which prevail for the given district and consider the main directions of lines of a radio communication.
Usage of aerials of the routed operation, which diagramme of a trend are partially overlapped, gives the chance to form a circular graph of trend BS. Possibility of change of radiated power and its auto control in everyone separate the sub channel Besides, is supposed.
The network of cellular communication optimal for research the model the Okamura-hut as the most universal for all types of district and conditions of distribution of signals is.
Literature
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2. Громаков Ю.А. Стандарты и системы подвижной радиосвязи. 5-е изд. - М.: Эко-Трендз, 1998
3. Кловский Д.Д. Теория передачи сигналов. – М.: Связь, 1973.
4. А.А. Зеленский, В.Ф. Солодовник. Системы радиосвязи – Учеб.пособие. Ч.3.-Харьков: Нац. аэрокосм. ун-т "Харьк. авиац. ин-т", 2003.–90с.
