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The propagation of Electromagnetic waves in millimetre wave band is very much affected by rain and dust particles resulting in de-polarization and attenuation. Due to the increasing number of terrestrial and satellite links in the regions that encounter sand storms and dust storms, interest is growing to study the effect of dust particles on the propagation of microwaves. To compute such effects we need to have knowledge of electrical properties of the scattering particles and climatic conditions of the region under study. When microwaves pass through a medium which has precipitations like dust and sand particles, the signals get attenuated due to absorption and scattering of energy out of beam by the sand and dust particles. The main aim of this paper is to discuss the phenomena associated with signal attenuation through sand storm and dust and to quantify the impact of these environments on signal propagation.
Wireless communication systems are influenced by climatic conditions. The performance of service of many applications such as cellular telephones, broadcast television, radio stations and differential GPS transmitters, CDMA and Wi-Max networks, that require RF or microwave propagation from point to point near the earth's surface, depends on many factors such as area of coverage and climatic conditions. The millimetre waves bands are in the short wavelengths range and shorter the wavelength the more attenuation will be induced by absorption and scattering due to rain drops, dust and sand particles in the radio path. The attenuation and de-polarization caused by sand and dust particles is one of the major problems in the using microwave and millimetre wave bands for terrestrial and space communication. The attenuation and phase shift constants for a medium with dust and sand particles depend on the frequency, visibility, maximum particle-size, complex permittivity, shape of the scattering particles, concentration and orientation relative to the wave polarization. The attenuation of electromagnetic waves due to dust is the function of moisture content of the particles.
Most studies regarding the attenuation of satellite communication signals from dust and sand are based upon the assumption that sand and dust mediums occur due to nuclear detonation. From the experience in the desert storm is concerned, it is shown that signal attenuation due to dust and sand must be considered for evaluation in satellite communications systems which are ground-supported where the sand and dust medium results from local storms. Sand storms are different from dust storms. What we generally call sandstorms, as they occur in arid regions, are actually dust storms. Places such as Riyadh or Sudan around Khartoum, where the surface is not sandy, storms will produce vast dust clouds which rise more than 1 km into the air and almost blocking the sun for long periods of time. Actual sandstorms hardly rise above 2 meters in the air. In a sandstorm, except at the beginning, the air will often be clear with the sun shining over the sea of sand. Due to the low maximum height of sandstorms, they are hardly a factor in propagation loss calculations in Satellite Communications.
Looking at the rain data available for Riyadh city shows that there is a very little rainfall for most of the year and hence rain attenuation may not be the dominant propagation factor. In contrast, sand and dust storms may occur several times each year. Particle sizes may range from a fraction of a micron to few hundred microns in radius. Due to the heavy particles of sand storms, which are always more than 0.04 nm in radius, the air 2 metres above the earth's surface could be clear of sand. Thus we may not expect that radio links to be affected by sand storms. On the other hand, dust storms consisting of much smaller particles (less than 0.01 mm in radius) may be found at as high as 100 metres or more. This will reduce the visibility as well as affect the propagation in millimetre wave band. In order to obtain the attenuation and cross polarization due to dust particles, knowledge of the particle's shape, the size distribution and the refractive index is needed. Multipath fading due to temperature and pressure gradients is also expected to affect the propagation in Saudi Arabia.
Dielectric constant of dust and sand:
It is very important to know about the dielectric constant of particles suspended in the atmosphere for radio communication. Desert and semi-desert regions often encounter sand and dust storms and hence it is important to investigate the dielectric constant of these particles.
Various models to estimate the dielectric constant of sand and dust particles are available. To calculate the complex permittivity of the composite component we can use the Looyenga equation given by:
Here, Ð„m is the complex dielectric constant of the mixture, Ð„i the complex dielectric constant of the ith substance and υi is its relative volume. Complex permittivity depends on frequency of operation and moisture content. Materials at microwave frequencies have permittivity given by:
ε =ε ′− jε ′′
ε ′ is the dielectric constant and ε ′′ is the dielectric loss factor.
Moisture causes the increase of real and imaginary parts of the complex permittivity which dependent on the chemical composition of dry soil samples.
Visibility during dust storms:
In meteorology, visibility is used as the measure of severity of dust storm. As the intensity of the dust in the storm increases, visibility decreases. Relation of visibility and the mass of dust per cubic meter of air given is by:
V is the visibility in kilometres, M is the mass of dust in kg. Relationship between visibility V (km) and mass density ρ (gm/cm3) is obtained from the measurement of dust concentration and visibility given by:
υ is the relative volume occupied by particles (m3 of particles /m3 of air), constants C and γ depend on the composition of soil, distance from the origin of the storm and climatic conditions at the origin. Visibility during sand/dust storms increase as the height is increased. Following is the relation for the variation of sand and dust mass concentration (kg/m3) with height (meter),
a and b are constants and they vary a bit from one year to another and they depend on meteorological factors, climatic conditions and particle size distribution of sand and dust. Substituting the value of M from previous equations, visibility equation is written as:
Let visibility at some reference height to ho be Vo and hence yielding:
Representative Dust Model:
It is necessary to develop a representative dust model to estimate the effects of a dust storm on the performance of satellite communication links. Models developed so far have primarily characterized dust in the dynamic and turbulent medium that results from the strong winds which follow the fireball phase of a nuclear event. Actual particles in a dust storm might differ a lot from that of a nuclear-induced region.
Dust particles having different sizes are represented by a power law probability distribution given by:
P(r) = kr-p (1)
radius of the particle is 'r', p is the power law exponent and k is selected so as to satisfy:
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This nuclear-induced dust cloud is modelled by five different regions which evolve independently in time, exhibit individual characteristics with respect to region upper altitude, region radius, particle density distribution and minimum/maximum average particle size. Eqn. (1) mentioned above could be applied to each region to get the particle distribution for that region. This distribution can be used to quantify the attenuation of signal per kilometre of path length.
Satellite communication signals that are incident on dust particles undergo scattering and absorption and the intensity of these would depend on the shape, size and complex dielectric constant of the particle and also the wavelength of the signal. Mie was the first person to derive the expression to determine the scattering and absorption from a dielectric sphere. When the size of the sand/dust particle is very small compared to the signal wavelength, a Rayleigh approximation is used. Mie expressions are used for dust/sand particles that approach the signal wavelength. The wavelengths of signals in the millimetre range are limited on the lower end by a millimetre. In desert dust storms, maximum size of dust particle is of the order of 0.2 mm or approximately 50 times smaller than the minimum signal wavelength in the millimetre wave range. Thus for operating frequencies in millimetre wave range, Rayleigh approximation is valid for modelling attenuation of signal in dust storms.
Altshuler applied Rayleigh approximation to get the results shown in Fig. 1 i.e. signal attenuation due to dust (dB/km) is shown as a function of operating frequency. To get results in Fig.1, power law exponent P was 3.5, maximum and minimum radii of the particle were 5 mm and 0.005 mm respectively, bulk particle density was assumed to be 2.6 gram/m3. Signal attenuation is a strong function of the operating frequency and it increases with increasing frequency.
Altshuler showed the correlation between signal attenuation and the maximum and minimum radii of the particles. It was observed that signal attenuation was a very weak function of the minimum particle radius. Fig. 2 shows that there is a strong correlation between the maximum particle radius and signal attenuation such that signal attenuation increases as the maximum particle radius increases. When the maximum particle wavelength is of the order of signal wavelength, signal attenuation reaches its maximum value.
Figure 1 Figure 2
Altshuler's data shows that the attenuation due to sand can be very high and it depends on the maximum particle size. The resulting signal attenuation can be upto 10 dB per kilometre. However, the particle region modelled in Altshuler's analysis because of a nuclear detonation would exhibit different particle characteristics from those resulting from a dust storm. Results generated by Altshuler in Fig. 2 shows that the signal attenuation caused due to dust is around 0.5 dB/km at 45 GHz and 0.2 dB/km at 20 GHz. According to Rafuse, the maximum particle size for dust storm is 0.2 mm and that the maximum height of the dust storm is 1-2 km. If we combine the results shown by Rafuse and Altshuler, we can expect the maximum attenuation due to dust storms to be around 0.5 to 1 dB for the millimetre wave band. Terrestrial communications links might exhibit a larger signal attenuation due to multipath fading which results from the extreme changes in the refractive index throughout the storm region.
Depolarization due to sand and dust:
Depolarization is defined as the change in polarization characterstics of the radio waves propagating in the atmosphere. Linear and Circular polarized systems are affected by depolarization. Polarization state of a depolarized wave is altered such that it results in interference and crosstalk between the two orthogonally polarized channels. Multipath depolarization is seen only for very low elevation angle space communications and is dependent on the polarization characteristics of the receiving antenna .
It is seen that for linear polarization the effect of dust storms is almost negligible except for storms with visibilities less than a few metres . As far as Circular polarization is concerned, cross polarization can be high when visibility falls below 100m over about 10km of path or when it falls below 10m over about 1km of path .