The performance of fixed satellite systems in the shared frequency band depends on the tolerance level of interference between them. Interference disturbs the functionality of the ground station and causes signal degradation. The knowledge of interference level must therefore be known for an optimal satellite design. In this work, we evaluate interference due to hydrometeors for a situation in which a satellite downlink signal is affected by the signal from a terrestrial microwave network operating at the same frequency as the satellite system in a subtropical station: Durban, South Africa. The evaluation of the transmission loss is based on the modified 3D bistatic radar equation and the exponential rain cell model for the scattering. The results of intersystem interference for different station separation over frequencies variation, terrestrial antenna gains, and exceedance probabilities are presented. The effect of the additional rain attenuation
Intersystem interference is caused by an undesired signal received from a transmitter of a different system. The cochannel interference arises due to the other carriers transmitted by the satellite to earth stations of the same system, at the same frequency, and at the same polarisation as the useful carrier. These interfering carriers are usually sent to earth stations located in a different spot beam as the useful earth station frequencydivision multiple access (FDMA) and timedivision multiple access (TDMA) but located in the same spot beam as the useful earth station in codedivision multiple access (CDMA) [
The media traversed by the electromagnetic waves and the spectrum used are shared. The need for the frequency sharing has been due to the congestion experienced at the lower frequency bands. This has resulted into the move to higher frequency to gain more bandwidth. However, at higher frequencies and most especially at frequencies above 10 GHz, the most serious problems in system design emanate from attenuation, depolarization, and scattering interference by precipitation particles along the radio path [
The study on the impact of interference based on the evaluation of bistatic interference on communication paths has drawn much attention at the temperate and the tropical region. The report of some researchers at the temperate region includes the works of Crane [
In this work, we evaluated the intersystem interference,
The rain scattering geometry with additional rain attenuation,
The transmission loss based on the Bistatic Radar Equation (BRE) can be expressed as [
The equation assumed the scattering process through the single scattering narrowbeam width antenna. The scatter crosssection per unit volume is estimated using the complete Mie solution or Rayleigh approximation. Hence, using the narrowbeam approximation equation (
At frequencies less than 10 GHz, the scattering crosssection per unit volume of precipitation,
The extra attenuation that the signal experiences along the path towards and from the common volume is also taken into consideration for interference prediction in this work. According to [
Power law attenuation parameters for thunderstorm rain type [
Frequency (GHz) 



4  0.0003  1.0325 
6  0.0032  1.0056 
7  0.0046  1.0980 
8  0.0043  1.3562 
10  0.0175  1.1443 
12  0.0285  1.1211 
15  0.0476  1.0698 
20  0.0998  1.0421 
25  0.1356  1.0312 
30  0.1161  1.0426 
35  0.2002  0.9910 
40  0.3234  0.9971 
45  0.3967  0.9423 
50  0.5284  0.8379 
60  0.5830  0.8307 
The knowledge of both vertical and horizontal spatial distributions of rain rate is also important in the estimation of interference level. A rain cell has been defined as any connected region of space composed of points where the rainfall rate exceeded a given intensity threshold [
This study assumes the proposed form of the simplified Capsoni 3D model for the characteristic distance
Power law parameters for the cumulative probability density for thunderstorm convective rain type and the path geometries for calculating transmission loss.
Location  Coordinate 




Durban, South Africa  29.58°S, 30.57°E  1.03 × 10^{−5}  484  12.8237 


Path Geometries 

Common volume height (km)  Common volume  

Station separation (km)  


50  50.7  1.245  1.556  3.125 
100  101.9  2.849  3.122  6.223 
150  152.9  4.810  3.887  14.719 
200  204.9  7.135  5.802  19.688 
250  255.9  9.812  7.965  24.528 
As already stated, an extra attenuation could also be observed along the wanted path from additional attenuation due to rain; this extra attenuation reduces the signaltonoise (S/N) ratio at the receiver’s end. The effective transmission loss
In this study, we used the subtropical lognormal raindrop size distribution proposed by [
The interference is produced at the input of the received earth station, by carriers transmitted by either the satellite of the considered system or a satellite of another system. In an ideal system, these carriers should be strongly attenuated, due to multiplex scheme, frequency and polarization plan, antenna pattern, and filtering issues. Several of these parameters are considered in this study as a result of interference caused by rain and meltingsnow. The input parameters needed consist of the geometric and electrical properties of the link, as well as the meteorological parameters. For the purpose of this work, we have used the Intelsat 17 (IS17) a geostationary satellite located at 66°E with its service footprint over Durban, South Africa. A GEO satellite has been known to offer a 24hour view of a particular area, which leads to its wide use as a provider for broadcast satellite services (BSS) and multipoint applications. These parameters are summarized in Table
The input parameters needed: the geometric and electrical properties of the link and meteorological parameters [
Station Name  Durban, South Africa 
Location  29.58°S, 30.57°E 
Frequency range  4–40 GHz 
Elevation above sea level  0.008 km 
Rain type considered  Thunderstorm 
0°C isotherm  3.8–4.25 km 
Transmitting antenna  
Elevation angle  1° 
Gain  26.3 
Beam width  1.5°, Gaussian radiation pattern 
Polarization  Horizontal 
Receiving antenna  
Elevation angle  38.5° 
Gain  37.14 
Beam width  0.15°, Gaussian radiation pattern 
Polarization  Horizontal 

461 
In this section, we will discuss the results of the transmission loss and the effective transmission loss simulated based on the SBRE [
Figure
Equivalent satellite to common volume distance and the corresponding station separation.
The variation of the total gaseous attenuation (sum of gaseous attenuation in the 1st and 2nd links) over the station separation is presented in Figure
Variation of the total gaseous attenuation (sum of gaseous attenuation in the 1st and 2nd link) over the station separation at different frequencies.
In Figures
Variation of the transmission loss with the terrestrial station antenna to common volume distance at some percentage of time and at frequency of (a) 12 GHz and (b) 30 GHz.
It is obvious from the results that as the distance from the
We also examine the influence of the terrestrial propagation path length on the transmission loss at frequencies 16, 25, 30, and 40 GHz and time availability of 99.9% as presented in Figure
Influence of the terrestrial propagation path length on the transmission loss at different frequencies and time availability of 99.9%.
Figure
Variation of the transmission loss with frequency at some percentage exceedance and for long and short propagation path lengths.
Figure
Variation of
Figure
Influence of the computed transmission loss with terrestrial antenna gain over station separation of 50 and 200 km and at frequencies (a) 12 and (b) 25 GHz.
If two stations operating at the same frequency are separated from one another by a specific distance, situations can arise when rain attenuation on the path from a space station reduces the signal sufficiently to allow interference from a neighbouring earth station into the receiving earth station. The joint effects of attenuation of both wanted and unwanted signals then have to be considered. In Figure
Comparison of the effective transmission loss and the transmission loss at different frequencies, path length of 50 km from terrestrial antenna to the common volume and at some percentage times.
In this paper, we have presented some results on the application of simplified Bistatic 3D rain scatter model on horizontally polarized SHF signal propagation in the subtropical environment. Results are presented for a geostationary Intelsat satellite downlink terminal receiving interference from a terrestrial microwave system operating at the same frequency. The result shows that when the station separation is longer than 150 km, the transmission loss curve becomes steep in a strong received power region due to the decrease in radar reflectivity factor in the ice region. Though there is a less interference (significantly higher transmission loss) at a relatively higher frequency of 20 and 40 GHz, the rain scatter interference problem may not be negligible for a subtropical region with high rainfall rate because, for a small percentage of time, the rain scatter power can be received even at a large distance from an interfering station. Further results obtained based on the additional attenuation of the satellite signal show that, for time unavailability of 0.01%, a difference of about 27, 33, and 42 dB could be noticeable between
The authors declare that there is no conflict of interests regarding the publication of this paper.