International Journal of Scientific & Engineering Research, Volume 3, Issue 11, November-2012 1

ISSN 2229-5518

Investigation of the Applicability of Garcia and ITU-R Models to Rainfall on Satellite Communication Link in Nigeria

Adeyemo. Z.K., Akande. D.O. and Ojo. F.K.

AbstractThis paper presents the investigation into the effect of rain on satellite communication link in Nigeria at a frequency of 30 GHz, using six cities in each geographical zone of the country. The annual rainfalls of the cities spanning a period of three years were collected from the Nigerian Meteorological Centre for proper analysis. The ITU-R rain attenuation model and the Garcia Model were considered among several models in existence. The average rain rate value was computed and substituted into the rain attenuation prediction models under study. The result obtained, indicates that value of attenuation increases as the altitude decreases. Also, the analysis of the results shows that the Garcia model is more applicable in Nigeria using both horizontal and vertical polarizations.

Index TermsRain rate, attenuation, polarization, satellite.

—————————— ——————————

1 INTRODUCTION

ne of the major problems facing the successful propaga- tion of communication between the transmitter and the receiver along a communication path is attenuation. Rain plays a significant role in the undesired absorption of micro- wave, milimetre and centimetre wave propagation in the low- er atmosphere. Beside rain, other major contributors are water vapour, liquid water clouds and fog [1]. Rain rate distribution is one of the most important factors for calculating rainfall attenuation at a specific location [2]. Rain attenuation becomes considerable at frequencies above 10 GHz [3]. The effect of rainfall is more severe in tropical regions which are character- ized by heavy rainfall intensity and presence of large raindrops [3]. Rain attenuation decreases the intensity of sig- nals sent through the satellite as rain has the ability of scatter- ing and absorbing signal waves. To achieve effective commu- nication, there is a need to study the effect of this natural phe- nomenon on the radio propagation path above 10 GHz, using the various rain attenuation statistical models. A lot of re- search work has been carried out in this area over the years globally [1],[4],[5],[6],[7] and [8]. In Nigeria, more research needs to be carried out to ensure reliable signal and appropri- ate model [3],[9] and [10] to mention but few. It is therefore, pertinent to investigate the effect of rain since Nigeria has suc- cessfully launched its satellite into the earth orbit. This re- search work focus on the NIGCOMSAT 1R propagation link and two prediction models that suit the tropical climate to
which Nigeria belong.

2 MATERIAL AND METHOD

The technical data such as position, elevation, latitude, longi-
tude and altitudes at six different locations: Ikeja, Kano, Lokoja, Enugu, Maiduguri and Port-Harcourt from each of the geographical zone in Nigeria were analysed using the ITU-R and Garcia models to determine the appropriate model for satellite communication in Nigeria..

2.1 RAIN RATE PREDICTION MODELS

2.2 ITU-R MODELS

To estimate the long-term statistics of the slant-path rain at- tenuation at a given location for frequencies up to 30 GHz the following steps have to be carried out using [11].

Step 1: Determination of the effective rain height, hR, for the latitude of the station φ:
for the northern hemisphere (1)
Step 2: For compute the slant-path length, , below the rain height from:

(2)

Step 3: Calculate the horizontal projection, , of the slant- path length from:

(3)


Step 4: Obtain the rain intensity, , exceeded for 0.01% of

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an average year (with an integration time of 1 min). If that information cannot be obtained from local data sources, an estimate can be obtained from the map of rain climates given in [12]. An alternative method is by applying the Chebil mod- el, which makes use of long-term mean annual accumulation, M, at the location under study [13] and is given by

(4)

Where and are regression coefficients. This model was shown to provide good approximation of the measured data. The regression coefficients are defined as:

and

Step 5: Calculate the reduction factor, , for 0.01% of the time for :

(5)
where is the equivalent path length, in km, given by, (km) (11)
The coefficients a, b, c and d are constants depending on the geographical area. Coefficient e is only a scaling factor. Taking e = 102, the “worldwide” coefficients are: a=0.7, b=18.35, c=-
16.51, d=500.

3 SIMULATION PARAMETERS

The simulation of the two prediction models were carried out using MATLAB software package. The parameter of the satel- lite under study at a frequency of 30 GHz is as shown in Table
1.
Table 1: Parameter of satellite under study

where (6)
Step 6: Obtain the specific attenuation, , using he frequency dependent coefficients given in [12] and the rainfall rate, , determined from Step 4, by using:

(dB/km) (7)

Step 7: The estimated attenuation exceeded for 0.01% of an average year is obtained from:
The parameters of the cities used study is presented in Table 2. Table 2: Parameters of cities under study

(dB) (8) (8)

Step 8: The estimated attenuation to be exceeded for other per- centage of an average year, in the range 0.001% to 1%, is de- termined from the attenuation to be exceeded for 0.01% for an average year by using:

2.3 GARCIA MODEL

(dB) (9)

4 RESULT AND DISCUSSION

Rain attenuation increases with increasing frequency, and with decreasing elevation angle. Nigeria being in the tropic region, rain at times is from convective rain-cells, with rela-
Rain attenuation for satellite link as obtained from [14] and
[15] as;

(dB) (10)

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Dr. Z.K Adeyemo is a senior lecturer in the department of electronic and electrical engineering, Ladoke Akintola University of Technology, Ogbomo- so Nigeria. E-mail:zkadeyemo@yahoo.com.

D.O Akande is a lecturer in the department of electronic and electrical engineering, Ladoke Akintola University of Technology, Ogbomoso Nige- ria. E-mail:akandedamilare@yahoo.com.

F.K Ojo is a lecturer in the department of electronic and electrical engineer- ing, Ladoke Akintola University of Technology, Ogbomoso Nigeria. E-

mail:festusoluseye@yahoo.com
tively small diameters often resulting in ‘heavy’ down pours
for short periods. From the study it was observed that Garcia model is more sensitive to elevation. Comparing the probabil- ity of two models, it can be concluded that the Garcia model for both vertical and horizontal polarisation revealed Lokoja to have the greatest attenuation from Figures 1 and 2 the greatest attenuation being the area with the lowest altitude with Kano having the lowest attenuation value, while the ITU-R model in Figures 3 and 4 for both vertical and horizontal revealed Mai- duguri to be the city with the highest attenuation value, while Port Harcourt has the lowest attenuation value; although Port Harcourt does not have the lowest altitude value and Maidu- guri is not the area with the highest altitude. This clearly sup- ports the fact that Garcia model is more sensitive to elevation and altitude of the location of satellite.

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attenuation distribution functions (Vertical polarization) R0.01 (Garcia model)

2

10

Enugu Maiduguri Port-Harcourt

1 Kano

10

Ikeja

Lokoja

attenuation distribution functions (horizontal polarization) R0.01 (ITU model)

2

10

Enugu Maiduguri Port-Harcourt

1 Kano

10

Ikeja

Lokoja

0 0

10 10

-1 -1

10 10

-2

10

0 1 2 3 4 5 6 7

Attenuation[dB]

-2

10

0 2 4 6 8 10 12 14

Attenuation[dB]

Fig. 1. Probability against attenuationfor Gracia model (vertical polarization)

Fig. 4. Probability against attenuationfor ITU-R model (horizontal polarization)

0.01

2


The exceedence curves for rain rate and attenuation showed

attenuation distribution functions (horizontal polarization) R

10

(Garcia model)

Enugu Maiduguri Port-Harcourt

clear breakpoints; implying changes in the rain structure and hence attenuation. From Figures 5 to 10 the breaking point is

1 Kano

10

Ikeja

Lokoja

0

10

-1

10

-2

10

0 1 2 3 4 5 6 7 8 9

Attenuation[dB]

close to 0.1% after which the attenuation decreases as the per- centage of time increases. This is because breakpoint is more
evident if the time rate is short. Looking through Figures 5 to
10, it was observed that ITU-R produces a higher value of at-
tenuation than Garcia model .For instance, in Figure 5, Enugu
at 0.1% of time ITU-R has an attenuation of 4.2 dB while Gar- cia model has an attenuation value of 0.8 dB. This still support the fact that Garcia model is better adopted in Nigeria. Hence ITU-R produces a poor attenuation and exceedence prediction.

Fig. 2. Probability against attenuationfor Gracia model (horizontal polarization)

attenuation distribution functions (vertical polarization) R0.01 (ITU model)

2

10

Enugu Maiduguri Port-Harcourt

1 Kano

10

Ikeja

Lokoja

0

10

-1

10

Fig. 5. Attenuation against percentage of time for Enugu

-2

10

0 2 4 6 8 10 12 14

Attenuation[dB]

Fig. 3. Probability against attenuationfor ITU-R model (vertical polarization)

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Exceedence Curve for Kano

7

6

ITU model

Garcia model

Exceedence Curve for Maiduguri

8

7

ITU model

Garcia model

6

5

5

4

4

3

3

2

2

1 1

0

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Percentage of time[%]

0

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Percentage of time[%]

Fig. 6. Attenuation against percentage of time for Kano

Fig. 9. Attenuation against percentage of time for Maiduguri

Exceedence Curve for Ikeja

6

5

ITU model

Garcia model

Exceedence Curve for Lokoja

6

5

ITU model

Garcia model

4 4

3 3

2 2

1 1

0

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Percentage of time[%]

0

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Percentage of time[%]

Fig. 7. Attenuation against percentage of time for Ikeja

Fig. 10. Attenuation against percentage of time for Lokoja

4.5

4

3.5

3

2.5

2

1.5

1

0.5

0

Exceedence Curve for Port-Harcourt

ITU model

Garcia model

5 CONCLUSION


In this paper, the effect of rain on satellite communication link in Nigeria using Garcia and ITU-R models has been investi- gated using Nigeria data. Evaluation was carried out using the probability and attenuation in both horizontal and vertical polarisation. The result obtained from the analysis showed that attenuation increases as angle and altitude decreases. Moreover, the analysis of the results also shows that the Gar- cia model is more applicable to Nigeria data, thus, recommend also for use in Nigeria. It can be seen that Kano with the high- est altitude has the lowest attenuation value while Lokoja with the lowest altitude has the highest attenuation the Garcia
model for both vertical and horizontal polarization. Also, Port

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Percentage of time[%]

Fig. 8. Attenuation against percentage of time for Port-Harcourt

Harcourt and Maiduguri have the lowest and highest attenua- tion value respectively for the ITU-R model.

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International Journal of Scientific & Engineering Research, Volume 3, Issue 11, November-2012 5

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