JOURNAL OF TELECOMMUNICATIONS, VOLUME 25, ISSUE 1, MAY 2014 6

Rainfall Variability and Impact on Communication Infrastructure in Nigeria

O.O. Obiyemi, T.S. Ibiyemi, and S.O. Akande

Abstract— Communication satellites remain a prominent backhaul communication infrastructure. It also provides a wide coverage across the rural, sub-urban and urban areas. However, rainfall invokes a number impairments on satellite communication links, particularly for operations within the microwave and millimeter wave bands. The resultant effect of its variability has been evaluated on the viability of maintaining availability objectives, especially over long periods within which satellites might be due for replacement and re-planning. 1-min rainfall rates has been deduced from 30 years daily rainfall data, sourced from the Nigerian Meteorological Agency (NIMET) for 19 stations across Nigeria. The equivalent decadal point rainfall rates (mm/hr) were estimated from the mean rainfall accumulations over three decades using the procedure proposed by Chebil, while the rain attenuation (dB) was subsequently predicted using the ITU-R P.618-10 model for direct to home (DTH) reception at 12 GHz. Results indicate that the slight variation observed on the point rain rate statistics only accounts for little variation in the rain induced attenuation over the link, through the years and across the decades. Hence the variation is negligible at higher percentages of the time (typically between 0.1 and 1% of the time) as is the case across the climatic regions ranging from Arid to the Coastal areas. Results indicate that effects at 99 and 99.9% availability is negligible across the selected sites. It also reveals a slight variation at 99.99% of the time, which indicates a variation of about 1 dB at Calabar (Forest belt) and Ibadan (Wooded Savannah), while variation remains negligible at Maiduguri (Sahel), Jos (Sudan-Sahel) and Ilorin (Guinea Sudan) over the decades. The variation of about 1.4 dB was recorded at Calabar, 0.8 dB at Maiduguri, 1.3 dB at Ibadan, while variation remain negligible at 99.999% in Jos and Ilorin across the decades. Although it is apparent that the variation observed in rainfall intensity produces corresponding variation in the rain attenuation across the decades and over the years, this has no significant effect on communication link design, particularly over any particular location and over the years. Hence due to the short lifespan of satellite vehicles in the geostationary orbit, availability objectives can be maintained, provided no technical parameter is altered when satellites are being replaced.

Index Terms— availability objective, communication infrastructure, rain attenuation, raifall variability

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1 INTRODUCTIONainfall is a natural phenomenon with non-uniform structure. Its variability in time, space, duration and frequency of occurrence dictates the need for the

knowledge of its dynamics, particularly as required for quantifying its effect on radiowaves at frequencies be-yond 10 GHz [1]. The scattering, absorbing and depolariz-ing impact on signal propagations in microwave and mil-limeter wave bands remain a major concern for satellite and terrestrial link designs.

Recent technological advancements such as the inter-net-of-things (IOT) actually suggests the need for in-creased broadband connectivity and penetration. This is a rising demand for the connectivity of personal devices and appliances. In order to meet this demand, a number of infrastructures are currently in use as backhaul for reli-able service delivery. Communication satellite links, ter-restrial microwave links and fiber optic links (undersea and hinterland) are typical examples. However, due to the wide and expansive coverage offered by communica-

tion satellite, it’s been a prominent infrastructure for communication backhauling globally. It has also been broadly explored in bridging the existing digital divide across the urban, sub-urban and rural areas, particularly for the deployment of digital television, even in Nigeria.

Seemingly, communication satellites has served and continues to serve a host of applications ranging from direct-to-home (DTH), broadband service provision with extended applications in telemedicine, e-learning (virtual libraries and virtual laboratories), e-banking, e-commerce, e-agriculture, e-governance, e-voting and in surveillance in community policing and military missions [2, 3]. In recent years however, its impact on home entertainment has been tremendous. It has enhanced television viewing experience with the provision of sharply defined images and quality sound to subscribers, either on the free or pay subscription platforms. Moreover, it also serve as the main backhaul infrastructure for the distribution of digi-tal television contents to cable TV operators and terrestri-al television operators for onward distribution terrestrial-ly within their respective coverage.

Interestingly, satellite links are designed based on de-fined availability objectives, which varies from one satel-lite operator to the other and from one application to the other. Since 100% link availability may be not be practica-ble at the microwave and millimeter wave bands, even for mission critical applications, link design is therefore de-

———————————————— • Obiseye O. Obiyemi is a lecturer at Osun State University Nigeria and He

is currently working towards his Ph.D at the University of Ilorin, Nigeria. • Tunji S. Ibiyemi is a Professor at the Department of Electrical and Elec-

tronic Engineering, University of Ilorin, Nigeria • Akande O. Samuel is a GIS Analyst with the Centre for Space Research

and Applications (CESRA) at the Federal University of Technology, Akure, Ondo State Nigeria.

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signed based on achievable availability objectives ranging between 99.999, 99.99, 99.9 and 99% - which corresponds to 5.26, 52.56, 525.6 and 5256 minutes in that order. This indicates the time in a typical year when the link may not be available.

However, the lifespan of a typical satellite ranges be-tween 12-15 years, thereby indicating early planning for replacement around the 10th year of a particular satellite in orbit. Hence in order to maintain designed perfor-mance for satellite links operating in the microwave and millimeter wave bands, the prediction of the attenuation induced by rain is important and it depends on the knowledge of the rainfall rate or the rain drop size distri-bution.

In this paper, we investigate the impact of the tem-poral variability of rainfall on link availability objectives for NIGCOMSAT 1-R.

2 RAIN RATE MODELS The attenuation due to rain can be modelled on a wireless link using the rain drop size distribution, rainfall rate or the cell size structure. The dearth of precipitation data, such as the drop size distribution and the cell size struc-ture often limits the modeling of rainfall attenuation to the use of the available rainfall data from meteorological stations. Hence, for locations where direct rain rate data do not exist, the knowledge of the rainfall intensity (mm/hr) can be predicted based on suitable rain rate model for a particular location. Several approaches have been proposed for the prediction of the cumulative distribution of the 1-minute rain-rate, as required for the estimation of the attenuation induced by rain on a radio path. Some of the prominent tech-niques are those proposed by ITU-R [4], Moupfouma and Martin [5], Kitami [6], Rice and Holmberg [7] and Chebil [8].

Although the Moupfouma and Martin model is found suitable for most tropical locations [1, 9, 10], the rain rate at 0.01% is a major input parameter for this model and it is not widely available over Nigeria. The input parame-ters required for the use of the Rice-Holmberg model are the average annual and the maximum monthly accumula-tions over a period of 30 years, as well as the thunder-storm ratio !, i.e. the ratio of thunderstorm rain to total rain over a particular location. However, the major draw-back to the use of the Rice-Holmberg model is the lack of ! from the local precipitation data over Nigeria [10]. The approach by Chebil however appears suitable for the es-timation of the point rainfall rate, since it mainly depends on the monthly rainfall accumulation, which is readily available from rainfall data over Nigeria. It has thus been broadly employed for the prediction of the rainfall inten-sity [1, 10, 11], as required for the prediction of the atten-uation induced by rain.

3 DATASET AND METHODS For this study, rainfall data measured by the Nigerian Meteorological Agency (NIMET) between 1975 and 2004 were obtained from a secondary source for 19 stations across Nigeria. The daily rainfall statistics for 30-year data were sorted for analysis. The years with more than three months of missing data (especially those within the rain-ing season - between March and October) were treated as missing years and thus excluded from the dataset. The detailed characteristics of the rainfall data is as shown in Table 1. The rainfall series for the observation period is broken into 10 years interval, so as to examine the decadal varia-bility of rainfall intensity and to establish the correspond-ing implication on propagation link designs, especially with respect to the possibility of maintaining availability objectives. For the rain attenuation prediction, one site

was selected in each of the five climatic regions across Nigeria. These locations are Maiduguri (Sahel), Jos (Su-dan-Sahel), Ilorin (Guinea Sudan), Ibadan (Wooded Sa-vannah) and Calabar (Forest Belt) and they serve as the reception point (earth station) for digital television down-link NIGCOMSAT-1R. Although NIGCOMSAT-1R is not currently delivering digital DTH services, link design is however based on digital DTH reception at 12 GHz. The monthly rainfall accumulation (mm) was also used to estimate the seasonal and spatial variability over the dec-ades and across the chosen sites. Months with zero rain-fall accumulation (mm) within the raining season were discarded and thus not considered for the monthly deca-

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Table 1: Characteristics of the rainfall data over Nigeria

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dal accumulation, which is obtained by averaging the monthly rainfall (mm) across the number of years in each decade. The point rainfall rate (mm/hr) was estimated from the average annual rainfall accumulation (mm) using Chebil’s model, which offers a simple and alternative procedure. It is expressed as [8]:

0.01R M βα= [mm/h] (1)

where 0.01R is the point rainfall rate exceeded at 0.01%

of the time for a typical year, while α and β are 12.2903 and 0.2973 respectively.

The rain induced attenuation on NIGCOMSAT-1R downlink is estimated using the global ITU-R P.618-10 [12], which is mainly based on the following distinct pa-rameters for each location: the point rainfall rate for 0.01% of an average year (mm/hr), elevation angle ϴ  in  degrees, the station latitude in degrees, the station altitude (km) and the downlink frequency (GHz).

The rain induced attenuation downlink NIGCOMSAT-1R at 0.01% of the time is therefore predicted using the

procedures detailed in [12]:

0.01 0.01 EA kR Lα= [dB] (2) where EL is the effective path length (km) – which is a

function of the horizontal reduction and the vertical correction factors while k and α are frequency and polarization dependent factors as defined in [13]. The   attenuation   exceeded   at   other   percentages   of   the  time  is  estimated  using  [12]:  

0.01(0.655 0.033ln( ) 0.045ln( ) (1 )sin )

0.01 0.01

p A p

ppA A

β θ− + − − −⎛ ⎞= ⎜ ⎟⎝ ⎠

(3)

 where   0;β =  if   1%p = or  

036ϕ ≥                                            (4)

Fig. 3: Monthly decadal rainfall accumulation in Ilorin

Fig. 1: Monthly decadal rainfall accumulation in Jos

Fig. 5: Monthly decadal rainfall accumulation in Maiduguri

Fig. 4: Monthly decadal rainfall accumulation in Ibadan

Fig. 2: Monthly decadal rainfall accumulation in Calabar

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( )00.005 36 ;β ϕ= − − if   1%p p and  036ϕ p and

025θ ≥

                                               (5)  ( )00.005 36 1.8 4.25 ;Sinβ ϕ θ= − − + − otherwise                  (6)  

4 RESULTS From the data analyzed over the climatic regions across Nigeria, results indicate that the highest monthly rainfall accumulation (mm) is observed at Calabar in the month of June over the first decade (1975 – 1984), as shown in Fig. 2. The lowest monthly rainfall accumulation is how-ever observed at Maiduguri in the month of April in the third decade (1994 – 2004). This is shown in Fig. 5. The monthly rainfall variability over the 30-year observa-tion period is as shown in Fig. 1 for Jos (Sudan Sahel), Fig. 2 for Calabar (forest belt), Fig. 3 for Ilorin (Guinea Sahel), Fig. 4 for Ibadan (Wooded savannah) and in Fig. 5 for Maiduguri (Sahel). The point rain rate distribution across Nigeria was devel-oped for each decade using the Golden Sulfer software, while the contours were plotted using krigging method. The areas with the darkest colour indicates the areas with the highest point rain rate distribution values (from 114 mm/hr to 129 mm/hr). The contour lines serves as the demarcation for a region (on the map) with its corre-sponding point rain rate value. Fig. 6 shows the point rain rate distribution estimated from the available dataset for the first decade (1975 – 1984), while the point rain rate distribution for the second and third decades are shown Fig. 7 and Fig. 8 respectively. This clearly shows the spa-tial variation of the rainfall intensity across Nigeria and its temporal variation over the 30-year observation peri-od.

Although the dataset employed might be different from those used in [1] and [10], results obtained for the rainfall rates are similar to predictions in [1]. This predic-tion is also not in sharp contrast with those presented in [10] for rain rates exceeded for 0.01% of the time, over similar locations across Nigeria. It is worthy to note that the technique employed for predicting the rain rate is similar to the procedures used in [1], while the results in [10] is based on the combined Moupfouma-Martin-Rice-Holmberg procedure.

As the point rain rates decreased from the coastal re-gions to locations in the arid region, predictions coincided at 129 mm/hr for the first and second decades at Calabar (coastal area), while it increased to 131 mm/hr in the third decade. Similar predictions were also observed in the central regions around latitude 100 North and Longi-tude 80 East over the first and second decades, while the estimates rainfall rate increased by 2 mm/hr in the third decade. Similar results were observed in the North-Eastern region, where the rain rate predicted for the first and second decades is about 79 mm/hr, while the predic-tion for the third decade is 81 mm/hr.

The corresponding impact of the rainfall variation is as presented in Fig. 9 through Fig. 13 over the chosen sites. Results indicate that the slight variation observed on the point rain rate statistics only accounts for little variation

in the rain induced attenuation over the decades. The var-iation is negligible at higher percentages of the time (typi-cally between 0.1 and 1% of the time) as is the case at

Fig. 6: Point rain rate distribution for decade 1 (1975 – 1984)

Fig. 7: Point rain rate distribution for decade 2 (1985 – 1994)

Fig. 8: Point rain rate distribution for decade 3 (1995 – 2004)

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Maiduguri, Jos, Ilorin, Ibadan and Calabar. Predictions

for Calabar and Ibadan however indicates an obvious variation at 0.001% of the time, especially between the first and the third decade and between the second and third decade. Interestingly, 99.99% availability (i.e. 0.01% unavailability) can be maintained at Maiduguri, Jos and Ilorin, with somewhat negligible variation across the dec-ades.

The availability objective of 99.99% can also be main-tained at Ibadan and Calabar, but with only a slight varia-tion of about 1 dB between the first/third decades and the second decade at Ibadan and 1 dB between the first/third decade and the second decade at Calabar.

5 CONCLUSIONS The impact of rainfall variability has been explored on

the possibility of maintaining availability objectives on satellite communication links. The rain rate estimates decreased considerably from the Coastal region to loca-tions in the Arid region and the results obtained are simi-lar to those presented in earlier contributions [1, 10, 14].

The corresponding impact of the observed spatial and temporal rainfall variability was quantified on digital DTH reception downlink NIGCOMSAT-1R at selected locations across the five climatic regions. Results indicate that effects at 99 and 99.9% availability is negligible across the selected sites. It also reveals the slight variation observed at 99.99% of the time, which indicates a varia-tion of about 1 dB at Calabar and Ibadan, while variation remains negligible at Maiduguri, Jos and Ilorin over the decades. The variation of about 1.4 dB was recorded at Calabar, 0.8 dB at Maiduguri, 1.3 dB at Ibadan, while variation remain negligible at 99.999% in Jos and Ilorin across the decades.

The spatial and temporal rainfall variability is appar-ent as shown in the results obtained and the impact of this variation has been quantified on radio communica-tion planning. Results therefore indicate that designed availability objectives can be maintained on satellite link designs, particularly over decades within which commu-nication satellites might be replaced due to the peculiar and short lifespan.

REFERENCES

Fig. 9: Cumulative distribution showing decade-decade rain attenuation

variability at Jos

Fig. 11: Cumulative distribution showing decade-decade rain attenua-

tion variability at Maiduguri

Fig. 13: Cumulative distribution showing decade-decade rain attenua-

tion variability at Ibadan

Fig. 10: Cumulative distribution showing decade-decade rain attenua-

tion variability at Calabar

Fig. 12: Cumulative distribution showing decade-decade rain attenua-

tion variability at Ilorin

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 OBIYEMI Obiseye obtained B.Eng. and M.Eng. degree in Electrical Engineering from University of Ilorin, Nigeria in 2006 and 2010 re-spectively. He joined the Department of Electrical and Electronic Engineering, Osun State University in 2010 and He is currently work-ing towards the PhD degree from the Department of Electrical and Electronic Engineering, University of Ilorin, Nigeria.

IBIYEMI Tunji Samuel obtained PhD Control Engineering from Uni-versity of Bradford, Bradford, England in 1982. He is a full professor of Electrical Engineering (computer & control) at the Department of Electrical and Electronics Engineering, University of Ilorin, Ilorin, Nigeria. His research interest is in biometric signal processing and satellite system development. AKANDE Samuel O. obtained a B.Tech degree in Meteorology from the Federal University of Technology, Akure, Ondo State, Nigeria in 2011. He is currently working towards the Masters degree from the African Regional Centre for Space Science and Technology Educa-tion – English (ARCSSTE-E), Obafemi Awolowo University Campus, Ile Ife, Nigeria. He joined the Centre for Space Research and Appli-cations (CESRA) at the Federal University of Technology, Akure, Ondo State Nigeria, as a GIS Analyst in 2012. He specializes in Geographic Information Systems and Remote Sensing.