CHAPTER ONE
INTRODUCTION In this chapter, the background to this research is introduced along with the dissertation overview, the methodology adopted and the significant of the study. The background explains the importance of communication link and the over bearing effect of rain on the link at frequencies above 10GHz.
1.1 Background
Much attention has recently been shown to the use of higher frequencies for terrestrial and satellite communication links for civil, military and mobile communication systems globally. The use of higher frequency band for communication systems is necessary for higher channel capacities and also to avoid the congestion in VHF and UHF bands. As the communication systems using higher frequencies are growing rapidly in tropical countries, there is an increasing need for the knowledge of propagation characteristics of microwave because atmospheric effects play a major role in the design of the microwave links operating at frequencies above 10 GHz.. Basically, communication links operating at these frequencies are severely affected by the presence of rain over the link path, more so, in the tropical regions because of the high intensity of rain. Raindrops absorb and scatter radio waves, leading to signal attenuation and reduction of the system availability and reliability. The severity of rain impairment increases with frequency and varies with regional locations. It is therefore very important to make accurate prediction for the rain induced attenuation when planning for both microwave and terrestrial line-of-sight link.
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The method for the prediction of rain attenuation on microwave paths has been grouped into two classes: the empirical method which is predicted from pointrainfall rate, drop-size distribution and other relevant parameters along the radio paths or through link measurements, and the physical method which is an attempt to reproduce the physical behaviour involved in the attenuation process. However, when a physical approach is used not all the input parameters needed for the analysis are available. Empirical method is therefore the most used methodology largely from the point rainfall rate and dropsizes distribution data (Ojo et al 2008). For the use of the empirical method, an appropriate distribution of rainfall rate at 1-minute integration time is needed for the site under studies in order to have accurate rain attenuation prediction for the location. Studies have shown that there is still dearth of rainfall rate at 1-minute integration time for the prediction of rain attenuation because all national weather bureaus and environmental agencies in the tropical countries of the world only record daily or hourly rainfall data. Very important effort made towards gathering moreinformation on the 1-minute rain rate was through the use of distrometer or the Tropical Rain Measurement Mission (TRMM)jointly developed by the United States and Japan, and the GlobalPrecipitation Climatology Project (GPCP) of the World ClimaticResearch Programme (WCRP). The data available from this mission cannot directly be used in system design, due to its long integration time.A method for converting the available rain rate data to the equivalent 1-minute rain rate cumulative distribution is therefore necessary. Even though the International Telecommunication Union (ITU-R) provides the global model, most attempts for the 1-minute rain rate involved extrapolation of measurements from one
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location to the other. However, the complex nature and regional variability of rain make this approach highly inaccurate. Consequently, the need for local experimentally determined parameters such as the received signal level (RSL) of the microwave link cannot be ignoredfor the reliable communication system because it will provide the knowledge of the link fade margin to be provided in the system design in order to overcome the losses in the signal strength due to rain over the path.Therefore, this dissertation presents the modeling and prediction of rain induced attenuation using empirical approach from the measured received signal level (RSL) of BSC radio over a period of two rainy seasons in Kaduna.
1.2 Research Motivation
Microwave links are used extensively for base station backhaul in the base station subsystem of mobile communication network of GSM in Nigeria. At present, more than 60% of all base stations are connected via microwave links because the majority ofoperators (Airtel) seek to minimize their operating expenses (OPEX) by owning their own transport networks instead of leasing capacity. These communication links being used in the Abis interface of the GSM architecture operate at frequencies above 10GHz. The utilization of this higher frequency band provides a number of important benefits, it relieves the congestion in the lower frequency bands and it also exploits the larger bandwidths available at higher frequencies so as to accommodate the high demands for broadband services. Despite the advantages suggested by the use of these frequency bands, the systems can be easily degraded by some natural atmospheric phenomena of which rain is the principal factor and needs to be appropriately quantified so as to enhance reliable communication (Freeman, 1997). This is not to assume that other factors
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such as crosstalk or inter-system interference have become irrelevant, but in a situation where the effect of rain is so severe that a communication links nolonger functions, then other factors can be considered as secondary. Rain being a natural phenomenon which displays a high degree of spatial and temporal variation along the signal propagation paths from location-to-location on yearly or monthly basis, causes substantial rain induced attenuation, which is a dominant impairment to successful delivery of both voice messages and data even with digital communication link at high frequency of propagation (Odedina et al, 2007). There have been extensive works done on the measurement of rain drop size distributions, radar reflectivity and rain rate for the investigation of rain-induced impairment to radio signals at super high frequencies. Most of these works were carried out in the temperate regions. However, some researchers have carried out similar studies in tropical locations. Such studies include the measurement of rain drop size distributions at Ile-Ife by Olsen and Ajayi, (1999),Ajayi et al, (1999) at Ilorin, Zaria and Calabar using the disdrometer, while the measurement of rainfall intensity was done using rain gauge. Presently, there has not been any measurement of the vertical profile of rainfall parameters in Nigeria. Therefore, the need to provide empirical model using the measured received signal level (RSL) of link when there is rain drops on the antenna. It is believed that the approach uniqueness would account for the accurate profile of the rain effect on a microwave link in an area from the measured signal fade margin of the link.
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1.3 Problem Statement The effectiveness of wireless communication systems to a large extent depends on the transmission medium. Due to new needs and developments, system designers are interested in frequencies above 10 GHz. At these frequencies rain attenuation is one of the key parameters considered in link design and budgeting. For this reason, the International Telecommunication Union (ITU) recommends an expression for rain attenuation at a given frequency and rain rate (Seker and Kunter, 2013). Although this simple model (aRb)is widely used in telecommunications, however,the model does not accurately account for rain induced attenuation everywhere in the world because the rain rate and drop size distribution changes and are different across the climatic regions of the world. Consequently, a modification of the ITU-R model is important based on the actual climatic condition of a region for a reliable design of microwave link. 1.4 Aim and Objectives The aim of this research work is to develop an empirical model for the accurate prediction of rain induced attenuation at 13GHz and 15GHz frequencies using BSC microwave link of Airtel GSM network in Kaduna. The objectives of this study are to:
(i) Conductperformance analysis of the digital microwave links used in the AbisInterface of GSM network under the effect of raindrops.
(ii) Determine the links worst faded signal level due to rainfall
(iii) Determine the rain induced attenuation from the links Received Signal Level
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(RSL) variation measurement on the 13GHz and 15GHz microwave links
(iv) Developempirical models for the accurate prediction of rain induced attenuation for microwave links based on the measured data
(v) Validate the developed models using the ITU-R model.
1.5 Methodology Thisresearch is based on the direct measurement of the radio Received Signal Levels (RSL) as a unique radio propagation process, which could increase the accuracy of conventional propagation model predictions by making use of direct on-air link data. Toachieve this the following methodology was adopted:
(i) Collection of rain fall data from Nigeria meteorological Agency, (NIMET)
(ii) Determination of rain rate Rp from the NIMET data.
(iii) Direct measurement of RSL on the 13GHz and 15GHz microwave links during clear sky (when there is no rain or dust storm) and during heavy rainfall (April to October) for the years 2009 and 2010 respectively.
(iv) Development of empirical models using Lease Square Method based on items (ii) and (iii).
(v) Evaluation of the developed model using the following:
(a) Chi-Square Test; (b) RMSE statistical analysis.
(vi) Simulation of the models usingMATLABSignal Processing tools boxR2008a to predict rain attenuation at 13GHz and 15GHz.
(vii) Validation of the developed model using the ITU-R model.
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1.6 Significant Contributions The following are some of the contributions achieved as an indication of the significance of this research work:
(i) Empirical models were developed for 13GHz and 15GHz microwave frequencies based on direct measurement of the link performance.The predicted models showed that the rain-induced attenuation is directly proportional to the rain rate.
(ii) The study showed that the ITU-R model under estimated rain induced attenuation at 13GHz and 15GHz frequencieswith mean error of 0.78 and 3.02dB respectively when compared with the developed models, and the model under estimation increases with rain rate and frequency increase.
(iii) The study also ascertained the links worst performance under heavy rain with signal fading of -77dBm and -80dBm for the 13GHz and 15GHz frequencies respectively and predictedexcess rain attenuation of 11.35dB at 13GHz and 18.78dB at 15GHz microwave frequencies. The ITU-R model cannot be used to determine the link signal fading.
1.7 Dissertation Outline
This dissertation is divided into five chapters. Each chapter gives a summary of what it contains. Chapter is an introduction that covers the background knowledge and motivation for this research work, the problem statement, aim and objectives, the dissertation overview and itscontributions to knowledge.
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Chapter two deals with literature review, which included critical review of basic fundamental concepts of wave propagation and propagation losses, as well as various existing rain attenuation models, especially, the few developed for tropical regions of the world. Detailed descriptions of some rain attenuation investigations that have been carried out in different parts of the world were discussed. This is the descriptive part of the dissertationand is intendedto present a general overview of some experimental and theoretical terrestrial rain attenuation models that have been proposedby different authors. Rain rate statistics in theform of rain rate cumulative distributions was discussed here as the starting point of all rainattenuation prediction models. The theoretical approach for the calculation of rain attenuation along a terrestrialcommunication radio link based on the assumption that the raindrops are non homogenous was discussed in this chapter, including the principle of effective path length for path loss estimation by the different authors. Lastly, the chapter gives account of some similar research works carried out in this area keen emphasis on the model formulation approach, validation of the predicted models and the limitations.
Chapter three unveils the material and method, for the analysis data collected and measured. It explains how data processing and theoretical empirical modeling using least square method generates different curve fitting functions for the experimental data point with the help of Microsoft Excel toolbox. Also it covers the evaluation of these curve fitting functions by statistical techniques using Chi-Square Test and Root Mean Square Error (RMSE) and simulation of the chosen functions usingMATLAB R2008a signal processing toolbox, as well as their validation with the ITU-R model by
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comparison.Chapter four gives detail analysis and discussion of the result obtained. Chapter five presents the conclusion and recommendations drawn from the study.
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