ABSTRACT
The quality of satellite communication link can be seriously affected by variable climatic phenomena such as rain, gases, and scintillation. As the frequency of operation increases
beyond 10GHz, the effect of rain becomes very severe with the growing need for high bandwidth frequencies such as frequencies in the Ku-band and Ka- band hence the need for better fade mitigation becomes necessary.
In order to identify the appropriate rain fade mitigation technique, there is need to quantify the amount of fading experienced due to rain.
This project aims to identify an appropriate rain fade calculation technique for Nigeria.
After the technique is identified, rain rate data is then obtained from Nsukka, Enugu State in
Nigeria and used to obtain the rain fade margin.
In performing a link power budget analysis, the fade margin (FM) is a critical component. Often times, the inclusion of a large FM is not cost efficient or technically feasible. Therefore FMTs must be integrated into the overall system design.
This thesis is aimed at presenting a comprehensive review of current and future satellite communication technologies and applications, paving the way for a comprehensive analysis of current and proposed FMTs catering for rain attenuation. This analysis spans techniques such as; Diversity, Adaptive Signal Processing, Adaptive Source Sharing, Uplink and Downlink Power Control, and others. These techniques are comparatively analyzed with their advantages and disadvantages qualified and quantified to deduce quantifiably the influence of rain attenuation on received signal power on Ka-band frequencies. This data is now made available to organizations interested in building earth-space communication links over Nigeria.
CHAPTER ONE
SATELLITE TRANSMISSION IMPAIRMENTS
1 INTRODUCTION
As the radiowave propagates through the earth-space link, the nature and composition of the atmosphere introduces impediments which cause a degradation of the signal. The effect of the atmosphere on the propagating radiowave is a critical concern in the design of a satellite communication link as it can cause variations in signal polarization, phase and amplitude [1]. These effects depend on the geography of the location, climate of the location and frequency of operation of the satellite link. The frequency of operation is a critical factor in determining what impediments the radiowave would be susceptible to. The complexity of the atmosphere means that the losses experienced are a combination of losses due to individual components in the atmosphere. The primary constituents affecting a satellite link are water vapour, gases, rain and clouds.
At frequencies below 3 GHz the ionosphere is the primary source of transmission impairment as a result of background ionization and ionospheric irregularities along the propagation path. Above 3 GHz, rain is the major impediment impacting on the earth-space link as the raindrops scatter and absorb the radiowave energy causing a reduction in the amplitude of the transmitted signal leading to a situation known as rain attenuation [1]. At frequencies above 10 GHz and spanning into the Ka-band and beyond, the effect is very significant leading to grossly ineffective links [2]. While clear air losses generally lie under IdB, the loss experienced as a result of rain can be as much as 20 dB depending on the operating frequency and rain rate for the particular climate and location, thus significantly impacting on the availability of the link [3].
This thesis took a look at the earth-space link, with the goal of measuring the level of attenuation due to rain to be expected on a signal transmitted from a satellite and subsequently received by a ground terminal or station. Actual rain rate information obtained from three geographical locations in Nigeria (Nsukka, Kogi and Abuja) were used to predict the possible attenuation due to rain expected over Nigeria in order to plan an appropriate fade margin (FM) or fade mitigation technique (FMT) to incorporate into the link to curb outage and achieve the percentage of time specification for a particular link design.
1.1 PROBLEM STATEMENT
Due to the presence of transmission impediments in the atmosphere and to ensure optimal quality of service (QoS), satellite links are usually designed with a certain link availability percentage performance criteria. This ensures that for a given percentage amount of time (usually measured yearly or on a worst month basis), the minimum carrier-to-noise ratio required at the input of the receiver is maintained or exceeded – without blinding the receiver – for proper operation.
The availability of the satellite link is a paramount factor in designing a link, and rain is the greatest source of signal attenuation to frequencies in the Ka-band [1]. If the outage due to rain is not properly managed by building an appropriate fade margin or employing an appropriate FMT, the satellite link would perform poorly and become unreliable in supporting the service(s) which it was designed for as it would be unavailable for extended periods of time and fall below its expected availability requirement.
The growth of the satellite industry has led to a congestion and scarcity in the lower frequency bands prompting the need to utilize the higher frequency bands such as the Ka-band [4]. Also, the growing commercial need for high bandwidth applications such as High Definition Television (HDTV) which can be served at higher frequencies, has sparked a keen interest in the utilization of the higher frequency
bands[5]. With the attenuation due to rain increasing as the frequency of operation increases, the growing demand for the utilization of the higher frequencies becomes impossible to meet as exceedingly great outages would be experienced on the link thus making it unable to accommodate the growing number of new and high bandwidth services being planned for. Also, Ka-band satellites would be unable to compete with terrestrial alternatives such as fibre optic networks which provide high bandwidth and decent availability. In addition, huge financial losses and inefficiencies would be experienced by businesses and organisations utilizing such services.
In tropical climates (such as Nigeria) with high average rainfall, the effects of rain attenuation are critical as it could completely disrupt the communication link for huge percentages of time leading to frequent and long outages. Mitigating the effect of this attenuation due to rain requires a quantitative knowledge of the average and mean amount of attenuation expected on the link and applying an adequate fade margin (FM) as well as the best-fit fade mitigation technique (FMT) to improve link availability and reliability.
1.2 MOTIVATION
In 1957 the U.S.S.R launched the SPUTNIK. This was the first artificial satellite [1]. The U.S.A followed suit with the launch in 1958 of the artificial satellite called SCORE [1]. A few other satellites were subsequently launched in 1960, 1962 and 1963. In 1965 the EARLYBIRD satellite which was later called INTELSAT 1 was launched. This launch heralded the beginning of the commercial satellite revolution [1]. These commercial satellites provided telephone and television signal transmission between continents, thus complimenting the services provided by submarine cables which were used to provide national trunk connections across continents.
Over the years, satellite technology services and applications have grown as a result of the satellites unique ability to offer a wide and peculiar field of view (footprint) and connect geographically disparate locations. An additional benefit is the ubiquitous coverage which enables a small constellation of satellites take advantage of orbital location and footprint to cover virtually the entire surface of the earth. Various organisations within the government, private sector and academia, as well as homes, not only utilize these services but have made them an essential part of their routine economic and daily activities due to the satellite’s ability to offer high link availability for a large percentage of time in a calendar year. The global satellite industry revenue grew from $ 121.7 billion dollars in 2007 to $189.5 billion dollars in 2012, marking a 55.7 percent growth [5]. Between 2011 and
2012, the global satellite industry grew by 7% outperforming the world economic growth rate which was 2.3% [5].
The particular frequency band a satellite or network of satellites operate(s) in is dependent on two attributes of the satellite(s). These are; the service to be provided by the satellite system and the location of the satellite system. As more services come online, more frequencies have to be allocated (within the currently useable portion of the spectrum) to the respective operators bearing in mind the need to optimize the radio spectrum while provisioning guard bands to avoid interference. The radio frequency spectrum is becoming a scarce resource as it is not inexhaustible. The growth in demand for services such as, mobile voice, data and entertainment have pushed the demand by operators for frequency allocation within the radio spectrum to new heights [6]. This is reflected in the scarcity of frequency in some lower frequency bands stirring up the demand for the design of new technologies with better spectral efficiency, as well as the design of cost effective equipment able to effectively and efficiently utilize the higher frequency bands (such as the Ka-Band) and thus provide higher bandwidth to compete with possible terrestrial alternatives such as fibre optic networks.
This thesis intends to contribute to efforts aimed at designing high margin cost effective Ka-band satellite links.
1.3 THESIS STRUCTURE
Following this introductory chapter, Chapter 2 commences with a review of the satellite systems architecture. The quantities that make up the received power on a satellite link are presented. A description of the various sources of atmospheric signal attenuation impacting links operating above 3
GHz is given. Rain attenuation and the two types of rain are presented. Four rain attenuation prediction models are presented and compared.
In Chapter 3, the experimental setup used for the collection of the rain rate data is presented. The rationale for input values to be used in the ITU-R P618 Rain attenuation model is presented. The attenuation due to rain is calculated and presented.
In Chapter 4, various power restoral and signal modification restoral techniques for rain fade mitigation are reviewed and compared. The most suited technique is chosen for application to the link.
Finally, conclusions are drawn and areas for future work based on this thesis are presented in Chapter
This material content is developed to serve as a GUIDE for students to conduct academic research
THE QUANTIFICATION OF THE INFLUENCE OF RAIN ATTENTUATION ON RECEIVED POWER AT KA BAND
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