Abstract-According to the increasing demand of offshore communication in Malaysia. There is a requirement of high-speed and reliable communication system, which can be used for over sea beyond-the-horizon communication. In this work, the feasibility of long-range communication using evaporation duct is studied. AREPS Software is used for theoretical assessments. The optimum frequency and antenna height are determined for Terengganu, Malaysia. The range and reliability of radio-link is also determined for off-the-shelf radio equipments.
Malaysia is surrounded by large water bodies. It has two distinct parts separated by South China Sea. There are many islands in its territory. It has many nature mineral recourses. To discover the energy resources, hydrocarbon exploration companies have an intention towards the oil reservoir in deep seas. The oil fields of Sabah, Sarawak and Terengganu have contributed much to the Malaysian economy. As offshore activities are increasing, the requirement of high-speed communication system is also increasing to communicate between ships, vessels, islands and rigs.
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Nowadays, most available and common resource for over-the-horizon wireless communication at sea is satellite. As there is reasonable enhancement happens in hardware and software technology which reduces the cost and increases the reliability but they still have limitations in sense of coverage, bandwidth and cost.
The Alternate methods for wireless communication at offshore sites are Line of Sight (LOS) and Non Line of Sight (NLOS) radio links which critically depend upon tower heights and/or aerial distance between two sites.
From observations, it is found that the performance of communication system highly depends upon the atmosphere characteristics. The non-standard conditions of atmosphere can cause an anomalous propagation. From many years, anomalous propagation has been under observation. Over and over again, Signals with good strength were received at far distances. This happens due to complex formation of the troposphere. Troposphere is a lowest atmospheric layer which starts from earth's surface and extends up to 13-15 Km above. The critical behavior of radio refractive index is very important to understand. In Section II-A, Radio refractive index is discussed in detail. Due to gradient of refractivity, ducts or layers are formed in troposphere. When signal is trapped in these ducts, it can travel over long distances. Types of tropospheric ducts and causes of their formation are described in Section II-B. These ducts can be used for the beyond-the-horizon wireless communication.
Radio Refractive Index:
Due to, changes of refractive index (n) in troposphere, the signal couldn't follows the straight path as in free space. In troposphere the deviation of refractive Index from unity is very small. So, a convenient way of expressing the refractive index is refractivity (N). N is defined by:
There are four refractive conditions which depend upon refractive gradient. In normal condition, the gradient of refractivity varies from 0 to -79 N-units per kilometer. When the refractivity gradient varies from -79 to -157 N-units per kilometer, a super-refraction condition is said to be prevailed in troposphere and the ray will refract downwards at a rate greater than standard but less than the curvature of earth. If the refractivity gradient is greater than 0 N-units per kilometer, a sub refractive condition exists and a radio ray will now refract upwards away from the surface of the earth. The trapping condition is occurred when refractivity gradient is even less than -157 N-units per kilometer. This will result in a ray that refracts towards the earth's surface with a curvature that exceeds the curvature of the earth. Refractive conditions are illustrated in Fig-1.
Refractive index measurements can be made by direct or remote sensing techniques. Commonly used direct sensing techniques are Radiosonde and Refract meter, and remote sensing techniques are Doppler radars, Lidars (Laser sounders), Sodar (acoustic sounders) and satellite-borne. B. Tropospheric Ducts:
Well-known Tropospheric ducts are Surface duct (Ground-based duct), Surface-based duct and elevated duct. In surface and surface-based duct lower boundary of duct is earth's surface, but in elevated duct both lower and upper boundary are above the earth's surface.
The major causes for formation of ducts are evaporation, nocturnal radiation, subsidence inversion and advection. The rapid change in vapor pressure in first few meters above sea surface causes the evaporation duct, the point where M-inversion occurs is called evaporation duct height, Fig. 2(a). And, the temperature inversion with height causes the surface-based and elevated duct as shown in Fig. 2(b, c).
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Evaporation duct is also a ground-based duct but it's found only on large water bodies. By virtue of their nature of formation, evaporation ducts are nearly permanent features over the sea surface. The height of an evaporation duct varies with spatial and temporal basis. The average height of evaporation duct is reported to be 13 meters .
Over the years, much research has been undertaken to explain the mechanism of radio wave propagation in evaporation ducts. A key reason why evaporation ducts are so important for radio communications is because they are often associated with enhanced signal strengths at receivers this ducting phenomenon can used to take advantage for beyond-the horizon wireless communication . Fig. 3 shows a schematic ray bending in the evaporation duct, which allows over-the-horizon radio wave propagation .
In this paper, we use Advance Refractive Effects Prediction System (AREPS) software for propagation pathloss calculations. AREPS was developed by Space and Naval Warfare Systems Centre, San Diego (SSC, San Diego). The main engine of AREPS is Advance Propagation Method (APM). APM is a hybrid model which contains four models. These are flat earth (FE), ray optics (RO), extended optics (XO), and the Split-Step Fourier Parabolic Equation (PE) Algorithm. PE Method is the primary model for which the other three sub-models are built around [Amalia].
AREPS software has an environment creator, which gives the modified refractive profile from different meteorological parameters. Approximate evaporation duct for selected region can be calculated from already given statistical Marsden Square environmental data.
To check the feasibility of radio-link in evaporation duct, investigations for optimum frequency and height were made. First of all, local environment was created for Terengganu, Malaysia Region from Marsden Square environmental data. The average height with percentage occurrence of evaporation duct for Terengganu, Malaysia region is shown in Fig. 4
Using local environment the propagation pathloss is calculated for various frequencies. Fig. 4 shows the plot of data obtain from AREPS software. This shows the propagation pathloss for 100km link with a range of frequencies from 1.7-25 GHz and a height of 0-30 m above sea surface.
The main objective of this work is to check the feasibility of radio-links for un-licensed band. In this regards, by using the local environment profile created from Marsden Square environmental data, pathloss prediction were made for selected Frequencies. Fig. 5, Fig. 6, Fig. 7, shows the pathloss predictions with antenna height of 15ft for 2.4, 5.8 and 24.125 GHz band respectively.
The Feasibility of high-speed radio link is investigated in this paper. It is found that with the suitable frequency and receiver antenna height signals can propagate beyond-the-horizon. The possibility of radio link using 2.4GHz and 5.8GHz is also examined. Using these low frequencies propagation can be extended to few Km but we cannot communicate beyond the horizon.
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