Improved analysis of 2.5 Gbps-inter-satellite link (ISL) in inter-satellite optical-wireless communication (IsOWC) system
Introduction
The application of laser technology to communications, particularly space communications, was envisioned in the very early days of laser development around 1962, described a method for secure communications between a satellite and a submarine. From the last 40 years to now, government agencies, companies, universities, and individuals in many countries have made tremendous technical progress in optical-space communication i.e. inter-satellite optical-wireless communication [1]. The present satellite communication system uses microwave technology for space-to-ground and geosynchronous satellite to low earth orbiting vehicles. In the future system, the satellite to ground links would remain in the microwave regime but satellite to satellite communication will be governed by optical-wireless links. The technology uses laser light of infrared wavelengths to transmit optical signals between two points via free space. This requires devices similar to those used for the transmission through fiber-optic cable, except that the signal is transmitted through free space and not via optical cable capable of transmitting data, voice or video. IsOWC can be used to connect one satellite to another, whether the satellite is in the same orbit or in different orbits. The data can be sent in IsOWC systems at the speed of light without much delay and with minimum attenuation since the space is considered to be a vacuum. The advantage of using optical link over radio frequency (RF) links is the ability to send high speed data to a distance of thousands of kilometers using small size payload [2]. By reducing the size of the payload, the mass and the cost of the satellite will also be decreased. Another reason of using OWC is the wavelength. RF wavelength is much longer compared to lasers; hence the beam width that can be achieved using lasers is narrower than that of the RF system [3]. Due to this reason, OWC link results in lower loss compared to RF, but it requires a highly accurate tracking system to make sure that the connecting satellites are aligned and have line of sight. However, the transmission of such transmissions is affected in different ways by the environment processes such as absorption, scattering and shimmering. All three conditions attenuate the transmitted energy, affecting reliability and the bit error levels. Satellites revolve around the earth at their own orbit, and there are three commonly used orbits for satellites. Satellite orbits with orbital height of approximately 1000 km or fewer are known as Low Earth Orbit (LEO). LEOs tend to be in general circular in shape. LEO satellites take from 2 to 4 hours to rotate around the earth. This orbit is commonly used for multi-satellite constellations where several satellites are launched up to space to perform a single mission. Satellite orbits with orbital heights typically in the range of 5000 km to about 25,000 km are known as Medium Earth Orbit (MEO)/Intermediate Circular orbit (ICO). MEO and ICO are often used synonymously, but MEO classification is not restricted to circular orbits. In Geosynchronous Earth Orbit (GEO), the satellite is in equatorial circular orbit with an altitude of 35,786 km and orbital period of 24 hours. Three satellites in GEO placed 120° apart over equator cover most of the world for communications purposes [4]. At present, there are 6124 satellites orbiting the earth and this number increases year by year [5]. At the same time, the optical-wireless communication (OWC) technology has grown and advanced throughout the year. Laser communication is now able to send information at data rates up to several Gbps and at a distance of thousands of kilometers apart. This has opened up the idea to adapt optical-wireless communication technology into space technology; hence inter-satellite optical-wireless communication is developed. In this work, we have presented the improved simulative investigation of Inter-satellite optical-wireless communication systems at high transmission rate of 2.5 Gbps over a spacing distance of 1000 Km by means of the square root module. The paper is organized as follows: section I contains the system description, section II discusses the results of inter-satellite optical-wireless communications system and finally, the section III summarizes and concludes this paper.
Section snippets
System description
The IsOWC system consists of three main communication parts which are transmitter, propagation channel and receiver as shown in Fig. 1 where the transmitter is in the first satellite and the receiver is in the second satellite. Optical-wireless communication uses light at a near-infrared frequency to communicate. The IsOWC system is not much different from free space optics and fiber optic communication where the difference relies in the propagation medium. The free space between two connecting
Result and discussion
An inter-satellite optical-wireless system is designed with the help of OPTI-SYSTEM™ simulator consisting of two satellites with a space- difference of 1000 Km. The two satellites exchange externally-modulated optical-data at 2.5 Gbps through the free-space medium at an operating wavelength of 1550 nm. Fig. 3 depicts the measurement of SNR at satellite 2 at different space-differences between the two satellites and different transmitted powers at an operating wavelength of 1550 nm. It has been
Conclusion
In this work, we have designed an inter-satellite OWC system to establish an inter-satellite link of 1000 Km length between two satellites at data rate of 2.5 Gbps with- and without- SM module. It is concluded from our simulated IsOWC system that the ISL link of 1000 Km with improved SNR ratio in conjunction with acceptable BER can be achieved by using the SM module. Also, less transmitted-power is required to transmit the externally-modulated data of 2.5 Gbps over an ISL link of 1000 Km at an
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