Introduction
In this era of 21st century one cannot imagine human life without mobile communication. From mobile handsets to computers, smart-phones to laptops, iPads and Cable Television systems everything is part of this mobile communication era. As the countries are progressing, competition among them is increasing all over the world. With the advancement in technology lives of people are getting faster and busier, businesses and industries are expanding globally, therefore, the demand for mobile communication is increasing immensely with every passing day. Scientists, researchers and engineers are always looking for ways to serve the world with this demanding technology in every possible way.
During the last 25 years there has been a tremendous growth in the field of satellite communication. The idea of using the Satellite Systems for mobile communication has appealed to many people in the past and it still continue to do so. There has been an extensive research going on in this particular area globally. A lot of progress has been achieved so far but it is a fact that advancement in technology can probably never end.
As the demand for communication is increasing, means and resources to carry out this communication are often limited. For instance if communication of information is to take place between the countries that are hundreds of miles apart, across the ocean, the typical wired medium cannot always be used. And also how are we supposed to communicate when we are not connected to the land communication systems by any meansHere satellite systems come into play. With the help of satellite systems we can provide mobile communication services even to very fast moving vehicles, to the aircrafts during flights, to ships and submarines in oceans, and also to remote areas of earth where there is no communication infrastructure. So in this way we are able to provide services to the areas where application of wired cable medium is not always practically possible.
Satellites are the object that revolves around the earth in fixed orbits. These satellites are at typically 400 km to 36000 km above the earth surface. Our purpose of communication over long distances is served by these satellites. Another advantage of using the satellite systems for communication is that they can cover a very large geographical area over the earth surface to provide communication means. When we are using a satellite system we are not limited by the problem factors that we encounter on earth such as laying hundreds of miles of expensive cables, space and land to store machinery and equipment to handle this sort of communication, buildings to handle all the infrastructure needed etc. Therefore, Satellite Systems are often given a thought as a better alternative for mobile communication.
Before we look into the details of how mobile communication takes place through these satellites, we need to first understand the satellite systems in general as well to have a better understanding of how the whole technology works.
Satellites are sent into space from earth. When in space above the earth surface, these satellites are made to revolve around the earth in fixed orbits with the help of gravitational force of the earth. To understand the phenomenon we can take a very simple and well known example from nature and that is of the Moon. Moon is the natural satellite and as it revolves around the earth, it shines over a huge geographical region of the earth. In a similar fashion, man made artificial satellites, though not as big as the moon but still, covers a considerably large section of the earth to provide communication.
These artificial telecommunication satellites can be in four different kinds of orbits above the earth surface depending on the purpose they were sent into space. These can either be geo-stationary orbits, elliptical orbits, medium earth orbit or low earth orbits.
In geo-stationary orbits, as shown in Figure 1, the satellite remains at a fixed location over the earth surface which means it covers the same geographical region of the earth.
Figure1: Geo-Stationary Orbit
Elliptical orbits are used when satellites are required to cover a certain geographical area of the earth for longer period of time than the other geographical region of the earth. Figure 2 displays in general a satellite in an elliptical orbit.
Figure2: Elliptical Orbiting Satellite
Medium earth orbit is between 5000 km to 15000 km above the earth surface. While in low earth orbits, satellites revolve around the earth in circular orbit at about 400 km above the earth surface. Figure 3 depicts the low earth orbits (LEO) and medium earth orbits (MEO) of a satellite.
Figure3: LEO and MEO orbiting Satellite
Three types of services can be offered by a telecommunication satellite. First is FSS (Fixed Satellite Services), which is for long distance telecommunication services provided by different telecommunication networks on earth stations. Second is DBS (Direct Broadcast Satellite services), which is used for direct Television signals broadcasting from large earth stations. And third is MSS (Mobile Satellite Services), which is used to provide mobile communication services to different stations on earth.
In the year 1976, Mobile Satellite Communication was started by Communications Satellite Corporation (COMSAT), which is a US based company belonging to the field of telecommunications. The communication satellites launched by them were called MARISAT (Maritime Satellites) and later an International Maritime Satellite organization (INMARSAT) was formed which now provides Mobile Satellite Communication Services [1]. Earlier satellite communication used to take place by routing calls and information from public landline to an earth station first, and then forwarding them to the satellite. But now mobile communication can take place directly between a satellite and a station or handset on earth.
This entire phenomenon fantastically sounds simple but there can be few problems as well in using satellites for mobile communications. For instance, keeping the satellite in its orbit is not an easy task. The orbital motion does not depend only upon the earth’s gravitational pull. When a satellite is in its orbit, its orbit is also affected by the presence of other bodies such as the Moon and Sun. Moreover, our earth is not a perfect sphere so its own gravitational force on a satellite can vary at different locations and the Moon and Sun have their own gravitational forces as well that affects the path of a satellite. Under all these circumstances, satellites do drift from their original path which needs to be adjusted in order to keep the satellite on track.
To transmit and receive signals, these telecommunication satellites have a number of antennas to receive signals from one mobile earth station and transmit it to one or more mobile earth station. There is a Doppler Shift as well in the transmitted signal which occurs because of the movement of the satellite and rotation of the earth about its own axis.
Mobile communication satellite systems can provide services to those areas that cannot get services from networks on earth. These systems can be of three possible forms [2]. First is that a direct link to the gateway of satellite station can be given to a mobile earth station to connect to the network. Second is that a mobile earth station can be connected to a translator station through a radio link which is responsible to transmit the data from a mobile earth station to the gateway station through a satellite link. In the third type of mobile communication satellite system again a direct link can be provided to a mobile earth station but a dedicated satellite system would be required for this purpose.
Different frequency bands are allocated to the satellites to perform mobile communication. Mostly used frequency bands are L-Band, C-Band, Ka-Band and Ku-Band. L-Band has the uplink frequency of 1.6 GHz and a downlink frequency of 1.5 GHz for commercial mobile satellite services (MSS). The long wavelength of this band allows it penetrate building structures and also get least affected by rain. Therefore, less powerful antenna transmitters are required. C-Band has the uplink frequency of 6 GHz and a downlink frequency of 4 GHz. Ka-Band uplink and downlink frequencies are 30 GHz and 20 GHz respectively for commercial use of mobile satellite mobile services and 44 GHz and 20 GHz of uplink and downlink frequencies for military use. This band has very large spectrum and high bandwidths available. But due to short wavelengths, it is largely affected by rain. Therefore, to increase the signal power very high power transmitters are required. On the other hand, Ku-Band has the medium range of frequencies. The uplink frequency is 14 GHz and downlink frequency is 12 GHz for fixed commercial use. Due to medium wavelengths, its signals can also penetrate many structures and are still able to provide high bandwidths but still they are affected by rain.
As the earth terrestrial networks, satellites are also required to serve a number of users simultaneously. So at a time when millions of users are accessing satellite services there is a need to have some sort of mechanism to differentiate and secure each user’s transmission. For this purpose, there are a number of multiple access schemes of which the three most commonly used are TDMA (Time Division Multiple Access), FDMA (Frequency Division Multiple Access) and CDMA (Code Division Multiple Access).
In TDMA, there are different time slots. Each mobile earth station transmits its data in chunks in a specific time slot at the same frequency. So the data of each user is differentiated in different time slots.
In FDMA, different frequency bands are allocated to different users for both the uplink and downlink channels. However, TDMA is a better because of its lower distortions of inter-modulation. In FDMA, downlink bandwidth is divided among a number of users whereas in TDMA full downlink bandwidth is available to all users during a specific time slot allocated. Also in FDMA, we may have to decrease transponder power by one half to minimize the distortion due to inter-modulation.
The third most commonly used multiple access scheme CDMA has no restrictions as in TDMA and FDMA. Each user can transmit its data at any time and can also use the same frequency bandwidth. In CDMA, each mobile earth station’s transmission is separated by a unique code. Signals transmission are separated by using spread spectrum technology that is why CDMA is also called Spread spectrum Multiple Access. Spread spectrum scheme assigns to each mobile station a unique code to generate a pseudorandom sequence to separate signals transmission and to spread the transmission across the whole bandwidth available from the satellite. When the signals transmission arrives at the receiver, it can be extracted by using the same sequence generated initially. The only limitation of the CDMA scheme is that it is very expensive to implement and can support a very limited number of mobile earth stations at a time.
As the science has advanced in technology, there are a number of mobile communication satellite systems now. Now we have a look at the different mobile communications satellite systems that exist today. These mobile communication satellite systems are divided into three groups namely: Geo-Stationary Systems, Big Low Earth Orbit (LEO) Systems and little Low Earth Orbit Systems. In Geo-stationary systems, INMARSAT and MSAT are the two satellite systems that we have. In Big LEO systems we have IRIDIUM, ARIES, ELLIPSO and ODYSSEY satellite systems. For little LEO systems we have LEOSAT, STARNET, ORBCOMM, and VITASAT satellite systems.
Of these satellite systems, International Maritime Satellite (INMARSAT) System is a very well known global mobile satellite telecommunication system. These satellite systems are connected to PSTN (Public Switched Telephone Networks) and they provide connectivity and communication services to a mobile earth station all over the world. Which means a user with a satellite mobile handset connected to the INMARSAT satellite system can receive services while roaming in any part of the world without being dependent on local terrestrial public networks. So a satellite connection proves to be really helpful in case something goes wrong with the terrestrial network such as a natural disaster etc.
INMARSAT is running its operation on four geo stationary satellites providing global coverage namely: INMARSAT-A, INMARSAT-M, INMARSAT-B and INMARSAT-C. High quality telephone, fax and high speed data services are provided by INMARSAT-A satellite system. In addition to dialling to a telephone or fax number directly, it can also provide image and video transmission services. INMARSAT-M provide services at a much cheaper rate as compared to INMARSAT-A. It has a fully digital and portable terminal to provide high quality cellular voice and data transmission services. Additional functionalities are added in INMARSAT-B satellite system compared to INMARSAT-A at a much lower charges. Along with new services, it provides very high quality voice services and very high data rates with a scope to increase them further in future. INMARSAT-C comes as a low cost communication system with a light weight terminal powered by a battery. It comes with a small personal computer to communicate with the network headquarter whenever needed.
The following Figure 4 shows the statistics of the INMARSAT satellite systems all over the world [3].
Figure 4: INMARSAT Satellite System Coverage and Service Providers [3]
Iridium Satellite System was proposed and developed by the Motorola Company in collaboration with a few other companies. This satellite has been purchase by the Iridium, Inc Company. In this satellite system, satellites are set into orbits in 6 different polar orbital planes with a total of 72 satellites at about 780 km above the earth surface [4]. Satellites are divided into groups of 11 with equal distance among them. These are all Low Earth Orbit Satellites. L-Band frequency band is used by mobile earth stations to access satellite link using TDMA or FDMA schemes. Access of the mobile stations need to be synchronized to enable them to transmit and receive in the same time frame slot. Each satellite can handle more than 1000 calls at a time. All the satellites can route traffic to each other as well. This satellite system is designed in such a way that global coverage is achieved and gateway stations, which need to be connected to public switched telephone network on earth, required are less in number.
Two other LEO satellites systems are ARIES and ELLIPSO which are set into circular orbits above the equator to provide low cost services.
Another very well known satellite system is GLOBALSTAR system which provided full global connectivity. This system consists of 48 satellites divided into groups of 6. These satellites are set into orbit in eight different planes at 1414 km above the earth surface and are inclined at 45degrees and 135degrees to the equator. There is no exchange of data among these satellites as in the Iridium System. Therefore, a mobile earth station can only get access to the satellite link when the satellite has a line of sight path to the gateway earth station. Mobile stations can access the satellite using L-Band frequency band. Code division Multiple Access (CDMA) scheme is used to separate transmission of each mobile station. Six spot beams are used to cover the same geographical area on earth as is required by the Iridium satellite system. Satellites in GLOBALSTAR system complete their lifecycle between 5 to 15 years.
OrbComm launched its first two satellites in 1995. This satellite system is capable of providing remote monitoring and mobile tracking along with many other commercial services. OrbComm also contains 48 satellites which are in Low Earth Orbits. Access by mobile earth stations to these satellites is made on VHF (very high Frequency) band. The uplink band is 148 MHz to 150 MHz and the downlink band is between 137 MHZ to 138 MHz. Signal and data transmitted from the satellites are not directly forwarded to hand held devices first, rather they are first diverted to the gateway station on earth or public switched telephone network and then they are forwarded to mobile handsets. Satellites in this system have a lifecycle of about four years.
Intermediate Circular Orbit (ICO) satellite system consists of ten satellites with the help of which it can provide full global coverage. These satellites are divided into groups of five into two orbital planes at about 10,355 km above the earth surface. They orbit the earth the earth in about 6 hours serving a geographical area on earth for 20 minutes. Each satellite in the Intermediate Circular orbit can provide up to 4500 channels for voice data traffic and access to these channels is made secure and reliable by using the Code Division Multiple Accessing (CDMA) scheme. The terminals ICO systems are capable of supporting dual mode which means that they can operate with satellite and as well as public switched telephone networks.
In this entire scenario, each type of satellite system has its own advantages and disadvantages. Geo-Stationary satellites can be useful for one form of mobile communication but in some scenarios it may be more feasible to use Low Earth Orbit satellite system. Like with the help of three Geo-Stationary satellites we can provide almost full global coverage. As the satellites remain at the same point above the earth in Geo-Stationary system, the transmitters and receivers can have the fixed antenna positions without the need to keep track of the satellite which at times can become a very tedious task. Because they are located at about 36000 km above the earth, they are less affected by the atmosphere around the earth during its orbit making its life cycle greater than the other satellite systems. On the other hand, there are certain disadvantages of the Geo-Stationary system as well. Besides almost having no coverage at the north and south poles, one of the biggest problems in this system is performing voice and data communication over these satellites. Because they are far away from earth’s surface, there is very high latency or delay in the transmission and reception which makes it unfeasible for voice traffic. And also because of its large distance from earth’s surface, very high power antennas are required for these satellites which consumes a significant battery power. Moreover, sending a satellite into a Geo-stationary orbit from earth is very expensive as compared to other Low Earth Orbit satellites.
While with the Low Earth Orbit (LEO) satellite systems, transmission rates of about 2.4 kbps can carry out voice data communication efficiently. Because these satellites are orbiting the earth at much lower altitude as compared to Geo-Stationary satellites, transmission power required for the antennas is much less which in turn saves the overall battery power consumed. And for the same reason, the delay or latency in the transmission is much less which can compete with the wired cable terrestrial networks on earth. Higher elevation of these satellites to the equator enables are better coverage at the north and south poles. And also because the geographical area covered by one satellite in LEO system is smaller; frequency reuse policy can be best utilized. On the other hand, the very obvious and possibly the one of the biggest problem with this satellite system is the need to have a large number of satellites to provide global coverage. As the satellites in this system do not stay at a fixed position over the earth surface, their serving time geographical is only about 10 to 15 minutes which require very complex antenna mechanism on earth to keep track of the satellites. Another disadvantage of having low earth orbits is the very short lifetime of a satellite. Friction from earth’s surrounding atmosphere can severely effect the lifetime of a satellite in LEO satellite system. If a satellite mobile user is roaming around the world, providing global connectivity to that user also require interconnectivity of these large number of satellites.
Mobile Communication Satellite Systems are evolving very quickly in this era to keep pace with the increasing demand of communication globally. The trend is shifting towards the LEO satellite systems from the Geo-Stationary satellite systems because of very less latency delays in LEO systems as far as voice data communication is concerned. However, these satellite systems cannot fully replace the need of terrestrial networks because of these systems are still very expensive for an average user and not all countries of the world are developed enough to cope up with these satellites systems.
References:
[1] Handbook of Antennas in Wireless Communications, Chapter 2, Michael John Ryan, University of New South Wales
[2] Requirements for a Mobile Communications Satellite System. Volume 1: Executive summary, Final Report, 15 Dec. 1981 – 31 Mar. 1983, TRW, Inc., Redondo Beach, CA.
[3] The Use of Mobile Satellite Communications in Disaster Mitigation, Eugene I. Staffa, World Conference on Natural Disaster Reduction Technical Committee Session C
[4] The Past, Present and Future of Satellite Communications, John V. Evans
