A satellite is a device placed in the space over the earth as communication equipment that can be used for different purposes such as telecommunication, television broadcasting, global positioning system etc. It produces the space segment part of the satellite communication.
Sputnik was the first human made satellite placed by the Russians in the year 1957. Launched by the United States in 1958, score was the first satellite for communication function. In 1960, the first regular satellite communications service was used by the Navy. To bounce teletypewriter signals between Hawaii and Washington, D.C. the moon was used. During the early 1960s, as a medium for passing messages between ships at sea and shore stations the moon was used by the Navy. This method of communications are survivable, reliable, secure, and a cost effective method of tele-communications [8].
Figure-1.-Satellite communications system. [8]
World's first true communications satellite, called Telstar, was launched in 1962 by the AT&T - American telecommunications giant [9]. Since then, countless communications satellites have been placed into earth orbit, and the technology is forever growing in different aspects. Figure1 illustrates a satellite handling numerous combinations of links concurrently.
Basic Elements
The basic elements of Satellite communications can be divided into 2 main components:
The Satellite: This is the segment placed in the space. The satellite consists of 3 disconnect units, they are the fuel system, telemetry controls and the last one is the transponder. The transponder is responsible for the boost up the signals of the receivint antenna from the ground station. It uses an multiplexer input, a frequency converter and a broad-band receiver to route the signals from end to end a high powered amplifier for downlink. Reflecting the electronic signals is one of the most important tasks of a satellite. This signal reflection can be used as a telecom satellite whose job is to gather the signals from a one ground station and transmit this signal to other ground station. The distance between those receiving and transmitting ground stations should be coverage within range of each other. Satellites can also be used in television broadcasts, where the ground station's uplink is down-linked over a long distance of coverage so that it can be subscribed by many viewers. Another use for satellites is inspection, where the security is the main issue and watching and monitoring the cameras and transmitting the data for controlling and prevention of crime.
The Ground Station: This is the segment placed in the earth. It has two jobs to do. In uplink direction, the baseband signal is propagated, undergoing baseband processing, converter, high gain amplifier and finally the output dish to the satellite. On the other hand, the downlink same operation as uplink is done.
Communication Satellite Systems
A communication satellite [1] known as COMSAT is an artificial satellite stationed in space for the purpose of telecommunications. Variety of orbits including geostationary orbits, Molniya orbits, other elliptical orbits and low (polar and non-polar) earth orbits used by modern communications.
Communications satellites [1] provide a microwave radio relay technology corresponding to that of submarine communication cables for point-to-point services. They are also used for mobile applications like vehicles, communications to ships, plane and hand-held terminals, and for TV radio broadcasting, for which application of other technologies, such as cable, is impossible or impractical.
In addition to serving as navigation systems, mobile satellite systems assist connect remote regions, vehicles, ships, people and aircraft to other parts of the world and/or other mobile or stationary communications units.
A satellite telephone is a sort of mobile phone that connects orbiting satellites in lieu of terrestrial cell sites. It is sometimes called as satellite phone or satphone. Coverage may consist of the entire earth or only specific regions, depending on the structural design of a particular arrangement.
Mobile Satellite Communication System
A numerous mobile satellite communication system [2] is available for business use. Most of the systems which primarily based on Low Earth Orbit (LEO) constellations are planned to be launched in the near future. For the Mobile Satellite Systems (MSS) only a small number of services commenced until now and are therefore commercially available. Inmarsat C, Omnitracs/Euteltracs and Prodat are the MSS which provide messaging and data communication. Inmarsat M/B and Inmarsat A are the MSS which not only supporting data capabilities but also indispensable for voice communication.
Inmarsat C
First regarded for maritime applications whose fundamental service is store and forward messaging. As it was born as a satellite extension of terrestrial telex network all the terminals and earth station provide at least this access. Other connections have been implemented because of abandonment of telex service. It can be regarded as a classic paradigm of open network where a significant part of the services offered by the terrestrial networks are extended through a satellite connection, to the mobiles.
The currently available services comprise telex, fax, E-mail, X400, value added services such as GPS localization, data gathering from mobiles etc. Many manufacturers due to Inmarsat terminal policy recommend standard-C terminals in direct competition among them. As a result both the terminal size and its price have noticeably reduced. Tariffs have also been greatly cut following the enhance in the number of Land Earth Stations (LESs).
Omnitracs/Euteltracs
The most extensive MSS [2] in terms of number of terminals presently in operation is Omnitracs and its European version Euteltracs. About 100000 terminals have been sold in USA in 5 years of operation and about 6000 in Europe in 3 years. It provides regional coverage and organized as a closed network that means for the physical connection between the fixed user and Network Management Centre (NMC) the terrestrial public infrastructure is only used, which routes the message to/from the mobiles. Mobile0to-mobile communication is not allowed.
Omnitracs/Euteltracs, conceived for the road transport, provide store and forward messaging and localization. It is generally operated by non skilled people, without specific knowledge in this field for easiness of use. Terminal prices are lower than the direct satellite competitors, however, only one model is available.
Prodat
Developed by ESA Prodat is a new MSS, which has been made commercially available in the beginning of 1994. Like Inmarsat, Prodat is an open network, designed to be an extension of the public X.400 E-mail networks supporting the Message Handling System (MHS). Two different kind of terminals manufactured by FIAR (Italy) and PESA (Spain) offers by Prodat, at a competitive price with respect to the other similar systems. The basic Prodat service is store and forward messaging among mobile and fixed stations.
Inmarsat A
In 1982, Inmarsat Standard A launched commercial operation. In 1994 the number of terminals both for land mobile and maritime applications reached to 24.000. It allows voice, fax, e-mail, telex and data communication by means of the about 30 land Earth station. It is continuing its growth in spite of its old technology.
Inmarsat B/M
Inmarsat is the digital evolution of Inmarsat A, which allows about a 30 percent reduction in traffic charges, however, the type and size of terminal are still as same as Inmarsat A. This is considered one of the reasons for its slow take off. On the other side, Inmarsat M,
its low speed version is conquering the market which is launched in 1992.
Standard M is currently and for future, the only possibility for global, low cost voice communication due to its reduced size and weight and also terminals price.
LEO Satellite / Example of Mobile satellites communication system
Low-Earth-Orbit (LEO) [7] satellites have polar orbits, altitude is from 500 to 2000 km, with a rotation period of 1.5 hours to two hours and speed is between 20,000 and 25,000 km/h. Analogous to the cellular telephone system, an LEO system typically has a cellular type of access. The footprint in general has a thickness of 8000 km. Because of the closeness to the Earth, the round-trip time propagation delay for LEO satellite is basically less than 20ms, which is acceptable for audio communication.
An LEO system work collectively as a network and made of a constellation of satellites where each satellite act as a switch. Links between two satellite that are close to each other are connected through inter-satellite links (ISLs). A user mobile link (UML) is required for mobile system to communicate with the satellite. A satellite can also communicate with an Earth station (gateway) through a gateway link (GWL). The following figure demonstrates a typical LEO satellite network.
Figure-2.- LEO Satellite system.
LEO satellite can be divided into three categories: little LEOs, big LEOs, and broadband LEOs. The little LEOs operate less than 1 GHz and are generally used for low data rate messaging. The big LEOs operate from 1 GHz to 3 GHz. Iridium and Globalstar systems are examples of big LEOs. The broadband LEOs provide communication alike to fiber-optic networks. Teledesic was the first broadband LEO system.
Iridium System
The Iridium system [7] was started by Motorola in 1990 with a 77 - satellite network. To materialize the project took eight years. During this time the number of satellite was decreased. Finally, the service was launched with 66 satellites. The original name, Iridium, came from the name of the 77th chemical element; a more suitable name is Dysprosium (the name of element 66).
Iridium has gone through rough times. In 1999, due to financial problems the system was halted; it was sold and started again in 2001 under new ownership. The system has 66 satellites which divided into six orbits. Each orbit has 11 satellites. The orbits are at an altitude of 750 km. The satellites in each orbit are separated by approximately 32o of latitude from one another. Following figure shows a schematic diagram of the constellation.
Figure-3.- Iridium Satellite system.
Since each satellite has 48 spot beam, the system can have up to 3168 (48 * 66) beams, however, as the satellite come within reach of the pole some of the beams are turned off. At any moment the number of active spot beams is just about 2000. In the Iridium system, communication between two users takes place through satellites. When a user calls another user, the call can go through a number of satellites before getting the destination. This means that relaying is done in space and each satellite desires to be sophisticated as much as necessary to do relaying which eradicates the need for many terrestrial stations.
Globalstar
Another LEO satellite system similar to the Iridium system is Globalstar [7] which uses 48 satellites in six polar orbits with each orbit hosting eight satellites. The orbits are situated at an altitude of approximately 1400 lan.
The most important dissimilarity between Iridium system and Globalstar system is the relaying mechanism. In the Iridium system, communication between two remote users needs relaying between several satellites where as Globalstar communication needs both Earth stations and satellites, which means that ground stations can generate more powerful signals.
Teledesic
Teledesic [7] is a system of satellites that provides fiber-optic-like communication which has broadband channels, low error rate, and low delay. The most important purpose of Teledesic is to afford broadband Internet access for users allover the world. It is sometimes called "internet in the sky". In 1990, the project was started by Craig McCaw and Bill Gates; later; other investors joined the consortium. The project is scheduled to be fully functional in the near future. Constellation Teledesic provides 288 satellites in twelve polar orbits with each orbit hosting twenty four satellites. The orbits are at an altitude of 1350 lan , as shown in following figure.
Figure-4.- Teledisic Satellite system.
Mobile Satellite Channel for Communication[3]:
The propagation-link-margin is required for design the mobile satellite channel. It is provided in order to assist the appropriate quality of marginable fading conditions. This can be grouped into wideband and narrowband characterization.
Narrowband channel characterization is primarily aimed at establishing amplitude (and phase) variations of the signal transmitted through the channel. It is implicit in such characterization that all frequencies contained in a typical signal would experience 'flat fading'. A number of channel models have been proposed such as empirical and statistical/analytical type models.
Empirical channel models provide easy-to-use expressions to directly determine 'link margin' as a function of system level parameters of interest. Various available empirical channel models are:
Empirical Roadside Shadowing (ERS) Model. The empirical expression is given by
A(P,θ) = -M(θ). 1nP + N(θ) dB
where
M(θ) = a + bθ + cθ2
N(θ) = dθ+e
A is the required link margin, 0 is the elevation angle (degrees) and P is the percentage of the distance travelled over which the fade is exceeded. The expression is applicable for P in the range of 1-20% and 0 in the range of 20°-60°. The fit coefficients are :
a=3.44, b= 0.0975, c= -0.002, d= -0.0443 and e= 34.76
Modified ERS (MERS) Model. The basic form of the model is similar to ERS model. The MERS model is as follow:
F(Pr,θ) = -A(θ) e ln(Pr) + B(θ) dB
A and B are defined by the following equations
A(θ) = alθ2 + a2θ + a3 ,
B(θ) = b1θ2 + b2θ+ b3
Empirical Fading Model (EFM).EEM is given by:
M(P,θ, f)=a.ln(p)+C dB
where a and C are given by
a ( θ , f ) = 0.029θ - 0.182f - 6.315
c(θ,f)=-0.129θ+ 1.483f+ 21.374
Statistical channel models are very useful in the software (and hardware) simulations and comparative analysis of different modulation, access and coding schemes. These models are particularly useful for communication system simulations and provide a more 'comprehensive' understanding of the nature of a communication link.
Wideband measurements primarily provide information on the excess time delays of the echoes received at the terminal due to multipath propagation of the signal through the channel. Very little data is available on wideband characterization of either vehicular based or hand-held channels. At CSER planning for a wideband propagation campaign is at an advanced stage to investigate excess delay and other wideband parameters at L and S bands.
Much of the available information in this area is more applicable to vehicle-mounted channels and may not necessarily reflect the severity of handheld (personal) type channels.
Developments in Mobile satellite communications
The IC0 system [4] will be the first such system to use the frequency band allocated for the satellite component of FPLMTS. The IC0 system is designed to provide very cost effective voice, data and value added services to handheld terminals. The system when integrated with the ICO-Net will provide a set of bearer, teleservices and supplementary Services very similar to those provided by GSM.
The ICO system architechue comprises of space segment, ground segment, and control and management segments.
The Space Segment
A constellation of 10 Hughes built satellites in two intermediate circular orbits provide global coverage, with some users having the capability of seeing more than 2 satellites at any instant. The constellation k arranged as two planes of 5 satellites each plus one in-orbit spare per plane.
The payload consists of (a) a communications transponder with narrowband digital beam forming and digital channelization. A total of 4500 voice channels can be accommodated on one satellite (b) a High Power Notification - HF" transponder and (c} an Inter-SAN C-to-C band transponder for satellite resource management, synchronization and HPN service support functions.
The IC0 System and Associated Ground Segment
Access to the public networks is via a set of 12 optimally located Satellite Access Nodes (SANS). CO-located at the SAN sites is the Mobile Satellite Switching Center - MSSC and the associated databases for mobility Management roaming. A total of 5 tracking antennas" provide multiple satellite connectivity. At some of the SAN sites the RF and antenna portion of the SAN fulfils a dual role of providing communications and TT&C functionality required for satellite operations. Access to the 12 SANs find to public networks is via a service provider owned Gateway (in most cases an ISC or a GMSC). There are expected to be over 100 gateways providing global and national services and these connect to the 12 SANs. The architecture of the SAN is similar to a BSS found in cellular systems - the BTS equivalent is the Satellite Access System -
SAS comprising of Channel units and the BSC equivalent is the Satellite Processing System. SPS comprising of channels managers. A high speed internal LAN is used for carrying user, signaling and administrative traffic within the SAN.
IC0 system will meet the challenges for Personal Satellite Communications services and
in particular the following :
1. System design compatible with universal and regional service via a low power hand-held terminal
2. Speech quality comparable with digital cellular
3. Service with sufficient link margin in conjunction with diversity operation and seamless handoffs
4. Inherent mobility management functions, Interworking and integration with 2nd generation cellular systems allowing
5. Technically optimized space and ground segments enabling very competitive pricing structures for a variety of voice, data and value added services.
Global Mobile N-ISDN Satellite Communication System
In Global Mobile N-ISDN Satellite Communication System (GMISC) [5] - the ISDN basic rate up to 144 kbit/s (2B + D) with notebook size user terminals and vehicular terminals, offering by a GEO satellite communication system, to provide a comfortable roaming communication environment. In addition, it also talked about the most appropriate satellite channel control scheme.
GMISC System [6]
The GMISC services with the GEO architecture can be started with just a satellite covering a region including numerous countries needed to create a big LEO constellation. The most important satellite part needed to realize the GMISC system is a phased array antenna system which generating a great number of multibeams with side-lobe levels.
System Configuration
The GMISC is a regional satellite communication system consisting of communication satellite, user earth stations (UES), gateway earth station (GES) and a satellite switch control station (SSC). To cover several nations for service links, the satellite will provide more than one hundred spot beams. Spot beams are allocated to match up to each country's geographical size. To support many GESs multiple spot beams for feeder links are also presupposed.
Access Scheme
It is very essential to recognize small user earth stations, low channel cost and efficient frequency usage. To accomplish these goals, SCPC (Frequency Division Multiple Access - FDMA) is the most appropriate multiple access scheme for service links , start better than Time Division Multiple Access - TDMA and Code division Multiple Access - CDMA. TDM would be the most appropriate access scheme for feeder links, if it had an on-board base-band switching function. On the other hand, this would increase satellite complexity, hardware and cost. As a consequence, for feeder links TDM can not be employed. FDM is the right for feeder link access scheme, and satellite switched FDMA (SS - FDMA) will be employed for the on-board process, not base band switching.
Satellite Channel Control System [11]
A satellite channel control system is appropriate for the purpose of the GMISC systems. There have to be measured some channel control system: few are describe as follows.
Reduce of connection delay time (used for approximate the performance).
High utilization efficiency of satellite channels (used for approximate the performance).
Arrangement of plural feeder links.
Realization of direct connection between user earth stations.
Channel Control Functions:
There are mainly four types of functions for controlling channel. Those are call control, satellite channel control, on-board switch control and mobility control. Call control connects or disconnects the end terminals. Satellite channel control manages channels assign to every earth station. On-board switch control operates switching the spot beams and feeder links. The mobility control certifies and manages the position and responsible for the hand-over control.
Channel Control Scheme
Among various channel control scheme, centralized, dispersed and layered are mentionable. Centralized control scheme controls the whole system with a single controller. Dispersed control controls every partial systems with multiple controller and layered control consists of regional controllers which is a master controller and it monitored the local controllers. The centralized control scheme is appropriate for SSC. It controls whole satellite channels regardless of its location.
Future Satellites[11]
Now-a-days, satellites are constructed to help various S-band satellites. Some are using onboard controlling and some with highly complex antennas like multibeam. The ACeS is using unfurlable-antenna which is almost twelve metre diameter. The TDRSS are also using the same. On the other hand ith Iridium satellites use onboard controlling unit for demodulation the signals into TDD or FDMA and correct the signal strength. Those techniques were combinedly used to implement the hand satellite phones in the early times. For example, NASA has funded JPL for L-Band mobile systems (M-;-SAT X) and now researching on mobile and personal terminals[10].
Conclusion:
To provide a large geographical coverage and a long distance wireless communication, satellite communication is necessary. Mobile Communication Satellite System plays an important role in this regard. This kind of satellites characterizes one of the most important role in space technology. This systems allow us to use radio, television and telephone transmission to anywhere in the space and earth. This system is playing an escalating role in communication systems where the land communication is impossible and very costly and unsecure. This system also allows to provide transmission of P2P voice and data services.
